Foam Board Insulation R Values

By Todd Fratzel on Insulation

Foam Board Insulation Types

I’ve written several posts about how to insulation basement walls in which I promote the use of foam board insulation as the first line of defense against moisture and mold. Because of this I often get questions about which type of foam board insulation to use and what R values these products provide.

There are three basic foam insulation board products on the market produced under several different manufacturer names. The basic types of foam board insulation include: polystyrene, polyurethane or polyisocyanurate.

They include expanded polystyrene, extruded polystyrene,  and polyisocyanurate unfaced or foil faced. DOW products has lots of information on their site about different foam board products here. I also recommend you read a recent article about Open Cell Vs Closed Cell Foam products to understand the differences between the two product types.

Expanded Polystyrene Foam

Expanded PolystyreneExpanded polystyrene foam (EPS) is the cheapest and least used foam board product on the market. This product typically has an R value of 3.6 to 4.0 per inch of thickness. Expanded polystyrene insulation is similar to the foam used for packing “peanuts” and it’s typically used for insulated concrete forms also knows as ICF’s. It is also sometimes used on commercial buildings for roof and wall panel insulation which is typically sandwiched between light gauge metal.

Cost = Cheapest of the foam insulation boards.

Extruded Polystyrene Foam

Extruded Polystyrene InsulationExtruded polystyrene foam (XPS) also known as blue board or pink board comes in many different thicknesses and edge profiles. This insulation board is probably one of the most widely used foam board insulation products in the residential construction industry. XPS has an R value of 4.5 to 5.0 per inch of thickness.

This is the product that I typically use to insulate basement walls. It’s reasonably priced, light weight and easy to use. This product is also used to insulate the outside of foundation walls and even under slabs.

Cost = This product is the middle of the road for these types of foam board insulation products.

Polyisocyanurate and Polyurethane

Polyisocyanurate foil faced insulation.Polyisocyanurate also known as polyiso is seen in all kinds of commercial building applications and more recently with residential building projects. Polyiso is typically used with a foil facing and it has an R value of 7.0 to 8.0 per inch of thickness. The reflective foil facing makes it an excellent insulation board when radiant heat is involved. The foil facing also makes it very easy to seal with good quality foil faced tapes.

Cost = Polyiso is the most expensive of the foam board insulation products however it’s the highest R value.

Polyurethane and polyisocyanurate are both closed-cell foams. They contain low-conductivity gases in the cells (usually one of the HCFC or CFC gases.) The higher R-Values (R 7.0 to 8.0) are the result of thermal resistance of the gases in the cells. This can lead to a couple of disadvantages including: off gassing of HCFC or CFC gases, and reduced R Value over time as the gas escapes.

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

All posts by Todd »

Not what you're looking for?

Search for more articles here. Enter keywords like, 'insulation' or 'kitchens' etc to find your topic.


  1. Wells says:

    Also Polyiso is the least moisture resistant. Not only is it subject to breakdown from extended exposure, it also loses significant insulation value when wet.

    Use expanded or extruded in any potentially damp locations.

    • John Stevens says:

      Mr. Wells is incorrect; the foil facing makes the Polyiso the MOST moisture-resistant of the products discussed.

      • Chris S says:

        Andy Engel from FHB did an article on this and EPS is the way to go. It allows misture to move back and forth through the board but protects 2×4’s from contact with damp concrete. Of course, you should eliminate any leaks first but vapor WILL pass through concrete no matter what you do – therefore – let it breathe in order to avoid rot.

        • Todd says:

          Chris – I respectfully disagree. The problem is it will hold water like a sponge. So it’s no different than using open cell foam and I’ve seen disastrous problems with open cell foam. I’m of the same opinion as the folks from Building Science who also steer clear of EPS foam.

          • Fred says:

            I have to agree with Todd on this one. The EPS products are one of the culprits in the synthetic stucco debacle. I have personally seen attic condensation on EPS that produced visible water flow and serious problems. Wherever EPS touches Wood you will eventually have mold, mildew, and rot.

            As to ISO and water absorption, that will depend on the type of ISO. Open cell will absorb water like a sponge and soft open cell ISO is used to make sponges. Closed Cell ISO is inherently the most moisture resistant.

            External applications should always use Closed Cell ISO. It is the most water resistant of the three but there is one caveat. External application of ISO must be protected from UV as UV radiation will cause it to break down. Hold a thin piece of ISO up to the light and think about all those cell walls breaking down from UV damage. Now you have open cell foam.

            What about a coating that says it is UV resistant? It depends on whether or not it means the product will weather UV or block UV. Two very different things. Look for coatings that are both UV “Blocking” and Resistant. The Foil is a blocking on the faces only but a silicone coat may not be UV blocking at all.

  2. All good info. Add to that the ability to purchase recycled/once-used rigid foam insulation boards of all types and sizes and you have a eco friendly solution.



  3. Robert Rein says:

    I am going to build a new home on the Brad’Or Lake in Cape Breton Island Nova scotia. It will be a two story with a furnished walkout basement (built on a hill facing the water). The lower walls will be poured 8 inch reenforced concrete. On the inner side I intend to use one inch of rigid foamboard then 2″X 4″ 16 in OC framed wall. Fiberglass batton insulation and 1/2 inch gypsumboard. My question is, is this a good idea, what type of rigid board should I use? My understanding is that the polyiso foamboard does not hold up to any type of moisture but has a higher r factor per inch than the extruded but the extruded holds up better to moisture. If the polyiso is foil faced does that protect it against moisture and act as a vapor barrier. And in either case what is the best to use for a vapor barrier and where??

    • Todd says:

      @ Robert Rein – Building a house that far north I would opt for a minimum of 2 inches of DOW extruded tongue and grove (other brands are fine as well), joints taped and sealed. Then go ahead and frame with 2×4 with R11 fiberglass insulation. No need for a vapor barrier if you install 2 inches of foam and seal all the joints really well. Best of luck!

      • Angelo says:

        Hi Todd,

        Your suggestions as very helpful re foam board insulation.

        Here’s my question: I’ve added about 12ft by 10 to a garage extension in northern NY and want to heat it so I can work in it during the cold winter months. The roof line is lower than the garage roof. I have 7/16 sheathing on a slight slope roof and want to insulate it with 2″ foam board – and then button it up with an additional 7/16 sheathing on top to “sandwich” the form board. I then want to shingle the top sheathing so it matches the existing garage roof.

        Would this work? Any specific suggestions?



        • Todd says:

          In theory it should work. I have a couple thoughts.

          1. 7/16″ plywood isn’t really sufficient on a roof in a snowy climate. In this situation you could argue that it’s ok.
          2. 2 inches of foam on a roof isn’t nearly enough in my opinion.

      • Bill says:

        Hello , i would recommend an Air Barrier anywhere but floors , and preferably closest to living space as possible , lets not confuse Air barrier with vapor barrier … We need to protect the structure from all the humidity generated inside a home …ie each molecule of air in a home normally has 45% to 55% water .

    • Paul says:

      @ Robert Rein:

      I’m presently insulationg my basement with 2″ foil-faced polyisocyanurate glued directly to the blocks. Then I’ll screw resilient channel through the rigid, to the foundation wall using 3″ tapcons. Then drywall. 2″= R12, minimum.

      • Ryan says:

        I’m curious how this project turned out. Its completely wrong and will most likely result in wavy walls and moisture trapped in between the concrete and the polyiso. don’t do what this guy did.

        • Todd says:

          Ryan – Trapped moisture behind the polyiso isn’t of concern in my opinion. It doesn’t matter if you spray foam, rigid foam, etc., you’ll trap moisture behind the foam. What I don’t like is the foil faced against concrete. The aluminum facing will react with the concrete and deteriorate.

          • Tammie says:

            Hi We are insulating and attic to make it another bedroom.Can we use 1″ isoboard and leave a 1″ gap between the second sheet isoboard?
            Thank you

          • Todd says:

            Not sure I understand your question. An attic that’s turned into a room typically needs quite a bit of insulation. Gaps are fine with foam insulation.

          • tammie says:

            We were wondering if we could layer 2 1″ sheets of polyiso to get a better r value.The insulation has foil on both sides.
            Thank you again

          • Todd says:

            Sure…but that’s only about R14….not really up to today’s minimum insulation standards.

          • Tammie says:

            So if you left a gap between the 2 sheets of isoboard would that give you a higher r value we were told it would then we read that there would be condensation between the 2 sheets.This is my last question.Thank you :)

          • Todd says:

            That really isn’t going to increase the R value. It does potentially create an area for condensation as well.

    • John Stevens says:

      Yes, the foil is moisture-resistant, but according to Dow, none of their rigid insulation is a barrier for water vapor.

      • Todd says:

        John – It’s proven that closed cell foam is actually a good vapor barrier when it’s installed at a minimum thickness of 1.5 to 2 inches.

    • Bryan Freeman says:

      Why not use ICF foundation construction. Far superior method than insulating from the inside and trying to seal out moisture.
      This method of construction is becoming more popular as the price of heat continues to climb. Very cost effective over the long haul. Don’t look at the inital cost of construction without doing the math on the energy savings ove time.

      • Todd says:

        It’s certainly a very good option. ICF does have some minor issues that need to be considered. You must cover the foam for fire protection from the very beginning unlike concrete where you can wait until you want to finish the basement. ICF is much harder to deal with electrical and plumbing (although certainly not impossible). And obviously there are cost issues. Certainly worth looking into though.

        • Kevin says:

          While ICF’s certainly have the issue of having to protect from sun and elements, I am not sure I agree regarding electrical and plumbing.

          On a conventional wall, you would typically stud it out and run your electrical and plumbing. You can do the same thing with ICFs, but you do not have to run conduit because the wiring is not exposed to concrete. You will not need to insulate the cavity either. So, the electrical in a studded cavity along an ICF wall is less expensive than along a conventional poured or block wall.

          If you choose, and this is where it can become more work, you can embed plastic conduit into the ICF wall while constructing. If you do it this way, I agree with Todd, because you are going to incur a higher cost of labor. But you gain square footage of living area because their is not any studded wall cavity. I would not recommend running plumbing this way though. This method will kill you on costs if you are not doing it yourself, as you may need to have the electrician working with the ICF installers.

      • Grits says:

        Personally, I would do what I am going to do when I build my next house. Use a thermo mass foundation – four inches of rigid foam sandwiched between two 4″ reinforced concrete walls. I don’t want to worry about off-gassing. The basement, if I finish it will be framed with 2×4 with the cavities filled with Roxsul. Vapor will move as it will.

        In this house there is 4″ of rigid foam on the OUTSIDE and the interior walls of the 8″ reinforced CIP walls are coated with two coats of Ultra DryLok followed by two coats of SW highly reflective paint. The floor is painted two coats SW concrete paint. Nice and dry and almost totally dust free. It is also warm.

  4. Debbie says:

    With not much room to build a 2×4 wall frame and insulate along the side of our basement stairs, we have opted for polyiso sheets with foil and plastic face. Now on the inside of the wall, which face (the plastic or the foil), goes against the block wall and do I still need a vapour barrier between it and the drywall?

    • Todd says:

      @ Debbie – I would place the plastic side against the concrete. No need for vapor barrier if you seal all the joints well. For foil faced polyiso i suggest you get a good quality foil tape (used for duct work…..but NOT duct tape).

  5. PeterL says:

    As long as you’re building new and have the luxury of doing it right the FIRST time, why not put the insulation, and the waterproofing on the OUTSIDE of the concrete?

    You get the advantage of the thermal mass inside, to moderate temperature changes, and it doesn’t cost you any more.

    Also, instead of conventional insulation, do superinsulation and cut your heating bill by 80%- you can save most of the cost of the insulation by installing a TINY (and much less expensive) heating system, and pay $300 a year to heat instead of $3000 (most of your heating would come from waste heat inside the building). A savings of $2400/year would pay for about $48,000 greater expense in construction.

    • Todd says:

      @ PeterL – Insulating the outside is certainly one option. However, it’s not as simple as that. The big issue with insulating outside is the detail where the foundation comes above grade. Making the transition from below grade to above grade and transitioning to the interior insulation without a break it difficult to do properly. Furthermore, many people in the industry agree that exterior insulation still doesn’t’ address the severe moisture problem that exists throughout the life of concrete products.

      Your point is well taken on spending up front to save in the long run. We’ve built several homes in the last two years that use less than 50% of the energy that previous homes use. Thanks for your input.

      • Tim says:

        Todd – I’m in the planning stages of building an earth bermed single story home. The basis for mine is the Earthwood home built by Rob Roy in New York. He’s had great success building (to all intents and purposes) underground homes with external extruded foam insulation, and also using bituthene membrane applied directly to the outside of his surface bonded concrete block walls. Of course, he also has the luxury of building from scratch, and he puts great emphasis on drainage rather than waterproofing, but his house was built in the mid 80s and is still dry. I intended making the transition from below grade to above grade (the south facing walls/windows will be exposed to maximize passive solar heating) by staying with exterior insulation and going with a natural or manufactured stone veneer. Any thoughts?

        • Todd says:

          Sounds like an interesting project. What will be the “structure” above grade? Below grade I assume these homes rely on the earth as the major structural component but you will not have that once you’re above grade.

  6. charlie says:

    I am planning to insulate a large basement, over 200 ft of outside block wall. It is damp in basement but no water problems.There are mildew problems, especially in summer. We run two dehumidifiers constantly, which uses alot of electricity. The floor is concrete. I am wondering how much good it will do to put two inches of extruded or iso sheet on exterior walls, if I don’t cover floor with anything.

  7. Mike says:

    I’m going to use the XPS on basement walls, but i’m not sure what to do about the space above the concrete walls between the floor joists. Currently batt insulation is placed there. Do I need to do something special in this space? I don’t want to spend the time and money on the XPS only to have the moisture barrier benefits negated because I didn’t address this space. Thanks!

    • Todd says:

      @ Mike – I recommend you replace the fiberglass with XPS. You can cut pieces to fit between the joists. Use spray can foam (Great Stuff or similar) to seal each piece in place.

  8. frank dicarlo says:

    todd….We are in need of any of your advice. We have a walkout basement, three framed walls,the other wall concrete. the framed walls have a paperfaced R 19.the basement is unfinished and has no drywall.The slab was poured over 2″ foamboard and plastic on top as a vapor barrier. In the winter, the framed walls have moisture and black mold behind the insulation. the dehumidifiers keep the basement @ 30% humidity. There is also a pellet stove which keeps the 1200 sq. basement @ 70 degrees. The exterior consists of vynil siding and Tvec. My husband dug the cellar hole,he stated, the ground is pervious, with no ground water. Any help or advice you could lend would be greatly appreciated. Thank you, Dawn Dicarlo

    • Todd says:

      @ Frank – From the sounds of it the kraft faced insulation was not properly sealed and moisture has been trapped between the fiberglass and sheathing. If the basement will not be finished I recommend a poly vapor barrier, properly taped and sealed. At this point it sounds like you need to remove the damaged fiberglass, clean the mold, and start over.

  9. tnt says:

    We live in the northern minnesota. The house I worked on has plaster walls, Kraft-faced R-ll insulation, shiplap exterior sheathing, and 3 layers of tar paper. We put 1 inch foil-faced polyiso insulation on the exterior of the house to get the most insulation for the price. With no poly vapor barrier, is this installation correct?

  10. Brent says:

    I live in Alaska and I am having a new build done this summer. The winters have gotten down to -70 in the past. I want the best insulation available. The slab will be radiant heated. I’ve seen previous posts about foam insulation on the outside of the concrete slab, how beneficial is it? What is the best possible insulation for the exterior wall? I’ve heard R-5 blue board on exterior of stick frame walls with vapor barrier and R-11 fiberglass on the inside. What would you do if it was your place and you were looking forward to that extreme of cold?

    • Todd says:

      @ Brent – I’d look at two options.

      1. Spray foam entire house inside stud wall cavities, I might even consider a 2×8 wall section for maximum insulation depth.
      2. Spray applied dense pack cellulose in the wall and a layer (1″ to 2″) of blue board on the exterior of the house.

      If I lived in Alaska I’d spend every dollar I could on insulation versus other nice to have things like nice countertops, etc.

      As far as the slab goes you’ll want to install foam board insulation on the outside of your slab/walls, down at least 4 to 6 feet.

  11. John says:

    Greetings, I’d like to get your advice on insulating my basement. We live in MA in a middle townhouse with a walkout basement. One wall is completely below grade, two side walls are shared with neighbor’s basement and back wall is completely above grade studded with fg/vapor barrier/door/windows. Area is approx 800 square feet. The concrete walls are poured. I’d like to:

    – glue 1″ XPS to all concrete walls, then stud
    – NOT use bat insulation on shared walls, only front below grade wall/possibly back above grade wall (unfaced)
    – lay 1″ XPS over slab, then carpet

    Questions: Should I:
    – Remove insulation/vapor barrier from above grade wall and use XPS
    – Fill in studs on shared walls w/ bat unfaced insulation
    – DryLoc walls before insulating. Is this necessary for the walls above grade/shared walls?

    We plan to tie into HVAC for heat and air return and run dehumidifier when necessary. I did the moisture test with the taped foil on the walls, no moisture on either side.

    Thank you, John

    • Todd says:

      @ John –

      1. I would leave the old grade wall insulation. However, check it and make sure there is no moisture or mold in it.
      2. Just foam is fine on those walls..however…i recommend 1-1/2″ min. foam board.
      3. DyLoc is just added protection…not sure how much it buys you in the grand scheme of things.

    • Grits says:

      I would generally agree with you with these exceptions. If you have a shared studded basement wall, I would be sure the insulation is Roxsul (mineral wool) which does not burn or retain moisture. On the concrete walls, I would use the Ultra DryLok. Yes, it is extra protection but it also controls the smell of bare concrete and the dust.

  12. James says:

    Can I use 2″ of foamboard (Polyiso) under my metal roof. foil facing up?

  13. David says:

    Hi. I building a new home in Ontario and will be installing hydronics in the basement floor. I will install 2″ of rigid foam insulation under the slab. Question is which rigid foam insulation is best “Pink or Blue”? I’ve noticed “pink” carries a premium price to the blue. Both have the same R value and the same compressive strength. Is there something I’m missing? Thanks D.

  14. Christine says:

    I live in Geneva, Switzerland. Temperatures range between 25 and 35 in the winter. I am insolating my basement but the Swiss tend to over-engineer their walls so I want to provide some guidance to my wall-guy. I don’t have a lot of space and would like to forgo framing and batting. Is it possible to use 2″ Extruded Polystyrene, attach furring strips and then gypson board? By screwing the furring strips through the blue board into the cement, does that not let in the moisture that seems to be the trouble with basements?

    Thank you in advance.

    • Todd says:

      Christine – That method actually works quite well. You want to be sure you’re using galvanized or stainless steel screws and be sure the screw heads are sealed well afterwards before the gypsom board.

  15. Todd, your article says polyiso is more expensive than XPS but is higher R-value per inch. Is polyiso also more expensive on a per R-value basis? OR is cost-per-R-value close enough that one or the other product may be less expensive per R-value in any given market?

    I currently only specify polyiso when the insulation is exposed, as in an unfinished basement or closed crawlspace. The foil face satisfies code officials. Unfortunately, I never get to see the costs.

    • Todd says:

      David – Good question, frankly I’ve never compared cost per inch. Next time I purchase some I’ll have to run those numbers. Good point!

  16. Jacob says:

    I’m currently finishing out my basement. I’m going to use 1 1/2″ extruded polystyrene on the concrete walls and frame and drywall against them. What tape is recommended to seal the joints? I read somewhere about tyvek tape, but I’ve never seen that in my area. Also, I’m considering drywalling the ceilings, mainly due to costs of drop ceilings. Are drop ceilings a better idea to encourage air flow? Will a drywall ceiling create a basement that is too airtight?

  17. Frank says:

    We have a two-thirds finished basement, and have stripped the finished side down to the studs. Currently there is no insulation nor vapor barriers. We would like to add some insulation to the finished side to help normalize the basement temperature to the rest of the house.

    1. Does it make sense to insulate the finished side, if we are leaving the unfinished side alone?

    2. Can we put foam board between the studs, since we don’t want to remove the studs and install the boards against the concrete walls?
    2a. Would this configuration still require some type of vapor barrier?

    We live in Ohio, if that helps, and have had no moisture problems in past.

    • Todd says:

      Frank – Thanks for stopping by the site.

      1. It certainly doesn’t hurt! You’ll probably find that the rooms above the finished/insulated area will feel more comfortable afterwards.
      2. This isn’t the best option but it’s the best one if you don’t want to move the framing. Be sure to seal all the edges with spray foam from a can.
      2a. If you install at least 1-1/2 inches of closed cell foam I think you’ll be ok without one.

      Good luck.

  18. Joe says:

    Thanks for this forum and the great information! I will implement the basement wall according to your Oct 12th post. Love it!

    But I have a foam board question for NON-basement walls.
    Is there any advantage with using EPS insulation for “non Basement” framed wall?
    Say 4 inches of solid EPS insulation stacked in a wall, then sealed with can foam (prior calking any sills and opening to prevent any air). I see no need for a vapor barrier Kraft paper. I haven’t priced it out – but since it’s “solid” though more work than then spray foam – if sealed well, it would achieve the same result – right?

    As closed spray foam installers say – batt is really just a thick furnace filter, walls have convection, and even if the R values appear the same – Solid (spray) foam is warmer in use.

    I’m building at 11,000 feet in Colorado. 2×6 construction and have the building weather tight but framing exposed inside.
    (I should have done 2×4, saved money and did foam board layer on the outside to prevent “thermal bridging” from the wall studs)
    But we are ready to start the cedar siding and the builder isn’t keen on adding layer(s) under the siding since the windows are in, tyvek is done, …) – he’s worried about moisture getting behind. So I’m pondering anything to beef up the walls
    But I have very low moisture in Colorado. R21 is recommended for walls.
    I likely will get the ceiling closed cell spray foamed but likely not spray foam the walls two story, plus a drive under garage is a lot of walls but not much roof.
    Plus, spray foam is pricy and at the High Altitude the formulation gets even more costly (supposedly) since the microscopic air bubbles could pop.

    I noticed one post above mentioned three layers of tar paper.
    Could that be for R value – just for another cheap layer. Does it really help? My builder would be fine with tar paper layers.

    Thanks in advance!

    • Todd says:

      Joe – Using foam inside the cavities is a fine idea. However, I have a feeling it may end up costing as much as spray applied when you consider the labor to cut all the foam board, install it, seal it etc.

      Not sure tar paper is going do to much of anything for you in this situation.

  19. Mark says:

    Hi Todd

    Here is my dilemma…My finished basement got flooded…12 inches up…I cut up 12 inches of drywall & removed insulation w/vapor…I am going to go up to 4 ft because (Drywall is 4 x 8…would make it easier to replace…unless you think otherwise)… My mancave was damp to begin with which I believe is totally do to the foundation walls (totally in ground) not having any Foam xps board attached

    I do not want to totally remove all drywall & studs…I will remove 4 ft of drywall from floor…This will probably be very difficult to fully tape foam boards together…but I will try my best…because I am sure XPS on the Foundation walls is better than none…Correct.

    also insulation in between my studs R15 I can use a non vapor backing if I foam board my foundation walls correct?

    And lastly I am installing new carpet…will the carpet pad be enough or should put down a thin foam board on the floor?

    Any other ideas would be great

    Thank you

    • Todd says:

      Mark – First off let me say I wouldn’t go through all this trouble until you figure out why your basement flooded. If it happened once it’s likely to happen again. 12 inches of water is in my opinion a basement that shouldn’t be finished.

      Having said that I also don’t think your approach will work very well. Unless you remove it all and create a good vapor barrier you’re doing to just push the wall moisture up higher and eventually into the remaining fiberglass.

      Have you investigated the flooding situation?

      • Mark says:

        Hi Todd

        Yeah, my sump pump failed in the 2010 Greatest New England Flood of all time…Just my luck…I never had water before…

        So the bottom line…I need to make sure I have a good vapor barrier…I plan on using foam board all the way up on my foundation walls some areas of basement I can make this happen easily…some other areas it will be tight but if I cut half my drywall out I may be able to reach it…How about putting foam board on my floor…or should I just use carpet pad & rug?

        Thank you

  20. Eric says:

    We are going to reside with vinyl siding on an older home with 2×6 framing that has fiberglass bating insulation inside. To add insulation it seems our only option is the “blue board” installed under the siding? If so should it be house wrapped too? Thanks for any ideas. eric

    • Todd says:

      Eric – The foam board should be installed first with all the joints taped. Then a house wrap should be installed over the foam prior to the new siding.

  21. Geo says:

    I live in Texas, the sun hits my converted garage and kitchen very hard, garage was insulated. I would like to know if I can put the polyisocyanurate over my outside wall which has stucco on it to reflect the sun rays and will it last?

    • Todd says:

      Geo – I wouldn’t really recommend putting polyiso on the exterior and leaving it exposed. It really should be covered by a siding product.

  22. Stan says:

    I just found your site, and like the posts. I am removing my Alaskan house’s beveled cedar siding to install rigid foam insulation on the 2×4 wall’s exterior, prior to reinstalling the siding. My concern(s) are that I do not want to add furring strips for solid siding attachment, as I want to secure the siding directly through the rigid foam. I also am concerned about totally sealing off the foam, as I think it may be in my interests to allow water vapor to move from inside to out. I cannot vouch for the interior water vapor seal quality under the sheetrock. My home has been “breathing” for 40 years without mold in the walls,” so I’d better keep letting it do so.

    What do you suggest as a maximum thickness for the foam, for use without furring strips? Should I really seal all foam sheet seams? I was even thinking of using house wrap on top of the foam just to provide the extra barrier for water moving into the wall. Also, what type of fasteners should I use for the foam and the siding? I admit to being new to this part of construction, and I’ve heard a different bit of advice from each person I ask.

    Thank you in advance for your time.


    • Todd says:

      Stan – You have lots of options and some of the options depend on the type of siding you’ll be installing. First of all I would strip the old siding, fix and damaged sheathing and then install a layer of housewrap. Next I would install a layer of rigid foam board (XPS would work fine). The thickness will depend mostly on how it affects trim details at doors and windows. Depending on the thickness you can use just nails to re-install the siding or you’ll need to use a foam product that accommodates strapping (like DOW Wallmate). However, a product like that will really only help with horizontal siding products.

      Does this help?

      • Stan says:

        I only mentioned once that I would reinstall the beveled cedar siding, so I do have a horizontal siding application. I’ll look at the strapping. The cedar siding was installed over celotex, which was directly attached to the studs. I’m putting OSB ontop to the celotex, except for one wall, which had the celotex removed. The XPS will be attached to the studs through the OSB. You recommend house wrap under the XPS. I was thinking over the XPS, but on second and third thought, your idea makes more sense.

        Window and door trim will be dealt with by using 3/4 ply boxes inside the window RO’s, and custom furring trim on the few doors, for finish trim nailing I’m leaning towards 1″ XPS, while finding some type of fastener that will give me the stud penetration depth I would get without the foam. I can understand your inability to safely recommend details in “print” from far away.

        Thanks for your time. I appreciate your thoughts on my problem.

        • Matt says:

          I’d be interested to know how it worked out. I’m in this same boat as you (were), and the options seem limited. From what I’ve read, if one applies thin insulation (<=1") in climates like yours (or mine, in VT), then it encourages moisture to form at the interface between that and the siding or sheathing beneath it. Add to that the foam board being a vapor barrier, and the breathing of the existing walls to the exterior is impaired. Finally, housewraps are miraculous, but not miracles. They pass water vapor both ways, and they trap water droplets on either side EXTREMELY well. So, moisture that forms on the inside of a housewrap will be trapped as effectively as the housewrap excludes water penetration from the outside.

          I've been looking/reading for days and hitting many, many, MANY blogs and advice sites trying to find a workable solution to my project that doesn't involve slapping 2 inches of rigid foam over the existing sheathing or siding.

          We've got 2×6 studs w/an average of 5+ inches of fiberglass or blown in insulation (min R-19 when built). Plywood sheathing, tightly shiplapped 3/4" vertical cedar siding. House is tight and well insulated. But, the siding needs replacing and we're doing the windows while at it. So, we're looking at options to claw back some of the insulation lost to the siding while avoiding too much of a moisture or water vapor barrier.

          There don't seem to be any ready solutions. If anyone has been through this and found one (for a cold climate home) I'd love to hear how it turned out for them!

          • Todd says:

            Matt – You are certainly facing a situation that many home owners face. It’s hard to get the insulation value you want in old walls without causing lots of other issues. If you’re replacing the siding and windows, then you have an advantage over some homeowners. You can “absorb” most of the 2″ exterior foam by pushing the new windows out further and using larger extension jambs. That’s how I’d probably approach your situation.

  23. Kyle says:

    Hi I’m looking to insulation my crawl space because my wood floors are cold in winter. What type should I use? I have a sump pump down there because sometimes have water.

    • Todd says:

      Kyle – Without seeing the space it’s hard to recommend a system. Typically crawl spaces benefit nicely from foil faced polyiso foam board.

  24. Dana Hanson says:

    I am reading the post’s and they are great! Here is my question. We have the foil backed insulation board on the bottom half of our walls in the basements. We have recently put in batting type insulation over the foil backed insulation board. We were recently told that we should remove the batting insulation as it will produce moisture between the two insulations. Is this true? What are your recomendations. (I hope this make sense)

    Thank you in advance for your advice!

    • Todd says:

      Dana – Thanks for the compliment.

      The batts could be a problem depending on the details. First off how thick is the foam board? Are all the seams taped? If the foam goes from floor to ceiling and it’s properly sealed then it creates a great vapor barrier and it shouldn’t be a problem. Of course that depends on it’s thickness too.

  25. Laura says:

    Todd, we are just starting to build a new house in Michigan. I asked my builder to add 2″ of rigid foam boards to the exterior before adding brick and cement board siding and he panicked. (We have to have brick on the full front and partial sides/back.) He’s never done it before and he is concerned about several things:

    1- how does it affect the foundation? He wonders if he’ll have to move the foundation wall out to accomodate another 2″ in addition to my planned 2×6 walls. (we have to have 2×6 on large side walls). of course, I need a brick ledge large enough to set brick on, so do we just move the foundation wall out or does it have to actually be thicker for my proposed 2X8 wall?
    2- and he’s worried about the soffits and interfering with their depth/overhang.
    3- I figured we could save money by eliminating the OSO sheathing but that worries him, too. He doesn’t know if it will pass code. Doesn’t know how to go about engineering it without sheathing.
    4- He is worried if we do or don’t need a house wrap as I also suggested that we could eliminate that if we use the right foam.
    5 – windows/ doors. He said he doesn’t know how to flash them with foam.

    I have read your site and have investigated a little on my own but wonder if you can answer these basic questions and/or provide us with a few resources to find good solutions to build it right.

    He asked me why I can’t ‘flash and fill’ inside my 2×6 wall cavity with spray foam and cellulose. From what I’ve read, and correct me if I’m wrong, here in Michigan with cold winters, I need enough spray foam to keep the dew point to the outside of the foam or risk condensation inside of my cellulose where it would get trapped. So, a little foam actually can do more harm than no foam. Exterior foam of 2″ will keep the dew point to the outside of the foam so the cellulose will stay drier and it will provide a thermal break and wind barrier over the whole wall, including the sill joists. Am I correct? Can I use both methods? ie: 1″ spray foam on interior and 1″ rigid foam on exterior? That may be more manageable for him…

    Thanks, Laura

    • Todd says:

      Laura – Thanks for stopping by this site. Your builder probably panicked as this is such a new concept to him. All of his concerns are valid, however, they area all manageable with proper details.

      1. Typical foundation around here is 8″ minimum concrete wall. So you can actually install the pressure treated sill plate to the inside of the wall and frame above that with standard framing, 2×6, 1/2″ sheathing. Then you can install the 2″ of foam board which will bring the foam to the outside edge of the concrete wall. You could actually use a 2×8 sill, so the foam sits on the sill with a piece of flashing above the sill to keep water from migrating under the foam and back into the house. All of the joints in the foam should be sealed well.

      2. By using step 1 above the face of soffit can still be in the original position.

      3. I would not omit the sheathing.

      4. If you tape it well the house wrap is probably over kill.

      5. All this does to windows and doors is require 2″ deeper extension jambs which shouldn’t be a problem.

      Installing exterior insulation is MUCH better than interior because it’s continuous. Spraying the cavities with and inch or so of foam could possibly work if you can get sufficient R value to prevent the dew point problem, but it’s hard to control. I think you’re on the right track.

  26. Michele Knapp says:

    thanks for sharing your knowledge on this subject. I am a contractor preparing to renovate a 1965 house with homasote sheathing. Plan is to leave the homasote add 1 1/2 foam board, 1/2 osb or cdx and cement fiberboard clapboard siding. This will bring me to a 6 9/16 jam for the new windows. Dealing with the 2″ overhang on the foundation is under discussion. We want to avoid air and insect penetration. Proposed are metal flashing (sort of a reverse drip edge and covering the foundatiion with a manufactured stone product or using a a Weatherwatch ice/watershield and then applying an azek cornerboard as a water table. your thoughts?

    • Todd says:

      Michele – Can you put an extension on the sill plate, then trim the bottom of the extension with a PVC trim board after caulking the joints? That would probably work pretty well.

      • Michele Knapp says:

        yes we would need to do the extension as a nailer for the corner board idea as well. Any preference on Dow vs. Owens Corning vs Celotex?

        • Todd says:

          Michele – Not really, the real issue is being sure it’s closed cell foam.

          • Mark Langdon says:

            These are great comments. Here’s my situation. I have an old 1920’s home with a brick exterior and plaster interior. I am renovating it and just took down the plaster and lathe. Now all I have left is the back side of the brick with 1X3 furring strips nailed on the inside 16″ OC so I only have 3/4″ or so due to the furring strips. I plan to install sheetrock attached to the furring strips. It never had any insulation. What product do I use to get some kind of wall insulation ? Do I have to stud this out further somehow to get the thickness I need or be happy with some sort of foam insulation panel (Which type do you recommend) that gets me only a R-5 or so wall and will get this passed the building inspector and this is a permitted project. I don’t want to use spray insulation contractor and would like to do it myself. Any help would be great !!

          • Todd says:

            Mark – Well it really does depend on where you live and what Energy Code if any is in effect. I’d start off by checking with your local building inspector to find out what the minimum (if any) required insulation value is. After that you can come up with a plan of attack. Most likely you’ll want to install a layer of foam board over the furring strips prior to installing the drywall.

  27. mike says:

    todd I live in upstate ny I am putting a 2nd floor on my house 2×6 walls can I use foam board along with 6in faced fiberglass to get higher R value and is it better to install foam board on the interior or exterior or both and what kind of foam board do i use

    • Todd says:

      Mike – Sounds like a big project. Couple of thoughts about your situation. First off are you looking to do all this work yourself or with contractors? For new construction (including an addition) I would consider having a contractor come in and fill the wall cavities with blown in dense pack cellulose insulation. I REALLY like this product for wood framed walls because it does an amazing job of air sealing and you can achieve a full R21.

      If you’re looking to do all the work yourself then you’ve got a couple options. I guess I would lead towards applying the foam on the outside of the house. If you’re going to do that you need to pay special attention to all the details, depth of window and door jambs, etc. You should use a closed cell foam specially designed to be installed under siding. You’ll probably want to hold the framing back from the outside of the existing house by the thickness of the foam so that the siding all lines up.

  28. Kim Wisdom says:

    Hello Todd. Your website is fantastic!! I read through all these posts and didn’t find my exact situation, so thought I’d post.

    I live in Jacksonville, FL and due to age and water damage, I had to replace the bottom sill plate on my 1940’s garage apartment. To do this, I had to remove the shiplap tongue and groove siding. Doing this showed me that there was no insulation and I’d like to add it. Can foam board insulation be used instead of batts of fiberglass?

    The lap siding was installed directly to the 2×4 studs with no sheathing. Can I use the foam board cut to fit between the studs for insulation and then re-install the lap siding directly to the studs? I could also use the spray foam (Great Stuff comes to mind) to seal the gaps between the foam and the studs. I would assume a layer of Tyvek house wrap should also be installed before the siding is replaced.

    I am willing to remove and replace all the siding if I have to but if you have a better idea on how to insulate it without removing the siding, I’d love to hear it.

    Thanks so much for any insight you can provide.

    • Todd says:

      Kim – Foam board insulation can be used anywhere fiberglass can be used. I assume you’ve got access from the inside? Another solution would be installing foam directly over the old siding then installing new siding. This creates great insulation value but it does require some special details at doors and windows to address the added thickness. Good luck!

      • Kim Wisdom says:

        Thanks for the reply. We do not have access from the inside without tearing out sheetrock. And since we had to remove siding to repair the damaged sill (and also termite damage) I thought we would just insulate from the outside rather than tear out sheetrock on the inside. Sheetrock may be cheaper, but it is a lot quicker and easier to just remove all the siding rather than try to get behind tubs and kitchen counters and, and, and….. :) I don’t want to double up due to the issue you mentioned regarding doors/windows.

        You didn’t comment on whether Tyvek would be needed over the insulation under the new siding. Is that required?

        Thanks again!

  29. brad roth says:

    can blue foam board insulation be used under a metal roof – I have an old cabin that has no insulation in the roof just metal roof on tongue and grove pine and need a new roof and thought this might be a way to gain some insulation

  30. Chris says:

    I started building a room in the basement. Part of the basement is above ground and was insulated with fiberglass. We moved back some of the insulation and found mold! In the process of removing and cleaning with vinegar.

    So would thick xps work well with the areas that are above ground as well? Is this stuff a fire hazard if not covered with drywall? I was hoping to leave the above ground walls open in the unfinished area of the basement for easier insect and mold inspection.

    Will a hepa vacuum filter work for cleaning up fiberglass particles after I’m done removing this garbage?

    thanks! been asking everywhere but not getting a good response.

    PS also bought a dehumidifier after finding mold.

    • Todd says:

      Chris – Foam will work well there as well. Frankly foam board will be no more of a fire hazard than fiberglass. Most building inspectors would definitely require that you cover it, but lots of houses don’t. Good luck.

  31. Jazz says:


    My contractor is installing what looks like polyiso panels 1.5 inch R10, behind new vinyl siding on 2nd floor exterior wall.

    I asked about Tyvek wrap and he said not necessary, but can they really seal that well with tape

    Wouldn’t it be better to put Tyvek over the foam insulation to protect the foam from water and stop airflow? He said Tyvek is expensive but I think I saw 1000 ft2 roll for $100, so maybe its labor intensive install.

    Anyhow, not sure I like the idea of the siding acting as the only rain and wind membrane.

    Any thoughts?

    • Todd says:

      Jazz – First off the foam can be sealed very well with the right type of tape. There are many tapes on the market now that stick very well..just be sure it’s not duct tape! Secondly there are lots of folks out there that argue about Tyvek (or house wrap) over foam board insulation under siding. I will say that if you think it’s worth the cost then it’s certainly not going to hurt anything. I say make your decision based on sleeping well at night.

      • Jazz says:

        Thanks Todd,

        1. So the Tyvek plus the tape sealed foam would not inhibit the wall’s ability to breathe.

        2. Another quick question about siding selection. The contractor is offering Procanna siding for the color I want whioh seems to be OEM’ed from an Ontario siding manufacturer, thickness is .044 and 25 year fade resistance. They don’t have any detailed spec sheet touting any kind of advanced technology or features.

        3. Are there big differences still in vinyl siding from different manufacturers?

        Is .044 thick enough

        If I could choose any brand/thickness of traditional double
        panels, what are the top brands based on reputations and advanced features?


        • Todd says:

          Jazz – Frankly you want to keep the water away from the framing. If you don’t tape those seems water will get in. Vinyl siding is only a facade and it’s far from a water barrier.

          In my opinion there is a huge difference in siding products. We’ve tried dozens of them and my preference is Certainteed. In fact, my first preference is Certainteed’s Monogram line that has a 0.046″ panel thickness. That extra bit really makes a big difference in performance especially in climates with large temperature swings like here in the North East.

          • Jazz says:

            Hi Todd, I could also choose ABTCo timber creek premium plus .046 thickness rather than the OEM .044 which we chose only because the green was slightly more appealing but the more I research Procanna the more I have doubts I think it is contractor grade, no info available, they won’t tell me who makes their panels.

            What do you think? IS ABTCO good?


          • Todd says:

            Jazz – I’ve never heard of them but doesn’t mean the product is bad. At least it’s a thicker panel.

          • Jazz says:

            ABTCO is KP Building products, Home Depot carries it here.


      • Jazz says:

        Jazz says:
        July 11, 2010 at 10:53 pm

        ABTCO is KP Building products, Home Depot carries it here.

        Do you know this manufacturer?

        thanks again

        • Todd says:

          Jazz – Haven’t seen them before but that doesn’t mean much. We purchase all of our building products from two large “contractor” supply companies and mostly deal with two or three large, well known siding manufactures.

  32. Pete says:

    I live in New Jersey and I’m currently finishing/renovating my basement and have several questions:
    1. Any recommendations on how to insulate around (or behind) an existing electrical panel and wall mtd water heater? Do you actually try to get the insulation behind these items, or do you just get as tight as possible to them? Obviously, it’s a large task to get behind them.
    2. When completed, my basement will have a “finished” area (that will serve as a kids play room & home gym)and an unfinished area (that will be a utility closet and tool area). The finished areas will be conditioned, and the unfinished areas will not be conditioned. In your opinion, can I get away with only insulating the “finished” portions, or would you recommend insulating all of the exterior walls? If I only did the finished portion, I was planning on insulating (w/vapor barrier)the interior dividing partitions between the finished and unfinished spaces.
    3. One of my foundation walls is directly below a “family room” that is a “slab-on-grade” construction. I’d like to install a thinner insulation along this wall due to this wall will encroach on an already tight stairwell. Any thoughts on going with a 3/4 or smaller board for this section of wall? It is somewhat insulated from the outside since there is a slab on grade installed at the top of the wall.
    4. For the detail at the top of the wall and into the joist cavities, do you run a horizontal piece of insulation between the top of the wall and bottom of the joist cavity so that it’s continuous, or do you just run the insulation to the top of the foundation wall, put the pieces in the joist cavity, and leave a gap?
    5. With the foam board insulations, I was planning on installing 3/4″ furring strips on top of the insualtion board, and then fastening the sheetrock into the furring strips. Is there any problem with notching the insulation around electrical boxes, or would you recommend deeper strips (possibly even a full 2×4) so that the electrical boxes don’t cut into the insulation?
    6. As far as adhering the insulation, do you just use construction adhesive to install the foam boards, and if so, how do you hold them tight to the foundation wall?
    7. When you reference “Energy Code”, is there any website that shows the requirements for different areas?
    Thanks for your time.

    • Todd says:

      Pete –

      1. In an existing situation most people would opt to insulate up to these type of utilities. Relocating or moving them would be a pretty costly task.
      2. I would recommend insulating all of the exterior walls. Your home will be more comfortable all around and it’s easier to deal with vapor at that point.
      3. In situations like this you’ve got to weight the pro’s and con’s. Using 3/4″ foam will most likely not provide a very good vapor barrier. However, that wall will likely not get very cold so if water vapor from the interior space hits the foam it may not condensate. I say do the best you can with what you’ve got.
      4. You want things to be as continuous as possible. Typically we’ll run up to the top of the wall, then install a horizontal piece above that, then insulate the rim joist, then tape all the seams or seal with spray foam in a can.
      5. I would just use 2×4’s or if code allows you may be able to use shallow boxes (if they will fit in the 1-1/4″ space you have. I’d try to avoid notching the foam.
      6. We have recently been using DOW GREAT STUFF PRO it works exceptionally well at adhering the foam to the wall quickly. Sometimes you do need to apply some temporary shoring but not often with this product.
      7. Every state has different rules regarding energy codes. Here in NH where I live it’s governed by the Public Utilities Commission (PUC). I would call your local building officials and inquire about it.

      Good luck.

  33. Earnie says:

    Todd, What a neat website!! I’m building a home close to Houston Tx and have 2×4 walls, wrapped 100% with 1/2″ sheathing and then with 3/4″ R Max insulation boards (R-5). I’m also planning to install house wrap before putting on 3×10 log siding. While I plan to use bat insulatin in the wall cavities, is there anything else I can or should do to improve the insulation effectiveness?

    I am also installing knotty pine T&G boards on the interior walls and was planning to use sheetrock as a backer on the inside to keep down any dust or airflow. Would it be better to use an insulation board and if so, what kind would you recommend?

    I doing this myself, with the help of my lovely bride, and could use any help you might provide.


    • Todd says:

      Earnie – Hot climates like TX are a different animal compared to here in the north. I would recommend checking out they have great information on all types of insulation. You just need to be careful with where there vapor barriers are when it’s warm on the outside and cool inside (air conditioning).

  34. Brian says:

    I have an enclosed(aluminum) car hauler. There is about 2 inches between the roof and the bottom of the supports. I would like to insulate the roof with foam board and hold it up with some thin wood board/screws. Which would be the best kind of foam board to get and proper thickness ? Foil faced or not ? Which way should the foil face ?

    • Todd says:

      Brian – is it curved? If it’s curved most of these products will be hard to curve unless you install it in several layers. Most of the foil faced products are covered with foil on both sides.

      • Brian says:

        The roof is arced yes. I would say it is arced about 2ft with a length of 8.5ft. I am thinking about using 3/4″ polystyrene foam board that has foil on one side. That would leave about 1.5″ of clearance between the foam board and the metal roof.

  35. Mary says:

    Great site! I am planning to insulate the walls of my Illinois basement with the 2″ XPS. I’ve heard that it should be 1-3 inches off the floor to catch any condensation from behind the insulation boards. Is this true? In the past water has seeped in thru the joint (cove?) between wall and floor. Can this be sealed? How? Digging up the floor and installing drain tiles is not an option. Thanks for your advice.

    • Todd says:

      Mary – Before insulating you really should address the water issues. Interior French Drains are a great way to intercept and remove water from basements. Typically that’s a job best left to professionals.

      If and once you that then you can bring the insulation right down to the top of the drain, any water behind the foam will drop down into the drain. It’s VERY important that all the seams in the foam be sealed. We’ve got several articles on the site showing how to do that.

      Good luck.

  36. Frank says:

    Hi Todd
    Great website and very helpful…I have a slight variation in insulating my basement interior walls.
    I was hoping you could provide some expert advise.

    I had to frame out my basement walls 8″ away from the concrete walls on all sides since the house has alot of conduit and pipes. I have installed metal studs 16″ OC and the basement is moist not wet. A dehumidifier keeps the basement dry. This 8″ gap also allows me to run a fan behind the studs for good air cirulation.

    Question: Can I install Extruded or polyisocyanuarate between the studs for my insulation needs then cover it up with 1/2″ sheetrock.
    I’m concerned about the combustible nature of these products since the interior part of this product will be exposed (between concrete wall and metal studs)

    Note: This unfinished basement without any insulation in the coldest season (NY) maintains an average temperature of 67 degrees, so I’m not that concerned

    Thanks for the advise


    • Todd says:

      Frank – I think I’d install a layer of foil faced polyiso OVER the metal studs then attach the drywall. I don’t have the specs handy but you could check online to see what the flame spread rating for foil faced is. I have a feeling it’s not bad at all. I wouldn’t put the foam between studs because you won’t have any thermal break.

  37. Brad says:

    Lots of good ideas!

    I’m refurbishing a 1950’s finished basement in Seattle. It had 1/4″ plywood paneling over 3/4 furing strips on 24″ centers. I’ve removed the paneling, and was planning on gluing 3/4″ foam to the concrete between the furing strips, then another 3/4″ layer over it all. Then screw 1/2″ drywall over it all to the furing strips.

    Does that sound like a reasonable plan? There’s no moisture problem, no mold found after removing the old paneling.

  38. Rich says:


    Thanks for this informative site. The company I work for bought a large commercial building. There are many 68″x68″ openings for wall exhaust fans. My boss wants me to design some insulated covers that we can install in the winter (the previous occupants just stapled up blue tarps and they said it was still freezing inside). My boss was thinking about making steel boxes and using foam board inside. The outside temperature regularly gets into the teens and single digits during the winter. We will use a good diameter rubber tube seal between the boxes and the exterior walls. The boxes will be bolted down to compress the seal. The boxes will be stored indoors when not in use.

    1. What R value of insulation do you think is right?
    2. Since the boxes will be metal, does it matter what direction any liners may face?
    3. Do you think open side of the box (foam side) should have a cover?
    4. Would it help to have air gaps between the box’s exterior sides and the foam?
    5. Do you see any problems with this idea?

    Thanks for any advice you can offer (this is not really in my bailiwick).

    • Todd says:

      Rich – Sounds like an interesting project. Here’s a few thoughts/questions.

      – Will the box be installed on the outside or inside of the building?
      – Do you know how much wall insulation the existing building has? Most commercial buildings have an R19 in the wall.
      – Assuming the cover goes on the outside, I’m thinking you could install 3 inches of foil faced polyiso insulation in the “box”. I would only have steel on the outside and sides, leave the side facing the building with just exposed foil faced insulation.
      – Remember that steel itself transmits cold very well, so the gasket will be key.

      Make sense?

      • Rich says:


        Thanks for fast reply!

        The boxes will be on the outside.

        I don’t know about the insulation in existing walls, but I can probably find out. The outside walls are a decorative type masonry block.

        Yes, metal tranmits cold very well, so that’s why I was wondering about having the foam actually touch the metal or not. I was thinking a little air gap would be good – ?

        Hmmm, we may want to use a treated wood, or probably plastic, frame around the face of the box to mount the gasket. The insulation could extend into this volume. Does that sound good?

        ps. My boss would love for these things to last a lifetime.

        • Todd says:

          Rich – I think you’re on the right track. Making them slightly larger, having the foam extend slightly deeper than the box, and a gasket, should take care of it.

  39. Rich says:

    Thanks – I think I have a plan.

    But … Thinking about it, there are four large rollup garage doors that I don’t think have much insulation. Three inches of polyiso may be a little overkill compared to three inches of blue/pink board. Well, I guess I’ll just price it out and see how the boss wants to go. Maybe we’ll see about insulating the garage doors too. In any case, this will be a lot better than blue tarps!

    • Todd says:

      Rich – I suggested the foil faced as a bit of safety insurance. Foil faced insulation should have a much better flame spread number than just foam board.

  40. jeol says:

    Great site!
    Simple question I can find nowhere on the net. I have a old dirt basement/crawlspace in my old farmhouse that is vented well. The house floor is also un-insulated so we get a lot of moisture in the house after very rainy spells. Time has come to insulate the underside of the floor and I wanted to go with spray closed cell foam for its vapor barrier and insulating qualities….Plus it will hide some ugly bits of framing, as this house was built out of what appears to be scrap wood from an earlier home.

    To save money I was wondering if I could just tack 2″ foam board between each floor joist and spray the seams, joints and hard to get areas with the spray. The DIY spray foam kits run about $1.50 sf. for the foam, so this would be a huge savings if it will achieve the same effect. Is this ok to do and is a specific type of foam board more compatible with the spray foams?


    Joel L.

    Franklin, NC

    • Todd says:

      Jeol – Not a bad idea if you can’t afford a complete spray foam job. For this application I would recommend at least 2 inches of foam. Before installing the foam board though I would get some samples and test the spray foam on it to be sure there’s no reaction that would damage the foam board. Under a crawl space I would prefer to use foil faced polyiso myself. Good luck.

  41. cw says:


    I live in Madison, WI and I am converting a screened-in porch into a room. There is a concrete slab and the roof is held up by three 6×6 cedar posts. Because of the depths of the posts from the edge of the slab, I have framed the bottom section of the wall with 2x8s. That way the siding goes over the edge of the slab. Then, so that I can leave the posts (which are pretty and expensive) exposed, I have framed the top half of the wall (containing the windows) with 2x4s. The ceiling rafters will be furred out to 12.” And to allow for heating ducts, the floor is raised about 14′ on 2×8″ joists.

    So what I’m looking for is the best and most cost effective way to insulate this monstrosity.

    I was thinking of putting 6″ of foam board on the slab (the inspectors want r-30) and densepack cellulose in the ceiling. Does this make sense?

    Then I was thinking of putting one inch ridged foam board over the outside of the slab and the lower 2×8 wall. Do I need house wrap for this? OVer or under the foam board?

    But my big question is the walls. The upper 2×4 walls have lots of small cavities in the framing betweeen and above the window. Can you put dense pack cellulose in those? Or would spray foam be better?

    What do you think?


    • Todd says:

      cw – Thanks for visiting the site.

      The 6 inches of foam board would work fine on the slab.

      The 1 inch foam on the outside of the 2×8 is ok but you’ll need more R value (planning on more insulation in the wall cavity?).
      House wrap is a good idea even with foam on the outside of the house. Over the foam.

      Dense pack fiberglass or cellulose will work great in those wall.

      • cw says:

        Thanks for your help. The one inch on the outside is to insulate the studs, break the thermal transferance or whatever you call it. THere will be some kind of insullation inside the studs.

        So they can blow dense pack cellulose into small cavities, say 1 1/2″ wide? Would that be better than spray in foam?

        Thanks again for your help.

        • Todd says:

          cw – dense pack can blow into voids that small. when the voids get down to like 1/2 inch then typically foam those while still dense packing the rest.

  42. mongi says:


    Roof on Steel deck in Ottawa.

    1- Is there a specific vapour barrier that goes under polyiso?
    2- How to fix the polyiso on the vapour barrier? do we need to reach the deck?
    3- I need one or two sheets of polyiso to get the 4 inches.
    4- Can I install torch applied membrane on the insulation, is there any primer?
    5- can isoply be tapered?


  43. Will says:

    I live in Michigan. I installed 1 inch DOW XPS tongue and groove. I caulked and sealed the seams with tape. Then built stud 2 x 4 walls. Reading the forums, I realize I should have probably went with 1 1/2 foam. Anyway, should I go with faced or unfaced fiberglass insulation? If building inspector requires the faced would that be an issue with double vapor barrier. Thanks

    • Todd says:

      Will – If it were mine, considering the stage you’re at, I would just fit additional foam between the studs and be done with it. It’s really too risky to go with fiberglass with only an inch of foam.

  44. Brian L says:


    I scrolled through all the above stories first but cant seem to find one that fits my situation. I live in northeast Georgia (summer hits 95 some times and winter goes down to freezing temps at times) and I am looking to finish my basement (aprox. 1500 sqft) which consist of three cement walls and one outdoor exposed wall. The exposed wall is already finished but the cement walls are not even close yet. The walls are already sealed and has no noticeable moisture issues. My concerns with the project is what will make “code” vs. what is recommended. My question is…

    After the walls are up w/the foam board between the boards and the cement is it still required to apply the standard insulation between the wall’s individual 2x4s? I understand that it will certainly help with the temperature control in the rooms and I know it is recommended for that point alone but, is it against building codes to not install it as well prior to drywalling?

    • Todd says:

      Brian – In many parts of the Country there are now energy codes in place that govern the level of insulation in homes. I’m not really sure if you’re area in Georgia has this type of regulation. Having said that, I would imagine that down there the insulation value that might be required for a below grade basement would be substantially lower than it is up this way. Also, with your high humidity I’d be inclined to avoid the fiberglass. If it were me I’d go with 1-1/2″ of foam board and call it good unless there’s a more specific code issue.

  45. Maurice says:

    Great site, I wish I found it sooner as I am midway through my project.

    I’m insulating the attic on my 1910 craftsman in Seattle. The rafters are 2X4 spaced 24″ apart. There is not a lot of headroom, and since it will be livable space, I don’t want to bring the ceiling down too far. I drilled 2 1/8″ holes in the soffits and installed a ridge vent on the peak of my roof. I installed 2″ foil faced poly panels between the rafters and kept the remaining 1.5 inches as the air channel between the soffits and ridge vent. In an effort to increase the R-value and decrease potential drywall sagging, I ran 1.5″ furring strips across the rafters spaced at 16″ and filled the cavity with 1″ foil faced poly.

    1. Was there a better way to approach this without sacrificing more headroom?

    2. Should the 2″ poly and 1″ poly be sandwiched together, or is it better to have an air cavity between them? (any risks with condensation/mold between them?)

    3. I cut each panels to fit snuggly between rafters or furring strips however there are gaps although they are smaller than the plastic nozzle ox expanding foam cans. Do I need to seal these gaps and if so, how? is there take wide enough to span over the rafters?

    4. what would estimate the R-value to be on this?

    Thanks in advance for your help.

    • Todd says:

      Maurice – for the most part I think your approach is pretty good. Basically you have areas of 3″ foam, 1″ foam and 2″ foam depending on where you calculate it. In areas with 3 inches you’ll have about R21 and the 1″ areas will be about R7. I guess one slight improvement would have been installing the 1″ directly over the rafters then the strapping bu that would result in another inch lost for head room.

      Small gaps are hard to deal with. You could tape at each rafter and overlap two pieces to completely seal it. Other than that I think you’ve done a pretty good job.

  46. Kris says:

    Can You spray foam over XPS foam Board?

  47. donna says:

    We are looking to insulate our pole barn to knock off some of the cold air for winter use. Would you recommend insulation boards? I read that one of the concerns is with bugs that will want to burrow into the boards as this will also effect the insulating values plus not to mention the fact of housing for lots of bugs.
    Thank you for sharing your vast knowledge on insulation.

    • Todd says:

      Donna – I think some insulation products will be attractive to bugs. However, I haven’t seen much trouble with XPS or Polyiso insulation…not sure bugs would want to eat that!

      • Peter M says:

        Like Todd, I originally thought that bugs would hesitate to chomp on polyisocyanurate. But experience suggests otherwise! Having added polyiso between studs in an admittedly inadequate exterior gable wall (the studs were 2x4s, sideways) I was intrigued a few months later to discover a pile of white foam bits at the back of a closet. Closer investigation revealed carpenter ants emerging directly above and dropping off their cargo, chewed out from the inside of this insulation. They clearly weren’t eating it (there must have been something much tastier in that wall), but they certainly chewed it. For what it’s worth, that wall and the problems it caused have now been eliminated.

  48. rRob says says:

    I have a project similar to “maurice from seattle” with an attic space I am finishing, and want to retain as much ceiling height as possible but I have 2×6 rafters.Can I sandwich two layers of foam board together to increase my R-value bewtween the rafters, I will leave an air space between the roof of 2-3in. Also I thought to cover over the rafters with 1/2in foam board to make it all air tight, and then install my drywall,thanks.

  49. garry says:

    I too have similar insulation questions as above with the foam boards. i have been told that I may not want to use the foam on the ceiling or rafters of a similar room due to condensation would build up between the drywall and foam and cause problems? I will only be able to put in about 5 inches of insulation and wanted to up the R value by adding 1″ of foam over the rafters then putting the drywall up.

    • Todd says:

      Gary – I’m not sure that’s much of a concern. In order for that to happen moisture would have to pass through the drywall and paint. Most latex paints actually create a pretty good vapor barrier. I say use the foam! You’ll have a much better insulated ceiling than not using it.

  50. Anna says:

    I’m thinking of applying 1″ foil backed foam boards to my rafters to radiate the heat back out in the summer. I’m thinking that it might not be necessary to do the north side or the northeast gable due to the small amount of sun exposure they get and the fact that none of this will be air tight. Does anyone have a reason this wouldn’t be a good idea?

    • Todd says:

      Anna – Are you doing this in an unfinished attic? Frankly I’d do it everywhere as the mid-day sun is strongest when the sun is directly overhead.

      • Anna says:

        Yes, it’s unfinished. I have heard so many conflicting things about insulation that I’m ready to do nothing! I was gung ho on spray foam until a friend was concerned about overheating the shingles. Then I read that shingle color is more important than attic ventilation.

        Can a foil really reflect the heat enough that it stays behind the rigid foam and exits the ridge vent? If I go with this idea, should I do anything to my gable wall?

        • Todd says:

          Anna – You’re confused because there are many different solutions and approaches. If you install the foam on the bottom of the rafters then air can still move from down low up to the ridge. Do you have a ridge vent? Do you have soffit vents? If yes to both this approach will work well.

          I’m not so sure you’ll cook the shingles any more than they already get cooked.

          Obviously if you insulate the gables it will be better than not.

  51. Alison says:

    I have a question about Blue Board insulation in a party wall. What is the NCR rating in such a situation? Are there ways to improve the noise reduction? Does condensation occur between the properties? What if there is a bathroom on one side of the party wall?

    • Todd says:

      Alison – Can you be more specific about your questions? NCR rating? Noise is best handled with independent framing. Moisture should not be a problem with party walls. Bathrooms should not matter either.

  52. bal says:

    Hello there,

    My bedroom over the garage is very cold. Normally it is 5-6 degree below the home temperature ie. if I set up the thermostat at 74, my room’s temperature is at 68-69. It has big window which is covered by the blind shade. I get the early morning sun. In summer this room is boiling and in winter it is frigid cold. My garage walls are insulated and so the roof by the builder. My question is if I insulate the garage roof with foam board, will it help to make the overhead bedroom warm? If so which one would be the best type (eps, xps or the foil facing?. Appreciate your answer. Thanks Bal

    • Todd says:

      Bal – Living space over garages are notoriously difficult to heat and cool. Are you planning on removing the ceiling to install additional insulation? If so then I would use 2 inches of foil faced poly-iso over the bottom of the rafters so you can maintain ventilation. It would also help quite a bit if you can insulation the floor below.

      • Pat says:

        Can you put foam insulation in the garage ceiling over drywall to gain better heat? We already have rolled insulation with paper backing in between the rafters, then drywalled. If so, what would you recommend? It is over a bedroom that is on the North-East side of the house.

        • Todd says:

          Pat – You certainly could do that but you need to check on the fire rating of that assembly. It’s likely that you’d need to cover it again with drywall. I’d use a foil faced polyiso then another layer of drywall (size per fire code).

  53. Pat says:

    can i put blue board over drywall in the garage ceiling to help insult it, without putting another layer of drywall over it.


    • Todd says:

      Pat – I’m fairly certain most building officials would frown on that idea. While you still have a fire barrier behind it you now have a pretty flammable material exposed in the garage….I’d drywall over that.

      • Tariq Hussain says:

        I am in a similar situation where the rooms above garage become freezing cold.
        Adding another layer of drywall over foamboard, would mean taking out the existing garage opener, garage door railing and everything. Thaen there would be a problem of how to secure the second layer of drywall through the foamboard. That seems to be a lot of work.
        Is there a better, easier and affordable way to insulate the celing ? are there any fire retardant rigid foam in the market that we can just nail/glue/fasten to the ceilings to add extra insulation?

        • Todd says:

          Tariq – It may be possible to use a foil faced foam board. However, as I suggested earlier you’d have to get an opinion from your local building official if it’s ok. The real danger is whether fire would ignite it, you’ve presumably already got a proper fire wall/ceiling assembly.

          The other thing is to focus on the ceiling in the space above as far more energy is lost through the ceiling compared to the floor.

  54. Dylan says:

    I only have 3.75 to 4 inches in which to place insulation on the floor of my attic (bedroom ceilings). I was thinking of pulling up the old “pink” insulation and laying down some polyisocyanurate board between my abnormally short joists. I was thinking of polyiso because you get the most R-value for a limited space and we live in the Philly area so it does get cold. My concerns are with fire hazard, building code, and general effectiveness. This is our 1st home & it is 100 years old. Do you think the polyiso will work? I was going to get some of Dow’s Super Tuff R polyiso board and cut it into 2 foot wide strips to fit between my attic floor boards. Do you have any feedback?

    Thank you,

    • Todd says:

      Dylan – Can you install it over the fiberglass or are there floor boards over the existing fiberglass? Installing the foam board in the spaces sounds easy but it’s a real pain around all the electrical and mechanical equipment. It would be far easier if you install a layer of foam over the existing plus you get more insulating value that way. As far as the fire code it really depends on the use of the space and local codes. Quite a few of the foil faced products have very favorable flame spread certification which should allow it’s use in that space. If the space has floor boards and minimal storage you might want to use foam board over the floor as it’s pretty tough stuff.

    • Timm says:

      Dylan, You’re MUCH better off blowing additional loose-fill fiberglass over your existing insulation. Preferably R49 or above. It’s much more cost-effective and efficient for attic floors. If you’re able to, find a contractor that uses CertainTeed Insulsafe or some other white colored blown fiberglass. Stay away from the pink or yellow for health and indoor air quality reasons. I hope this helps.

  55. gerald says:

    I have and old house with double walls built out of lath which was covered by plaster. I am thinking of using foam in between the walls. You know the type that comes in those spray cans. I wonder if that spray in foam is equal to extruded polystyrene in r value an permeability of water. Also if I use 4 by 8 polystyrene compared to the foam which is more economical?

    • Todd says:

      Gerald – I’m not sure how far you’d get trying to use spray foam from a can. First of all it would be cost prohibitive and secondly it would be hard to get consistent results. 4×8 sheets of XPS foam will be FAR cheaper.

  56. Peter Brozek, AIA says:

    1- Per BSAF, 1#/cu.ft. eps has an r value of 3.85 AT 75 degrees F. Important for people living in the north, such as Minnesota, is that the r value at 0 degrees F is 4.55. The R Value goes up as the temperature drops.
    2- The exact opposite is true for polyiso. Also it is much more expensive. The Iso starts, per the NRCA, at 5.8 at 75 degrees F and drops to 5.0 st 0 degrees F.
    3- The polystyrene is recyclable and the Iso is not.

  57. jASON says:

    Nice site. I recently encountered a problem with foam board and the batt insulation on a below grade block wall. I had 1 inch pink board and glued it to the block wall taped it all off then put R-19 batt insulation with plastic stappled it up for a vapor barrior. All this was done in the fall and two days ago I had to access a drain pipe while doing some plumbing and the batt insualtion on the backside (what was touching the pink board) was soaking wet. I took all the batt down and the pink board looked as though it was sprayed with a garden hose. WE live in northern michigan so it does get cool it is a new home built last year… What should I do? Any help would be appriciated. THanks

    • Todd says:

      Jason – Well you’ve got a couple issues happening. I’ve written several articles on this situation that will provide some insight (see list below).

      Here’s the short of it.

      – One inch of foam board isn’t sufficient for two reasons. 1 – You need a minimum of 1-1/2 inches (2″ preferably) of foam closed cell XPS to create a vapor barrier. That vapor barrier stops moisture from the foundation concrete from getting into the framed wall assembly.
      – One inch of foam board only provides a minimal insulating value. So, the cold block wall can transfer that cool temperature through the foam. If any damp air gets into the wall cavity it now has the chance of that vapor turning into liquid condensate. At that point it gets trapped in there because of the poly you installed.
      – You’ve in essence created a double vapor barrier that wants to trap water in the stud wall.

      So the solution really needs to involve more foam and avoiding the plastic outer barrier. You may want to consider insulating it with just foam board to really eliminate the problem all together.

      For more information see:

      Good luck.

  58. Tom says:

    I hae just started the process of conerting an area of my unfinished basement into an office/den. I am trying to lessen the noise of people walking around on the hardwood floors upstairs while in the basement and was thinking that gluing some foam board insulation in the areas between the joists and then using Great Stuff to seal the cracks along the edges might do something to help.
    Afterward I plan on doing drywall.
    What do you think of this? Good idea or waste of time/money?
    If it might help, what thinkness foam board would you recommend?
    Also, I plan on putting recessed lights in the ceiling so they would be in pretty close proximity to the foam — I know you can get the “IC” lights, but is this still a fire hazard?
    Any adice you could give would be appreciated.


    • Todd says:

      Tom – I’m not sure that method will work very well and here’s why.

      – Noise is really the transmission of vibrations. If you install stiff foam board and glue it to the joists I think you’re creating a very nice “drum”. In essence I think you might amplify the sound.

      – If it were me I’d do this: Install fiberglass insulation in the cavities. Then purchase some resilient channel to screw to the bottom of the joists. Only ONE leg of the channel gets screwed to the joists, the other leg is left loose. Then you screw sheetrock to that. The channel makes for a VERY good sound deadening detail.

      Good luck.

      • Tom says:

        ok thanks Todd — that makes sense.
        Would you recommend faced or unfaced fiberglass insulation?

        • Todd says:

          Tom – It’s such a toss up. This hybrid approach is a “give and take” situation. It really depends on the basement. If it’s super dry, humidity controlled, I’d use faced as it’s easier to install. If there’s any question about moisture I’d skip the faced.

  59. skip says:

    I purchased a steel enclosed trailer and want to add insulation to the inside walls and roofs. I am thinking aboutblue or pink foam board. Any suggestions?

  60. Jackie says:

    I have a bedroom wall that is exposed to the attic area above my garage. I planned on just insulating with standard faced fiberglass batts. However air still will work its way through the fiberglass and to the bedroom wall. Could I add 1″ foam board on the 2×4 frame(essentially on top of fiberglass) simply to ensure that less air is hitting the fiberglass insulation? Should I be concerned with creating a double vapor barrier by doing this or is the unfaced 1″ foam board sufficient?

    • Todd says:

      Jackie – I’m having a hard time understanding exactly what you want to do.

      Is the wall a wall between a finished room and attic space? Do you have access to both sides of the wall? If so I think I understand the situation.

      You could do the following:

      From the attic side, install the foam board, tape all the seams very well.
      Fill the stud bays with unfaced fiberglass.
      No vapor barrier.

      • Jackie says:

        Sorry for the confusion…yes, wall between finished room and attic but only have access from attic side. Sounds like I was on the right path…I’ll fill with fiberglass first then cover with foam board (taped) Thanks!

  61. mike says:

    we have a 55 year old house, brick..our bedroom is in one corner of the house and the walls are FREEZING in cold weather (no insulation to speak of I guess) it possible to put a foam board (which one?) on the inside walls? If not, any ideas? thanks

    • Todd says:

      Mike – Have you cut a whole in the wall to see what the wall section looks like? Before proceeding I would do that. Is it a row of brick, air space, sheathing, framing, plaster? The more you know about the wall section the better of an insulation assembly you can use.

  62. Evelyn says:

    Like Skip (December 27, 2010 at 6:25 pm) I have also purchased a steel beam, enclosed cargo trailer. I’ll be using it as an RV. It will be replacing my motorhome, because I can’t fit my two 12vdc chest freezers into the motorhome.
    The trailer is smaller than I wanted, so the loss of even a few inches is critical. My son can’t stand up in the trailer as it is. The meeting of the ceiling & wall is curved, so that has to be taken into account. The ceiling has a slight curve as well.
    Cooling in Summer won’t be such an issue. I have solar electric panels on a rack above the roof (which will provide shade) & there will be vent fans.
    I’ve thought of using packing peanuts (good way to recycle them!) I’m thinking of using Polyisocyanurate w/ foil on one side. I’m just not sure I’ll have the air space needed. I’m also not sure about what to do to deal w/ the steel supports. I’m sure I’ll ave to loose inches inside the trailer, in order to get insulation onto those beams.


    • Todd says:

      Evelyn – You’ve got me stumped. Using it as an RV really means you’ll also need to cover the foam so that there’s not a flame spread problem. Seems like you’d need some type of spray foam and then a custom wall covering.

  63. Stan Galvin says:

    I live in a 90 year old home with minimal insulation. The attic has a wooden tongue and groove floor, but no insulation. I do not want to remove the attic floor to insulate. Can I install 2″ foil backed rigid foam board on top of the wood floor? I am concerned that I may trap moisture between the rigid board and the wood floor. Since I use this area for storage, I was thinking about putting plywood over the rigid board. Your thoughts?

    • Todd says:

      Stan – Frankly I’d blow in cellulose between the T&G and the ceiling below it. With that foil faced foam you want the foil on the warm surface.

  64. Stan Galvin says:

    Todd, another insulate the attic question. The attic I wish to insulate has minimal ventilation. It has two gable vents. If I wanted to insulate the rafters with fiberglass batts, would this create a moisture problem since there is little attic ventilation?

    • Todd says:

      Stan – Are you implying that you already have ceiling insulation and then you want to insulate the rafter bays above? I wouldn’t recommend doing that at all. Ventilation is essential to a properly functioning attic.

  65. ed says:

    We have a third floor condo in Chicago with Cinder block-Firring strip Wall construction. Since firring strips are used in place of 2×4 studs I believe there is 3/4″-1 1/2″ wall cavity for insulation and fiberglass bats are probably not going to be of any use What is the best product for insulating the room. Also would I need a vapor barrier when using rigid foam board?
    Thanks in advance.

    • Todd says:

      Ed – Are you planning on gutting the space and starting over? If so there are many different approaches. Can you explain the project further?

  66. Mike P says:

    Hi Todd – Very good info here! I have a 90 year old home as well in NJ. The walk up attic has existing batt insulation under wood flooring (partial original tongue and groove, partial plywood. The insulation is not consistent and looks dirty in places that I can see (where the original wood is broken). The attic has a gable vent on either end; one with an exhaust fan. No insulation on the rafters and no soffit vents. The wooden door to the attic (from a bedroom) leads up an uninsulated stairway (walls are plaster with lead paint) to the attic.

    My questions:

    1. How can insulate the door to the attic? I’ve already weather stripped it so it seals tightly. Does the door qualify as a fire-stop, therefore allowing me to use foam board on the attic (cold) side?

    2. How can I insulate the walls of the stairwell, and effectively cover the lead paint and protect it from damage (chipping) in the future. Can I use foam board without covering with drywall, since its not in livable space, and behind the door?

    3. I want to retain the storage space in the attic, but want to add more insulation to the floor (so blown-in won’t work). Was thinking about building up the floor with additional rafters and adding another floor since the roof is very high.

    Thanks for your help and suggestions. Mike

    • Todd says:

      Mike – Thanks for visiting the site!

      1. I’d use foil faced foam board on the back side of the door, the thicker the better. No real fire issues that I’m aware of (assuming you don’t have any fuel sources up there).
      2. I’d use the same approach on the walls.
      3. How about installing 2 inches of foam directly over the flooring, then a new layer of 1/2″ plywood screwed down with 3″ screws so it bites to the lower wood?

      Good luck!

      • Mike P says:

        Awesome – Thanks Todd.

        One follow up. Any tips for securing the foam to the painted plaster? I think I’d go with screws on the back of the door, but not sure about the stairway walls… And I have no fuel sources in the attic.

  67. Bill says:

    All the informations are well said and helpful. We don’t have to ask questions all over again. Full of information and this will help us a lot. Thanks for posting this one. We surely learned a lot about this.

  68. Dick says:

    I am looking to use this XPS as a thermal break between the concrete floor in my basement and the subfloor. Is there a specific grade of XPS I should be considering for compressive strength?

    • Todd says:

      Dick – The normally stocked XPS foam boards are fine unless you have a very heavy point load like a pool table. Even then it’s probably not required.

  69. Jim Proffitt says:

    My foundation was constructed using 12″ block walls.The front wall is completely below grade and the rear wall is above grade. The sides taper front to rear. I am finishing the gameroom side of the basement. Currently the game room is studded with 2×4 walls. I planned on insulating with fiberglass batts that are Kraft faced and have an R-13 value. The home is 10 years old and has never had a moisture or humidity problem. I was extensively involved in construction and really do not anticipate any water problems in the future. I have a concern about the space between the back of the studs and the block wall. In some spots it is as much as 3/8″. Should I be concerned or do you think I will be okay as described. I wish I would have used the internet more during construction. I think I would have had more and better options before the walls were built. Thank you in advance.

    • Todd says:

      Jim – Check out my article on insulating basement walls which shows how I think it should be done.

      I do not recommend having fiberglass in contact with concrete or block.

    • Jim Proffitt says:

      I will install blue board between the studs then insulation over top. It will be to much work to take down the framed walls at this point. What about the walls that are completely above grade? Should blue board and fiberglas be installed on them as well?

      • Todd says:

        Jim – You really should try to cut those walls free and slide them forward if you can. If you install blue board like that you’re going to concentrate moisture from the block wall against the studs. They will develop mold.

        Are the above grade walls still block?

        • Jim Proffitt says:

          All the foundation walls are block. It will be nearly impossible to move the wall against grade because of a stair landing. If the stairs will slide on the landing I may try to move.

          • Jim Proffitt says:

            The stud walls have been there since constructed and I see no signs of mold at all.

          • Todd says:

            Jim – There’s no mold because you haven’t sealed the framing in with XPS foam. Obviously there are times when it’s no practical. I’m just saying that if you use this approach you risk a very serious problem.

          • Jim Proffitt says:

            Todd- Thank you for your help, unfortunately the wall that concerns me the most is the one that is hardest to deal with. Hopefully I can come up with a suitable solution.

  70. Ted says:

    I am planning on using foam board for a slightly different approach. I want to build a portable bar (on locking wheels) and include a large enough cabinet that will hold 2 soda/homebrew 5 gallon kegs with 2 taps out of the front. The cabinet in the bar will need to be insulated to keep my already cold kegs from my basement kegerator outside while throwing a party. Is there a better foamboard to use? I will probably have some kind of tray or bucket to put ice on to help keep the kegs cold and so possble water will end up dripping on whatever foamboard I use under the floor of the cabinet. Hopefully this is not too goofy a question for this forum, but I see dual picnic taps (coolers with either coils or coldplates that have the beer lines run so ice cools the beer as it passed through the coils in the cooler) costing around $250. For that price I can pretty much build a moveable bar, though maybe a bit rustic, with an insulated cabinet and a couple of cheap beer faucets coming out the front, serve by brew cold and not worry about 100F weather heating up my kegs when having some folks over.

    • Todd says:

      Ted – Great idea! If it were me I’d use some 2″ thick XPS foam (blue or pink) then line that with some fiberglass tub surround, or cheap vinyl flooring, something like that. You can glue it all together. I think it would work great. The lining will help protect the foam from chipping, denting, etc and also make cleanup easier.

      Good luck!

  71. George says:

    My wife and I live in Atlanta are getting ready to break ground on a 3/2 2,700 sq ft modern house (think two 25Dx60W’rectangles on top of each other with a flat roof) with the public space at grade and the private sleeping quarters below (open to daylight). We plan on installing solar panels and want the house to be very energy efficient. The lower sleeping level will be poured 8-10″ concrete walls with 2×4 framed & finished. Since the lower level will have the bedrooms and the concrete slab will be polished and left exposed, we want the floor to be as comfortable as possible (no radiant only heatpump(s)). We plan on blowing in cellulose for the majority of the house, but would appreciate advice on insulating slab and exterior walls, plus roof (i.e. thickness, XPS, etc).


  72. Jeremy says:

    I wish I would of found this site about 2 weeks ago. I have a all concrete room under my garage and I am planning on finishing this room. I have framed the walls with 2×4’s and used 2×6 for the ceiling. In order to square the walls the 2×4’s are anywhere from 1/2″ to 1 1/4″ from concrete walls and the 2×6 ceiling is about 1/2″ to 2″ from concrete ceiling. I have already cut 1″ polystyrene sheathing to fit in between the 2×6 joist for ceiling. There is no duct heating in this room and it will be closed off. What do you recomend I do to best insulate this room . This room is 15 degrees colder then my unfinished basement. The rest of the basement is going to be my next project ofter this one. Thanks for your help.

    • Todd says:

      Jeremy – Thanks for visiting the site.

      First off you may want to read this article: How To Insulate Basement Walls

      What will this room be used for? Depending on the use there are different approaches.

      • Jeremy says:

        I forgot to mention that were in Ky which if very humid at times. This room will be used for storage mostly but may be a office one day. Its about 400 sq ft. and will have a steel door closing it off from the rest of the basement. Thought was to keep valuables in this room in case of fire or storms. Will heat with portable electric heat if need be. This house is 5 yrs old and basement has never shown water leakage. This room only has 1 wall that is above ground. We did use treated wood on the top and bottom of the framed walls. Do you think that will be ok? I did read the article on insulating basement walls and did not use trex or simular material under treated wood.

        • Todd says:

          Jeremy – I guess my concern would be the same one that I have for all basements. You really need a proper insulation/vapor barrier between the concrete and framing. If it were mine I’d recommend taking down the walls (you can most likely do that without demolishing them, just cut the top and bottom free, lay them down), insulating with proper foam board (or hiring a spray foam contractor), then putting the walls back up.

          If you don’t do that you will have moisture/mold problems in the future.

          • Jeremy says:

            Todd I appriciate your help on this. So if I had it sprayed behind and in between studs on the walls and ceiling it would be fine instead of taking down the walls? Do you know about what would it would cost to have 380 square foot sprayed?

          • Todd says:

            Jeremy – Yes spraying it in place will work. Word of caution – DO NOT use Open Cell spray foam no matter what a contractor tells you. I would imagine it’s going to cost probably around $1,000 just because the mobilization, clean-up on a small job is the same….just my gut feeling.

  73. Jeremy says:

    Todd just had a contractor stop by and give me a quote on the closed cell spray insulation. He mentioned 1/2″ of closed cell and 3″ of open cell on top of that on the walls. What do you think? The price was about 500.00 cheaper if I went with that instead of just 2″ of closed cell on the walls. Also I have one wall in that room that is concrete but on the other side of that wall is my basement. Should I spray that wall also or just put 2″ of foam board inbetween the studs?

    Thanks, hope I’m not bothering you to much, just want to get this right so I dont regret it in the future.

    • Todd says:

      Jeremy – No worries….this site is all about helping folks out. I just hope you come back often!

      I really wish contractors didn’t offer such “poor” options. At least poor from my point of view. 1/2″ of closed cell foam is NOT going to stop water and I would NOT recommend that approach. Go with the 2″ of closed cell and save yourself a lot of aggravation later.

      Not sure I understand your other question. Do you have a concrete wall that has rooms on both sides?

      • Jeremy says:

        Yes, one side of the concrete wall I’m referring to is my unfinished part of my basement and the other side is the room i’m finishing now. The room I’m finished now is under my garage where most people just backfill with dirt or make a cistern out of. I decided to make a storage room out of it instead of backfilling.

  74. Scott says:

    I am planning on installing Owens Corning FOAMULAR F250 4x8x2″ sheets to concrete basement walls (is this brand ok?). Then installing 2×4 walls with a PT base. Should I still fill cavity in between studs with R-13 fiberglass insulation? faced or unfaced? House is in New England. thanks for the info!

    • Todd says:

      Scott – The answer depends on your local energy code and what R value you need. Here in NH we have a pretty strict Energy Code that requires the entire building to be analyzed which determines the level of insulation.

      2″ foam does a pretty good job!

  75. john says:

    I have a rustic cabin ,3 season usage, with log rafters and 3/4 shiplap cathedral ceiling ,shingled no insulation. I”m installing a metal roof ,planning on roofing felt on wood ,2×2″s parallel to ridge with foil-faced foam (1 1/2) laid between and metal roofing over. To keep the cabin look original, I want to keep the roof as a low profile (ruling out 6-8″ roof rafters). This is a northern cabin . will there be problems with foam (summer heat, direct contact with metal roof)?

  76. Jeremy says:

    What do you think about the do it yourself handi foam closed cell kit? Is it as good or the same stuff contractors use. How many board foot would I need if I was to purchase this and use for a room 20’x19’x7′? I would spray 2″ on the four walls and 2″ also on the ceiling.


    • Todd says:

      Jeremy – I haven’t used one of the kits and I hear they only work well for small areas like a rim joist. Again I haven’t tried them so I’m not sure what advice I can give you. I also hear they are getting harder and harder to acquire do to VOC laws.

  77. Jeff says:

    I am remodeling an old farmhouse in Pa. The exterior walls are currently double plank with 1 inch E.P.S. under the aluminum siding.
    I removed the plaster and lathe from the interior and want to fir it out 1-1/2, using blue board between the studs. Would this cause moisture problems using 1-1/2 blue board or should there be an air gap. Do you have any suggestions or comments?

  78. Karen says:


    I’m planning my basement build out, (concrete pour, with 2/3rd below ground level). I plan on using the blue(pink) board, 2×4 walls, and fiberglass to get to the proper R value for Missouri.

    My question: What is the minimum blue board thickness I can use for the vapor barrier? I have to add fiberglass for R value, is it resonable to put a 1/2 inch or 1/4 blue board without decreasing the effectiveness of the blue board?

    You Rock!

    • Todd says:

      Karen – You have to use a minimum of 1-1/2″ of blue board in order for it to act as a proper vapor barrier. Good luck!

      • Chris says:

        I have a similar situation as Karen. I’m in central Indiana with a very dry poured wall basement. Do I still need 1-1/2 or would 1″ work? If not, can I do 1-1/2 with the studs tight to the Xps?


        • Todd says:

          I always recommend 1-1/2″ as the minimum. If you’re confident that the basement is dry then installing studs up tight to it is not a concern. Just be sure you’ve sealed all the joints well.

          Good luck.

  79. Loyal says:

    I live in Southern California where it ususally is warm, but my exterior walls do not have insullation in them. They are cold or hot to the touch depending on the outside weather. There is a Stucco wall outside, a 30 lb felt then drywall nailed to 2×4’s.

    The house was built in the early 70’s.

    I was thinking of getting some 1/2 ” blue board & nailing it directly to the drywall on the interior. NO muss no fuss for about $100 per room to add a little R value (I know it is very little, but perhaps better than nothing?).Then painting to match the room.

    Would putting a 15 or 30 lb felt in between make any difference?

    Your thoughts please,


    • Todd says:

      Are you suggesting to just paint the foam board and leave it that way? If so I’d HIGHLY advise against that. It would be a huge fire hazard and frankly 1/2″ of foam isn’t going to do much of anything.

  80. Harvey says:

    Great site. We have a basement in Baltimore that is damp in the summer. We are torn between 2″ XPS attached to the cement block wall with 1×3 furring strips covered with Mold-resistant drywall versus 1″ XPS, 2×4 wall with R13 insulation between 2x4s then covered with Mold-resistant drywall. The latter seems like it may be overkill since we don’t need to hang anything on the walls. If we go with the first option, will the gap between the foam and drywall (separated by the furring strip) be a problem for moist air? Thanks.

    • Todd says:

      Harvey – You’d be far better off with option #1. You need a minimum of 1-1/2″ of XPS foam to seal out water vapor from the concrete. If you use 1″ it’s really risky. The air space is actually a good thing.

      • Harvey says:

        Thanks Todd. One more question, any experience/opinions on the GreenGuard XPS (Pactiv)? It seems the Dow blue or OC pink are the boards of choice but the only 2″ XPS that our local Lowes carries is GreenGuard and I was wondering if you think it would be worth special ordering 2″ blue or pink?

        • Todd says:

          Seems like the same type of product. I wouldn’t special order anything. Most good lumber/building supply companies carry some type of 2″ XPS foam board. I find that Lowes and Home Depot don’t have a good selection of these type of materials.

  81. Ed says:

    Todd – I live in Colorado with a 1978 home. I am looking to add insulation to the above grade bedrooms on the North side of the home. The exterior has 1″ XPS covered with vinyl siding. There is some old siding board behind the XPS. The bedrooms are plenty big enough to build out an interior wall. If I remove the existing drywall (stud cavity filled with R13 fiber glass), add 2″ XPS foam and build an interior stud wall against the XPS and fill this new wall cavity with R13, then hang drywall. I know there may be alternatives, but this seems fairly straight forward way of adding R with minimal effort. Thanks.

    • Todd says:

      Ed – Your method is certainly one approach. However, there are probably some easier methods and possibly some cheaper ones that will yield the same result. Here are a couple thoughts.

      1. Have the wall spray foamed after you remove the drywall. This is probably the best approach and it will surely seal things up, stop vapor and air infiltration and likely be the quickest.
      2. Install a layer of foam directly over the old wall, pad out windows and doors with extension jambs, drywall over the foam. This would at least eliminate the need to frame a new wall.

      Good luck.

  82. Dean says:

    Hi Todd. Thanks for the great blog. I’ve read this whole thread, and would like to get advice.

    We are converting our 21×28 garage into a family room. It is exposed on 3 sides (s,w,n) and currently only has r11. The room will have a lot of glass (great views even here in Kansas). Our contractor recommended sprayed in foam ($1000+) into the exposed 2×4 wall cavities. But I found your blog and it seems to me it would be cheaper and better in the long run if we just put r13 or r15 into the cavities and then 1″ xps board over the studs. We would end up with somewhere near r19, without having the added expense of him furring out the studs, etc.

    When I suggested this to him his eyebrows raised about all sort of problems the drywaller might have with his screws popping through the paper on the drywall, or the drywall sheet not lying flush because the screws compress it into the foam. Have you heard of this happening when drywalling over foam board?

    Thank you!

    • Todd says:

      Dean – Thanks for the nice compliment.

      First off I’d be shocked if you can get spray foam in that quantity for that price unless it’s open cell foam. I DO NOT recommend open cell foam at all. It has many problems including acting like a sponge if you ever get water in there. Having said that there are quite a few options you can look at.

      1. Closed cell spray foam – works wonderfully, definitely most expensive option.
      2. BIBS (blown in blanket system) – Blown in fiberglass insulation is a great product and it does an amazing job air sealing and frankly provides the same R value as most spray foams, you can read more about it here:
      3. You could do the R11 fiberglass followed by an inch of XPS foam. There are a couple issues you’d need to deal with. You would need extension jambs on the windows and doors to come out the additional 1″. In my opinion there would be no issues with the screws. I say that because XPS foam has a very high compressive strength when compressed uniformly. Not sure this option will be much cheaper as XPS foam is pretty pricey and there will be more labor. If you do this approach be sure all the seams are taped with Tyvek or DOW Construction Tape.

      On another note I’d love to ask a favor. Without going into a huge long dissertation I’m asking for some help from readers. You see Google has recently changed the science on how they rank sites when you do a search. This site was adversely effected by those changes which means Google things the site is just “Spam”. So in order to try and fix that I’m asking folks to share the link to the site on Facebook, Twitter, etc to help spread the word and prove those crazy engineers at Google wrong. So if you enjoyed the site any help promoting it would be “GREATLY” appreciated.

      Thanks and best of luck!

  83. San Ball says:


    I am redoing a butler’s pantry in an old house and need insulation for the walls –I am interested in using insulation board, not batts. Advice about installation appreciated.
    Details: Zone 6 climate. Triple brick construction with concrete inner walls (plaster has been removed). Space will be used as traditional butler’s pantry, i.e. cabinets and counters.


  84. Saurus says:

    Todd, in need of your help. Just want to say after reading all the posts on this excellent page I have a good idea of what the problem may be. btw I found your site using Google

    I live in a 2nd floor north west coast condo and one external wall in the condo is poured concrete (i believe) and is insulated with well taped pink fiberglass and then covered with vapor barrier and then drywall. The wall has metal studs looks like 4 inch. Below my condo is a parking garage – the tiled areas of floor are generally cold. Outside winter temps norm range from 1 to 8 degrees Celsius, summer 15 to 30. Winter indoor RH is between 50-70% when occupied. The outside concrete wall is painted on the outside only.

    The problem is that during the winter months in which I am resident in the condo it appears that the entire concrete wall is moist – it is so wet that the floor up to 1 ft away from the wall is also damp/wet. I have been assured by engineers that there is no water coming in any external direct source such as rain and since the problem alleviates when I am away for extended periods I assume it is a condensation problem.

    I believe the moisture is coming from cooking/bathing within the unit itself or from the outside through the concrete, (if this is possible under what situations does this occur?) and is getting in the wall somehow and cannot escape due to the vapor barrier.

    Can you suggest the ideal type of insulation materials / method to use here to fix the problem.

    From reading other posts I think I should use 2 inch XPS foam sheets (or closed cell spray foam) directly against the concrete and then drywall with nothing else. What about using Open Cell foam to allow the concrete to breathe more or is this nonsense? A builder also recommended moving to 2×6 studs – what do you think?

    Also, I turn the heat off in the condo when I leave for work and back on in the evening. Will this help or hinder in any way?

    • Todd says:

      It sure sounds like condensation to me. Likely when the RH is high in the condo and the wall is cool/cold then warm damp air hits the surface and condenses. There are a couple of things that will help. First of all controlling the RH will always help in these situations. Secondly I would insulate that wall, you are correct, 2″ of closed cell foam (XPS). You can install it several ways.

      1. Glue to wall – frame a wall in front of that – drywall over that, be sure to seal all seams
      2. Glue to wall – Drill/shoot strapping over the foam – drywall over that.
      3. Use metal Z studs which allow you to attach to the concrete and fit the foam in between them. Then drywall to the metal flange.

      DO NOT use open cell foam.

      Good luck.

      • Saurus says:

        Thanks Todd, We are running a large dehumidifer 12 hours per day in the problem area and the RH never goes below 45%.

        Construction on the new insualtaion/drywall begins in 3 days so I’ll try to post with the results once it is all done. The builder wants to use 1.5 inch XPS, sealed at all the seams, I will push for 2 inches.

        • Todd says:

          Saurus – 1-1/2″ XPS technically qualifies as a vapor retarder which may work fine in your situation.

          • Saurus says:

            Ok, I am reporting back for the benefit of others who may be having this problem.

            As I wrote above this time last year we had big problems with dampness in our concrete construction condo during the winter months starting around early October when it got colder.

            To remedy this during the summer our contractor replaced our fiberglass / vapor barrier insulation with 2 inch thick (at least) of XPS sheets in between metal studs and then spray foamed the gaps around the ceiling and elsewhere.

            I can report that since the job was done there has been zero signs of moisture at all in our bedroom so far. I will continue to check throughout the winter.

            It seems amazing the difference insulation can make in a concrete condo. There was so much moisture we had 1 building engineer conclude that it was the bathroom fan venting leaking into the wall cavity and another told us it was certainly water leaking from the outside. Both missed the insulation problem… and our condo wasted thousands on these “experts”

          • Todd says:

            Thanks for reporting back and sharing your success. It is amazing to me how many people throughout the industry do not understand the science of this problem. Keep us posted!!!

  85. PaulV says:

    I live in the pacific NW mild/damp climate.
    The roof on my home is 2×6 T&G over 4×12 beams. I plan on replacing the roofing this spring. I intend to apply vapor barrier on the exterior, then frame the roof with 2×6’s similar to a wall and fill the cavities with one 3″ layer of foil backed (both sides) polyiso foam board. Then sheet with OSB, felt paper and shingles.
    I was also considering applying this foam board under the the car decking between the interior beams and covering it with 1×6 T&G to retain the appearance of the wood.
    Would this be a bad idea? Your opinion would be appreciated.

    • Todd says:

      PaulV – Why are you framing additional structure on top? You can just install ISO foam board insulation, then a layer of plywood with long screws. That will provide a much better overall insulation value and be cheaper.
      The second part sounds fine.

      GOod luck.

      • PaulV says:

        I’ll check again but, I was told local codes require that I vent below the sheeting. I agree that it would be much cheaper and less labor intensive to simply attach a layer of OSB directly over the ISO with long screws. As of July 2010 our county adopted a new energy code that requires a minimum of R38 for any re-roofing applications. I figured that I would add 3″ on top of the roof and 3″ below as the ISO is rated at 19.9 for the 3″ thickness. I figure I could seal up the interior well enough to stop any air flow to the exterior using caulk and tape. I want to make sure that I do not set myself up for moisture problems down the road.

        • Todd says:

          Many many commercial roofs are not unvented with 6″ of iso on top. Frankly I can’t see how they can argue the unvented roof with R38.

          • PaulV says:

            Got it, I will draw it up and present it to the premit center. I’ll let you know what I find out. BTW, when butting the sheets of ISO together, would you recommend an adhesive/sealant and then taping the joints? Or would a dry fit and taping do the deed?

          • Todd says:

            The later is fine. Best of luck!

          • PaulV says:

            The project took an unexpected turn. I found that I could remove the existing 12″ standing seam sheet metal roofing without damaging it. We removed the chimney penetrations and replace any damaged T&G. 30lb felt was used as a vapor barrier over the exterior side of the T&G. Next we created a 3 inch perimeter along the eaves and gable ends with 2 layers of 2×4’s. Sheets of 3″ polyiso was anchored with screws and covered with Feltex vapor barrier(feltex is similar to house wrap except that is rubberized on one side and is impermiable). Then we reinstalled the standing seam sheet metal panels with 3-3/4 screws. New sheetmetal facia and ridge cap was installed along with snow brakes. We also installed 4 fall protection anchors with D-rings exposed just under the ridge cap (2 on each side of the ridge). In addition we had a bit of snow brake left over so we installed three 4′ sections spaced 4 feet appart to make a permanently installed ladder on one corner of the roof to allow access to the peak. Overall, the project cost a whole lot less by reusing the existing metal panels. Had to purchase only 14 new sheets of standing seam. Only issue is that the new sheets obviously look newer than the 13 year old panels that were previously installed. Installed the newer panels (all in a row) on the side of the house that is not so observable. It resulted in an immediate impact on heating this winter. No more cold air cascading out of the loft area. The split mini heat pump heads now can handle the load.

            Thanks for your advice.

          • Todd says:

            Glad to hear the success! I hope you’ll become a Newsletter Subscriber and remain a member of our site. Good luck on future projects.

  86. Corky says:

    I have a big stack of 1/2″ eps foam boards in my basement which I was hoping to glue in three or four staggered layers against my basement walls. My winter basement temperature is 50-60 degrees and I have had small, isolated water leakage problems a couple of times over 6 years when too much water got close to the outside walls (downspout problems).

    City building code requires a 1/2″ gap between the basement wall and a framed wall, with R13 in the frame. My plan was to glue at least 1 1/2″ of foam board to the concrete walls, leave 1/2″ gap, then frame a wall with R13 fiberglass and cover with drywall.

    1. Is there any reason to use xps instead of eps against the concrete wall?
    2. Would it be better to use xps against the wall and use the eps (rather than fiberglass) for insulating between the studs?
    3. If I have another isolated water problem problem, will it simply run down between the wall and foam board, out the bottom and onto the basement floor, slowly evaporating up the 1/2 gap and through the floor boards?

    Thanks for your help and your wonderful site.

  87. Corky says:

    Thanks for setting me straight. All this time, I thought EPS was closed cell. It’s not like open cell I’ve used in other applications.

    • Todd says:

      If you cut through EPS you’ll basically see lots of small balls, water can fit around those balls. Whereas XPS is a solid, closed cell foam product.

  88. jason says:


    I have question not sure if it will be a problem but…We are in a cool climate in michigan I have glued 2″ of pink foam on the concrete wall then framed the wall 1′ off the foam creating a air way between the wall and the 2″ of foam. I noticed when I was mounting the tv jack that the cooler air was being drawn out from behind the wall. I did not like the idea of using fiberglass insulation in the basement wall cavities but I am wondering now if I should have? thanks.

    • Todd says:

      Sounds like you have that “air space” potentially connected to cooler air. In other words is that space completely sealed off? or does it connect to an area that’s not insulated or open to outside air?

      • jason says:

        It is completely sealed off. It can pull air from the utility room through the floor joists. The room is heated as well as the floor is infloor heat which has been off for a couple weeks now. Possibly could be a negative pressure issue? The air is not freezing cold but you can just tell it is a few degrees cooler than the inside room temp.

  89. Ron says:

    I am building a new house up in Maine and currently the inside is unfinished. I want to put 1″ insulation board right up against the exterior sheathing followed by regular faced insulation. I have 2×6 walls. Will this work for added insulation and will I run into any vapor problems?

    Thank You.

    • Todd says:

      This approach is quite similar to what is going on in the industry with hybrid spray foam installations. There are more and more folks spraying a thin layer of foam inside the cavity followed by fiberglass. This approach helps with air sealing. The key here is when you install the foam, be sure to seal it very well to the framing.

      Good luck.

  90. Jarrod says:

    I am replacing my Tar & Gravel Roof on my home in sunny californa with 4″ polyiso and a roll out cool roof product to reflect the sun. With both products is the an over kill waist of money? Suggestions?

  91. Mark Schmitz says:


    Here are my plans for insulating my home during a roof tear-off here in Madison, Wisconsin. Please let me know what you think and if there is a better way of insulating and attaching the gutter and fascia in your opinion. I want to keep the ventilation space I have proposed between the polyiso and OSB if possible (and including a ridge vent) to help reduce any moisture issues. The rafters are 2x4s on this 1920’s era home and I don’t want to lower the height of the room-in-attic ceiling if at all possible.

    I plan on using 2″ Polyiso insulation on top of the rafters, a layer of foil as a radiant barrier (unless I can get my hands on some foil-faced 2″ polyiso), 1″ or 1 1/2″ blocking to create an ventilation channel and at least 7/16″ OSB (maybe plywood) sheeting on top of that. I’ll also use 3 1/2″ of Polyiso between the rafters but I plan on installing that from the finished side most likely (unless we have time to do it from the top side after tearoff).

    I plan on using a ventilated drip edge for my air intake between the polyiso and the OSB sheeting. Will this work? Can I go with thicker foam on the roof if I want to without running into major issues with the drip edge ventilation intake and/or fascia board and gutters? Should I include a vapor barrier on the underside of the interior foam before I insulate?

    Thank you for your help and I look forward to your opinion.

    • Todd says:

      Well let me start with this and I’m assuming your local building official will also evaluate. Any time you tear off a roof it’s likely you’re going to invoke some new code provisions. 2×4 rafters are extremely undersized for any roof in a snow area which you clearly live in. If you’re going to spend that kind of money on a new roof I’d address those rafters at the same time.

      7/16″ OSB or plywood is again extremely undersized for modern roofs that experience snow loads. Almost all roofs that we work on today with 24″ rafter spacing includes 5/8″ T&G sheathing as a minimum. 7/16″ OSB will warp, sag and look pretty horrible within a few years of installation.

      Is the attic a conditioned space? If you end up with 5″ of polyiso you’ll be around an R35. Again this is fairly low for homes in snow areas.

      So before assessing your proposed details I think you need to focus on structure and insulation values as they are all on the “low” side of what normal code would require.

      • Mark Schmitz says:

        Thanks for the quick response Todd. Unfortunately, 2×4 rafters must have been the norm in 1919 when the house was built but you’re right, I feel that beefing up the rafters would be a good thing to do but I was trying to avoid the expense in materials and labor involved in doing so.

        The attic is a conditioned space, it’s a 1.5 story home with the room-in-attic addition and I will be building gabled dormers in the middle of the roof line on each side of the house that will take up about 1/3 of the total roof area.

        Do you think that I should just sister some 2×8 rafters next to the 2x4s that are currently there, install a ventilated drip-edge and keep the insulation at least an inch from the sheeting and install a ridge vent or is there a better way of doing this? I would like to avoid having to replace all of the sofit and fascia if possible but it doesn’t seem that this will be the case if I bump the rafters out that far.

        Also, just as a side note, why the heck do companies that manufacture ventilated nail base use 7/16″ OSB on their produce and then feel that it’s worth $111 a sheet (for 4.5″ Polyiso) if it’s going to be garbage in snow-loaded areas?

        Thanks for your input once again Todd.

        • Todd says:

          The dormers create an even bigger reason to evaluate that roof structurally. Depending on how you frame the roof you’ll need MUCH larger sistered rafters on either side of the dormers. Honestly most building code officials are going to want that type of structural renovation to be designed by a professional. Have you spoken to your building code official yet? I’d start there!

          Without seeing details of the existing roof it’s really hard to say how to ventilate it. The dormers are doing to complicate things even further. You might consider an unvented roof, highly insulated, to simplify things.

  92. Sue Brooks says:

    Hi there,

    I need help!! I have constructed a metal frame bldg. As you know it can get extremely hot in the south. I need to know what to use when installing styofoam insulation to keep it attached to the metal roof inside. We tried a type of glue from Lowe’s Hardware and it just would not hold. Can you give me any assistance on what to use in regards to insulating a metal roof.

    • Todd says:

      Sue – Typically on metal buildings the roof insulation in done during construction with one or two approaches.

      1. The most typical approach for insulating a metal roof is to install the metal deck, then install a layer of rigid foam insulation board, then the roof membrane. This is the most efficient way to insulate as it isolates the steel below the insulation and also doesn’t allow for thermal transfer.

      2. Another approach which is used on projects with a tight budget is installing batt insulation on top of the structural framing and then installing the metal roof deck on top of that. This isn’t as good as the insulation gets compressed at each structural member.

      Insulating after the fact is going to be MUCH harder. In that situation I’d recommend spray foam. Otherwise, efforts to adhere insulation to the bottom side of the structure will be difficult at best.

  93. Jill says:

    Hi all! Does anyone have any experience with the spray on cementitious foam called AirKrete? It sounds fabulous, albeit expensive and a less toxic form than other sprays that off gas.

  94. Mary says:


    Just wondering what is the life time of polyiso foam board insulation. We had polyiso insulation under existing tar and gravel roof (17 years old). Recently we want to re-roof. when removing the tar and gravel roof, will the original polyiso foam be damaged? If not damaged, can we still use the original foam and will there be any problem arise to consider.(ex: will old insulation die down in the half life of new roof) Please advice.

    OR is it ok to buy new insulation lay on top of old one to increase R-value.


    • Todd says:

      When properly covered from the sun and environment it should last indefinitely. You can certainly install another layer over the old after it’s inspected.

  95. James says:

    I plan to remove the existing aluminum and wood siding on an old house. The walls were blown with fiberglass insulation years ago. I plan to check that all the cavities are properly filled. I would like to use a 3/4″ or 1″ insulation board over the entire exterior before installing new steel siding and windows. My question is what is the best foam to use, do I need a housewrap, if so where sould it be placed, under or over the foam. Thanks.

    • Todd says:

      A closed cell foam is best in my opinion because it will not absorb moisture. The housewrap issue is up in the air with this type of application. I would install some type of wrap whether traditional or even tar paper over the old siding before the new foam. Then I’d tape the foam insulation seams. Good luck.

  96. Steve says:

    Hi Todd. Great site you have I came across it and will check your tool box buzz site out also.
    My garage gets really hot as it gets to 110 or more outside here in southern AZ. My attic can reach 135 deg.
    My garage is a attached garage to my home and is not insulated in the attic ,but the rest of my home is. Also the garage roll up door is metal/aluminum. The garage faces the western sun.
    I am not sure why it wasn’t insulated as it is a newer home ,but what can I do to insulate that garage door ? What about using blue board and is it OK to use that blow in insulation in the attic above the garage.
    PS. I did put in a attic fan to help get rid of all that extra heat in my attic .
    Thankx for any info……

    • Todd says:

      Steve – I’m not surprised that the garage wasn’t insulated. Many builders skip that to control costs. You can certainly insulate the garage attic with blown in insulation. In hotter climates some folks insulate the roof deck with reflective radiant barrier insulation to help reflect the heat back out.

      Insulating the doors might be a bit of a challenge. If you use foam then you’ll have exposed foam which is typically a code/fire violation.

      Good luck.

  97. John says:

    I am building a new house and putting radiant heat in the concrete slab, which will be the finished floor. I plan to put 2″ XPS under the entire slab, but I’m debating whether to put an additional 1/2″ foil faced polyiso board on top of that. Will the foil faced give me a real advantage in reflecting the radiant heat upward towards the slab? Will the polyiso hold up under the slab? I plan to put the radiant tubing in the middle of a 4″ reinforced slab. Thanks for your input.

    • Todd says:

      John – We’ve done dozens of radiant slabs and every one of them has been done differently based on ever changing recommendations from the HVAC industry. I definitely think the 2″ blue board is a very good idea. Not sure how I feel about polyiso under the slab, I’d probably just use one of the reflective bubble wrap type products if you want extra reflective properties.

      The bottom line is fairly easy, heat will move in the direction of least thermal resistance. So if you put insulation under it, the heat will be forced up.
      Good luck.

  98. Robert Nemoyer says:

    Hi Todd,

    I just found your web site and am impressed. I have a dream of building my own semi passive solar super insulated home. My wife has early stage Limb girdle MD so our current two story no longer meets our needs. I am hoping to orient the house 60 feet east west and 40 feet North south. I want a continous insolated sunspace along the entire south side with heat rising into the attic like and then desending along the north wall into the basement with tube openings into the south semi isolated sunspace. The air with then continue the same cycle. I figured I would use earth tubes to prewarm the air before it enters into the heat exchanger. I want to put insulation between the concrete. With this kind of sipcrete wall I thought I would put up 4 inch studs on the northwall and use the empty space before the gypsum as the ductwork. This sort of makes it a sort of high thermal mass semi-envelope home. Does this make sense. I would like to make a raft with 4 inches of board insulation below both the basement and the footings. The walls are solar-crete models 7 inches of foam between 2 1/2 inch inside and outside concrete shell.

    My first question is what psi would I need for the xps under the footings.

    My second question is can I use polyso as the board between the shell instead of xps because it has a higher r value. I am thinking of using shotcrete with an admixture of Xypex to waterproof the walls.

    • Todd says:

      Thanks for the kind words. Sounds like a very interesting house design! I’m certainly not an expert on passive solar homes so the design is something I wouldn’t be able to comment on. With regard to the compressive strength of the foam board there are several you can choose from. DOW offers compressive strengths in 25 psi, 40 psi, 60 psi and 100 psi. The required strength will depend on the foundation loading from your home design. I think the XPS is better suited for your shell application. Foil faced polyiso is a definite no as it will react with the concrete.

  99. Todd, been reading all this information and it has been great. I am trying not to repeat much of what has been asked already. I have a similar attic project as some others, finishing a 1900’s attic for additional living space. Summary, 2×6 raters, I added or am adding ridge and soffit venting with baffles between each rafter. Would like to add 2 inches of foil faced ISO on the bottom of the rafters, then blow in cellulose in the space above the ISO, below the baffle. Now my question is, rather than strapping the ISO and adding drywall, could I add some sort of beaded board (cedar, pine, beaded plywood, etc) to cover the ISO for the ceiling rather than using drywall. I have read that a fire barrier needs to be added such as drywall. I am looking for a rustic look, and was thinking if I cannot have to strap the ceiling for drywall I could be saving some $$, space and labor. Second question, I have heard of the do-it-yourself, blow-in insulation foam options. What are your recommendations or thoughts on these? Easy to do? Better off hiring it out? Thanks for your time and I look forward to reading your response.

    • Todd says:

      Thanks for visiting out site. I hope you’ll consider signing up for our FREE weekly newsletter so you can stay up to date with all kinds of home improvement projects.

      Sounds like a fun project! First off let me put no my engineering hat and at least point out that if you live in an area with any snow your existing 2×6 rafters are far too small by today’s standards. If that’s the case you should consult with a structural engineer before moving forward with your project…enough of that…

      2″ of foil faced polyiso on the ceiling is a great start, combined with blown in cellulose it should be a really good option. You really should check into the local building code and see if there are any energy code requirements as you might need an even higher R value. Your proposed insulation would be 5.5 inches of cellulose at 3.5 per inch + 2 inches of polyiso at 7 for a total of R33. In many locations today you need upwards of R50 for attic ceilings.

      Almost every code requires a 15 minute thermal barrier. T&G can be used but 1x typically doesn’t make that requirement. I’ve heard of some 7/8″ thick material in T&G being approved. You’ll need to discuss this with your local building code official. You might want to add a layer of 7/16″ OSB then the wood which will likely make the rating pass.

      DIY foam is really difficult and not something I recommend. DIY cellulose is much easier and typically something a home owner can handle.

      Good luck.

  100. jeff says:

    Todd, I have a below grade,uninsulated,mostly dry, basement in N.H. that I would like to insulate. What do you recommend?

  101. John says:


    Thanks for the very informative website!

    I am doing 2″ XPS in my basement, which has no water problems but is slightly humid. Measuring to insulate my rim joists, I have about 6″ in from the rim joist to the interior edge of the sill plate.

    Would best practice be to:
    a) put one layer XPS sealed directly against the rim joist (thereby creating an air pocket between the insulation and interior framing)
    b) put one layer XPS sealed on the edge of the sill plate (thereby creating an air pocket between the insulation and the rim joist)
    c) double or triple up on the XPS to eliminate any air pocket from rim joist all the way to the edge of the sill plate
    d) none of the above

    No grade for this quiz, just everlasting gratitude when I’m enjoying my nice dry basement!


    • Todd says:

      John – Thanks for the kind words. I hope you’ll consider signing up for our FREE Weekly Newsletter!

      You want the insulation to be as close to the outside as possible. Obviously you’ve got a tight space to work in but I’d get at least one layer of 2″ foam against the exterior rim joist. Then you can add more foam or supplement with fiberglass.

      Best of luck!

  102. Sara says:

    We live on the main floor of a duplex and have high heating bills in part due to old (inadequate) insulation. The apartment above us heats her unit. Since insulation cannot be redone at this time, I was thinking of stapling foil faced kraft paper over our drywalled ceilings in hopes of reflecting some of the hot air back down.

    Is this temporary fix a feasible solution ? And could it cause any moisture problems on the ceiling ?

    Thanks for your help

    • Todd says:

      First of all your proposed solution is likely not legal. Most foil faced products must be protected with a 15 minute thermal barrier. It’s far more likely that your heat loss problems are due to poor windows and wall insulation. If your neighbor above is heating the space then your not loosing as much up as you are likely out the sides.

  103. Kit Beveridge says:

    We are currently building a new home in Indiana. It will be a 2 story home with a basement with the first floor 2200 sqft, 2nd floor- 2000 sqft. and the finished basement with 1500 sqft. They have so far poured a 10 inch thick foundation and placed tar waterproofing on the outside with some sort of non-rigid insulation on the outside. They plan on pouring 4 inch concrete over visqueen vapor barrier and granular fill after the plumbing stuff. We have a 400 sq ft sunken by 4 inches -section designed for the theater that Im concerned about especially with moisture since immediately upon digging-there was water pooling but eventually desipated but occasionally is there anywhere from 2-4 inches. We added a sump pump in that section. My question to you is what would you recommend for us to do? My builder is reluctant to use the 2 inch rigid xps below the concrete saying that he is worried that over time it would not be structurally sound with the 2nd floor weight. and claims that he has not had any condensation issues in his basements. What you would do if you lived here in Indiana and roughly how much would your recommendation cost? Our climate is not as harsh as Canada and I obviously am trying to be cost effective. He plans on insulating the walls very well upto R13 with a space between studs and the wall as a thermal break. And if you advocate insulating below the concrete does that mean above the footers too or level with footers?
    Thank you so much for your time and GOD BLESS YOU!

    • Todd says:

      Sounds like a great project.

      1. First off I’d be sure that the area under the slab is excavated down to at least the bottom of footing elevation. Then backfilled with clean stone material with a complete perimeter drain to drain any ground water.
      2. Placing 2″ of XPS foam on top of the compacted fill and directly below the slab is definitely a good idea. This will do two things; it creates a good insulation layer to help keep the slab warmer, and it also helps stop moisture from coming up through the slab.

      3. I would HIGHLY recommend you read another one of my articles about properly insulating a basement. DO NOT agree to have your basement insulated as you stated above. It doesn’t work and there’s tons of good information out there to prove why. Read this:

      I hope this helps clarify some issues for you. Investing in insulation is by far one of the best things you can spend money on in your new home. Fuel costs will only continue to rise as time goes on.

      • Kit Beveridge says:

        Thank you so much for your quick response. But just to clarify, does the 2 inch XPS go on top of the footers as well as the gravel or does it go on top of the gravel but level with the top of the visible footer? Last question is assuming that we will place the 2 inch XPS uder the concrete, do you still recommend something to place below the wooden studs so that it is not in contact with the cement floor? If so, what material would you use? Thank you so much again. You have no idea how much my husband and I appreciate it.

        • Todd says:

          The answer to your question really depends on the slab elevation with respect to top of footing. In some situations the bottom of slab is an inch or so below the top of footing, in that situation the insulation would stop at the edge of footing. If the slab is entirely over the top of foot by more than 2 inches then you could put the insulation over the footing. Either approach is acceptable.

          If you insulate below the slab then some folks don’t actually insulate above. If you install a wood sub-floor and it’s in direct contact with the concrete they you’ll want to use pressure treated lumber. I’d use pressure treated even if it rests on top of the foam.

  104. John Taylor says:

    Dear Todd,

    I have a very old, partially frame farmhouse in PA. I’m presently remodeling the master bath and would like to add some insulation to the framed area. It is aluminum siding over minimal foam over wooden siding. There is an air gap, then lathe and plaster. I was thinking of adding 1/2″ polyiso foam covered with dry wall to the interior of the two ezposed walls. I read you comment about BIBS and wondered if this can be blown into the wall cavities? I’m trying to avoid ripping out the plaster and lathe.

    Thank you for your help,

    • Todd says:

      Insulation contractors have been blowing cellulose into wall cavities for years. They drill small holes in each stud bay from the outside or inside. Shouldn’t be a problem at all.

  105. Mike says:

    I will be insulating my basement concrete wall. My house is split level, so half the basement is above ground (already insulated) and half below. Can I glue the foam board right to the concrete? And should the foil be facing out? ALso, will I then also need another moisture barrier like plastic sheet before drywalling? Lastly, is it ok to insulate an area (under stairs) that i will not be drywalling?
    Thanks in advance! I am also in NH, gonna be getting cold soon wanna make sure I insulate properly. Thanks!!

    • Todd says:

      I would not use foil faced on the concrete. I prefer XPS foam board on concrete. Yes it can be glued in place. All foam boards should have a thermal covering to protect from fire.

      • Mike says:

        Thank you! I have Dow foamboard with one side foil… will this be ok? Also, what is a suitable thermal covering on a place i won’t drywall and how do i fill the foam board seams, will a tape be ok? SOrry for all the questions, i appreciate your response!! Very helpful . thanks, mike

        • Todd says:

          Yes on the foam with only foil on one side. Most codes require a 20 minute thermal barrier such as 1/2″ drywall. Some wood products do meet that requirement. Products like plywood, OSB or T&G siding. You have to check with your local building code official.

          Tape works great.

          • Mike says:

            Will the foil act as a vapor barrier or will i need something additional? Also, I assume the top of the concrete (sill) will need to be insulated as well?

          • Todd says:

            Typically we like to see at least 1-1/2″ of closed cell foam in order to have an effective vapor barrier. How the foil effects that I’m not really certain. Yes, the top of wall up to the sill plate should also be insulated.

          • Mike says:

            Also, do the outlet boxes need to be filled around with foam air tight?

          • Todd says:

            If you install the foam board behind your wall then there is no need. If you install the foam between studs in the same plane as electrical boxes you’ll need to seal them well.

  106. Demetrios says:

    I have a 1st level room (above grade). The wall are cement block. The 2×2 studs are nailed in the block. So I only have 1.5″ space for insulation. I was thinking of glueing 1.5″ blue or pink rigid insulation between the studs. Do I need space between the studs for expansion or can it be a snug fit? Do I need a vapor barier over the insulation or can I just use 5/8th drywall directly over it?

    Also I have a garage (cold room) next to the room and a kitchen above the garage. 2″ board on the garage ceiling glued to the plywood? or 1″ board plus unfaced fiberglass insulation.I have plenty of space for insulation 18″. I want to limit air leaks between the garage and kitchen with the rigid insulation. Closed cell foam spray too expensive will open cell sprayfoam be ok?

    By the way half my 2 car garage is under the deck and the other half under the kitchen. also water pipes are in the garage cieling near the garage and farther from the kitchen floor above. Worried about freezing. Kept warm with open heating duct blowing into the cavity between the drywall and kitchen floor above.

    Any advise is welcome. Thank you.

    • Todd says:

      First of all I would remove the 2×2 and install a continuous layer of insulation. Any other approach is prone to serious problems.

      For living space over an unconditioned room like your kitchen it’s best to use either closed cell spray foam or some type of blown in cellulose with a layer of foam board. Open Cell Foam is a really bad idea in my opinion.

  107. Thomas says:

    I am wanting to re-insulate a vertical wall which exists between a finished ‘attic’ and an unfinished attic. The finished side is actually the upper pitched end of a living room with a cathedral ceiling.

    The drywall to the finished side is attached directly to a truss. The opposite side of this truss is in the unfinished attic. It currently only has faced fiberglass batts which have been installed between the voids found within the truss, which is actually 2×4’s laid flat; only 1.5″ thick.

    I want to add more insulation to this triangular, center peaked wall. In a perfect world I would simply add 4″ of polyiso, seal it up and be done with it, but there are complications.

    My scuttle can barely accept a 32″ piece of 3/4″ polyiso (1/3 of a 4×8 sheet). A 3 or 4″ panel would probably have to be cut even smaller.

    Originally, I was planning on putting up the 3/4″ polyiso (which I can easily get at Lowes where they cut it in thirds vertically so it fits in my car..I’ve no truck). That would give me an R-5 for now (plus anything I might add in the 1.5″ void provided by the flat 2×4’s of the truss). Then I was going to later add some R-19 or R-30 batts or rolls on the side facing the unfinished attic.

    The point being to seal off this area asap before it gets really cold because about 30% of the fiberglass which was there before has fallen out. So even adding the 3/4″ and sealing it would be a big improvement.

    This is a 12:4 pitched roof and not much fun to work in so I want to keep the work up there as simple as possible. I am especially concerned about the increased difficulty in 1) getting the 3-4″ through the scuttle, 2) cutting it, both before I get it into the attic and also once it’s up there, 3) attaching the 3-4″ to the truss.

    My gut is telling me to get the 3/4″ installed and sealed now, basically because it would be so easy to get it into the attic and especially just using not very long drywall nails to attach it. Then seal it and solve the additional R-19 or 30 later. Possibly by stacking batts or rolls against it.

    The height of the truss is only 50″ at the center so it would taper down as I would work towards the ends.

    Not knowing how difficult it is to cut and attach the 3-4″, my dilemma is: do I want to fight the 3-4″ in the cramped area or put on the easier 3/4″ polyiso and figure out how to add unfaced (I think) rolls to it later. I plan on blowing in quite a bit more insulation to the existing 3-4″ which is in between the trusses now, once I get the prep work done.

    I do have a friend with a truck to get some 3-4″ here. I think I would need 2 or 3 4×8 panels to finish it.

    Not being able to pull the trigger about which thickness of polyiso is really bugging me. Something just keeps spooking me about working with the thicker pieces.


    I am located in an uppermost county of Zone 4, just south of Zone 5. Southwest Ohio.

    • Todd says:

      I can certainly understand the cramped quarters and not wanting to spend much time up there. I have a different solution that you might want to consider. When we build this type of wall that divides conditioned and unconditioned spaces we treat it as though it were an attic floor, meaning we want at least R38 or more if we can get it. Obviously that’s not easy with foam board. So the insulation contractors typically have us create a 2′ wide void between that wall and the next adjacent truss. We do that by attaching Tyvek or Typar to the next truss and then nailing 1×3 boards over the truss members to help reinforce the building wrap. Then we blow that cavity completely full of cellulose of fiberglass. The beauty here is you don’t have to fuss with all those attachment details and you get a super insulated wall. Blown in insulation is cheap and you could even do it yourself. Make sense?

      • Thomas says:

        Hi Todd. Thanks for the quick reply.

        I understand your idea. It’s very similar to what I was thinking of doing with rolls of R-30. The difference being your elimination of the ‘sealing’ part provided by the 3/4″ iso board, which I thought would help as I’ve noticed that some of the horizontally blown in stuff seems to have been blown around at least in one nearby corner, which is at one end of the truss I’m trying to insulate.

        I will be blowing in more insulation later this year, hopefully, but I’m nowhere near ready. I need to install attic baffles (which is a pain, I’m working on trying to pre-glue and pushing them into position at the soffit ends) but that will take time. I also need to cover some voids created by dropped ceilings over the kitchen cabinets (unless it’s ok to simply fill them with the blow-in).

        So I guess my question now is will the 3/4″ iso sealed board help? Would it offer a vapor retardant? Do I even need a vapor retardant?

        I have been spending my morning trying to figure out a way to keep a stack of R-30 rolls in place. The stack would only be 50″ tall at the peak and shorten pretty quickly afterwards. The lowest 2 runs would take it up to 30″ (using 15″ widths) which seems to me could be held in place with wire or a similar framed rig that you mentioned. I don’t know if the rolls/batts would want to settle if only stacked 2 high, 3 at the most. The R-30 rolls would measure about 9.5 inches wide (when stacked vertically).

        I think I’ve talked myself out of using the 3-4″ iso. Just too much attaching work I think.

        • Todd says:

          The 3/4″ will certainly help “seal” up drafts. The reason we often do the blown in is it can really seal things up very well when pumped into a cavity like that and packed full. Even if you do the 3/4″ iso I’d still use this approach when you blow the rest of the attic.

          • Thomas says:

            Good advise.

            At least now I have a plan of attack which can be modified along the way. I really couldn’t start anything until I had decided whether of not to try to use the thicker foam boards or not.

            I’ll go ahead and put up the 3/4″ iso and see how that goes. Should help and can’t hurt. I’ll be looking for some 24″ ‘cavity designs’ in the meantime.

            Great website.


  108. Trent says:

    I’m trying to insulate the completely uninsulated attic in my 1926 house, in Seattle. The ceiling is lath and plaster with 2×4 joists. Insulation would probably be blown in cellulose or fiberglass rolls. Except the area where the roof is highest, which I’d like to use for storage.

    Obviously a 2×4 cavity full of cellulose with a plywood floor on top isn’t going to have an R-value anywhere near the R-38 I should be aiming at. So one suggestion I’ve gotten to is place 4″ of polyiso sheathing on top of the cellulose filled cavity, and put a bit of cheap OSB or ply on top to protect the surface of the polyiso from dents.

    Is the polyiso strong enough to support the weight of someone walking on it, if there is a some kind of covering?

    Will the polyiso sheathing trap moisture in the cavity under it?

    • Todd says:

      The polyiso is more than strong enough. That’s what’s used on flat roofs under rubber/pvc roof linings. The polyiso does act as a bit of vapor barrier which in this case should be close to the warm surface. However, if the joints are not perfectly sealed then it’s probably ok.

  109. Brian says:

    Hi Todd,
    I just watched your video on Youtube, Nice job.

    I Have a small 900 sq ft single family house with a walk in cellar in the Boston,MA area.
    It was built in the 50’s. All 4 walls of the Foundation are 7 ft poured concrete, except for cinder blocks by the doorway.
    The house is heated with forced hot air.
    I am wondering, since it has a walk in basement,
    (the front foundation wall is back-filled, the rear is bare).
    Does this affect how you insulate?

    There was a family room in the cellar when I first bought the house.
    The paneling had mildew stains on it, so I tore it all out.
    On extremely humid days I have seen moisture spots on the concrete floor.
    I am thinking of insulating the cellar so I can work down there comfortably. The furnace doesn’t seem to be able to handle heating the cellar & first floor.

    At what point is moisture a problem?
    Will a vapor barrier keep the moisture out or will I have to use a de-humidifier?
    Do I need to insulate and seal the entire cellar & floor in order for the insulation to be useful?
    Will insulating “only part” of the cellar” help?

    In your video, you used 2″ foil faced polyiso at the rim joist, then you used 2″ Blue foam board on the wall.
    Why did you use different insulation at the 2 locations?

    Thanks, I have been enjoying reading your answers.

    • Todd says:

      Brian – Glad you found the video and article helpful. Here are my thoughts on your situation.

    • Adding 2 inches of closed cell XPS foam to your walls is going to make a huge difference in the humidity and comfort of that room. Be sure you seal all the joints well and it runs continuous from the slab up to the bottom of your floor deck above (rim joist area).
    • I use XPS on the basement walls because it works great in direct contact with the concrete. Foil faced isn’t made for direct contact with concrete. I use the foil faced polyiso in the rim joist because it has a higher R value and better vapor barrier properties.
    • You don’t have to insulate the floor in order to make a big difference. Obviously if you do things will be even better down there but it’s not mandatory.

      Good luck. I hope you’ll consider signing up for my FREE Weekly Newsletter so you can keep getting great Home Improvement Tips.

  • Brian says:

    What is your opinion on sealing the concrete with Drylok (or something similar)?

    Thanks again

    • Todd says:

      Not sure I understand your question. Can you be more specific?

      Products like Drylok are ok for helping reduce moisture release from concrete. However, they are not sufficient to act as an effective vapor barrier for basements that need to be insulated.

      • Brian says:

        I went to Home Depot the other day to get an idea what types of foam insulation they carried. I asked one of the workers there if they could tell me the pros & cons of what they carried. He said he was a contractor and that I needed to seal the concrete before I put up any insulation. He showed me the Drylok. I always become a little suspect when somebody tells me how to do something when they don’t listen to my question. He never did answer the question I asked. But since I was not familiar with concrete sealers, I wanted to learn more about it.
        One place I looked said that sealing the concrete could lead to other problems. So I was curious what your thoughts were.


  • Roman says:

    Hello Todd,

    Hi Todd,
    I am very confused and concerned.
    We are renovating our home and basement walls are troublesome.
    The following is what we have.
    Concrete Block Wall that is drained from the inside (yes inside) of the house using weepers around the perimeter of the basement with Delta MS attached to the wall (up to ground level) which extends over the weepers under the basement floor. The bottom course of block wall have 1” holes drilled into the cavities so that the walls drain into the weepers.
    Also there is a brand new weepers system on three walls on the exterior of the house which also has Delta MS which covers the exterior from ground level to the footing.

    I want to insulate the interior walls. I started using 1” DOW Clad mate and was going to frame and continue insulating using batt insulation and cover with drywall. (No vapour barrier)

    I have a few concerns.
    Is 1” DOW OK to use if I’m adding further insulation between the studs after framing. Space is an issue.
    When attaching the DOW product to the wall, it isn’t perfectly flat. The Delta MS is a little bit further away from the wall and this leaves a little gap under the Clad Mate where the Delta is connected to the block wall. Also the Delta product is covered with divots.

    Thank you,

    Roman from Toronto

  • Theresa says:

    Hi, Todd. I am wondering what your opinion is on the following.
    We are building a 24×38 detached garage in Illinois and trying to determine the best insulation option for our use and budget. We have the building all framed and closed in and now ready to insulate. Other than housing vehicles, we entertain a lot and want a shelter /entertainment space for guests at our outdoor parties. Also, we plan on keeping left over paint etc in a part of the area and don’t want it to freeze in winters. We initially planned on using fiberglass batts to insulate it and maybe some blown in for the attic space. (2×6 walls and 8/12 pitch roof- storage option in attic) However my husband really wants to do closed-cell spray foam (professionally installed). I am not really sold on the cost / benefit of it’s use in this type of application. I don’t want to heat or air-condition the space and think fiberglass could meet our needs at close to 1/2 the cost. Do you think spending twice as much to insulate an un-heated/un-conditioned space would make sense? In my mind, its a garage, not a house. Thanks.

    • Todd says:

      Theresa – I sure don’t want to get in the middle of a house hold feud but I’ll give you my two cents.

      I agree that spray foam is probably over the top for this space. However, I don’t think fiberglass is such a great option either. I’d pick a solution in the middle which will cost less than foam but perform almost as well. I’d suggest you look into some sort of blown-in fiberglass or cellulose similar to this:

      We are using more and more of this type of insulation and the results are amazing. It costs slightly more than fiberglass batts but it’s probably twice as good at sealing and insulating.

      Good luck!

  • Dave says:

    I have an aluminum frame sliding glass door going out of my back bedroom that I intend to remove wall off and put a window there next summer. I am looking for something to insulate the door for the winter and do not want to spend very much as it is a temp thing. I was wondering if spray foam to seal all the edges and 1/2″ cheap foam board insulation would be wise or if bat insulation would work just as well?


  • Dave says:

    Sorry I forgot to tell you that I am on the southern Oregon Coast.

  • mattgenton says:

    hi to all at i thought i had sent this newyears eve but it didnt send so i have sent it again all things good for the new year to all of you
    – matty g

  • Mark Z says:

    Hi we are interested in moving our double wide out of a park and on to our own piece of property. We live in upstate NY and we have to either have a 4 foot frost wall slab which I have been told is very expensive. I was also told by our town code enforcer that I can do a floating slab with high density foam board around it to protect the slab from the frost. Can you please tell me what the thickness should be and if it has to be on the bottom of the slab along with around the sides and it should all the the same thickness or the sides different from the bottom. Also how would I go about pouring the slab on top of the board? Thanks for any help you are able to give me.

    • Todd says:

      Floating slab details vary greatly around the Country. You’ll likely need to work with your local building code official to work out the fine details. “Floating Slab” actually implies that you let the slab move up and down with the frost. Technically what your inspector is asking for is NOT a floating slab.

      Most people in the industry consider each inch of foam board to be approx equivalent to 1 to 2 feet of artificial cover. So if the thickened slab is a foot below grade, and you have 2 inches of foam under it, you’ll likely defeat the frost.

      Most thickened slabs are roughly 12 inches thick at the perimeter and 4 to 6 inches thick for the remaining slab. You’ll need to put the foam under the slab, then pour directly on top of it.

  • Don says:

    Hi Todd,
    I am interested in putting down a temporary floor in a garage (aprox. 300 sq ft)for a studio space (used for yoga)that can be removed in the future to convert back to garage space. I know that the correct installation involves laying rigid board with sleepers and subfloor. However could you lay 1 1/2″ rigid foam board without sleepers but rather two layers of plywood? That is: 3/8″ sheathing grade with a second layer of 3/8″ (G1S grade) layed perpendicular to each other, screwed well together, but just resting on the rigid foam board? All of this bounded by a perimeter of 2x4s that the plywood would be screwed into? It is a well levelled concrete garage floor. Thanks.

  • BEN says:

    Hey Todd, im going to be siding my house this spring . I would like to know a couple of things before starting. I live in north central connecticut, my house was built in the 50’s with a Frank loyd Wright design in mind (not original Frank L Wright).It s 2 stories about 3600 sq ft outside. I have original cedar clapboard and shingles on there now. My question is do i demo the existing wood siding to the plywood sheathing or do i go over the wood siding with rigid foam polyiso. Then if i do demo the existing wood siding do i put tyvek first then rigid foam or rigid foam first then tyvek over foam. Also being in New England what thickness of foam should i use 3/8”, 1/2”, or 1”.

    • Todd says:

      Ben – Great questions. The answers vary depending on whom you ask.

      1. In my opinion the best approach is removing the old siding. This allows you to inspect the sheathing and framing for damage and make appropriate repairs.
      2. Most house wrap manufacturers recommend installing their product over the foam. This gives another layer of protection against air infiltration and also water.
      3. Thickness = Money. This topic is purely cost vs performance. More is better…but it also has implications at the door and window trims.

      Good luck.

  • Robert says:

    Hi Todd, I have no real knowledge about construction, so this may be a very ignorant question. I am in Florida – so heat is the big issue. I just bought a house built in 1964. A large hole was knocked out of the interior drywall on the second floor, and there is ZERO insulation between the drywall and exterior siding. (The inspector somehow missed that.) The first floor is concrete block, the second story wood frame. Blown in foam insulation is out of my price range. If at all possible, I want to insulate room by room (pay as I go)and I need to know if I can put up 1 1/2 inch of pink board on top of the existing drywall, and then put up another layer of drywall? Home Depot only carries 3/4 inch tongue and groove pink board, so I would have to do 2 layers to get the 1 1/2 inch (or special order thicker board from somewhere?) Tearing off the outside siding is not financially an option. I would consider tearing out the interior drywall (if truly necessary) and inserting the foam boards between the studs, and replacing the drywall, as I could do that room by room. Two month electric deposit was $1,000 – so cooling bills will be outrageous. What can I do, working from the inside? Which foam board would you recommend for this area? Humidity is also an issue. Any advice is greatly appreciated. Thanks.

    • Todd says:

      Robert – Sounds like the 2nd floor is the real problem correct? and that’s the wood frame section?

      Here are some thoughts.

      1. Installing foam over the drywall would be bad. DON’T do that!
      2. My expertise is more geared toward cold climates. One of the best resources for hot humid climates is:

      3. My best assumption for your wood framed, 2nd floor, is blowing cavity insulation into the stud cavities. Again, you’ll need to check this to the document above and also local insulation contractors. If that’s the case it’s still something you might be able to DIY. Some Box Stores rent insulation blowers which you might be able to use yourself.

      Not sure foam board is the best solution for you considering your parameters.

      • Robert says:

        Hi Todd,

        Yes – exactly – the problem is the second floor, and it is the wood framed section. The link you provided had a lot of information about new construction in hot humid climates. Not so much on retrofit repair that I saw (or understood…)

        I understand your specialty is cold climate. Even so, I am certain that you provide unbiased advice, and know more than these salesmen I have spoken with. I have hot and humid (Florida).

        Just found out large parts of the exterior have to be replaced. So, it will all be replaced, wall by wall. So the ‘blow in’ is not an issue anymore. There is not even any plywood under the exterior sheathing / cladding / siding. The DIY spray kits all needs to be used at once – and this project cannot be done all at once, and it is the second floor.

        #1) **** Now, working from the outside – would the foam panels be a viable option? The layers from the outside would be the: A) exterior cladding / siding, B) plywood or whatever is recommended, C) insulation panels – or roll insulation if absolutely required, and D) the existing interior drywall. By viable for the panels, I mean possible at all. No settling, no gradual loss of R value.

        #2) **** The panels also provide more *acoustical insulation* and that is a BIG plus in my neighborhood.

        #3) **** Do you recommend roll style insulation in addition to the panels (assuming I can use panels at all?) Which layer would the roll insulation be? I am trying to avoid the $500 a month plus cooling bills the previous occupants had.

        Thank you for your time, and sharing your knowledge. This is one of the only forums where there is no “agenda” to sell a specific item – and that is great.

        • Todd says:

          Robert – Thanks for the kind compliments. I hope you’ll continue to visit this site for advice and share it with others if you feel it’s a valuable resource.

          Your proposal seems solid. The issue ultimately will be price and how much R value can you get on the panels. Here are a few things to consider.

          1. If you’re suggesting that you put just a layer of insulation on the outside studs it will likely be quite thick. This can cause issues with windows and doors. If you put it in the stud cavities then it won’t be a continuous layer. I’d probably recommend some insulation in the stud cavities and and then a 1″ layer continuous over the studs before the sheathing.
          2. It’s important that each sheet of foam be sealed with an approved taping system.
          3. I would avoid the roll insulation.
          4. I’m not sure about termites…we don’t have them up here…so that might influence what type of foam to use.

          Good luck!

          • Robert says:


            Thanks for the advice. Now I have a plan. Clear, concise and direct – exactly what I could not get locally. Avoiding the roll insulation is a big plus – my helper is allergic – so I don’t have to handle everything myself.

            Thanks very much, and yes I will recommend this forum to all the DIY people I know.


  • Dave says:

    Todd–I am interested in mechanically fastening and gluing xps boards to a metal wall. I saw some “plastic nails” (that is what the article called them) which were exactly what I had mentally envisioned as working well. I thought I had invented something new until I saw them on the internet. The ones I saw appeared to be constructed of a thin wire stud covered by a barbed plastic sheething. The idea is to weld the metal part of the plastic nail to the metal wall (probably with a regular stud-welder) and then push the xps board onto the protruding plastic nail which were not long enough to go all the way through. Using this way of attachment the xps board could not come off because of the barbs but there was also a good thermal barrier between the wire and the foam–no chance of a thermal bridge. I can’t find them anywhere. I know they exist because I got a picture of them. Can you give me any directions as to where to look?

    I also was hoping to find xps board pre-manufacture with foil on one side (for radiant purposes) and a rough fiberglass mesh surface on the other side (stucco ready). It seems like a no-brainer but do you think I can find it? Any ideas on this?

    Looking forward to your response.

    Thanks in advance,


    • Todd says:

      Dave – Sounds like a plan but heck if I know where to find such an odd product. Mind me asking “why” or what application this will be used for? Is it really necessary to use studs?

      There’s one foam board product out there (name escapes me) that I believe has foil on one side and some sort of plastic on the other.

  • Dave says:

    The whole point is to mechanically fasten and glue xps board to a painted metal wall without creating a thermal bridge. Glue by itself would only be as good ad the paint and I don’t think it is cost effective to sand blast. In high winds glue may not hold. Plastic nails seem perfect but if you have other ideas, I am open! I will keep looking.

    There are some foam panels with a smooth finish on on side but that would not be good for stucco nor would it make financial sense to cover a surface meant to already be the finished surface. Maybe I will have to get someone in China to make it.

    If you have new ideas or some across something let me know.


    • Todd says:

      Why not use an insulated metal panel. Lots of metal buildings today are built with them and they have a nice finish on both sides.

  • Ursula says:


    What a fabulous website! I have just purchased a small (962 sf) single level home in central VA. Built in 1933, it appears to have no insulation, anywhere. There is sheet plywood nailed over lath and plaster interior wall which I plan to pull off. What would you recommend for under the drywall? Money is an issue. Outside is aluminum siding over the original wood which will need to be dealt with later.. Gable roof had tin which was shingled, this is being replaced with a metal roof next week. I would appreciate any ideas you could give me on the insulation. Thanks

    • Todd says:

      Ursula – You have a couple options.

      1. Hire a contractor to drill holes and fill the framing voids with insulation (usually cellulose).
      2. Remove all the interior plywood, lath, etc and install insulation yourself.
      3. You could install a layer of foam board over the framing, then install new drywall. This creates issues at door and windows with sills, jambs, etc.

      Good luck.

  • Terry L. says:

    I used some R tech 3/4 inch to put up in our windows that have west exposure here in Phoenix. Our 2 story heats up like a greenhouse with many larger windows on the west side of the house. The afternoons and evenings are brutal. We notice if we use foil it cuts down on the heat buildup dramatically. I decided to try the R tech in one of the windows, but I had some concerns about a fire hazard. It seems like it would be made for heat and would repel the sunlight, but I was not sure of the risk. Foil will not burn, but it does heat up. The sun gets pretty hot and I was concerned if the insulation got to hot and dry day after day it might combust.

    Any thoughts on this?

    • Todd says:

      Terry – Foam is flammable. You’d have to check the exact specifications for that foam to see what the flame temperature is. That might give you a better idea of the hazard. You could also check with the manufacturer.

  • Joe says:

    Hey Todd,
    I’m buying 6″ 4×8 EPS panels from a guy for $20 each. (They were removed from a walk-in cooler.) I plan to cut them into ~16″ strips to us in my floor, walls and roof of this tiny cabin I’m building.
    Any advice? I’m most worried about the “sponge’ factor you mentioned and some say the off-gassing can be less than healthy but not much specific proof there. I think I’m getting a good price for some 26-30 R-Value stuff. (Walls will be rough-cut 6” or 2×4 with furring strips added inside.
    Thanks man, Joe

    • Todd says:

      Joe – Ideally you wouldn’t cut down the insulation. Ideally you let it run continuous over the framing. Obviously that poses some issues on siding or interior wall finish. That’s a great price! The sponge factor is definitely going to be an issue. You’ll want a really good vapor barrier on warm side. You’ll also want to be sure there’s no place for a leak from the roof/windows or doors. Once water gets into that stuff it will be trapped and cause all kinds of problems. I think the off gassing is a much smaller problem.

  • John says:

    I am planning to build a room over my attached garage. Can I use foam board behind the knee walls and fill the stud cavity with fiberglass insulation? Also can I use the foam board in the roof rafer area while leaving a space between the underside of the roof and the foam board cut to fit between the rafters due to ridge vent system? Also over this insulation I want to put fiberglass? Would this setup cause moisture problems? I live in North Carolina south of Raleigh?

  • John says:

    How should I handle moisture problems? A vapor barrier, How and where to install one?

  • John says:

    The foam board I am looking at has a one side with it seems to be thin plastic on it. If installed between the rafters which side is the roof side?

  • Dan says:

    I am insulating my Seattle basement . Using pink foam on concrete walls and caulking all infiltration areas first. My question is about the concrete slab floor. I would like to find a high density foam to put on top of slab under floating pergo-type floor. The floor has been very dry the three decades I have lived here. Am trying to locate a foam 1/2 inch or less to just give a bit of thermal buffer, but not loose much height.
    I see the Dow HPU extruded poly (blue) that is 3/8 thick and used for under siding.

    I see commercially, or for roofing, some foam is 40 or up to 100 psi density.
    Is there a better product I can get my hands on ???

    • Todd says:

      Your best best is the high density DOW products. The trick will be finding it in a thickness you want. The best place to find it is a contractor supply depot. You might have to special order it.

  • Tammy Carpenter says:

    Recently purchased a 4 level, approx 3000 sq ft home with a sandstone exterior in upstate NY. Last year’s winter was a mild one and we went through 1,000 gallons of oil! Cut a hole in drywall in upstairs bedroom last week to find that owner placed drywall directly on concrete block with firn strips – no insulation whatsoever. Contractor friend advised foam board over existing drywall, then re-drywall over that. Your thoughts? Do I use EPS? XPS? or polyiso [that latter not looking good based on all these blogs I’ve read about moisture and gases]. Though I must admit I’m still not positive over existing drywall is acceptable, though much easier than tearing it all out! Again, the entire home is concrete block with a sandstone exterior – nothing but a conduit for the cold. We are anxious to start insulating but I need to do it right. Please lend your advise. Thank you so much!!!

    • Todd says:

      Tammy – Wow that really stinks! In my professional opinion you really should REMOVE the old drywall first. Otherwise it’s likely to absorb moisture and eventually mold. I would recommend removing the damaged materials (inside and out), installing closed cell XPS foam board, taping seams, then drywalling.

  • Todd says:

    Hi Todd,

    I live in southeast PA. I’m planing on finishing my basement. Right now it is a 10 inch block wall, all underground. I was going to put 1 inch foam instulation over the entire wall, then do a 2×4 wall maybe 1/2 inch away from the foam and insulate it with Fiberglass. I read concerns in another inquiry that maybe I should not go with fiberglass if I only have an inch of foam. Please advise.


    • Todd says:

      My concern is you shouldn’t even attempt this unless you use a minimum of 1-1/2″ and preferably 2″. You need that much to stop water vapor from leaving the block wall and getting trapped in the new wall cavity. Fiberglass is fine after the minimum foam. Good luck.

  • glen says:

    Bought a cabin northern Wisconsin. Cold lower floor wooden floors on concrete slab. No foam insulation around the slab when built in early 90’s. Now would like to put it around the foundation. Is on a slope so earthen coverage varies. Half the house is only accessible by going beneath the closed off decks. Should I use that blue foam/pink foam? Does it glue on? Approx how thick?
    Thanks for any info. Not that handy but the energy audit guy thought it would help.

  • Joe koehler says:

    I want to make my own hot tub cover because all of the manufacturers only use expanded polystyrene because it assures them you will buy a buy a new cover every three years. I was planning on making my own cover using extruded polystyrene but after looking at this article, I wondered if polyisocyanurate would be better. Which one is less likely to absorb moisture?

  • Sarah says:

    Hi Todd,

    Sorry if this is a repeat of another question. I’m having a contractor finish my basement and he’s recommending 1″ foam board adhered to the concrete walls with no further insulation. I live in Kansas so its hot in the summer and cold in the winter. Is this method sufficient? If not, would 2″ foam board do the trick or do I need 1″ plus bat insulation.


    • Todd says:

      Sarah – 1″ is definitely not enough. 2″ is enough to keep things warm and dry expect when it’s exceptionally cold. 2″ might meet your local code but you should check.

  • John Gagnon says:

    Hi Todd,

    What would be the best way to insulate our vacation home in the Philippines, where average daytime temperatures are usually 25 – 32 Degrees C? House is 2 stories, solid cement with up to 6″ thick walls. Heating won’t be necessary, just cooling. I was thinking 2″ XPS on all of the interior sides of all the exterior walls, strapping with something, then dry walling over it. The problem with wood strapping is termites, but the real problem is the XPS sheeting, I’m having a hard time finding any suppliers here. I spent 2 weeks finding spray foam insulation to finish installing my windows, they just haven’t heard of that stuff there. I’m going to need about 3000 sq. ft. So I might end up getting it shipped from China. The other method would be using steel studs and insulating with R12 glass wool and vapour barrier, but you still would have to keep the steel studs off the cement walls with something to prevent thermal transfer.

    • Todd says:

      John – The best approach would be to use light gauge metal hat channel, then 2″ foam, then additional metal furring, then drywall. Good luck.

      • John Gagnon says:

        Thanx for the answer Todd,

        So I take it you would screw the hat channel to the cement wall,(Raised part towards the cement, or the feet?), then sandwich the foam between the next hat channel and screw them together? Then screw the drywall to the raised part of the exterior channel? This would result in an air gap of 7/8 (or whatever size channel I use), on both sides of the 2″ foam?


        • Todd says:

          John – You’ve got the idea. I’d “shoot” the hat channel onto the concrete with some type of pneumatic nailer or powder actuated fastener.

  • Ryan says:

    I am replacing my peeling wood siding with vinyl on a 60 year old 1500 sqft ranch house in Wisconsin this summer. I currently use a dehumidifier upstairs in a bedroom and also a large unit in the basement because we always have frost on the windows and have a hard time keeping it below 50%RH inside. We use our bathroom fan for 20 min after showers and do all we can to keep the moisture down. I am replacing all the 60 year old windows at the same time and want to do the best I can to avoid mold or moisture problems in the future. I have textured and painted drywall inside, craft faced insulation inside the 2×4 walls and 7/8″ thick fiberboard outside that will stay. the wood siding will be torn off.
    There is plastic between the drywall and ceiling joists on most if not all of the ceilings in the house, 6 inches of open face insulation run between ceiling joists, 3-1/2 inches of open face insulation run accross joists and 11 inches of celulose free blow in insulation on top of that.
    I have hip roof ends with 2ft vented soffit on all sides and a ridge vent along the entire length of the house.
    The basement exterior joist spaces are all insulated with open face insulation and exterior top 2 ft of foundation has 1-1/2 inches of pink board insulation and ground breaker over it.
    What would you recommend using under the siding and why?

    • Todd says:

      Ryan – I would recommend using foil faced polyiso under the siding. The more the better. Because you’re installing new windows this shouldn’t be too difficult to deal with. I’d install at least 1″ of the polyiso, tape the seams, then install vertical strapping to create an airspace, then the siding. This means pushing the windows out a bit further and making deeper extension jambs but the reward will be well worth it.

      Good luck.

      • Ryan says:

        I am worried about moisture sandwiched between the insulation and the fiber board sheeting from the inside moisture trying to escape.
        Vinyl siding already has an airspace behind it, what would be the point of the furring strips when it already has drainage and breathability?
        After some more reading I am leaning more towards putting 3/8″ furring strips between the fiberboard and 5/8″ pink board taped well at the seams and bug screen wrapped around the bottom to allow any trapped moisture behind it to drain yet prevent insects and rodents as well as prevent any rain penetration from the exterior.
        My new questions are- Should I use house wrap over the fiberboard before the furring strips or leave bare?
        Do I put wood extension under the window mounting flange to make them even with the insulation or run the insulation under the flange or over the flange?
        Do I flash the windows before and under the pink board, or over, or both?

        • Todd says:

          Ryan – The most important issue is controlling the location of the dew point across the wall section. The more insulation you install on the very outer edge the less likely you are to have a dew point problem inside the wall cavity. So exterior R value should be the first priority.

          Properly sealed foil faced polyiso will stop exterior moisture from penetrating into the wall cavity.

          Having considered some of your statements I guess I’d rethink this a bit, I’d certainly put Tyvek or Typar over the fiber board first. Then Ideally you’d install some type of drain plane (strapping or rain screen material), then the polyiso, then the siding. The bottom of the drain plane should have screen of some sort as you pointed out.

          Most builders will place blocking behind the window flanges, 1×4, or something similar so the windows have a good solid connection.

  • Ryan says:

    If I put extension jam under the window flange, how do I seal it properly?
    House wrap up and over the extension or under and caulk it good? I’m afraid I could get water under the top corners of the extension. Or have water get behind the foam more easily being that I will only be able to caulk it against the window extension jam. I’ve been searching everywhere and can’t find a single picture, diagram, video or how to with an extension under the flange. Seems like going over the foam and using that as the extension would be safer and caulk the underlying furring strips well to keep them from allowing the moisture under the foam from penetrating. Even then, what do I do with the top flap of house wrap? Wish I could find a picture of how to do it correctly before I start and run into problems from guessing and hoping it won’t be a problem.

    • Todd says:

      Ryan – The extension jambs are installed first, then they get taped and sealed just like any window or door opening, being careful to work from the bottom up so that laps are staggered to prevent water entry. Then you install your foam and seal to the previous sealing membrane.

  • Mike says:

    Does the foil side face up if installing 6 inches below a metal roof ?

  • Chidski says:

    Dear Todd. Thanks for sharing the knowledge, appreciate it. I have gained confidence after watching some of your work/guidance. I am in NJ, my basement is made of hallow blocks. I have three questions (1) can I use Polystyrene / Polyisocyanurate Insulated sheathing (1/2 inch or 1 inch) to on block (2) have French drain & sump-pump, it gets busy when rains, any special suggestions (3) do I have to put framing and insulation, and drywall???

    • Todd says:

      1. No…it really needs to be XPS foam when in contact with basement walls. You need a minimum 0f 1-1/2″ of foam, preferably 2″. Anything less will result in moisture problems.
      2. Check drainage around exterior of house, install properly working gutters.
      3. You have to protect foam with a thermal barrier. Most codes vary but they all need at least 20 mins (1/2″ drywall). Check with your local building official.

      Good luck.

  • ivo says:

    Hi Todd,
    We just bought an old brick house in Maryland built in 1920. We are working on one of the rooms on the top floor (3d floor)we gutted everything and what is left right now is three brick walls. All three are exterior walls but two have the old furring strips and plaster and once has only the brick. I believe all walls are 2 bricks thick and the outside is painted brick. Since this is the top floor right now there is no ceiling in this room and we can see the slightly pitched roof. As you know Maryland is hot and humid in summer and cold and wet in the winter. Since the room is only 15 x 15 we are trying to maximize space as much as possible. One contractor suggested removing the plaster and going down to brick because plaster stays cold, do you think we should remove the plaster or leave it? Now, what kind of insulation do you recommend? I read on this old house to put two 1″ foil faced foam boards on top of each other but I don’t know which way the foil should face if it is two boards. Do you think we can do away with just one 1 1/2 inch foil faced board with the foil facing towards the room? How about the ceiling above this room. Since it is currently open we can pretty much explore any option. Thank you very much for your help!

    • Todd says:

      Thanks for your question. Seeing that you’ve gutted the space you have lots of options. First and foremost you really need to find out what if any code requirements are involved with minimum insulation values. At a very minimum I would recommend at least R19 in the walls and R30 in the ceiling in order to be comfortable and somewhat energy efficient.

      You have quite a few options but each one in my opinion needs to start with a closed cell foam product. DO NOT put any type of fiberglass in contact with the masonry. I would recommend that you either use a closed cell foam board product or you use a closed cell spray foam. In either case I highly recommend that you frame walls inside of the brick (yes, you’re going to lose a foot of width most likely but it’s VERY important to get things set up correctly).

      If you use foam board you need to use a least 2 inches of XPS foam or Polyiso. I’ll warn you, foil faced foam really should not be in direct contact with masonry, I would use XPS foam. Once you install 2″, you can frame a wall, then you can use fiberglass if you wish to get the desired R value. You can use the same methods we use for basements found here:

      You can use the same method with spray foam. Anything short of this and you’ll have serious moisture and mold problems.

      The ceiling area can be done with spray foam or if you have height, possibly fiberglass but it’s not the best solution.

      Good luck.

      • Ivo says:

        Thank you Todd for the quick and informative answer. What do you think we should do with the plaster that is covering the brick? Do you think we should remove the plaster and glue the XPS boards directly on the brick or we should glue on top of the plaster?
        I am thinking of using 1″ Dow Tuff-R blue board with R vlaue of 6.5. Framing on the inside and finishing it with fiberglass R-13. That will give me the R19.5 which is what you recomended. We do have room in the ceiling so we will probably go with R-38 fiberglass.

        Thank you again!

        • Todd says:


          You really need to use 1-1/2″ foam at a bare minimum, preferably 2″ (see )
          You need to do this to provide a proper vapor barrier or moisture will get into the fiberglass and cause mold. This is ESPECIALLY true in the more humid air you have down there. I would recommend removing the plaster and cover over the brick.

          I’d also recommend a small air gap between the framing and the foam if you can spare another inch, any room that allows for drying in case of moisture or water is a good extra step.

          Good luck.

  • Russ says:

    Hi Todd,
    Thanks for all your information. You seem to have the best understanding of a confusing topic. I am in Nebraska and I am finishing my basement. My basement seems damp in the summers so I have to run a dehumidifier. I started to put up 1″ XPS when I found your information. You strongly recommend 1-1/2″ minimum for a vapor barrier. Since I have about 1/3 of the foam already up, could I glue another 1/2″ sheet of foam to the 1″ that is already up. Then complete the rest with 1-1/2″ sheets. It may be tough to tear down what I have done, or should I just finish with 1″. I am using Owens Corning foamular 150.

    Thanks again

  • John says:

    I’m going to be doing the inside of all the exterior facing cement walls of our vacation home in Philippines. There is no basement, just 2 floors above grade. I was thinking on about 75mm of XPS foam board glued directly to the cement, should give me about R12. Could MGO board or drywall be glued directly to the foam board? Strapping with wood first isn’t recommended because of termites.


    • Todd says:

      You’ll have a tough time getting a good finish. I’d recommend buying Z-Channel. Basically you install a piece of foam, then attach the z-channel to the concrete, it holds the sheet in place and gives you a place to screw drywall to. It’s used in commercial construction all the time. Good luck.

      • John says:

        Thank you for the reply. By using the steel Z channel fastened to the concrete, isn’t there going to be some thermal transfer through the channel creating hot spots where it attaches to the drywall or MGO board?

        • Todd says:

          Technically yes, however, it’s SO light gauge that it doesn’t transfer much. Concrete typically doesn’t get really that hot either.

  • Philip Foster says:

    Hello Todd,

    I have a 1938 home about 1/2 hour south of Minneapolis, MN. Needless to say it gets cold here. The old construction of this house does not lend itself well to energy efficiency. The upper level has attic space that is used for storage with exposed roof trusses. Unfortunately, most of the roof (2×4’s with 3/4″ boards under felt and shingles) has old tar backed paper insulation, yes literal paper insulation (cool articles from 1937). Some has been replaced with batt insulation but i am getting terrible ice dams on the edge of the roof. My thought is to use 2×2’s as nailers in between the roof trusses and add some 2″ rigid insulation between to make it flush with the bottom of the roof trusses. Then i would add a layer of 6mil poly and then sandwich this with another layer of 1″ rigid. I want to get the maximum R-value here and create the cold air space between the wood sheathing and the insulation, along with small vents in the soffit to allow air in between this area. Is Polyiso the best way to go here or is there a better solution to insulate the roof? I’m reading some postings about potential moisture issues with Polyiso and some off-gassing. Thoughts?
    Thank you in advance for your posting.

    • Todd says:

      Philip – Hard to give advice without seeing/knowing more. However, what you need to do is either stop the heat from getting to the attic (insulate the attic floor) or insulate the roof completely on the underside (typically with spray foam). The eave area is always a problem because you typically can’t get enough insulation in the tight area and proper ventilation. Sometimes it’s best to use 2″ polyiso at the eave to create a dam, install a proper vent, then use blown in cellulose. The idea is to create the maximum amount of R value to keep heat from below from reaching that roof surface.

  • Tammy says:

    I havea fifth wheel camper I would like to enclose around the bottom to save on propane heat costs. What is the best way to accomplish this???? Don’t want to spend a fortune its a temporary thing
    Maybe 2 yrs at most .Thanks Tammy

  • james says:

    Here is another great use for the panels. Install them on top of a finished garage wall that is shared with an interior wall.

  • Ryan says:

    I posted earlier in May above. My siding and windows are done. I installed house wrap then 3/8 furring strips then 1/2″ pink insulationboard with vinyl siding over and new vented soffit. I am now having lots of moisture problems even with a dehumidifier in my master bedroom keeping it a constant 40% humidity. I have water on the bottom 1″ to 3″ of every window in the house. I can deal with wiping that up. My bedroom is on the north east corner of the house and I have about 20 cold spots on the ceiling along north and east walls with water. Its like 25* outside now, I’m worried what will happen when it gets any colder. I was having problems with my shingle roof nails popping from no vented soffit, but now that I vented it, Its obvious I don’t have enough insulation above the corners.
    The problem is, the rafters are 2×6 and notched halfway leaving only enough room for the batt insulation that was in when house was built.
    Short of taking all my newly installed soffit down and packing in as much batt insulation as possible and re-installing, I don’t know what to do. I tried pushing the insulation down into the corner but can’t reach that far down with my 3/12 or 4/12 roof very well. I was thinking of drilling holes in center of all the spots on the ceiling and filling them with great stuff or similar and patching the holes but would probably create more mess and problems than solve. I am broke from doing siding and windows for a while. I may need a roof in about 5-10 years, but not sure the ceilings will last that long with the spots. If I tear off the roof and lower boards, could I take out the batts and install something higher r value per inch? what would you reccomend? So far that’s the best Idea I’ve come up with, but how long do you think I can wait? Next summer? a couple years?
    thanks for the help,

    • Todd says:

      Ryan – You have two main issues. First and foremost you don’t have enough insulation. You only installed 1/2″ XPS insulation which only provides about R2.5 at best. As I mentioned back in May you really should have installed at least 1″ foil faced polyiso and preferably more than that. Also, your attic insulation is way below the recommended minimum for your area. You need at least R30 in the attic and preferably more than that if possible. The eves are always tough especially on a house with shallow rafters. Those tight areas are typically best dealt with using spray foam, and then switching to something more economical when you get away from the soffit area.

      Your other issue is humidity levels in the house. You’ve got to address why the levels are so high (identify sources of humidity, showers, dish washing, cooking, improper vented appliances, etc) and deal with it. Your humidity is so high that as soon as it hits a cold surface it condenses into water.

  • Harry says:

    Hello I would like to insulate my garage door Iam in central Florida which would be better Polystyrene or Polyisoc. Quite humid during the summer months. Foil side towards the door or away from it.

    • Todd says:

      Harry – Part of it depends on what kind of doors they are, how they are made, how you will attach it, etc. Polyiso is really the best solution typically for this application.

  • Joyce says:


    I have an older home in WI that has just had the second floor redone. The exterior of the home is brick and the dormers are going to be vinyl sided. Where the old walls were taken down you can now see exposed areas of brick (there is no exterior wall sheathing beneath the brick in some areas). Where the walls were rebuilt, the walls have 1/2″ OSB that was covered in Tyvek. Trusses were installed on the roof with a 10″ energy heel. The roof insulation will be an r-50 (typical blown in insulation). We are going with a closed cell insulation in the walls but we would still like to put some rigid foam board insulation outside under the vinyl sided areas. Question is wheher or not this can still be done and if so, should the foam board be installed over or under the tyvek house wrap? I am wondering if the addition of the outside foam board will create a double vapor barrier and cause condensation issues?

  • Joyce says:

    Thanks Todd,

    However see the article listed by DuPont:

    The article basically states that the rigid foam board can be installed over or under the Tyvek but that if it installed over the Tyvek, that Tyvek DrainWrap should be used. If the rigid foam board is put under the Tyvek then Tyvek HouseWrap can be used. I am not sure if this is just a selling point or not? However, note also that the rigid foam board is installed in the horizontal position in this article. I wonder if they would receive the same test results if the seams were installed vertically? They also state that seams should not be taped (at least in the horizontal position)?

    There also appears to be a difference with the type of insulation being used (ie XPS vs polyiso etc). The polyiso can have a tendency to breakdown and retain water possibly causing condensation (ie vapor barrier) issues since the Tyvek can pass water both ways it would allow the condensation back in the wall which could rot the insulation (fiberglass) if it were to get wet. I don’t think closed cell foam rots if it gets wet?

    Finally, I also found another article by building science management which basically states that the same comments in the article that you posted above?


    • Todd says:

      First let me address the foam type. In this application I think XPS is the preferred type as it is much more forgiving with respect to water. The orientation of seams will not make any difference. The topic of whether to tape or not tape is debatable by industry experts. I think it’s far more important to seal the seems (most often specified this way by architects) to prevent water from getting from the outside to the inside. It also helps create a far better air barrier. Is it necessary? No…just my preference.

  • Ryan says:

    I have batt insulation in the walls and 1″ fiberboard exterior sheeting under my 1/2″ xps. I couldnt put more than 1/2″ on without bumping out my existing windows and doors which i didnt really want to do as they were already aluminum wrapped a couple years prior. I run a 110 cfm bath fam before and to 1/2 hour after after each shower. I have boiler heat and run two dehumidifiers keeping the house at as close to 40% as possible. I don’t know what else to do to prevent the moisture besides not running our dishwasher or cooking wich isn’t much of an option. If you have any ideas, please let me know. I’m trying to find someone to give me a quote on spraying in insulation in my corners but haven’t had any luck yet in my area.

  • Joyce says:

    Do you have a vapor barrier on the exterior wall over the batt insulation?

    • Todd says:

      Actually when we do this, typically we have spray foam insulation or dense packed fiberglass. Then wall sheathing, then Tyvek (or Typar), then the foam board.

      • Joyce says:

        Sorry – I guess my comment was not very clear. I was asking if he had a vapor barrier on the inside of the exterior wall (i.e. from the interior to the exterior – plastic, batt insulation, OSB, tyvek, foam board insulation).

        • Todd says:

          As I stated before we’ve only done it with spray foam and dense pack fiberglass with no vapor barrier. I think if you’re using batt insulation, with a vapor barrier, I’d probably NOT tape the foam.

  • Joyce Thomma says:


    The contractor is claiming that because of the closed cell foam(about 2.5 inches or about an r16)in the interior walls that they do not feel that it would be worth the investment to install the XPS on the outside of the house? In other words not worth the investment? What are your thoughts? Again I am in Wisconsin.

    • Todd says:

      I disagree. The reason for putting additional insulation on the outside (besides the obvious increased insulation value) is how it helps move the dew point closer to the exterior of the wall section. This is important with regards to condensation. We are seeing this detail more and more as designers realize the importance of this critical issue.

  • Kevin says:

    I am purchasing an old house in Louisville, KY. Built around 1900. It still has the original wood siding. Not sure if it was ever insulated, but I was thinking of applying 1/2 foam (probably Polyiso) directly to the plaster of the interior of the inside walls, then dry-walling or using a new plaster that sticks to foam, over it. I suspect I will need to install mini vents high and low on the outside to ensure no moisture entrapment.

    Or, would I be better off having foam shot in through small holes drilled into the stud cavities?

  • Grant Schumaker says:

    I built a conservatory off of the dinning room many years ago with the intent to heat it
    with a cast iron radiator. I poured a concrete slab in the bedrock for the floor with no
    insulation. Now I would like to install pex for an in floor radiant heat installation and then tile over it all for the finish floor. What product should I use for a thermal heat break and insulation to keep all the heat being sucked into the slab an not into the room?

    • Todd says:

      Let me preface my answer with saying I’m not a heating specialist by any means. Having said that it really comes down to how much “space” can you allow for insulation? Ideally you’d place a couple inches of foam board before the tubing. Not all is lost if you can’t do this, the heat will still heat the slab/rock and radiate heat from the surface.

  • Joyce says:

    Hi Todd,

    Thanks for the answers. Outside rigid foam will be installed. However, I have another question which I hope you have seen or can provide some insight to the subject. As I explained above, this is a 2nd floor remodel. During the remodeling, there were areas of which the outside brick was exposed. The brick is a veneer wall (by veneer I mean a full brick (not the thin brick) but that it is a nonstructual wall). Therefore when you walk in the interior, you can see the outside brick wall as some areas of the outside boards and tar paper were removed. So right now, we have a 2 x 4 stud, and then a 1.5 ” drainage/air gap (there is no outside exterior wooden boards or tar paper as they have been removed). The thought was to close this with closed cell spray foam. However, a few people have said that I do not want to close the air gap behind the brick. Please note that this area of open exposed brick is in the middle of the 2nd floor wall so that there are more bricks up above this and also more below this on the first floor. I would like to make sure that spraying closed cell foam directly on the back of the brick is the way to seal this? If not, do you have any suggestions? I do not want to do anything stupid. Your insight is appreciated.

    • Todd says:


      I’ve never crossed this exact situation. However, we’ve built brick veneer and having an air space is a critical component. Is there anyway to get some sheathing on the back of that 2×4 wall so when you spray it the air gap will stay?

  • Matt says:

    Hi Todd,
    Question for you regarding insulating the ceiling in my 30x40x12 shop with scissor trusses. I’d like your opinion if it be acceptable to install 3″ polyiso fitted between trusses (as you did above) but have them in direct contact with underside of roof sheathing. On top of the polyiso I’d like to spray 1″ of closed cell foam. Soffit vents will be blocked and the space will be turned into an unvented attic. Sheetrock will be placed as fire barrier on lower truss chords and I plan on conditioning the entire garage (unvented attic included) with a ductless mini split and 4 60″ ceiling fans. Will this arrangement possibly cause moisture issues between the roof decking and polyiso? The shop is only 7yrs old and the roof is in good shape. The polyiso + closed cell provides R26 protection for the same price they’ll charge for 2″ of closed cell spray foam……..basically double the R value for the same price. If this will cause moisture issues though I’ll go a different route. Thank you for any advice you can offer.

    • Todd says:

      Matt – It’s an ok solution but not the best. The problem with this approach is the insulation isn’t continuous because it’s interrupted a each truss. The better solution is the insulation on top of the roof sheathing or applied to the bottom chord of the trusses so it’s continuous. While your solution will likely work ok, there will be quite a bit of surface area that’s not insulated at all.

      • Kevin says:

        This sounds like it would be essentially a “hot roof” which is not uncommon in Canada. I agree that you would lose benefit due to the non-continuous effect. Also, in hot climate, it could reduce the life of shingles.

  • Matt says:

    My apologies I forgot to say that I’m in Oklahoma City.

  • Mel says:

    Hi Todd, we just bought a house with a 32×24, 10’wall metal pole barn garage, uninsulated, it has 1×6 horz. runners that the metal siding is attached to. The posts are 8′ oc. I know spray foam on walls and roof would probably be best, but finances are in short supply. I have been looking at 3/4″ Poly Shield foam board foil on 1 side sold at a local Home Depot. I can’t find much information on it but I think if I attach the foam board to the runners, this would give me 3/4″ air gap/vapor barrier between the metal siding and the foam, the R factor for 3/4 is 3.2. Would it be better to put the foam next to the runners or the foil side.
    I live in Indiana and we have a large difference in temp seasonally.

    • Todd says:

      Mel – Poly Shield is an EPS foam (closed cell) so that’s good. However, I do question only using 3/4″. It won’t really amount to much other than sealing out some air infiltration. You really need to get a couple inches installed if you’re hoping to make any improvement in the insulating value. Also, products like these need to be protected from fire. Good luck.

      • Mel says:

        Thanks for your reply, I was wondering the same thing, the other option is 2″ foam board for about the same price, I wasn’t planning on putting sheet rock up right away if at all, I plan on heating next year, wood?
        Just found your web site, like the look of it and some of the projects you have done in your shop, hope to retire this fall and work in my garage/shop as I improve it. Baby steps.

  • Mel says:

    Hi Todd,
    Well, after reading your reply, I got to checking I think I’ll go with R-13 Kraft faced for the walls and R-19 for the roof. I’m planning on installing sheetrock over the walls later, maybe later something on the roof for looks, and light refraction.

  • Tim says:

    Thanks Todd from everyone.

    Hi, Mid michigan home, bonus room above un-insulated garage. Just need to keep it above freezing/boiling but still be nice looking area. Gambrel roof. I will be dense packing floor joyces with cellouse no vapor barrier 6 inches. I will paying strict attention to air sealing walls, knee board space, rafters (they are 24″ on center).
    Questions from top working down:
    – 2″ air space between sheathing and 1st Polyiso board for ventilation from eaves to vented cap, any moisture issue?
    – Can I stack three 1″ on top of each to create (say R21)?
    – Do I get a reflective bonus without the dust on the foil from the second or third board down during the summer?
    – Spray canned foam around seams to the wood rafters best?
    – 1.5″ air space from bottom of last Polyiso board to 1/2 inch or more sheetrock (heard this will give R3 increase)?
    – Latex paint vapor retarder paint on sheetrock?

    Any issues whatsoever or points or anything you can do or throw at me to consider would be appreciated. Again thank you…

    • Tim – That approach will work just fine. However, there’s a better way. The best approach would be getting some or all of the foam installed over the rafters so you have a continuous insulation layer. If you don’t want to lose much height, then install some of it between the rafters and the last layer over the rafters. Definitely seal the foam to the rafters with spray foam. You can stack multiple layers. Reflective bonus?….honestly not sure.

      Good luck.

  • Dennis says:

    Dear Todd:

    I have an oddball construction project. I’m looking to build a “tiny home” – roughly 8′ x 24′, or about 200 sq.ft. (Google “Tumbleweed Cyprus” to get an idea.) Most folks are building these in temperate areas – I’m looking to build one as a permanent residence in Vermont. In order to keep it “RV classified/moveable,” I have an 8’6″ max width to work with, so I’m wondering if a combination of exterior Polyiso boards and sprayed insulation on the interiro 2″x4″ framing will keep me warm. I intend to do radiant floor heating (via “Warmboard” technology.) What would you recommend in terms of “best bang per inch” for insulation in walls, floor and ceiling?


    • Todd Fratzel says:

      Well the best foam is going to be a closed cell foam. Spray foam tops the list, polyiso would be next. I think you’re certainly onto the approach I’d take.

  • John says:

    Hi, I am insulating our basement in southern Ontario. It is 100% below grade with concrete block walls. We were planning to install 1 inch XPS (this step has already been completed), frame it with 2×4 and apply dry wall. Also there is an interior water drainage system installed along the walls. Should we be considering a vapour barrier and/or fiberglass insulation.

    • Todd Fratzel says:

      John – You really need more foam. I recommend you read this.

      The vapor barrier is created by having the proper amount of foam. A vapor barrier alone is not going to help. Good luck.

      • John says:

        Thanks Todd for the quick reply, the info you provide on the site is great (wish I had found it before I had started with the 1 inch rigid foam!). At this point should I tear down the existing 1 inch of rigid foam we put up and replace with 2 inches and then install the frame/fibreglass/drywall or can I put up a 2nd layer of 1 inch rigid foam up? Is there any other alternatives?

  • Alan Ochieng says:

    Hi Todd,
    I recently purchased a home built in 1965.the previous owners have been numerous plywood panels laying on top of loose insulation. The insulation looks to be in the 4″ – 6″ depth range, and thin in several areas.There are 18 recessed lights all over the place, and the cans are therefore sticking up over the loose insulation. I have the mind to remove all the panels and blow in some fresh new insulation.I am however concerned that by doing that, I will not be able to carry out maintenance of attic fan, lights, etc.
    What would you advise me to do, and if I went with new insulation, what R value should I aim for? Thanks

    • Todd Fratzel says:

      Alan – Blown in insulation is a great option for attics. First and most importantly you’d need to verify that the recessed fixtures are rated for “in contact” with insulation. The fixtures should be stamped to indicated if they are indeed rated for that. The depth of insulation depends on your location. Here in the Northeast we need to have at least R38 but more is preferable.

      Access does become an issue, we typically build some sort of elevated cat walk to reach areas that need occasional maintenance.

      Good luck.

  • Norman Landry says:

    Hi Todd—

    Sorry Todd my last comment didn’t have the correct e-mail address.

    I recently raised my house because of a high water table in the area. I lifted my house 7′ and framed a wall from the existing foundation up to the new height of the house. I just finished installing R-21 Batt insulation and I noticed that I have a 2″ lip in which I can install up to a 2″ foam board against the wall before I start my new wall for the basement. There will be a space from the new framed basement wall to the foam board. My concern is that if a put the foam board up I will introduce a condensation problem behind the foam board inside the wall where the batt in is. I live in southern New England. I don’t want a mold issue. If I can use a form board which one would you consider. Thanks Norman

    • Todd Fratzel says:

      Norman – Could you send me a photo? I’m having a hard time understanding your detail.

      todd “at”

  • Norman Landry says:


    I have the picture, I just don’t have a email to send it to?


  • My Gatlinburg condominium has 8″ block walls with zero insulation. Copper water lines touching the inside of the block on one bathroom wall will freeze when temps get in the mid to low teens for an extended time. I have been given the job of finding a solution to the problem. I propose placing 2″ closed cell foam board on the outside face of the block wall. I will cover this with Hardy cement board and trim it with wood for esthetics. Are the pipes likely to freeze anyway, or do you think my solution has merit? Any suggestions? Thank you so much.

    • Todd Fratzel says:

      Gary – That solution will help. Is there any way to insulate the pipes themselves as well? or get some insulation between the block wall and pipes? Concrete can transmit cold easily so if the entire wall isn’t insulated well it could still be an issue if the temps get really low.

  • chris says:

    Hey todd
    What type of board would you recommend to lay down on the ground for pex tubing with 2″ sand cover this would be below a poly pool used for raising freshwater aquatic life in orlando area of floida

    • Todd Fratzel says:

      Interesting question! Do you just need a board to attach the tubing to? If so, I’d probably use some PVC decking material or PT lumber.

  • Joanye says:

    I am about to insulate my 4000 sq ft ceiling with 3/4 inch tyvek foam which comes in sheets of 4×8. It is to prevent heat, this is a butler building metal roof never insulated. Any suggestions?

    • Todd Fratzel says:

      3/4″ foam won’t do all that much to stop heat on a metal roof. You need at least 4 inches in my opinion.

  • Micho says:

    Hi Todd,

    I love in Montreal, Canada and I had water infiltration in the basement cause by a foundation crack which has been repaired. I want to re insulate the entire basement using a mix of 1″ Polystyrene along with 3.5″ fiberglass and I was wondering if that combination makes sense and if I need to put a vabor barrier?


    • Todd Fratzel says:

      I’d NEVER use polystyrene in a basement as it’s an open cell foam product. ONLY use closed cell, and you’ll need more than 1″.

  • Micho says:

    What is cheapest way to build a subfloor in the basement for laminate flooring? I live in Montreal

    • Todd Fratzel says:

      Cheapest way is installing the laminate directly on the concrete. Next is using a system of floor panels.

  • Tina T says:


    I am about to insulate my attic and create a vaulted ceiling.
    The problem i ran into is that the rafters are just 2×6 inch with actual dimensions of 2×5.5 inch.
    Since i am afraid of compromising the structural integrity of the roof i don’t want to sister 2×10 onto the existing rafters to create more space for insulation.
    My idea now was to put air-vents underneath the roof, stuff the space between the rafters with faced Fiberglas insulation (might bring it up to R-19) and add a layer of 2 inch foam boards with an R-Value of 13 on top.
    Would that be a good solution? Do i have to put a vapor barrier down or are the faced fiberglas batts enough of a vapor barrier?
    Second: Can i put drywall on top of the foam insulation and just screw it in?

    Any other ideas how to get an R-Value of at least 30 in a 5.5 inch rafter space without increasing the rafter depth?

    Thank you so much in advance. I am trying to find answers for hours now and it is quite confusing.

    • Todd Fratzel says:

      Tina – Are you creating living space? What part of the country do you live in? In most areas there are minimum required insulation values for cathedral ceilings and attics. 6″ of fiberglass even with 2″ foam won’t likely get enough r value. Sistering a 2×10 will only make the roof stronger, not weaker, and that will get you closer. Today even with 2×12 rafters it can be hard to get enough insulation value for a sloped ceiling application, and we often end up spray foaming the roof in order to make it work.

      • Tina T says:

        Todd, first of all thank you for your fast response.
        I live in Middle Tennessee. The house i live in was built in the 50’s and the room i want to remodel was a former garage. That means it is attached to the house, but has no crawlspace underneath. That situates the whole room about 2 foot lower than the main house. The same applies to the roof. It is a separate roof situated lower than the roof of the main house. The whole area is about 13×20.
        The main reason i wanna do that is because the ceiling in the room now is too low and the windows are higher than the ceiling meaning they are partially enclosed by the ceiling.

        My plan is to put some 2×4 as rafter ties in. If i sister some 2x10s as you suggest do they have to go all the way from the wall up to the ridge? Would it be fine to start at the knee wall and go up to the rafter ties? Do i need to put a ridge beam is to have more support?
        The rafters are 14″ apart. Can i just sister every second rafter with a 2×10? It would be a lot of wood otherwise. I would have to put 20 2x10s in. That’s some serious weight.
        Would you suggest consulting with a structural engineer?

        I am sorry about all the questions. I just wanna do it right and i am afraid the whole roof might come crashing down on me.

        • Todd Fratzel says:

          Tina – Glad I can point you in the right direction.

          – If you don’t need more rafter capacity for the room loading (not much snow there?), then you could use a 2×4 framed out away from the 2×6 rafters to get your 12″ depth for insulation. These would run parallel to the existing rafters, from the knee wall up to the new collar ties (no new ridge needed). Much cheaper and less weight. You are creating a deeper void and something to attach drywall to.

          Make sense?

          • Tina T says:

            That is a fantastic idea Todd thank you so much! Here in Tennessee we rarely have snow. If so maybe an inch or two so the snow load should be minimal.
            Just for clarification the 2×4 will not touch the existing rafter it will run parallel from the knee wall to the new rafter tie.
            That is definitely something i can accomplish and makes me more comfortable doing. After picking up some 2×12 i was really concerned about the weight. 2x4s are the better choice.
            I can’t tell you how happy i am finally having a solution for this particular problem.
            Thank you again.

          • Todd Fratzel says:

            You are correct. Best of luck, glad I could help.

  • joe says:

    Hi Todd- I’m building new construction in the fall and was wondering what your thoughts were on an ICF basement vs concrete block.

    • Todd Fratzel says:

      Joe – ICF is a much more energy efficient foundation. It does come at a cost, more money to build, needs exterior finish (stone, brick, stucco) for the exposed portion, and the interior needs to be finished in order to protect against fire. So like most things in life it comes down to money :)

  • David Slaven says:

    Hi. I am building a new home in Long Island, NY. The basement walls are studded out without a vapor barrier. Is my best bet closed cell to control temperature/moisture? Is 1″ sufficient with fiberglass inside it?

  • Hello,
    my name is Annette in Colorado, I am financially not well off. Wanting to add a 2nd Mobile Home to my existing one which has No insulation. I found 30 sheets of 4×8 in various thicknesses of foam sheets on CraigsList but don’t know the R-Value. How can I determine if they would be useful to me, please before I buy them. Thank You so very much for Your reply.

  • Michael says:

    My name is Michael and we are building a new house in Central IL, zone 5. You said you end up using spray foam in cathedral ceilings to get your r-value, do you use another insulation with that? Also, what do you recommend for our 2×6 walls?

  • Brad Allen says:

    what’s the thickest individual sheet / board insulation?

  • Eric says:

    I am looking for ideas to insulate a cathedral ceiling. I only have 2×8 ceiling joists and was wondering if I could put either foil faced polyiso or blue board poly directly against my roof sheathing underside, then spray foam and seal all of the gaps?
    There isn’t sufficient space to properly vent the area. My roof pitch is shallow around 4/12. Can either of these boards come in direct contact with the roof sheathing if I seal it well with spray foam? Also can I double or triple stack sheets to get added R Value? Or maybe even put fiberglass beneath it? If I can which is the better choice 2″ polyiso boards or blue boards?

    • Todd Fratzel says:

      Eric – What you are describing is a “hot” roof (unvented). We do these from time to time and typically with just spray foam. If you’re going to use foam board, then be sure each layer is sealed to the joists VERY well. Multiple layers is good, adding a final continuous layer over the bottom of the rafters is even better. Good luck.

  • Daryl says:

    Hi Todd, my first post seems to have disappeared, I will try again. I want to convert my 2 car garage to a man cave/entertain room. Will remove doors and add walls. It is attached to house, exterior has vinyl siding inside has drywall. The interior walls sit on about 18 inches (2 rows cinder block). My concern is moisture buildup from the 2 rows of block. Although the garage is above grade and I see no signs of moisture right now, I will be storing some antique furniture in there and concerned with any possible moisture. The room will have an AC running in the summer and likely a electric heater in the winter.
    Would you insulate or seal the block? If so how?
    Would I need a dehumidifier?
    I live in Nashville, Tn. Generally very moist most of the year. Thanks, Daryl

    • Todd Fratzel says:

      Daryl – I’d insulate that block with foam board, then frame in front of it to cover the foam board. Definitely want to watch the humidity from the slab at a minimum, so a good dehumidifier is essential. Good luck.

  • Gary says:

    I live in Georgia. The humidity here is high. I have humidity getting into the basement (unfinished). I have a dehumidifier. I have tall poured concrete foundations walls, and shorter ones (knne?) with stud walls above grade like one of your videos. Can I remove the batt fiberglass & put foam board between the studs like you did with the rim joists? I am afraid moisture will be trapped between the OSB and the foam board.

  • joann says:

    Will be building a home in Vermont soon and one of the builders is telling me to use roxul insulation and also put 1 inch
    dow foam on the exterior of the osb board. Is that something I should agree to ?

    • Todd Fratzel says:

      Joann – That’s not a horrible detail. Certainly that’s a very nice upgrade compared to regular fiberglass with no exterior foam.

  • Mike says:

    I have a pole barn in central Michigan that I would like to heat. It has a ribbed steel exterior, 8 ft spacing between the uprights. The poles are 5.5 inches thick. I figured I would use an extruded polystyrene sheet 4×8 to insulate. I am unable to find building size sheets in more than 2 inch thickness, so I guess I will have to double the amount of sheets to obtain maximum insulation value. Does anyone manufacture thicker sheets of extruded polystyrene? Also, who sells the material at a reasonable price?

  • Linda says:

    We are renovation our living room and kitchen in a 1950’s ranch home. My husband wants to use the rolled insulation on the exterior walls and then add 2″ foam board w/aluminum cover over that and then also on the ceilings. The ceilings do have the rolled insulation already. Is this too much insulation, and could it end up causing problems in the future?

    • Todd Fratzel says:

      If you’re going to use foam and fiberglass, the foam needs to be closest to the exterior…not the other way around…your detail will cause moisture issues.

  • KL Smallwood says:

    I’m going to build a house on St. Croix US VI. This is not only a warm environment (75-85 degrees year round) but it is also humid. 80% plus. It will have cathedral ceilings with open rafters. The contractor is suggesting a roof with foil backed plywood on the rafters, then an air space and then the plywood sheathing under the felt paper and shingles. Basically no insulation in the roof like I’m used to here in the states. What is your opinion?

    • Todd Fratzel says:

      Interesting….I have zero experience in that climate, but I’d think you’d want to insulate to keep cooling costs down. I’d ask around on the island and get some other input and suggestions.

  • Ken says:

    Hi Todd,

    I’m building a pole barn I intend on insulating, I’m thinking 1-1/2″ ISO or XPS between the purlins and another 2″s of ISO vertically on top of that. Then I would stud it out from there and cover with T & G pine and steel in some places. I wish I would have use house wrap, but too late. I have considered weaving it between the exterior steel and the insulation. Would ISO against the exterior steel be OK? Any thoughts would be appreciated.

  • Kevin Patterson says:

    I have read that foil faced foam does not perform well at temperatures below -20C (=4F). Is this your experience? Should I explore options?

  • Will says:

    Hi Todd,

    I’d love some advice on a small insulation project I’ll be starting up in the late spring early summer. The back of my raised ranch has a one room addition over a ground level framed shed. The walls and ceiling of the shed are not insulated so cold air seeps through the plywood and tile floor leaving the floor of my house super cold in the winter time. I was thinking of starting by installing closed cell foam board to the roof of the shed with spray foam around the gaps. My goal is to raise the floor temp of that room and decrease my energy bill. I’m currently living in upstate, NY. Thanks in advance.

    • Todd Fratzel says:

      Insulating all of that shed will certainly help. The floor of it likely being the most important, as that is what is likely transferring cold to the house floor.

  • Thomas says:

    Hello Todd,
    I have a side door on one of the corners of the house, stairs are made out wood leading to the basement, I noticed during cold days below -13C the wall under the stair sweats, the whole area under the stairs was used to build a small shelf but all around the stairs was enclosed with wood panels, leaving a small door to crawl in.
    having the stairs 2×4 already in place,I was thinking if installing and trying to squeeze rigid form behind the 2×4 and then install frames of rigid foam and seal them like you suggested, would installing XPS rigid boards would help at all?

  • Sandra says:

    I have a big issue. We bought my father in laws 70 year old home. The exterior wall are framed out of pieces of board. So anyways we put new siding on 13 years ago. Before we put siding on we used tyvex the we put the 3/8 robsinson one side poly bead shelterwrap. So we put a new basement in this year so we had to take siding off will my god almighty when we started removing this shelter wrap we almost passed out with the musty smell. We can’t figure what went wrong. Now we have no idea what to do with our exterior walls

  • Raj Rajasekar says:

    Hare Krishna!

    Hi –

    I live near a creek and my basement was flooded with 4 inches of rain water for 3-4 hours. I had to remove the carpet, pads, and the pink insulation between the studs. I am looking for ways to protect my house from future flooding.

    I plan to use a backer board, vinyl flooring, and PVC baseboards. The undecided part is the insulation. Could you please suggest the best insulation that is waterproof?

    Thank you,

  • eric kane says:

    hey Todd!

    you might have answered this already but im curious about vapor barriers. I live in a zone 7 area in a old house built in the 60s- the siding needs replaced and I wanted to do XPS foam board 1.5″ then 7/16 osb wall sheathing then a wood siding. I’m thinking foam boards directly against studs, then OSB, then house wrap, then the exterior siding. I also want to spray foam the inside between the 2×4 studs, with closed cell foam. Question is am I messing up the vapor/ air barriers by using the house wrap or with its location. does is make more sense to put the foam on the exterior of the sheathing? thanks for your ideas, LOTS of great information on your page! thanks again. eric

  • Leave a comment

    Be nice. Keep it clean. Stay on topic. No spam.

    This site uses Akismet to reduce spam. Learn how your comment data is processed.

    Copyright © 2009-2023 Front Steps Media, LLC. All Rights Reserved. Home Construction & Improvement™ is a Trademark of Front Steps Media, LLC.