Basement Insulation Guide

By Todd Fratzel on Basements, Insulation

Guide To Insulating Basements Properly

Over the last few years I’ve written many articles about different basement insulation topics. Readers have left hundreds of comments and questions about specific basement insulating topics. In an effort to streamline all of that information I decided it was time to create a comprehensive basement insulation guide. This guide summarizes the basic concepts in one single source to make your search for information easier.

If you take the time to understand the basic concepts of insulating basements you’ll be able to develop an insulation system that provides both warmth and comfort without creating an unhealthy environment. This may sound simple but the reality is that hundreds of thousands of basements are insulated incorrectly and lead to serious mold problems. I hope you’ll find the following information informative and helpful on your next basement renovation project.

Understanding Vapor Barriers

Probably the single most important concept when insulating basements is the vapor barrier. Once you understand vapor barriers in basements you’ll be able to insulate your basement without causing a serious mold problem.

In my one of my previous articles, Basement Vapor Barrier, I pointed out that you need to realize that concrete walls (or concrete block) are a huge sponge full of water vapor. Even concrete walls that “appear” to be dry are always full of water vapor. Without going into some serious chemistry and physics just take my word that on a microscopic level concrete has millions of tiny voids full of water.

Once you understand that all foundations are full of water vapor you start to realize that the primary objective of the vapor barrier is stopping moisture from the foundation from entering into the finished framing where it might lead to mold. As you can see in the adjacent sketch the basement wall is insulated in a fashion that forces the drying to the outside of the foundation.

Ultimately basement vapor barriers are best accomplished with closed cell foams, either spray foam or foam board products. Definitely go check out the Vapor Barrier for Basements article for more details.

Insulating Rim Joists

Another really important aspect of insulating basements is the rim joist. Rim Joist Insulation is often overlooked when finishing basements and it can be a crucial component of a properly insulated basement. The Rim Joist Insulation article discusses several options for properly insulating your rim joist. Today most quality insulation contractors recommend only using foam insulation products for the rim joist.

Fiberglass just doesn’t do a good job insulating this critical area and it’s prone to mold problems. As you can see above I’ve insulated my rim joist using foil faced poly-iso insulation board sealed with spray foam. Another great option is having the rim joist completely spray foamed. It’s worth pointing out that I DO NOT recommend spray foaming the rim joist with open cell foam. While some may argue that it’s not a problem I’ve personally seen many issues with it and cannot recommend it. For more detailed information check out my Insulating Rim Joist article for more details.

Insulating Basement Walls

I can’t tell you how many poorly insulated basement walls I’ve seen over the years. Almost all of them are due to the use of fiberglass insulation installed directly against the foundation wall or up against poly that’s directly against the foundation. If you learn three things from these articles it should be this:

  • NEVER install fiberglass insulation against a foundation wall.
  • NEVER install a layer of poly against a foundation wall followed by fiberglass insulation.
  • NEVER install moisture barrier type paint (DryLok or similar) followed by fiberglass insulation.

If you read my article on Basement Insulation you’ll see literally hundreds of questions from readers trying to insulate their basement. Over and over readers continue to ask if they can put fiberglass insulation against the foundation wall because they think the wall is dry. The answer is simple, NO! Then they ask if they put up some poly first would that be ok? The answer is simple, NO! Lastly I get the question what if I apply DryLok then fiberglass and again my answer is NO!

What my video on how to insulate your basement walls:

While the poly and DryLok might stop moisture from leaving the foundation and hitting the fiberglass it won’t stop warmer moist air from the finished basement from contacting the cold surface of the foundation. If that happens the water vapor in the warmer air will condense and turn into water. If that happens it will be trapped in the fiberglass and possibly start mold growth.

In my opinion there are two (and one modified version) ways to insulate basement walls and they are:

  • Spray Foam – Spraying foundation walls with a good closed cell foam is the best approach in my opinion. While this method is expensive it does provide the best vapor barrier and tight insulating system.
  • Foam Board – If you want to save some money and do-it-yourself then I recommend insulating your basement walls with foam board insulation. The basement wall insulation detail using foam board creates a good vapor barrier along with an insulation layer that won’t promote mold growth.
  • Foam Board & Fiberglass – In some situations you can create a hybrid approach that uses a minimum layer of foam (1-1/2 inches) to create an effective vapor barrier followed by some fiberglass insulation for increased R value. This approach can be used to save some money while maintaining the basic concepts that are needed to prevent mold problems.

Insulating Concrete Floors

Lastly I want to share an article that I wrote about insulating concrete floors. There are numerous ways to insulate a concrete floor and help provide a nice warm finished surface. If you can afford the extra height that an insulated floor requires the payback is surely worth the effort.

As you can see in the adjacent sketch one method for insulating a concrete floor involves a layer of foam board insulation followed by sleepers and a plywood sub-floor. The foam board insulation provides a great vapor barrier and a quiet floor surface.

Other modifications of this detail include eliminating the sleepers and attaching the plywood sub-floor directly to the foam board. Regardless of which detail you choose the keys involve sealing the foam well and being sure to have a good connection to the concrete.

Final Thoughts On Basement Insulation

My biggest piece of advice for insulation basements is to take your time and do your research. If at all possible completely eliminate fiberglass insulation from your plans. And lastly take your time and do a good job. A shoddy basement insulation project will likely result in serious problems down the road including potentially dangerous mold growth.

If you follow this advice I’m sure that your basement renovation project will be a success.

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.

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  1. Carrie says:

    Hello – Thank you for all the useful information. I live in PA and am experiencing frost/dampness in our basement, much like others have posted. My house has a walk out basement and the only framing in the basement is on the outside wall. My concern is that we will not be able to get a vapor barrier between the studs and the plywood. Is this a problem? What suggestions do you have? Thank you in advance for your response.


    • Todd says:

      Carrie – Not sure I know what you mean. Can you explain which portions of the basement have framing and which portions are just foundation walls?

  2. Steve says:


    Thanks for the website!

    Four questions:

    1. What’s the best way to finish around a basement window?

    2. My house has 4 feet of insulation wrapped with vapour barier. Should I remove this and foam the whole wall?

    3. Does it make a big difference to use foil faced or the same foam used on the wall for the floor joists?

    4. I was considering using a material like superseal(rubber membrane) on the floor and finish with laminate over top? Or, tile the whole floor. I’ve read that as long as the laminate is 12 mm it can be placed over this type of subfloor. Any suggestions?


    • Todd says:


      Thanks for visiting and the kind words!

      1. Most basement windows are best dealt with by installing an extension jamb around the existing jamb to extend it out to meet the new wall surface.
      2. First off have you cut open that insulation and examined it? If not that’s the first thing you should do. Secondly, you need to make a decision about what the costs are if a mold situation develops after you’ve remodeled the space. It’s always the situation of do it now or do it later.
      3. Foil faced foam typically has a different R value (see, improved radiant barrier properties and likely better vapor barrier properties. There are some publications that indicate reduced R values for foil faced polyiso with drops in temperature so that’s a concern. Foil faced insulation does offer a better flame resistant rating if you’re leaving it exposed. Ultimately these are all pros and cons that must be weighed.
      4. When it comes to flooring I say following the manufacturers recommendations. The rubber membrane sounds like it might work well if the laminate is ok on top of it.

      Good luck.

      • Steve says:

        Thanks for the response.

        The basement window sweats around the perimeter of the sill. The window is a white vinyl product. Should I cover the sill with an extension jam? What material is best for the jab? Why is it sweating so much?



        • Todd says:

          Steve – It’s sweating because the frame is cold enough that when water vapor (moisture in your basement) hits the cold frame it condenses into liquid. The same thing happens in finished living space in houses when there is too much humidity in the air. See this article for more information:

          I would build wood extension jambs that can be painted or sealed. The important thing is to control humidity in that space.

  3. Diane says:


    we have basement walls that are not the same depth from top to bottom, the wall is flush with the framing for about 3 feet then a “Shelf” then the wall is bumped out about 1 cinder block to the celling of the basement.

    My questions is how do we insulate with foam board? Do we make it flush with the framing or cut it out to fit each section and glue it in place?

    Your site has been a huge help!



  4. Mike says:

    Todd: I purchased an 85-year old home in Windsor Locks, CT in October 2009, with a partially-finished, but very cold basement. The house is a 2 1/2 story, on a 24′ x 28′ foundation. The below-grade portion of the foundation is approx 4′ of concrete, and the above-grade portion is approx 3′ of cement block (up to the rim joists.) The below-grade concrete walls are quite a bit colder to the touch than the above-grade cement blocks. The house has a hot water, cast-iron radiator heating system, and there is one radiator in the basement, but it’s insufficient to overcome the significant cold. I plan to insulate the walls and rim joists per the instructions in your articles but have the following question: in the interest of saving a few bucks, will I accomplish my purpose of warming up the area by only insulating the below-grade portion of the foundation? Thanks for your help.

    • Todd says:

      Mike – The answer isn’t all that straight forward. First off think of un-insulated walls as having a window that’s very cold to the touch. The more windows you have the colder the room will potentially be. Sure you’ll save some money but it’s likely that the money you do spend is not working very efficiently. The reason the “blocks” feel warmer is they are hollow and air is a fairly good insulator.

      If it were my home I’d bite the bullet and insulate it all. It will make the room MUCH warmer and MUCH dryer as you’ll be locking out the moisture that the foundation walls hold.

      Good luck.

      • Mike says:

        Todd – that’s pretty much what I thought. Thanks for your quick reply. I’ve been doing a lot of research on these whole issue and I wanted to tell you that your site(s) and the info you provide are the most helpful I’ve found.

        • Todd says:

          Mike – Thanks for the kind words. I truly appreciate that and hope you come back often. We give away some great tools on the site and try to keep the site up to date on a regular basis. Happy New Year.

  5. Scott says:


    I am going to start finishing my basement in the near future and I have a question regarding foam insulation. I decided to do foam insulation vs fiberglass but it is very expensive. I went to my local home improvement center and noticed it is $30 plus per 2″ 4′ X 8′ sheet. I have come across EPS styrofoam sheets of the same size for $17 a sheet which is faced with foil on both sides.

    My question is can I use the EPS styrofoam sheets to insulate my basement?

  6. Jason says:

    I have a poured concrete basement. We are planning on finishing soon. We have dryloc on the walls already. Is there any issue with putting the blue board over top of dryloc. If not what would you recommend attaching it with.

  7. JohnR says:


    Looking for 2 in. Polyisocyanurate Rigid Foam Insulation and I am having trouble finding this thickness. I live in St. Louis, MO. Any idea where I can find it? Lowes & Home Depot only carry 1/2″ & 1″.

    If not can I use 1 in. Polyisocyanurate and put 1″ Polystyrene behind it to get the 2 inches of thickness?

    John R

    • Todd says:

      I would use 2″ XPS for the basement walls, 2″ polyiso is good for the rim joist.

      Not sure you’ll find it as the Box Stores, but most large lumber yards carry it. If this is for the rim joist I would just use two layers of 1″.

      Good luck.

  8. John says:

    I plan to use this same method but I have a few quick questions.
    One, the thicknees of the rigid foam HAS to be 1.5″ to be a vapor barrier? I was thinking that I would do 1″ foam, frame the wall and then to unfaced within the studs but sounds like the 1″will not be enough of a barrier. Is this correct?
    and 2.I have the pony wall and I plan to make a ledge in some areas. Is it ok to us a paper backed insulation like krafts on the walls about the pony wall?

    • Todd says:

      You really do need a minimum of 1-1/2″ of XPS foam in order for it to work as a vapor barrier. 2″ is even better.
      Fiberglass will work fine above on the studded walls.

      • John says:

        Ok thanks Todd,

        I understand.

        I was trying to avoid having to go thicker with the xps. That stuff is not only expensive but hard to find at 1-1/2 and 2″.
        The big box stores around here don’t carry it. I found it in one place and it was 28 bucks a sheet. Ouch.

        thanks again for your help

  9. John says:

    I have found some 2″ “GreenGuard” xps.
    Its not Corning or Dow. Its “GreenGuard” and the board is actually green. I’ve gone to their site and it looks like this is the good stuff. Like I mentioned, its not Blue or Pink. Do you know much about this brand?

  10. Ted says:


    I live in MD and have a fairly wet basement. I treated the walls with Drylock about 5 years ago and its basically falling off and not doing a great job. I patched several cracks with hydraulic cement but those areas seem to still let in a little bit of water. My questions is I had a “Mr Basement” trench system installed around the entire perimeter of my basement. Between the trench, Sani-dry dehumidifier and 2″ foam board do you think finishing the walls with drywall is a bad idea? I am sure water will still come in through the block walls but my hope would be that the foam board will divert the water to the floor trench channel and the dehumidifier will dry the space enough.

    Any help would be appreciated,


    • Todd says:

      Did the Drylock come off after the trench system? Honestly if that’s the case I think your basement is one of those that I wouldn’t recommend finishing.

  11. Michael Anthony says:

    Todd –

    Your info re: below grade cellar insulation has been GREAT. Thanks for all the time and effort you devote to this website. You’re a maven, for sure, and it shows.

    I’ve read almost every article AND every comment/question & answer on the topic but haven’t yet seen anyone with my exact dilemma so I really hope you can help advise:

    I fully appreciate your strategy of 2″ FoamBoard glued to the foundation wall, then a pressure-treated Bottom Plate (perhaps with some “sill-seal” tape on the bottom to avoid direct contact with the concrete floor), Framing (we’d use metal studs to avoid the risk of mold), polyethylene vapor barrier, and lastly finishing with a dense-armor wall board.

    BUT we live in a 140 year old town house (New York area) built with a fully below-grade rubble foundation wall. (Cellar height: approx 9 feet. Floor is poured concrete.)

    Since the foundation wall is made of large uneven rocks, boulders & mortar that protrude out at irregular intervals and depths, we have no even surface and so are unable to glue up the 2″ foamboard. Since this is the first step in the process, we got a prob’m, as they say…

    How do you recommend that we insulate OUR unique situation?

    • Todd says:

      Typically with old stone foundations I recommend framing a wall a couple inches away from the stone then hiring a spray foam contractor to fill the cavity. ONLY USE CLOSED Cell foam.

      Short of that I guess if it were mine I might try framing the walls on the floor then attaching the foam board to the walls (probably have to use a combination of adhesive and screws + fender washers. Tape the seams real well, then stand the walls up an inch or two away from the stone. This approach would likely work (not so well with metal studs).

      • Michael Anthony says:

        Thanks Todd. Don’t think we’ll use the closed cell foam as it will be too expensive

        Here’s what I was thinking…

        1.) Hanging or tacking some rubber dimple board or some other non-molding waterproof membrane (well taped at the seams) to the uneven foundation walls. This would hopefully serve as a humidity barrier and divert any water leaking in through the rocks after a hard rain (there is a teeny bit) down the rock-wall and into the 2″ french drain that we dug into the corner where the wall meets the concrete slab floor.(leads to a sump pump)

        2.) Just in front of the french drain: Frame the wall. We intend to use PT wood bottom plate and galvanized metal frame studs (to avoid mold)

        3.) Stick 2″ XPS foam board inside the framed studs. (we don’t need crazy insulation – The cellar is completely below grade so it never really gets too too cold down there) We intend to seal carefully.

        4.) PolyEthylene plastic covering over the framing & foamboard to serve as a 2nd vapor barrier after the foamboard (we REALLY don’t want any musty smells down in that cellar)

        5.) Finish with DenseArmor mold resistant wall board, hung 1/4 or 1/2 inch off the floor so it never touches the concrete slab

        What do you think?

        • Todd says:

          It’s an ‘ok’ approach but there are a couple of concerns to at least acknowledge.

          1. The metal studs don’t have a thermal break so they will transmit cold to the drywall.
          2. The metal studs will definitely have condensation issues as they have no thermal break.
          3. The metal studs will eventually face corrosion problems. Galvanized will certainly prolong the life but its still an issue.

  12. Tom says:

    Great info on these websites. I think I am on the right track, but wanted to get your thoughts on my approach.
    First I put Drylok on the walls (before I read your comments on whether it is needed). Then I glued 2″ extruded foam board directly on the walls. I plan to put up 2×4 wood wall about an inch or so from the foam with R13 fiberglass insulation. The sill plate on one of the walls is directly over the inside edge of the concrete block. I put the foam board up to the floor joist and plan to put 2″ foiled polyiso against the rim joist (sealed with great stuff).

    The sill plate on the other wall is about 2″ from the inner edge of the block wall. I put the foam board up to the top of the concrete block and then a small piece on top of the 2″ of block before the sill plate.

    A few questions: Which Great Stuff should I use (windows/door, regular, fire rated)?
    Should I put the 2″ polyiso on top of the sill plate?

    Do I need to cut the foam board away from the floor joist to install a fire block, i.e., does foam board need to be covered with the 2x at the top of the wall?
    Does the polyiso on the rim joist need covered or fireblocked?

    Thanks for any advice.

    • Todd says:

      Thanks for the nice compliment.

      The type of Great Stuff doesn’t matter much as far as insulating value. Depending on how you deal with fire code issues you might need the fire rated version.

      The key to a successful job is being sure to insulated as much as possible and have a continuous layer of insulation up the wall, over the top and up the rim joist.

      You’ll have to check with you local building code official as to what they will require with regards to fire code. Typically you’ll have to meet a certain “fire spread” rating on any exposed materials.

      • Tom says:

        Thanks Todd. I will check local fire codes. I was originally planning to cover wall from floor to top edge, over sill and up rim joist so it would be one continuous layer. The reason I asked about covering the wooden sill plate is that I thought I saw on one of these sites that you suggested keeping it open to help with drying?

  13. Johnathan L. says:

    Hi Todd – 1st of all, you ROCK!

    We have an 8.5 foot tall completely sub-grade cellar in a 150 yr old townhouse in the NJ area. The problem is that the foundation walls are made of uneven rubble & mortar.

    I read your reply to Michael Anthony above to 1st frame the WOOD stud walls on the floor, THEN to fasten the 2″ foam board, and then stand the walls up BUT we ALREADY have galvanized 3″ metal framing up (approx 3″ from foundation wall) and many of these stud cavities are partially filled with piping and plumbing…

    What do you suggest for our situation?

    (We can try to cut out large (2″)foam board pieces & fit it in as best as possible between each stud, BUT, we will have many gaps (which I suppose we can fill with GreatStuff)… but will this solve the condensation issues? (PS: 3 of these 4 below-grade walls are NOT exterior walls, if this makes a difference…)

    Also, what should we do for a thermal break before attaching our dense-armor paperless wall board to the (cold) metal studs ?

    Would REALLY appreciate your reply !


    • Todd says:

      Thanks for the nice words.

      My first and best suggestion for an old home like yours is to hire a spray foam contractor. While it is likely too expensive the spray foam would be worth every last cent and then some. If you go that route INSIST on closed cell foam, no exceptions.

      It’s going to be really hard to get foam board back there. I guess in your situation I would probably take a different approach. I would likely screw the 2″ XPS foam board over the metal studs. This would require the electrical and mechanical connections (if any) to be extended out past the foam which isn’t a big deal. The one downside to this approach is the studs will eventually degrade (maybe 20 yrs or more). The nice thing about this approach is having a nice seamless insulation layer, good vapor barrier and a thermal break.

      This approach would require long screws and fender washers for the foam. Then long screws for the drywall or you could attach firing over the foam then the drywall.

      Good luck!

      If you found this useful I’d REALLY appreciate if you could LIKE our Facebook page or share the URL with your friends on Facebook. Thanks!

      • Jonathan L. says:

        Thanks Todd.

        Let me ask you a question that no one here has everasked:

        How about hanging one of those crawl space
        “dry-space” liners (like a thick pool liner) over the whole rubble wall first? (Say between the rubble and the metal wall with foam board in it?)

        Will this work?

        • Todd says:

          It certainly sounds promising. As with most things the devil is in the details. The key will be sealing the top and bottom of the liner so that there’s now way for moisture to get into the wall. Typically most of those liners run along the floor as well and seal the entire room.

  14. Jonathan L. says:

    To add: I suppose that if you’re ok with the waterproof crawlspace liner, then I should use unfaced batts between the studs, right?

  15. Steve m says:

    I recently had my basement finished and they put the studs directly on the concrete with fiberglass insulation also directly on the concrete and finished with drywall. O am wondering now what is the best way to fix this?

    The house is in new York city, it’s an old house with the foundation about 90 years old.

    Can I have a foam insulation sprayed onto of the fiberglass? Perhaps I was thinking to dig up around the old foundation and place a vapor barrier there?

    To bad I didn’t find this site before so much work was done, looking forward to your advice!

  16. Tom says:


    Thanks for the great information on these web sites. I am using 2″ foiled polyiso foam board on my rim joists. Is it ok for the polyiso to contact the hot water pipe that goes to my baseboard heater on the 1st floor? I assume the pipe won’t be hot enough to cause a problem with heating the foil.



    • Todd says:

      The heat isn’t a problem however there is a slight potential for corrosion of the aluminum that covers the Polyiso. When two dissimilar metals come in contact they can cause a corrosive reaction. In this case if that were to occur (you need water as well) the aluminum would be the loser in the battle. Frankly if it were mine I’d at least cover the pipe with some thin piper insulation so that the two can’t come in contact with each other.

      • Tom says:

        Thanks for the response. One place they touch is right on tyvek tape covering a seal. In 2 other places, I carved out a little of the polyiso just so they wouldn’t touch. In another place, I have Great Stuff foam around the pipe – is the non-fire rated foam ok around the hot pipe?


        • Todd says:

          Look up the spec’s on the foam and see what temperature. Frankly it shouldn’t be an issue, the boiler water for baseboard heat is typically a maximum supply temperature of 180 degrees F and a return around 140 to 130.

  17. timmy g says:

    Todd Building a new house in Ohio and want to know if 2″ (10R) foam board is enough insulation or should I have more?

    • Todd says:

      Not sure what the code is in Ohio for energy ratings. However, I can tell you that a basement with 2″ of foam will perform very well in the winter. In some locations you will have to have more than R10 though. So it’s best to check with your local building code official.

  18. S D says:

    Hi. Thanks for great articles on insulating basements. I have a crawl space that I am enclosing and semi-conditioning. I have followed many of your ideas and want to share some bumps in the road as a resident of Georgia.

    Termites are a southern fact of life. Codes require a 3″ viewing strip at the top of the wall for termite inspection. I have agonized over this because 3″ of exposed cinder block essentially defeats the purpose of sealing everything else tightly. Finally as a compromise I mounted the 2″ blue board so there is a viewing strip 3″ above the dirt floor. I chose that solution so that the viewing strip will be located under the vapor barrier. Using a clear plastic vapor barrier, one can see if any termite tunnels are present.

    Another challenge as been adhering to fire codes. After purchasing the blue foam board I realized I preferred to meet fire codes after all. The thought of installing drywall over the blue stuff … well, sucks. The scope of the project has doubled! I just learned (too late for me) about Thermax, a foam product that is fire-rated. That would would have saved a lot of time and labor. I thought about adding a layer of fire-retardant plastic over the blue foam board as a solution (easier than drywall). However not sure if I create vapor barrier problems doing that. Do you have any opinion on that?

    Thanks again for writing extensively on this subject, I’ve learned a lot.

    Marietta, GA

    • Todd says:

      Glad you found the articles useful. Thermax is an interesting option and likely a decent compromise if you’re local building official accepts it. I’d be curious what they say about sealing the seams and cut edges. My understanding was the fire retardant properties come from the covering over the foam.

      Add a layer of anything over the foam is fine. The foam itself is semi-permeable to impervious so adding another layer that might be a vapor barrier is fine. Good luck.

      • S says:

        Hi, thanks for your reply. I have run into a couple more challenges:

        First question deals with combustion oxygen. I discussed my plans to seal the crawlspace with my HVAC specialist. He pointed out my gas furnace requires adequate oxygen to burn properly and safely. His suggestion — leave 2 or 3 of the vents open. Really? I checked into this and it appears I have a conflict. I asked him if I can run an air supply duct from the exterior and attach it to the side of the furnace. He wasn’t thrilled with the idea, saying something to the effect that it’s a “natural draft” furnace and wouldn’t recommend piping air to it. He said OK to run a piece of duct and stop in a foot or so from the furnace. But this contradicts everything I’m reading from your site, Hartey’s book, Building Science, etc. As you can imagine this has thrown me curve. If my HVAC guy is on target, it’s frustrating that I’ll end up having to insulate the subfloor which I had hoped to avoid if I made the crawlspace walls part of my thermal envelope. But the fact remains: gas appliances need oxygen. Have you run into this before? Any suggestions?

        Second question appears easier… at first. I’ve purchased a high-clarity clear plastic for the dirt-floor vapor barrier. As previously posted I left a 3″ viewing strip between the bottom of the insulation and the dirt floor to aid in detection of termites. There is a possibility that I may get some condensation on the vapor barrier b/c of the 3″ of exposed cinder block (viewing strip is mostly at grade or slightly below grade). More on that: it’s a semi-conditioned space. I don’t plan on conditioning the crawlspace. I figured the leaky ducts would provide some conditioning in addition to thermal energy coming from the sub floor. All that to say the temp variance between the outside and the crawlspace will be less than it would be for living space.

        So now the question (finally!). I plan on attaching vertically-mounted furring strips to the insulation using a long concrete screw and then installing drywall. In order of assembly, should I glue the vapor barrier to the foam? followed by drywall? or put up dry wall, and glue vapor barrier to the drywall? The latter creating unvented airspace between the foam and drywall and the vapor barrier?


        • Todd says:

          Part 1 – Have you already purchased the furnace? If not then I’d look into a new condensing boiler. These boilers actually have fresh air piped directly into them.

          Part 2. I’d put the plastic/barrier behind the sheetrock and caulk that seam.

          • S says:

            Part 1: Yes, the furnace came with the house. It was replaced last year by the prior owner. It’s practically new so I must design around the installed gas furnace.

            Unless you have a better idea, the patch solution I am considering is to install a air-supply duct from the outside to within a foot of the furnace. At least I can direct where the cold air is coming in as well as minimize the volume of cold air. I thought of using an accuated damper that opens when the furnace comes on, but then worry about the requirement for a failsafe that dependably works should the air supply fail. The simpler and cheaper solution is lightly stuff the end of the duct with fiberglass insulation to impede a draft but allows adequate airflow. Other than that, I’m at a loss for a practical way to “tightly” seal the crawlspace AND provide combustion air supply to the gas furnace.

            Gas furnaces in basements/crawl spaces are ubiquitous so others must have run into this problem. I’ve read about air-supply chases from basement to attic in Harley’s book, specifically be careful not to seal them. So far not finding information about how they solved the problem of “tightly sealed” vs required air supply. I’m at a loss. Any suggestions?

            I don’t know of anyone who has a boiler. I may be mistaken but I don’t believe they’re common in residential dwellings in the south.

            Part 2 — thanks, will do.


          • Todd says:

            I think you”ll be stuck bringing fresh air into the space. Short of building another room inside a room I can’t think of another solution. Not sure the fiberglass thing will work all that well.

  19. Joe says:

    For the Foam Board & Fiberglass hybrid approach, you recommend a minimum layer of foam (1-1/2 inches). Does it matter whether the wall is earth bound or stucco faced? One would be exposed to wind and the other would not. Is one colder than the other?

  20. Gillian says:

    We are in the process of finishing our basement and followed your suggestions with insulation.

    We are now ready to possibly insulate the ceiling for sound. Our upstairs is wood flooring and we can easily hear our dogs walking around. We are creating a media room in the basement. Do you have any suggestions- possibly and different price points?


    • Todd says:

      The best approach is using acoustical separation materials that “un-couple” the hard surface above to the finished ceiling below. Floor/ceiling assemblies act much like a drum. When you walk on the floor above the vibrations are passed through the wood flooring, to the wood floor joists and finally transferred to the ceiling below. To stop this the best approach is using a combination of two products:

      1. Batt insulation to help deaden noises. This can be unfaced insulation installed in the floor joist bays.
      2. Secondly and most important is using light gauge metal “hat” furring channels. Basically it’s a light gauge piece of metal that looks like a top hat. You install it by screwing through one of the side legs into the floor joists above. Then you screw the drywall into the top surface of the “hat”. This allows the metal to vibrate and reduce how much of the floor vibration reaches the drywall. This isn’t expensive and it’s VERY effective.

      Some folks install insulation and then do a suspended ceiling. This too works fairly well. Both approaches are somewhat similar in price except that suspended ceilings can be easier for some DIY’ers.

      • Gillian says:

        Thanks Todd-

        So the local hardware store sells a 7/8″ by 12′ 25G furring channel- I assume this will work- correct? I see some places sell clips to secure the channel in but they seem to be pricey and, from your comment, unneccesary.

        I just want to make sure I have this down- so I just screw in one side of the channel leaving the other side unsecure or “hanging”? When I screw the drywall onto the flat top of the channel should I make sure the screws do not go into the joist above?


  21. Steve m says:

    A contracted recently installed wood studs directly to my basement foundation followed by regular r19 insulation and Sheetrock. I know this was a terrible job but it’s done. Should I cut a hole in the Sheetrock and pull out the insulation and fill with a spray foam that may work better, any recommendations in that direction? I can’t afford to redo the whole job. What about possible adding a vent to each framed section so that the modifies can vent out and let the wall dry? I’m desperate for some kind of correction here that won’t break the bank.

    Also what would you recomend for a cement slab basement floor with radiant heating – do I need a vapor barrier? Some companies push a vinyl tile that is raised a half inch above the cement which acts as a vapor barrier. I am not sure if I need any of this since I have the radiant heat.

    Thanks in advance.

    • Todd says:

      First off lets start with the slab. With radiant heat I don’t recommend anything. You can use a number of flooring products with varying degrees of efficient use of heat.

      1. Tile the floor (use an underlayment like Schluter)
      2. Vinyl Composite Tiles
      3. Wood (glue direct)
      4. Carpet (you need a special pad and carpet)

      As far as your walls are concerned there’s really nothing short of tearing them down that I recommend. Your contractor frankly wasn’t qualified for the job and he’s the one to blame. Wish I had better advice.

      Good luck.

  22. MJ says:

    What’s your opinion on using Prodex instead of 1-1/2in XPS on the basement concrete walls with a 2×4 batt insulated framed wall in front of it ? Lot less expensive than XPS and as effective both with insulation and moisture barrier.

    Thank you,

    • Todd says:

      I wouldn’t use it for a couple reasons.

      1. It has an aluminum facing which will react with concrete. Aluminum will oxidize and deteriorate quite quickly when i contact with concrete.
      2. While the radiant aspect sounds great the reality is the R value has to be very low with such a thin layer of foam.

      Might be cheaper…but it won’t work effectively in this application

      • MJ says:

        You are correct about the R value. In order to get the R value they advertize about, it needs to be in the center of a wall cavity 2×4 with 3/4in plywood on each side.
        I have been looking for alternatives to 1.5 -2″ XPS foam board for basement since it is $25-$30 here in NJ, but haven’t been able find any. If anyone knows a good location (good price) to by this stuff in NJ please let me know. I am looking for 50 4×8 sheets.

        Thank you,

  23. MJ says:

    At Home Depot now and i see a bat insulation made by Roxul from stones. It is water and moisure resistant and could be put against concrete. Do you have any experience with this? Seems that this could be used and no need for the xps foam boards against the wall.

    • Todd says:

      It is moisture resistant but it’s NOT a vapor barrier and it won’t protect framing and the basement from moisture. I wouldn’t recommend that approach.

  24. Tim Mauriello says:

    Hey Todd, big fan here! thanks for all your advice.

    I’m planning out my basement finishing and will be able to do most walls with 2″ of XPS. The problem is the bathroom is plumbed already and the toilet flange is set so I can only frame the walls 1″ off the concrete. What would be your preferred approach? Or approaches in order of preference? Some ideas I had were…

    1″ of XPS with another inch or two between the studs?
    1″ gap and spend the extra money to spray foam it?
    2″ of XPS behind studs and go 10″ toilet
    2″ of XPS 2×3 framing with blocking for anything that needs it.

    I was already planning on using foam it green DIY foam for the rim joists, so I could just order more and spray that area.

    Thanks for any advice!

    • Todd says:

      I’d definitely use a 10″ rough in toilet for starters. Then from there get as much foam behind the studs as possible. You could even go with 1×3 strapping installed over the foam board and shot to the wall. I think that’s what I’d do versus spraying such a small area.

  25. Tim says:

    Thanks! Another question for ya…

    I got black and deckers basement remodeling book and they recommend that along with insulating as outlined here that we should also put 2″ of xps on the outside of the basement wall to a depth of 6″ below grade as well.

    I was wondering your thoughts on that. If the inside of the walls are covered with 2″ of xps do you need some exposed concrete on the outside to allow for moisture to exit that way?

    Just wondering as we are working on landscaping and could do that pretty easily as we get ready for the big basement project.

    Thanks again for this site and your newsletter! So helpful!

    • Todd says:

      With the approach of sealing up the inside I would not use the foam on the exterior for the very reason you mentioned.

      Good luck and thanks for the kind words.

      • Ryan says:

        Thanks for all the great info. My radiant floor guy is telling me to insulate the outside of the concrete so the concrete walls will act as a heat sink. Have you heard of this.

        • Todd says:

          No…and frankly it doesn’t make sense. Typically we want a thermal break between the slab and walls so we’re not “losing” heat to the walls. We want to keep it all in the slab.

  26. Joe says:


    Great article and using the methods you menton in the planning for my basement. Unfortunatly the design of my house has a slight variation to your diagram and not sure how to properly insulate and vapor barrier it. I have a poured concrete foundation that goes up about 7 and a half feet. They then put a 2X8 sill on top with my floor joints then sitting on top of that. So when I put the 8 foot foam up towards the top there is a gap behind and still above the foam. How do you recommend to insulate this area? Thanks in advance for any advice.

  27. Barbara S says:


    I am getting ready to use 2″ foam board on my basement walls. Problem I have is I have a window right in the middle which I just had put in the window frame in level with the foundation wall. When putting up 2″ foam board around window not sure how to? then I planned on putting up a 2×4 wall infront of foam board. How close should I put the foam board to the window? Should I put up extension framing around the window framing before putting on foam board? I am stuck thanks in advance for your expertise.

    • Todd says:

      You’re on the right track here. I would run the foam up to within 3/4″ of the inside of the window frame, just enough space for an extension jamb. Then frame the wall leaving the same 3/4″ gap. Then you can install a finished extension jamb after and it will all look nice.

  28. Barbara S says:


    Oh I forgot I was going to ask your opinion on using DRICORE? As a subfloor on basement floor which is concrete?

  29. Brandon says:

    Great advice!!

    I was wondering what your thoughts would be on using Tyvek against the poured concrete wall and then framing your wall 1″ away and using fiberglass. Then using a poly vapour barrier covered with drywall.
    Thank you

    • Todd says:

      If you want my frank opinion….it’s an awful detail. That still won’t stop condensation from forming when warm damp air hits the cold Tyvek. Foam insulation is really the only proven method that works.

  30. David says:

    Thanks for the info Todd. I have a walk out basement with poured concrete walls. The area near the walk out portion is my concern. I plan on insulating with XPS foam, building new stud wall, adding fiberglass insulation and then drywall. In the walk out portion the lower half of the wall is concrete but the upper half (above grade) is a normal wood stud wall (the two are flush with one another). Should I insulate the concrete portion with xps foam and the uppper stud wall a poly vapor barrier and fiberglass OR could I just attach xps foam over the entire wall (concrete and stud wall) (xps over entire wall would be my preference) Thanks!

  31. Mike says:

    Hi Todd,

    Great site, been on here for a while soaking in the good info. One question, I have an exterior basement door that is 6ft below grade with a poured stairwell exiting at a right angle up to a patio. I need to replace/rehang this door, but it is framed by the block wall. Where do I install the new door? On the block or on my new stud wall 6.5″ away and 1″ higher (2″xps, 2×4 wall, dricore subfloor)? Either way, I’ll need to build an extension jam, but I wasn’t sure where to place the door. From what I can tell, if I put it in the block wall I won’t be able to open it wide enough to get large items through with the stud wall in the way, but putting it in the new frame creates a “breezeway” between the door and storm door. What’s the best way to do this?



    • Todd says:

      Mike – Welcome to the wonderful world of compromises. If you didn’t have a storm door (if you buy a good exterior door you might not need one) then I would suggest possibly having it open out, and installing it at the block. Typically the door wants to be flush to the inside finish of the exterior insulated wall. In your case that can be kinda confusing.

      If it’s going to open in then I’d certainly install it flush with the new wall/finish just like it were new construction. Then outside you’ll have deep extension jambs. Then you can raise the exterior floor/walkway height up to transition the threshold. Make sense? I guess then you would install the storm door further out.

      Good luck.

  32. Wendy says:

    Hi Todd,

    Is it okay to put spray insulation over top of waterproofing dimple sheet on the inside basement walls? Will the spray insulation adhere to it okay? We are currently rectifying a wet basement, by trenching for an interior sump pump water control system and also excavating and waterproofing the exterior. Overkill maybe, but we want a good dry basement. Can we still use spray insulation after the interior walls are covered with dimple sheet?

    Thanks, Wendy

    • Todd says:

      I would check with the product manufacturer of the dimple material. However, I am fairly confident that it should be ok.

  33. David Preuss says:


    Thanks for all the great advice and your dedication to this site. I’ve followed your advice throughout my basement finishing project. I’m near completion and couldn’t be happier with the results so far.

    I have a question if priming or sealing drywall will behave like a vapor barrier. I’m in southern Michigan and I’ve finished my basement using XPS on the subfloor, 2″ on the walls and rimjoists with a 2×4 stud walls built in front of the walls. In addition to XPS on the walls, I used the hybrid method with fiberglas insulation between studs on the exterior walls to add a little more insulation. The drywall has just gone up and I’ll be priming/sealing in a couple days. I’ve read that sealing drywall in a basement creates a vapor barrier where priming alone will not. I’m concerned about the effect on the fiberglas if it is trapped between the XPS vapor barrier and a second vapor barrier created by sealed drywall.

    Could this create the potential for mold and do you have any advice on priming vs sealing in this situation?


    • Todd says:

      David – Most all latex paints today create a semi-permeable vapor barrier. Semi-permeable is the key, the paint will slow down moisture movement but it won’t trap it. Because you’ve used the foam board you shouldn’t get water vapor from the foundation side, so it will only be moisture from the framing materials or moisture from the room space if the humidity gets too high.

      I’d use a normal primer and latex paint combination. Good luck.

  34. Marc says:

    Hey Todd – great article. Our contractor has proposed using rock wool to insulate our basement. Any thoughts on that? We are in eastern Massachusetts. The previous owner put up reflectix against the interior of the foundation many years ago. Contractor said it’s fine to leave it up. Thanks!

    • Todd says:

      Rock wool is good in the fact that it won’t mold, however, it can still collect and hold moisture. So the problem is any moisture that gets in the wall cavity is going to collect in the rock wool and then get into the framing and wall board. I would stick with foam board as I’ve pointed out or spray foam.

      Rockwool also won’t create a proper vapor barrier to stop the water vapor from the foundation from getting into the framed wall and/or finished space. The Reflectix certainly can’t do that.

      Good luck.

      • Marc says:

        Thanks Todd!

        Do you have any recommendations on flooring?

        We were thinking of doing cheap wall-to-wall carpeting, which wouldn’t be terribly expensive to replace if needed. Also it would feel warmer underfoot. We’ve also thought of doing laminate, but that’s rigid, and our concrete slab is not flat; it’s higher in some places, and has areas that are kind wavy/lumpy.

        What about ground moisture? If we put down a moisture barrier won’t moisture get trapped in the concrete? Will our radon fan help to get rid of any moisture trapped in/under the slab??

        Finally, to compound the problems, we removed the old vinyl tile (it had asbestos in it) and there’s a kind of sticky residual of mastic everywhere. (no asbestos in the mastic) Whatever we put down is going to STICK to that stuff. Though perhaps it is already serving as a moisture barrier of sorts… LOL

        • Todd says:

          Moisture in the slab will exist forever. Concrete isn’t concrete without water….so the only issue is making sure the moisture that does escape into the flooring material doesn’t damage it. Carpet is a good choice but is the most prone to water damage. I like tile in basements because it can handle water problems.

  35. Walden says:

    Hi Todd. Great site!

    I just concluded an entire renovation/demo with the exception of my basement. I live in NJ and recently installed a perimiter drain inside my basement. It pumps water every couple of hours (even when it doesnt rain). The basement is dry – as fas a I can tell. But I am not sure how it will be in the summer and whether there will be a lot of humidity. Here is my question. How do I finish my basement walls? I dont think that foam board will work since it may trap moisture between the board and the masonry cinder block wall. The water moisture might leak into the perimiter drain but I dont think this will prevent mold and must. I also dont think a vapor barrier is a good idea for the same reason. Can I just put up drywall without insulating? I was thinking of insulating the coldest part with foam board (the floor joists to the outside) and just putting up drywall. This would keep the basement cool and would allow the walls to breathe. What do you think? How would you do it? I dont mind having a cool basement in the summer or winter. Thanks for your time!

    • Todd says:

      What’s the goal? Finished space? Just better conditioned space? Foam insulation (spray or boards) is the best way to keep the moisture in the foundation wall away from the space. Drywall by itself will mold most definitely.

      • Walden says:

        Hi Todd, thank you for your response. I would like to finish the basement and have a living space. There is definitely effervescence in certain parts of the basement (Maybe four spots max.) Like most, my main goal is to not have mold. If I use foam board, should I put it right up against the masonry or leave a small 1/4 inch air space (this would allow any water that accumulates on the masonry to drip into the perimiter drain with out any obstruction.)I would then frame out the wall and possibly use faced insualtion, facing the warm side. Thanks!

  36. Walden says:

    Hi Todd, thank you again for your reply. The moisture is not too bad – I think once I redirect a few gutters all will be fine. Quick follow up question. I will be using 2 inch insulation board. I have the option of using Owens Corning Foamular 250 which is $30 per sheet for a 24 inch wide board, vs Polyisocyanurate foam which is the same price but twice the size (4ft wide). I understand that you can not use the aluminum faced foil against masonry. If only one side is covered with aluminum foil, do you reccomend that I still use it for the coast saving rather than the pink foam? Thanks!

  37. Mike D. says:

    Todd –

    First thank you for answers that you have given on other items. The projects have worked out great. I have a question about my basement wall before I start insulating them. I did have some spots in our basement that seeped. I currently have gotten them taken car of from the outside, but I know that I have specific spots that could cause issues. Our basement has poured concrete with these metal dividers between each section (the sections are about 4 feet and teh metal is about 1/8 inch.) Where these metal divivers are have been the common spots. Anyway like I said currently there is no seepage. **My question is should and would it be ok (two fold question) to use hydrolic cement over those seams then paint the walls with dry-loc? After this is done or not I am insulating the wall with that foam board the XPS stuff (about 1.5 to 2 inches) and also the floors (with about 3/4 to 1 inch foam board) and taping up every joint like you suggest. Then plywood for the floors and 2 x 4 framing for the walls.

    Currently I finished the rim joists… used closed cell spray foam about 2 inchs or so maybe 3…. then put the fiberglass batting back up. Just doing that I have started to notice a difference.

    Thank you again for your input and time.

    Mike D.

    SO what do you think about doing that

    • Todd says:

      First questions is this, if you get seepage, where will it go? Do you have a perimeter french drain or a floor drain? How likely is it to be substantial water? If this happens it may ruin your floor if there’s not place for the water to go. Just be cautious…some people force a wet basement to be finished space and regret it later.

      Hydraulic cement and Dryloc certainly won’t hurt, but they won’t stop water from penetrating concrete, they are far better suited to stop moisture and water vapor.

      Good luck.

      • Mike D says:


        Thank you… sounds like my best is to take care of it all from the outside… first. This has seemed to stop all the spots that I worked on. I guess I will use every precaution, it’s just like you say people tend to forgot its a basement and water will come in period… the question is when. So I just figure take care of every possible issue and it just reduces the problem. I also know a lot of people in our area (all the same builder) have finished basements so it must not be to bad, I just dont want to put the cart before the horse.

        Anyway… the seepage I got last year was it just spotted the wall it never really went anywere else. But drainage wise there is exterior tile and sump pump one floor drain in about the center of the basement.

        Thank you again.

        Mike D.

  38. Rachel says:

    I wish I would have read your article a month earlier!!!! We just bought a 5 year old house in the st louis area and started finishing the basement upon moving in. So… our contractor is using fiberglass insulation and has already drywalled most of the area 1400 feet. My son has severe mold allergies and while we were just finishing the basement to have a safe place to go during storms and a nice guest bedroom/bath and extra office we didnt want anything elaborate… we were very adamant about keeping the mold out as much as possible. We do live on the top of a hill and it doesn’t seem like there has been any water issues thus far, but my question is… is it worth it to have them tear down the drywall and put up the foam board???? So frustrated right now!

    • Todd says:

      Rachel – Unfortunately over time it’s likely you’ll have a mold issue. Will it happen right off or next year? Probably not..will it happen within a few years? probably. I can’t give you an answer because I can’t predict the future with something so complicated. What I can tell you is I see far more basements with mold (insulated like yours) than I do with no problems.

      Good luck.

  39. Kris Casello says:

    We have a 63 year old house and we just had a drain tile system installed as we have had water issues at various times. We were thinking that it would be a good idea to insulate the ceiling to keep the floors above warmer in the winter (we live in MN) but after reading some of the tips I am feeling confused. It says there is no point in insulating the ceiling if you don’t do the walls but then it says that in older houses you don’t want to insulate more than 2′ below ground level. Our duct work hangs a bit below the bottom of the joists as well. What should we be looking at doing? We are not planning to finish the basement but we will be storing items there. Is there any point in painting the blocks in the basement? The walls were painted when we moved in and then had waterproofing paint put on but should we just leave them “as is” at this point? Thank you for your help.

    • Todd says:


      Basements are certainly confusing! If you’re not going to heat the basement, and if you’re only using it for storage, then the best solution is insulating the floor/ceiling. Having said that the hard part is deciding what type of insulation and how to protect the insulation from any fire hazards that are identified by your local building code.

      In many locations it’s a simple as insulating with fiberglass insulation. Some codes may require a 20 min fire barrier. Building soffits around duct-work is the best way to deal with that situation.

      Good luck.

  40. Dave Miller says:

    I’m planning on finishing most of my basement, and I’m in the research phase right now. I’ve stumbled upon your site, and I’m very impressed with the advice you give. I want to first insulate the rim joists with 2″ of polyiso foam board sealed with Great Stuff foam and then use 2″ rigid XPS foam board against the poured walls. Since XPS needs to be covered by a fire rated material, what do I do in the case where a closet in the finished section is opened exposing the XPS-covered poured walls? This closet will contain the water softener and water meter. Is the fact it’s in a closet good enough, or should all XPS be completely out of sight? Also, as I said earlier, I’m not finishing the whole basement. The finished wall will extend along the poured wall and end before the poured wall does. The finished wall will then take a 90 degree turn to form another finished wall. This will leave the edge of the XPS insulation between the poured wall and finished wall exposed. Does it need to also be covered by a fire rated material? Thanks for your advice, and I hope you’ve understood my description.

    • Todd says:

      Dave – Glad you’ve found the site useful. Most building codes require all foam (even foil faced) to be protected from living/storage areas by a minimum of 20 min rated materials. This can be drywall and even some types of plywood. Each location has inspectors that all read and interpret the code in different ways. The closet is one of those situations where some may let it go, some may not.

      The rim joist is a great example, it doesn’t necessarily need to be covered if it ultimately is behind some sort of wall/ceiling assembly that provides the 20 min rating. So I’d think the closet is similar to that.

      The 2nd situation is pretty clear, that end of the foam should be covered. That’s easy enough to cover with wood or drywall.

      Good luck.

      • Dave Miller says:

        So for the rim joist part, my plan is to remove the fiberglass and install 2″ of polyiso up tight against the board. Then use 1/2 inch drywall cut to the same size as the polyiso as the cover over the polyiso. Finally, I’ll foam the entire perimeter of this with Great Stuff to seal it up and replace the original fiberglass if room permits. Is securing the drywall with Great Stuff good enough, or should I install the drywall after sealing the polyiso with Great Stuff? If this is the case, what method of securing the drywall inside the rim joists would you recommend?

        • Todd says:

          Dave – That will work, but when you put the fiberglass in then you’ll be back to square one. Fiberglass also must be covered.

          • Dave Miller says:

            I wasn’t aware that fiberglass insulation had to be covered. I wonder how this home passed the code inspection with the fiberglass insulation in plain view covering the rim joists. I guess I won’t replace the old fiberglass then and just leave it with the polyiso and 1/2 inch drywall.

          • Todd says:

            It gets missed/ignored all the time. I’d certainly recommend that you remove the fiberglass and install the foam first, then put back the fiberglass. Otherwise, you run the risk of getting water trapped in the fiberglass. Good luck.

  41. Ron says:


    It recently stormed here and our basement leaked. We never had rain come in before. The rain just came so fast and flooded the yard. So it came in our basement. I started to roll up the wet carpet and noticed the old asbestos had moisture around the perimeter of the each tile. I’m am guessing this is condensation on the floor because it is in the center of the floor away from where any water could come in. We have a height problem in the basement. What would the best way to insulate the floor if we can go maybe a maximum of about 1.5 inches worth of subfloor? Also if for some reason it would storm bad again would that compromise the subfloor? Should I install an internal french drain?


    • Ron says:

      Ceiling height is roughly 7 ft to 7ft 4. the floor slopes in places. I have 1.5 inch foam around the walls. Just nothing on the floor. My main concern is if it leaks again.

      • Todd says:

        Ron – First off I’d spend some time outside trying to find ways of preventing the water from getting to the basement in the event of another storm. Keeping it away is easier than dealing with it once it’s in the basement. This type of situation is why I typically recommend some sort of tile in the basement (if theres any chance of future water). Tile can withstand an occasional “wet” event without a problem. With only 1-1/2″ of space you’re pretty limited. If it were me, I’d install some Schluter and then tile. The Schluter will help create a bit of a thermal break which should help keep the temperature of the floor warmer. Good luck.

        • Ron says:

          Would the condensation be an issue with the tile? Ived use Schluter before and its quite expensive. Would 6mil poly with 1 inch foam followed by 1/4 inch ply wood work?

          Thanks Again

          • Todd says:

            I’ve never had trouble with water as long as you’re using thinset and NOT mastic.

            By time you install the poly, put down 1″ of foam and then try to get it all flat with 1/4″ plywood…I think the tile will but up quickly….it will flex and move alll over the place.

          • Ron says:

            So carpeting is a no go then? even with the 6mil poly with 1 inch foam followed by 1/4 inch ply wood.

          • Todd says:

            I wouldn’t recommend it…if that gets’ll never get all that dried out. I’ve seen it too many times unfortunately.

          • Ron says:

            Thanks Again,

            Just want to make sure the tile wont condense because we have had issues with condensation in the basement. Thats why i ripped the walls down and added the 1.5 in Foam.

  42. Dave Miller says:

    My project of insulating the rim joists is coming along nicely. I’m using 2″ of polyiso foam board with Great Stuff foam around the edges and then putting back the fiberglass insulation that was there originally to make the R value about 26. However, there are a handful of rim joists that are extremely difficult or impossible to reach because they stick out past the foundation of the house. They are for a bay window, fireplace chimney and above a doorway area leading to our basement. What do I do with these? 2 of the 3 have quite a bit of fiberglass insulation in the opening. 1 of them doesn’t have anything. Some of them are in the part of the basement I plan on finishing. Do I just keep them stuffed with insulation? Do I remove the insulation and put in the polyiso board / fiberglass combo at the sill plate area? This method will leave a big gap of air between the polyiso and rim joists. Do you suggest something else? If I leave the fiberglass, how do I seal from the basement floor to the subfloor with XPS and polyiso if fiberglass is in the way?

    • Todd says:

      Dave – You point out a fairly common situation that’s difficult to deal with. In fact, bay windows are notorious for poor insulation and being COLD and DRAFTY! During construction we typically stuff the cavity with fiberglass, then install 2″ of foil faced foam to the bottom of the joists, then sheath over it. This creates a really tight well insulated situation.

      After the fact is more challenging. It really comes down to what can you do that’s most effective, how can you get the most R value in that tight space and do a great job SEALING it off after. Preventing air movement should be the top priority as it drastically reduces the effectiveness of fiberglass insulation. Each location will likely have it’s own solution.

      Good luck.

  43. corey says:

    Excellent site! I recently built a new home, 9’poured wall foundation and my problems are stumping me. Basement walk out is sweating in the winter to the point water runs down the wall onto the floor. I have 1″ blue board glued to the wall, then studded with r-13 batts in stud cavitys. Condensation is forming on board, soaking batts and running to floor. If I replace 1″ with 2″ board will that solve my problem? Also, contractor spray foamed (DOW brand) the band board (3/4 to 1″) and then batted it, and it is also sweating, espically below my patio door where so much condensation has my sub floor wet if looking up from basement.

    thanks for your reply,


    • Todd says:

      Corey – My first question is, what’s covering that insulation? If nothing is covering either location than the biggest problem is humid interior air is getting in the cavity and hitting your thin foam insulation. That insulation is cool/cold at or near the dew point temperature and the humid air is condensing.

      – You need more foam insulation.
      – You need a finished wall that will prevent the humid air from getting to those surfaces. Painted drywall makes a good vapor barrier and stops most of that.

  44. Matt says:

    Hi Todd,
    Awesome site. Truly appreciating all the valuable info you provide.

    I’m beginning to scope out my basement finishing project. I plan on following your guide to insulating the walls and floor. My question…I plan on finishing 80% of the basement. What should I do with the unfinished space? Should I insulate that area too? My hot water heater and furnace are tight to the wall. It would be tough to get xps behind them. If I don’t insulate the unfinished area what do I do with the exterior of the finished wall that will face the unfinished area?

    • Todd says:

      Matt – Thanks for the kind words.

      Your situation is not unique, most people leave an area unfinished. Ideally you’d insulate that space but many cannot because of equipment conflicts like your situation. I would recommend you insulate the partition wall that separates the spaces. You can use either foam board insulation or fiberglass. If you use fiberglass, be sure the vapor barrier is on the WARM side. Good luck.

  45. Mark says:

    Hi Todd,

    God bless you for being so patient with us…..I’ve just read all posts, just to find out that non of it is in regards to a new foundation (in progress) so I have to ask.

    Building my addition to an existing 1849 house. I poured my foundation 38’x18′ last year, used 3 coats of Aquaseal Eco-flex (rubberized water proofing) and topped it of with 2″ Fomular 250. It is 8′ deep. Installed outside perimeter drain on W & N side, as there was a little stream discovered with a flow rate approx. at bucket per hour. On those two sides I used 2″ Dow blue Styrofoam board with pre-cut vertical grooves to prevent or minimize possible hydro-static pressure. Made a mistake and forgot to create a capillary break between footings and the basement wall. Footings are 12″ high, all 12″ on the interior side of the basement was filled with 3/4 blue stone and compacted. When we have a heavy rain…West corner is being wet up to around 4-5 inches, crushed stones on some spots are discolored because of the high content of moisture. There is no standing water though.

    Unfortunately, I had to use shelve on the top of the concrete wall to accommodate 11-7/8 I-joist to I have the same floor height as the existed house. Therefore, basement height from footings to ceiling joist is 7’3″. I wanted to use 2″ Fomular 250 foam boards on top of stones (and around perimeter of the basement), than 8mil poly on top, and radiant heat in fiber mesh reinforced 4″ slab. Total thickness of this is 6″. If I add another 1″ for strapping and sheetrock on the ceiling, I am left with the minimum of 6’8″ for basement. Not ideal but here in MA it can still be officially used for like an entertainment purposes (not a living space though unfortunately).

    Since I am so close to the minimum height and concrete might easily be over poured by 1/4 inch… you think I could use 1″ Fomular underneath the slab?

    Also, do you think, that having this set up will help with the moisture I can see now after rain? What could I do at this point? And should I somehow try to insulate the walls since I already have the foamboards on the exterior?

    If you see anything wrong and/or could suggest how to improve this, I would very much appreciate it.

    Thanks for all the hard work. It is nice to have someone like you, willing to help homeowners.


    • Todd Fratzel says:

      Mark – My pleasure, glad you visited our site. If the slab hasn’t been poured yet then you definitely have the option of changing the interior grade to accommodate the thickness of the insulation. Removing an inch of stone/sand backfill may be difficult, but the end result will be much better.

      Also, while the exterior insulation is certainly helpful, you really need to insulate the inside to properly protect against moisture and achieve a good insulation value.

      Good luck.

      • Mark says:

        Hi Todd,
        And thanks for getting back to me on this. I agree, I still have the time to properly adjust prior to pouring the slab. Neverthles, what confuses me is this: If I take/ remove 1″ of stone, than yes, that would allow me to use 2″ foam board. But, that applies only to the space in between footings. I obviously can’t make footings 1″ lower so the 2″ insulation on top of the footings would have to be 1″ anyway. So 1″ on top of the footings, 2″ elsewhere. Isn’t the footing perimeter the most critical though? Having 2″ in the middle of the foundation is certainly helpful but isn’t the heat loss more likely to occur around the perimeter?
        In terms of inside wall insulation, what would you suggest? 2″ exps?
        Lastly, I learned that, XPS on top of the stones, 8 mil poly, radiant heat tubes and slab is the way to go….unfortunately, nobody specifies, hot to attach/secure the tubing to the XPS. If 8 mil is on the top, I can’t staple tubing down through it as it would, in my eyes, compromise the vapor barrier by introducing many holes where staples would secure the pex tubing. Mostly, in this application, people would zip tied the pex to the rebar or mesh, but I wanted to use fiber reinforced concrete instead. Cost of mesh/or rebars is much greater than fiber reinforced cement. To spend a lot more for rebars just have something to tie pex to seems to me unnecessary expense….any idea how to around this? Thanks a million.

        • Todd Fratzel says:

          Mark – In most cases we pour 2″ of concrete over the footing and 2″ down below the footing and seldom have insulation over the footing. The heat loss is really a function of the entire space, and the perimeter doesn’t really matter anymore than the center because of the exterior cover. I wouldn’t worry about that.

          Whenever we do radiant, we install welded wire fabric and tie to that. There’s simply no other good option. Radiant is expensive, and this is just one more cost associated with it. I prefer XPS on the walls.

          Good luck.

  46. Mark says:

    Thanks Todd…that sounds great as the ceiling height would be than at 7’1″ or so. What confuses me is that all over the internet is written, that there should be a thermal bridge between the slab and the walls. Usually XPS. Otherwise heat from floor would try to go up into the walls as well what leads to an extra heating cost….wouldn’t pouring 2″ directly over the footings do exactly that? That way, the heat from slab can transfer to the top of the footing and to the inside side… that what you mean?
    Unless, I put 2″ XPS on the wall first, all the way down to the footings. Than 2″ XPS on top of the stones. This way, moisture or water from the wall would drain down on the footings. In this application, where do you terminate 8mil poly? On inner side of XPS on the wall?
    Sorry for bothering you, it is just I have only one shot to make it right. Thanks

    • Todd Fratzel says:

      Mark – You’re over thinking this a bit. Several issues at play. Underslab insulation helps force heat up, so the radiant is as efficient as possible, it’s really a big thermal transfer problem, the heat can go up out of the slab easier then going down through insulation. The small (really small) amount lost by contact with the wall is too small to matter in my opinion. The vapor barrier, helps keep moisture from under the slab from moving up through the slab into the basement. Again, do the best you can, small areas along perimeter probably don’t amount to much, and you’ll have 2″ coming down the wall sitting on top of the slab.

      It certainly wouldn’t hurt to put the foam first, then pour slab, but it may be hard to hold the foam in place. Don’t beat yourself up on small details, you’re way ahead of 95% of people doing this!

  47. Mark says:

    Todd, thanks for such a lovely comment. It really boosts my confidence. And yes, you are absolutely correct, I beat myself to death with all the details that at the end it is either over thought or overkill…..or rather both. So thanks again for your feedback and your suggestions. I will do it as you’ve said. Although taking out 4″ of stones won’t be easy, it is worth it at the end. Thanks a gain, you’ve helped me more than you think. God bless you. Regards, Mark

  48. Jack Krieger says:

    Todd, just purchased a townhouse. I want to finish the basement. I have a units on both sides of me. Meaning I only have two walls exposed to the outside. Should I put a vapor barrier on the walls that are not exposed to the outside ? The basement is dry, no sign of moister in the basement at all. The neighbors all have finished basements with no sump pumps. This place was built in 1986. Also no sump pump. Would it be overkill or just extra protection ? Also the walls are cinderblock, do they need to be painted ? Thanks, Jack

    • Todd Fratzel says:

      Jack – I’d definitely insulate the two exterior walls with foam as I detail in the article. If it were my place I’d do the other two walls as well with foam so that there’s no issue with which side is the hotter side with regard to moisture. I don’t see much point in painting the blocks.

  49. Curt says:

    My basement walls have a textured paint surface (possibly a sealant?). And, in some locations where plastic or cardboard bins have been close to or against the wall, there has been some mold growth.

    Is rigid foam still the best applicant in this case? I am a little concerned that dampness could still get in through the wall and the paint could serve as food for mold (essentially, would the same thing that happened behind my plastic bins happen behind the rigid foam?).

    • Todd Fratzel says:

      Curt – Hard to say what that paint is, mold/mildew needs water, air, and food. I’d clean it really well, then be sure that when the foam goes in it’s sealed really well. Most likely it won’t be an issue. Still the best choice unless you strip that coating off the wall. Good luck.

      • Curt says:

        Thanks for your feedback – your website is invaluable.

        A couple quick follow-up questions.

        Mold needs air, water, food. Can paint serve as food?

        Also, because it is textured paint with maybe 1/8 or 1/4 inch bumps, it will never be perfectly sealed to the wall. Am I setting myself up for disaster because all 3 are present – small amounts of air behind the rigid foam, paint for food and small amounts of moisture naturally present in concrete?

        I’m a little gun shy due to the mold that seemed to naturally grow behind those plastic bins that were against the wall.

        • Todd Fratzel says:

          Typically paint shouldn’t be a food source, but I can’t say that for sure. I’d reverse the situation and ask yourself, can the paint be removed easily? Can it be sealed with a better product? I’m not a paint specialist, so I’d ask a paint person if there’s something you can seal it with that won’t promote mold.

  50. Geoff Soard says:

    I have a small lower level condo in the Atlanta area I’m remodeling. I’m dealing with block walls on a couple sides with a portion adjacent to a crawl space, a portion about 1/2 under the ground and the rest above ground.

    Do I still need 2″ of foam insulation board in this climate and situation? My primary goal is creating the vapor barrier as I think I’ll have enough insulating with the fiberglass added between studs. Am I able to use 1/2″ foam board, the Tyvek tape, and spray foam method for the joist area? Since the walls will be finish only, with no load to speak of, am I able to use aluminum studs at the floor level against the concrete floor?

    A small part of what I’m looking to finish is adjacent to a bathroom area. Am I able to accomplish the same thing here (assuming I’d not use the fiberglass between studs)? Am I able to use the aluminum studs in this area both at bottom and vertically?



    • Todd Fratzel says:

      Geoff – I’d say you need a min of 1-1/2″ of foam (XPS) to get the vapor barrier you need. You can definitely use metal studs (they are galvanized steel, not aluminum). Steel studs can transmit cold/heat more than wood do to thermal properties, but they are used in residential and commercial construction every day in contact with concrete.

      Good luck.

  51. Mike L says:

    Todd; Thanks for all the valuable info;….I’ve been repairing a basement wall and pulled out all kinds of moldy fiberglass insulation. You were spot on with this advice. I want to cut pieces of rigid foil faced foam to fill in the stud space and cover the wall with greenboard. Will this be a problem as I’ve heard somewhere that one cannot use greeenboard over a vapor barrier.

    • Todd Fratzel says:

      Mike – I wouldn’t use foil faced insulation in direct contact with concrete, it will react with the concrete. Use Dow or OC foam board. Unfortunately, unless you’re doing the entire wall behind the studs it won’t be fully functional. However, this is far better than replacing the fiberglass with more. It won’t hurt the green board at all. Good luck.

      • Mike L says:

        Thanks; upon closer look the foundation I’m working with is 3 layers of brick (older house) and some kind of skim coat of stucco- concrete…I’ll try to locate some dow or oc foam board….Thanks again…

  52. Mike L says:

    What if I apply red gard over this wall, then this will be between the foil face foam board and the wall?

  53. Jennifer says:

    I have 2″ white styrofoam insulation sheets, no vapor barrier on it, can I use it to insulate the rim joists in my basement? Was going to spray foam around edges to seal it.

  54. Len says:

    Hi Todd,
    I ended up buying the 2×2 zips panel for the bathroom. I tried putting them up yesterday but the bath walls are old cinder block so the Pamela are very uneven. What do you suggest? The house was built in the 50’s and there wasn’t any mold when they demoed the previous bath. I’m thinking if I can’t get the panels even it is a waste and I should go with dry lok and call it a day as far as insulation/waterproofing. The contractor spray foamed the entire perimeter so I feel confident its sealed right from the rim joist and up.
    Thank you.

  55. Matt says:

    What are your thoughts regarding sound barrier methods in basement ceilings (fiberglass or Roxul Safe n Sound)? Assuming the foundation walls have been insulated properly, is this still discouraged?

    • Todd Fratzel says:

      Matt – I think it’s fine to do. The results will vary depending on the finish ceiling assembly.

      • Matt says:

        Any concerns about blocking heat transfer between floors when doing this? Basement is heated by gas fireplace and whatever radiates from hydronic boiler. Thanks!

  56. Damien says:

    Had a mold problem in our basement because of high humidity. Had two contractors come and give two different opinions and how to solve the problem. One suggests a spray foam on all the walls, the other suggests Prodex and an industrial grade humidifier. Which do you think would be the safest bet to keep the humidity down and the mold from returning?

    • Todd Fratzel says:

      Damien – Definitely spray foam (but is MUST be closed cell, DO NOT use open cell). Prodex isn’t a great idea in my opinion (i’ll leave it at that).

  57. Justin Heffner says:


    Thank you for the great guide. We live in Pennsylvania and are considering finishing our block basement. Our plan was to drylock, 1.5″ XPS, stud framing (no r13 fiberglass insulation), then drywall.

    Do you think the drylock is necessary / would it harm anything?

    Do you think 1.5″ XPX would be sufficient?

    Are we ok not also adding R13 fiberglass insulation?

    Thank you in advance

    • Todd Fratzel says:

      The Drylock is a good extra step. The fiberglass depends on the R value you’ll need to meet minimum code.

  58. Alex V says:

    Hi Todd, I love the information on this site, your doing a great job. I’m in need of some advise.

    I have a partially finished basement. It’s all poured concrete. I plan on renovating the entire basement one section at a time. I already planned on using 2″ XPS foam (R10) glued to the wall with the joints taped, and a 2×4 wood frame, and finally 1/2″ drywall. This site confirmed that I’m on the right track, thanks! I haven’t decided if I will also fill the wood frame with batt insulation, I’m trying to keeps costs down, is it worth the extra cost? (I live in Montreal, it gets cold here).

    My real problem is the concrete floor. I have a drain in the center of the basement, the slope of the floor between the wall and the drain is almost 3″. The slope is not even, it starts gradual and then gets steep faster closer to the drain. I have been told that leveling the floor with concrete is the best option. I haven’t had any quotes yet but I heard that this can be very expensive. Also it would require me to empty my entire basement which will be a great pain (I have a lot of stuff down there). I was considering leveling the floor using 2×4 “sleepers” (with shims) placed 18″ apart with 1.5″ XPS in between, taping all the joints. Then I would cover with 1/2″ plywood. Basement will probably be laminate flooring except for a bathroom/laudry room which will be tile. I know I will lose about 2″ head room (compared to leveling with concrete) but the advantage is that I can do the work myself, I can do one section at a time, and it will be cheaper. Will my plan work? Do you have any suggestions? Are there other options? Any advise is welcome.

  59. Jason Smith says:

    Hi Todd. Really great site thanks for the info. I am working on finishing my basement and i am not putting in a ceiling. 3 questions

    1. Will the air flow be enough to keep my sheetrock from accumulating mold or do I still need foam board to act as a moisture barrier.

    2. If I use foam board do I still need a vapor barrier. I have seen this different ways

    3. I saw something that noted foam board is not code in the joist rims if there is no ceiling to protect it due to its toxic gases it gives off when on fire. What have you seen and what would you suggest for this circumstance.

    • Todd Fratzel says:


      1. NO definitely need the foam.
      2. No vapor barrier needed.
      3. Most jurisdictions, typically require foam insulation to be protected against flame spread. Drywall, some types of wood, and sometimes even a product like Roxul can be used. I’d talk to your local inspector.

  60. Pete Kaczenski says:


    I have basement walls that have effervescents forming on them. I would like to finish my basement but am concerned about moisture and mold. digging the foundation is not an option. my question is can I spray the walls with closed cell foam and safely sheetrock afterwards? is foam board an option in this situation?

  61. Brian Restivo says:

    Great site thanks for the info.

    I am looking to have an interior drain system put in my basement to handle small seepage in my concrete block walls. All of the water proofing Companies are suggesting a poly sheet or dimple board to channel the water into the system. Would there be issues adding 2″ rigid xps to this system. Maybe bypass the poly sheeting and dimple board and put foam up. If i do i dont get the warranty. Other option is to exterior waterproof for about 2x the cosr and do rigid oam inside with no interior drain. Thanks

    • Todd Fratzel says:

      I’d put the foam OVER the dimple/drainage system. That will work just fine. Having that drain plane is important.

  62. Susanna says:

    Hi Todd! Thanks for this great information. I have a question about my basement I’m hoping you can help with. I live in an 1867 Victorian in Boston, which has been converted into condos (one each floor). Our basement is unfinished with bulkhead access only, and the first floor owner complains of cold floor and a cold apartment, so we are looking in to insulation options. More about the basement: one gas furnace, 3 water tanks, and some electrical panels are down there. The floor is mostly concrete and the foundation walls are stone. The ceiling was once plaster and lathe, but most has fallen. A previous tenant put some fiberglass insulation between some of the ceiling joists, but most of the ceiling is uninsulated. We recently insulated the rim joist (foam board and spray foam) and the basement feels warmer and stiller but the first floor resident still feels cold. Is there something we can do to make her apartment more comfortable without posing a mold danger?

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