Rim Joist Insulation – Insulating Options

By Todd Fratzel on Insulation

How To Insulate Rim Joists

We’ve written a number of articles on insulating basements including; insulating basement walls, insulating basement floors and basement insulation options. In this article I want to focus on rim joist insulation and the options that are available.

Rim Joist Spray Foam

Rim Joist InsulationSpray foam insulation is the best way to insulate a rim joist in my opinion. However, spray foam insulation comes at a serious price point and it is best left to professional installation. Spray foam does a great job not only insulating the rim joist but it also seals all the possible air infiltration points.

Rim joists are typically riddled with nails from siding, trim, decks and porches. The foam also helps encapsulate the nails and screws which helps eliminate any condensation problems from the cold fasteners.

Foam Board Rim Joist Insulation

Rim Joist InsulationIf you’re looking for a good way to insulate your rim joists that’s easy to do and relatively inexpensive then foam board insulation is your best option. When I insulate rim joists I like to use a minimum of 2 inches of foil faced polyisocyanurate foam board insulation.

Installation of the foil faced polyiso is very easy. All you need to do is cut (you can use a utility knife or hand saw) the foam to fit the space between floor joists, the bottom of sub-floor and the top of sill plate. Rim Joist InsulationTypically we will cut the foam about 1 inch smaller so it can be fit into place easily.

In the adjacent photo you can see a piece of the insulation that was cut to fit between I-Joists. The insulation doesn’t have to be cut perfectly because of the next step.

Once the insulation is in place we use a can of spray foam (like Great Stuff) to seal the edges of the foam board to the framing. As you can see above in the photo the spray foam fills the voids and also helps hold the products in place.

Fiberglass Rim Joist Insulation

Rim Joist InsulationThe last method for insulating rim joists is to use fiberglass insulation. Over the years I’ve come to dislike this method. In fact, our new home was built this way (this photo comes from my own home) for the simple fact that I had too much going on to focus on this detail.

I’ve recently started replacing all the rim joist insulation in my home with the foam board that is shown above. Fiberglass insulation doesn’t work very well in this application. As you can see from the photo it’s very difficult to get the fiberglass insulation installed evenly around all the framing members.

I’ve also seen this application result in mold problems. As I mentioned early nails are very common along the rim joist. Those nails are always very cold and act as moisture condensation magnets! This application is also very hard to seal up and create vapor barriers. The bottom line is it doesn’t work well and I’d recommend you replace your fiberglass rim joist insulation with some type of foam product.

Rim Joists at Bay Windows / Deck Cantilevers

Many readers have asked about special situations where the deck cantilevers out past the foundation (typical at bay windows, fire places, and deck overhangs). If you have a situation like that, then I recommend reading my article: How To Insulate Bay Window Floors.

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. Tony Hoch says:

    If you are using I-joists consider the following: precut 1×2 material for the depth of the web of the josit. Install these on both sides of the web vertically with a dab of construction adhesive or a drywall screw. This will create a rectangle space that needs the insulation with out needing to notch the corners of each piece. Finish with the Great Stuff after all the pieces have been installed so you don’t waste the straws or clog the can. Saves time cutting the foam board and increases the load carrying abilities of the I-joist end. This idea came from a class offered by Willam Hanlon with Flint Hills technical college in Emporia, KS on energy saving building techniques.

    • Todd says:

      Tony – Although your idea would work I don’t see the value in this approach. First of all the wood blocking doesn’t have nearly the insulating value as the foam. This alone to me is reason enough not to use that approach. Second, the time and effort required to cut blocks, glue and screw them is MUCH more work than cutting the foam. Finally, if your joists are designed properly there is no need for increased shear capacity at the bearing point.

  2. Jose Cunha says:

    Great web site todd!
    I don’t have access to the 2 inch foamboard foil faced product . Can I just use the 2 inch foamboard without the foil faced? And, also use the great stuff in a can to fill any gaps like you suggest? If so are there any problems that may occur?

    • Todd says:

      Jose – Unfaced foam will also work, it just has a lower R value.

      • John says:

        Todd, I thought you couldn’t use unfaced foamboard without using a barrier like sheet rock.

        • Todd Fratzel says:

          John – Every code on this issue is a bit different. In most places it must be protected with some sort of fire barrier, plywood, drywall, etc. But that doesn’t mean it has to be directly applied, a properly installed ceiling below is enough to protect it. Recently some of the foam board manufacturers have been getting testing data to support leaving it uncovered, but it’s important to follow local codes.

      • SaraBeth says:

        Helping a friend encapsulate her crawl space — 30-year old home.
        We have put the pink foam (we’re in GA) in all of the rim joists and are ready to foam around them.
        She is now nervous about using Great Stuff after reading a “precaution” label on the Dow site about cutting all the pilot lights off, etc.
        The crawl space abuts the basement (concrete wall) and has a 3×3 door that we have been climbing in/out each time. She has a gas furnace and water heater approximately 10-15 feet from the door of the crawl space.
        She’s been reading about the fumes and flammability of it. THe crawl space has six vents to the outside and we plan on putting a fan in the crawl space doorway to “blow” fumes outside.
        We have all of the protective gear, and would like to get this done this weekend.

  3. Craig says:

    I recently insulated my basement. I used 2×4 studs and left a space away from my cement pored walls. Used Roxal insulation thats mold free, moisture free, fire free etc. then VB it and accostic seal as well in ther with the headers etc. Question I have is should I have used Rig board insulation first on the walls the stud it?

    But I am doing my wheeping tile outside and I have blue skinned it and used R-value drainage board that acts as a insulation as well pretecting the blue skin etc. I just wonder if this was ok to leave out the rig blue board inside the house? If not what a easy fix now that I have the walls up but there not dry walled yet. Any help would be appreiated. Thanks.


    • Todd says:

      Craig – Roxul Insulation is a really good commercial product that we almost never see on a residential project. Having said that, it’s certainly a good product to use in a wet environment (basements certainly qualify as wet!) because of it’s water and moisture resistance. Honestly I’m not familiar with it’s specifications so I can’t say for sure if it’s going to be ok but I’d guess it will be fine. It’s often used in masonry wall construction so it should do well.

  4. Craig says:

    thanks Todd I do Appreciate the info. Also just a reminder that outside my house I did the wheeping tile and also blueskinned it and used a drainage board that is roxal but for ouside stuff cause I know that some people use Rig board foam and herd it decays later and fills with water so defeats the purpose anyways…..you should look into it I just learning and researching but thanks i have another idea looking at for my floor….using on my floor delcore is like a bubble vB but breeze underneath for basements that don’t leak or leak it avoids water from damage the wood that you put on the floor. anyways…..I put that down the rig foam board ontop and then plywood and a foam base with hoodward flooring or limate etc if ya understand what i am explaining to you lol hopefully….anyways check it out let me know if you think its good idea? thanks


    • Todd says:

      Craig – Not sure I’ve heard of Delcore but it sounds like a feasible solution. If you use closed cell foam I think you can do away with the Delcore….either way you’re on the right track.

  5. Craig says:

    Hi Todd lol sorry I gave you the wrong name I from the correct name on this site. Its Delta-floor.

    “•Install Delta-FL plastic subfloor system. The Delta-FL plastic subfloor system can be installed under a plywood subfloor to create an air-gap membrane to keep moisture from contacting the flooring materials. With this system you’d intall the Delta-FL first, tape the seams, then install a layer of 1/2″ plywood and screw it to the concrete with Tapcon screws. I prefer this solution if you plan on using carpet because it’s all screwed down.”

    Question is after putting this down can I use a 1 inch foam board on top then put the plywood down and after that put carpet or tile etc ? I thinking this would give you air space underneath to breath and plus insulation foam board would keep the room even warmer and your feet etc. What do you think about this idea?


    • Todd says:

      Craig – Honestly I’m not sure. You may run into a problem with the foam sitting on the Delta-FL. You may end up with high bearing pressures between the foam and Delta-FL on it’s “ridges”. I would actually contact them and get their opinion.

    • Tom says:

      Did you find out about the foam on top of the Delta-FL? Looking at finishing our basement. The contractor framed the poured basement walls, and already insulated them (fiberglass), and has installed the vapor barrier. My concern is that the fiberglass is making contact with the concrete poured wall. Looks like I will take that down and put up the rigid foam (two 1 inch layers staggered to provide a better seal). Looking for solution for the floor. Was going to just epoxy/urethane the floor (no mold problems), but I suspect the floor would be just too cold. Again, let me know on the foam on top of the Delta-FL. Thanks.

  6. James says:

    Hi Todd,

    Great site, I am finding it very useful. Question for you on our home.

    Our house is 6 years old, we bought it foreclosed and someone had broken in and stripped the electrical in the basement. We had that repaired and that is working well.

    Some of the electrical wires/ground wires are running along the rim joists next to the sill. There are also several junction boxes against the sill in certain areas. I want to insulate these cavities before winter. I had considered 2″ foam board however I don’t have a lot of room to play with that due to the wires, etc. The other option was fiber glass insulation but am not sure this is the best application.

    I am thinking about buying the Froth-Pak spray foam kit and insulating that way however is it OK to foam over the wires? I know the junction boxes need to be accessible so how would I problem mask them off when foaming? How much room would I need to give them around the box?

    Here’s an example, you can see junction boxes in the cavity with wires, etc. This is probably the most difficult area.


    • Todd says:

      James – Wires get foamed in place all the time with houses that are completely spray foamed. I would just mask the front of the box, foam, then remove the foam in front of it.

  7. Chris says:

    I’m having a new home built. My spray foam insulation contractor has completed his work and left, and has informed me the rim joist insulation in the basement and between the first and second floor (two story house) were not included in his bid.

    I plan to use the 2-inch foam you mentioned above in the basement. Do you recommend fiberglass between the first and second floor, or some other type of insulation?

    • Todd says:

      Chris – It really depends on whether you want sound proofing and if one of those floors in an unconditioned space.

      • Chris Johnston says:

        Both floors are conditioned. I am assuming if I use fiberglass between the first and second floor (both conditioned) the condensation risk not be there.

        However, it appears you are saying foam would insulated and reduce noise. Is that correct>

  8. Chris says:

    Todd – one followup question – thanks for being patient with me. I need to install rigid foam in my rim joists in the basement. The walls in the house are insulated to R15 in accordance with where I live (southern Illinois). A contractor has told me I don’t need to install 2-inch thick foil face insulation in the rim joits and can go to one 1-inch because the wood rim boards are so thick they give added insulation. That sounds fishy to me. What are your thoughts?

    • Todd says:

      Chris – Sorry but I completely disagree. Rim joist is 1-1/2″ thick. Wood is only about an R =1 per inch for a 1.5 in your case. Most building envelopes today require at least R19, if you get R=7 per inch of foam you’d end up around 8.5 total with the wood assuming it has no leaks, drafts, etc. Spend the time and money and you’ll be far more satisfied.

  9. Vikram says:


    Good information. If one uses foam board w/foil, wont that leave a space between the rim joist and the non-foiled side of the board where condensation problems can occur? Same for non foiled insulation also. Or will proper sealing of the foam board prevent hot air from home reaching the cold rim joist – no condensation issues.

    Any thoughts?


  10. Jeff C. says:

    When you say – “cut the foam about 1 inch smaller”, you mean so that there is 1/2″ clearance on all sides?

    The reason I ask is because the great stuff foam spray (red can) is normally only to fill 1/2″ gaps.

    Also when spraying the foam, do you place the tube all the way to the back of the joist and then spray? Or do you start about a 1″ back (middle of the 2″ foam board) so that the spray foam expands all the way out to the front facing end? Just trying to get an idea of the technique since these spray foam cans are not cheap.

    • Todd says:

      Jeff – Great stuff will expand much greater than 1/2″. If you’re worried about how much spray foam you use you can certainly cut them tighter. I’ve found that joist spacing does vary a bit so trying to cut each one perfectly is nearly impossible. So by making it 1/2″ clearance that template will most likely work for all joist bays.

      Yes I put the tube to the back and just slowly out-line the foam and it will expand and seal nicely.

      Good luck.

      • Jeff C. says:

        Thanks for the quick reply! Do you also air seal the space between the where the sill plate meets the foundation? I have seen some contractors and energy guys on the web to suggest just using a good caulk like OSI quad for this rather then the spray foam. What is your opinion on this?

        • Todd says:

          Jeff – There are several places where caulking is a better choice for air sealing. When we do an Energy Star house all the plates get caulked, so between the sill plate and foundation, between the wall plates and sub-floor, etc. I would do that before insulating the rim joist, then seal the insulation to the sill plate as well.

  11. Elaine says:

    My House was built in 1920. I would like to insulate the rim joists, but we will not be finishing the basement. My understanding is that I can only use foam board insulation if it is going to be covered. Does that mean that fiberglass is my only option?

    • Todd says:

      Elaine – That’s not necessarily the case. In most cases (check with you local building code official) the issue is whether a material meets the building code flame spread rating. If you use a foil faced foam it will most likely meet that requirement. Fiberglass is a really poor way to insulate rim joists.

  12. Michael says:


    I installed a a shed dormer recently that houses a bathroom – with exhaust fan. The pitch is 4:1, nearly flat. I installed a very large sofit vent ridge vent. I stacked 2″ thick polystyrene between the 2″ x 10″ joists leaving a 2″ air gap to mimic proper vents just under the plywood. The polystyrene doesn’t quite make it to the bottom of the joists so I have to fill that near 1″ void with something before I install 1″polystyrene to the bottom of the joists to reduce heat sink or thermal bridging. I assume I can use un-faced bubble insulation, but is there any harm using foil faced bubble insulation or does the foil need to be adjacent to an air space to work?


    • Todd says:

      Michael – I’m having a hard time understanding your detail.

      Do you have multiple layers of foam between the studs? Not sure what the “bubble” wrap is that you’re speaking of, why not use a layer of 1″ foam?

      • Michael says:

        Sorry Todd.

        I sandwiched 3 layers of 2″ along with a 1″ layer of polystyrene between the rafters. (I left a 2″ air gap above this sandwich to allow air to move from the sofit to the ridge vent.)

        The 7″ of polystyrene do not fill the vertical void between the joist completely, there is 1/2″ – 3/4″ of space left to the bottom of the joists. I need to fill this uneven space before attaching one more layer of polystyrene to the bottom of the joists. I thought about using foil faced bubble wrap or plain bubble wrap.

        Looking at this in section from the inside of the ceiling to the underside of the plywood sheathing we have 1/2″ sheet rock and 1″ of ploystyrene screwed to the bottom of the rafters. Then we have a 1/2″ – 3/4″ air gap then 7″ of ploystyrene then a 2″ air vent between all joists.

        I am asking about bubble wrap because that last 1/2″ – 3/4″ is not uniform. A 1″ piece of ploystyrene will extend down beyond the bottom of the joists creating an uneven cieling condition…

        I can draw this but don’t know how to het it to you.

        I hope this is better?


        • Todd says:

          Michael – No need to draw, I get the idea. I’m not sure the air space is a big issue to be honest. Air is a decent insulator so if it were mine I’d just leave it. I’m assuming you’ve sealed the sides of the foam to the rafters so no drafts can get in or air can get to those voids?

          • Michael says:

            Thank you Todd…I actually wondered about that. I also assume, given the polystyrene is a pretty good water vapor barrier, that air in its high energy state will not exist in that air gap…

            Thank you for your help!


  13. Mainer82 says:

    Hi Todd,

    Can you tell me if using the same foil faced material on the basement “cieling” will help insulate the 1st floor? Most people use the fiberglass insulation but I don’t like it that much. Also, was thinking about the 1 1/2 – 1 3/4 for thickness.

  14. Brent says:

    Hey Todd,

    I recently insulated my rim joists. I had alot of extra fiberglass insulation so I put 2 layers of R20 and finished with the 1.5″ blue rigid foam and spray foamed the cracks. Is that overkill? Can it lead to problems? Mold?

    Also, there are some spots that are nearly impossible to get the rigid foam in and spray foam it. Can I just insulate it and not vapor barrier it? What are some of the possible downfalls?

    • Todd says:

      Brent – Hard to say on your first question. I guess it really depends on if air can infiltrate from outside through leaks in the rim joist, get trapped in the fiberglass with moisture.

      Second question…there are places where you just do the best you can.

  15. Steve Hall says:

    Having not done my homework, I’ve put fiberglass in all my rimjoists within the past yr. No moisture problem so far. I do plan to get the materials and get at insulating with your system. FYI, I’m in a 1900 farm house, balloon framed, without much insulation. When the wind blows, it’s very drafty. It has a two-board hardwood floor and air seeps up thru the baseboard/corner round badly.
    My question is where is a good place to use my fiberglass insulation? Walls are horsehair plaster and lath. I hate to just dispose of it. It really wasn’t cheap.



    • Todd says:

      Steve – There are several uses. One could be as simple as re-installing it over the foam board afterward to create even better insulating value. You can always use it up in an attic for an additional layer of insulation.

      Hope that helps.

  16. Kieran Wolfe says:


    Recently bought a place and they insulated the rim joists with fibergalss (two batts so it’s tight against the outer wall, then covered with poly.)I opened it up today and found ice and water on the insulation that touches the outer wall of the house, along with frost on the outer wall (I live in a cold climate). Is there a proper way to install the fiberglass insulation to avoid this? If not I waS THINKING of replacing it with your rigid foam method described in this article. Does the insulation sit flush with the sill plate on the inside basement cement wall? This will then leave an air pocket behind to the outer house wall, aprox 4-6 inches. Is this OK?



    • Todd says:

      Kieran – I’ve found that using fiberglass is a futile effort. I would remove the fiberglass and insulate with foam. Put the foam up tight to the rim joist.

      • Kieran Wolfe says:

        Thanks for the reply Todd. So are you saying the rigid board is tight to the outer wall sheathing? Your picture looks like it’s flush to the inside cement wall. Sorry I’m confused about this part, and a rookie. I know if you did spray foam it will fill the whole cavity (can’t afford this method).

        • Todd says:

          Kieran – Yes, tight to the outside. Typically there is a “Rim Joist” that sits on the outside of the floor joists, then sheathing.

  17. Rae says:

    I would like to insulate my rim joists. Currently, there is only fiberglass there (very old and turning black due to wind blowing through). The upstairs walls are quite cold and I’m thinking this may be the problem.

    The issue I have is that my basement has a suspended ceiling in one part and has an unfinished ceiling in the laundry room and the furnace room.

    My understanding is that I’m not able to have spray foam done to the basement because of the exposed ceiling. Cost is also an issue with professional spray foam.

    I could try the rigid board, but that too must be covered due to fire issues, yes?

    Anyway, now I’m thinking of using the Dow crack foam myself and then stuffing the joist area with Roxul after that. Roxul is a fire retardant. It also comes in an R-20 batting.

    Does anyone see any problems with this?


    • Todd says:

      Rae – First off let me say this. Most codes deal with rating materials for flame spreadability (sp). Most foam boards actually meet the spec. and are not a problem. Do you have a reliable code official near you that can check into this for you? If not then foil faced usually does work.

      Your other method is fine as well, just won’t insulate as well :)

  18. John says:

    I have put rigid insulation between the floor joists and on top of my concrete walls … and now I am building 2×4 walls (about 1.5″ away from the concrete walls) and am screwing them to the bottom of the joists. My question is … how do I prevent the cold air that is in the space between the concrete and the insulation from coming up

    • Todd says:

      John – You need to seal all the foam board insulation to the foundation and framing with spray foam from a can.

      • John says:

        I’ve done that along the upstairs subfloor, floor joists, and the top of the concrete wall. However I only put foam board insulation up to the inside edge of the concrete wall … now I’m framing my 2×4 walls and I’m told I need to leave them 1.5″ away from the concrete … so this means that I will have a 1.5″ gap of “cold” air that is not sealed off from the room (it can go up between the floor joists and into the room). Not sure if I’m explaining it correctly … I guess what I’m asking is … how do I seal off the top of this 1.5″ cavity that I’m creating between my 2×4 walls and the concrete walls.

        Hope this makes sense … let me know!


        • Todd says:

          John – Any chance you can take a photo and email it? Why the 1-1/2″ gap?

          • John says:

            If I am not putting foam board on the walls … just using fibreglass bats … I was told that I am supposed to leave a gap between the insulation and the concrete wall … is this not necessary?

          • Todd says:

            John – I’d NEVER recommend that approach. You should never use just fiberglass in a basement application. Trust me…i’ve seen it done that way and it ends badly! MOLD!

  19. John says:

    So … I should use foam board on the walls and then fiberglass in the stud cavities? Do I leave a gap between the studs and the foam board for air movement? Or do I build the walls right up against the foam board?

    • Todd says:

      John – We have several articles on the site that show you different ways of using foam board. The short answer is choose a foam board product that fits your budget. You’ll need at least 1-1/2″ of foam board to create a vapor barrier. Two inches is much better. After that you need to decide what R value is required for both code and comfort. Once you know that you can decide on how much if any fiberglass.

      You need to be very careful about details like sealing all the seams, etc. Leaving a space is optional in cases where there is a risk of moisture. In some cases we frame tight against it.

  20. Steve says:

    Todd, how would you insulate cantilevered joist bays? I have a 3 joist bays that stick out over the foundation to support a gas fireplace. I used rigid foam on the bottom of each bay (3), exterior joists (2) and the end (3). The builder had filled all 3 bays with fiberglass with kraft paper against the floor above (supposed “warm side” but this is pretty much outside the house). The kraft paper already had evidence of mold after 9 years. Should I put rigid foam on the top of each bay or joist fill the entire remaining space with batt (Roxul).

    • Todd says:

      Steve – Insulating cantilever joists is always a problem. Probably the best way to do it is with closed cell spray foam. I’m assuming that’s not an option for your situation though.

      I would use a minimum of 2 inches of rigid foam up against the sub-floor, with the edges sealed with GreatStuff. Roxul will work fine for the rest.

      Good luck.

      • John W says:

        We have several cantilevered joists also – just a clarification on your response:

        The rigid foam goes against the subfloor (top), but fiberglass bat is ok below that (the “cold” side)?

        What about the space where the joist is over the foundation? Should that be sealed with rigid foam just as if there was no cantilevered joist?


        • Todd says:

          John – I would put a piece of rigid foam over the top plate, between the joists, and sealed. Then but the outside foam under the sub-floor up tight against the first piece of foam. Make sense?

          • John W says:

            Yes, but I think I have to put the subfloor foam in place first – I don’t have access to the space from directly under the cantilevered joist (it’s already covered), I have to install from the end that faces the basement.

            Don’t worry about using fiberglass bats at all with this configuration?


          • Todd says:

            John – I think you’ll be fine. Just be sure to seal things well.

  21. Steve Hall says:

    Todd, I posted Dec. 24th and have been trying to get the materials lined up and the time to do things. In that period of time, I’ve been told be two contractors that I should not use spray foam because of the possiblity of carpenter ants getting into it and then going from there to work on my house. We have had just a few sitings of carpenter ants in the past. We live near a wooded rural area. I’m totally confused now and don’t know what to do. One side of my rim joist isn’t very accesible and is a straight run. By this I mean the joists run parallel with the rim joist, so I don’t have much room at all to work and very little visibility. I was going to get the highly expandable spray foam, encase the rim joist about 18 inches at a time and with a spot light foam it full. Now I’m very concerned about the carpenter ant situation and possibly termites. I have read the concrete housing people, using the ICF forms, have had this problem and it’s brought that type of building to a stop in this area. Any help would be greatly appreciated.

    • Todd says:

      Steve – I’ve never dealt with an ant problem as you describe so I’m not sure where you should proceed. For starters I would suggest speaking with a local exterminator to find out if there’s something you can do before hand to stop this or if there’s a certain product that would work best.

      Also check out:

      As I suspected the real issue is wet wood and not the foam. Ants don’t eat foam, they seek out wood to eat and then the foam becomes an easy area to hang out.

      • Steve Hall says:

        Thanks Todd, I’ve been going back and forth on this. I wonder to myself why they recommend spray foam for insulating around replacement windows and weight boxes, and why this area would be different or as prone to draw ants as that type of situation. All I know is you are very right about fiberglass, it’s a waste of time and money. I’m thinking I will talk to an exterminator and plan to continue with the project as I had planned and watch it. Thanks for your time and advice.

  22. Steve Hall says:

    Todd, In follow up to my concern with carpenter ants, etc. I spoke with a local exterminator. $500 initial inspection, treatment, plus $50 per mo. afterwards. A friend linked me with a guy who works for an exterminator and has all the licensing, etc. He said if I saw one carpenter ant there is a nest somewhere. For DIY’ers he recommends Bora-Care sprayed on the wood. Any hand sprayer will work he says. About $.10/sq.ft. if you DIY. Then baiting yourself. Don’t want to use this as an ad so won’t say anymore names, but he gave me several you can buy online. There are two types of poison and carpenter ants may ignore one for awhile so he recommends using both. The ants want either sweets or protein. They will walk around sweets if they want protein and vice versa. Then carry the poison back to the nest. So my plan is to use this method when it warms up. The ants go dormant in the winter. Anyway,I’m going to spray the rim joists and then foam them full. This guy said they will gravitate to foam because it’s a lot easier to get into than wood. But wet wood will draw them. That’s the first place they(the exterminators) look and look for trails. Just thought I would get back to you. This exterminator has foam board in his house and uses spray foam and isn’t worried. BTW the poison isn’t all that expensive, in today’s world, that I looked at. Thanks for your time.

    • Steve Hall says:

      Sorry to be a pest(no pun intended). One other thing I was told is the perpetual monthly return isn’t necessary. Watch the poison you put out and when it stops being taken you can stop treating until you might see one again. He said a good precaution would be to bait every spring and see if the poison is being taken. Thanks again and keep foaming.

  23. Greg says:

    We are a young couple renting an old home with very little, if any, insulation in the walls. It is terrible to heat. Would insulating the rim joists as you suggest help or would it push the cold air into the walls even more? My Dad is afraid the cold air will just go to the walls. The rim joists are freezing to the touch and air is coming in around the seams. There is no insulation at all in the rim joists. The landlord will not discuss doing any insulating and we have a yr. lease agreement. He will let us do this if we show him what it looks like after trying a couple joists. We don’t want to go to the expense if it’s going to just push the air to another place. Thanks in advance, love your site.

    • Todd says:

      Greg – First off sorry to hear you live in a house that the landlord won’t upgrade. Certainly insulating the rim joist will help keep a house warmer. One of the most important parts is sealing out drafts and creating a more tight building envelope. Insulating the rim joist will not adversely affect the walls. Will it make a big difference on your heating bill? that’s hard to say. A majority of heat loss is through attics, windows, doors and drafts. Just not sure how much difference it would make in just a year.

      Good luck.

  24. McSpud says:

    Nice website, great to see the comments.

    We’re using Bonded Logic’s Ultra Touch denim insulation (http://www.bondedlogic.com/) to do our rim joists as I didn’t want fiberglass in the inhabited areas and had not yet seen your foam board & Great Stuff concept.

  25. kieran says:

    Hi Todd,

    I’m trying to wrap my head around the difference between foil faced polyisocyanurate and extruded Polystyrene rigid foam board. Can you give me a layman’s definition. And I was wondering if the foil faced stuff meets fire code. I know the rigid board is flammable…



    • Todd says:

      Kieran – First off check out this article: Foam Board Types / R Values

      Because of the foil facing it has a higher R value. It’s typically used in commercial applications but it is really making inroads to the residential market. Frankly either one will work, I just prefer the foil faced in this application to give the most R value possible.

      Does it meet fire code? That’s a loaded question. Fire code can mean many different things. Most of the time basements will not require a fire rating per se. However, most codes today promote materials with low flame spread ratings. In that case foil faced insulation certainly ranks better than regular foam. Trying to answer fire code questions here is certainly not something I can do. I would recommend speaking with your local building code official to see what if any requirements are pertinent to your situation.

  26. Peter says:

    Todd – The information you provide is amazing. I’v heard bits of this before but you put it all together and explain why it works! Thank-you.

    I’m going to use your wall insulation recommendations in my 50 year old basement. I’d be grateful if you could answer a couple of questions.

    1. The inner edge of my sill plate is recessed about 1 1/4” in from the inner edge of the foundation wall. Do you recommend brining the foam board from the wall tight, up to the bottom of the floor joists and then laying foam board in between the floor joist from the top edge of the wall foam board over to the foam board in the rim joist?

    If the answer to #1 is yes, which I think it is, doesn’t this trap moisture from the concrete and transfer it to the wood sill plate and rim joist? I thought we are trying to keep the wood away from the moisture?


    • Todd says:

      Peter – Thanks for the compliment, I’m glad to help.

      You should bring the foam board up to the top of the concrete. Then install a narrow piece from the front edge of foam back to the sill plate. Then insulate the rim joist. While it may seem like you’re trapping the moisture the foundation can still dry to the “outside”. Be sure to seal all the joints up well with spray foam and/or tape. While this method isn’t 100% full proof it is the best approach that can be done in a DIY situation.

      If you hire a spray foam contractor they will spray it all, up the wall, inside the rim joist area and seal it all off. I think leaving the sill plate exposed definitely helps with drying.

  27. Kevin says:


    I live in a 5yr old house in Central Wisconsin, so the winters are cold. None the less the builders put fiberglass batts as insulation over the rim joists, and as I started pulling some of the batts back to see if there was air coming in, to my surprise there were a couple large gaps where cold air was coming in along with some mold starting to show up. As I continued to pull batts out there was some signs of mold starting in areas. I plan to clean up the mold and let it dry and then continue to put foil faced foam board back over the rim joist and seal as you suggest. My question to you is do you think there is benefit to caulking the top and bottom of the rim joist before putting the foam insulation in? It seems foam insulation would still allow cold air to creep in at the bottom and then push up into the walls, ultimately ending up with cold air on the first floor of the home. Thoughts?

    Thanks for all your help, this is a fantastic website.


    • Todd says:

      Kevin – Thanks for visiting the site. I hope you come back often!

      Anytime you can caulk joints in the framing you’ll be doing your home a huge favor when it comes to air infiltration and therefore energy use. On higher end homes the insulation contractors are now air sealing all framing joints, between wall plates, framing members, etc. So if you have the time and patience it certainly will help.

      Having said that, if you cut the foam just a bit smaller and then seal around the perimeter with spray foam from a can you’ll probably end up doing the same exact thing.

      Good luck!

  28. Tom says:

    Great site! we are preparing to finish the basement of our 1963 home and plan to install rigid foam on the rim joists as you’ve described. We are also getting ready to encapsulate our 500 s.f. unvented crawlspace with a dirt floor and block foundation wall. We will be installing a 20mil sealed vapor barrier on the floor and up the walls and sealed to the foundation wall a few inches from the top. With everything I’ve read, it is not a good idea to insulate the rim joists of a crawlspace with rigid foam. They are saying it could prevent a proper inspection if we ever sell. We are thinking about using urathane caulk to seal the rim joist, sill plate and around the joist ends. Then install and caulk treated 1×4 lumber to cover the top portion of the foundation wall that the sill plate doesn’t cover. Once that is completed we were planning on insulating the rim joists with fiberglass insulation. I really would like to just do rigid foam but don’t want to have to rip it out in the future. Fiberglass insulation is food for mold and really don’t want it in my crawlspace. Also,with a closed and encapsulated crawlspace. Do the floor joists above need to be insulated? There is kraft faced fiberglass between them now but I would really like to remove it. Please let me know what you think? Sorry for writing a book here!

  29. Miguel says:

    Todd, excellent site – I learned a lot

    My question is regarding the preparation of the cement blocks before the installation of the rigid foam. Do you apply a waterproof coating and if so, do you apply it to the entire wall or only to the section below grade? Thanks, Miguel

  30. John says:


    My township inspector recommend that I fill my entire rim joist area with roxul for the fire code. I live in the suburbs just outside Chicago.

    I read on the manufactures site that roxul is water resistant and is tested for fungal grow and passed with zero growth as it is made of volcanic slag there is no food source for mold growth.
    I filled my joists with 6 to 9 inches of roxul, as it is very dusty stuff I then cover each joist with a square of poly plastic.
    I think I should be ok – do you agree?

    I wish this stuff wasn’t so expensive I would do my whole basement with it.


    • Todd says:

      Roxul is a good option in many situations. However, as you pointed out it can be quite expensive. Your inspector was helping you out by suggesting Roxul which can be left exposed and not be a fire hazard. However, you pretty much destroyed the fire benefit by installing plastic. Typically most codes will not allow plastic to be exposed as it present a fire hazard. Did the inspector see that?

      • John says:


        The inspector did not see it yet. I assumed as the roxul in the rim joist was not being covered with drywall, as it will be above the drop ceiling, that it would still need a vapor barrier, so I added the plastic. Is there another fire proof material I could cover it with? I don’t mind leaving it open and above the drop ceiling if no vapor barrier is needed but in my work room (where no ceiling will be installed I would like to have something over it, as roxul dust tend to fall when drafts or vibration disturb it.

        Also, I have another question that I asked you about in your section on basement insulation. I asked about my Dricore system and I will take your advice and try to get foam board behind my walls. However, I am still confused about one thing regarding the air gap between the foam board and the insulation. I have read most your articles and a lot of your FAQs and I found what seems to be two different replies, they were totally different cases so I see why you offered two different solutions but I don’t know which one I should follow. In one case a guy had a big air gap (about a foot I think) and you advised him to install more foam board to fill the air gap but in most cases you say a small air gap between the foam board and insulation doesn’t matter. I have a drain pipe running along the ceiling so I just build my wall out about 5″ from the foundation, therefore, if I put in 2″ foam board I will still have an air gap of about 3″ from the insulation. Is this too much? Is there a max air gap space?

        Thanks for all you help.

        • Todd says:

          John – There’s no such thing as too big of a gap. What I was suggesting the other person do is take advantage of the extra space and use more foam if he could afford it. If you use 2″ of foam and seal it well there’s really not much need for the air space. It’s kinda like belt and suspenders if you know what I mean.

          • John says:


            What about covering the plastic that is over the Roxul?
            Does the rim joist area need a vapor barrier?
            I can remove the plastic and form in place some foam board but I thought I heard somewhere that form board and or form spray also need to covered too as they can release toxic fumes in a fire. I guess I could put drywall in each bay over the roxul and plastic this would give me a vapor barrier and cover the plastic with drywall, a fire rated material. What do u think?

            Thanks again.

          • Todd says:

            Yes you need a vapor barrier and yes in almost all cases flammable materials must be covered with a 15 min rated product like 1/2″ drywall. This is true for foam, plastic, etc that don’t meet certain flame spread ratings.

  31. Matt Beaton says:

    Todd –

    I have a 100-year old brick house (loadbearing brick walls) on a stone foundation. The unfinished basement is freezing in the winter, causing the first floor to be cold as well. What is the most effective way to warm up the first floor? Insulating the rim joists in the basement, or insulating the entire basement ceiling? The basement gets humid in the summer, even with a dehumidifier. Since the foundation walls are stone, moisture always finds its way in. Please advise, thanks.

  32. Doug says:

    Regarding rim joist rigid foam board insulation – I just read on another site that drywall/sheetrock must be placed right on the rigid foam board (within the rim joist). I will be putting drywall on the walls and solid wood tongue & groove ceilings. Is that necessary? Thank you.

    • Todd says:

      Most building codes require a thermal protection for flammable products like foam insulation. Each code is different you you’ll have to check it. In most cases it must be protected with a minimum 20 minute rating. This can be accomplished by 1/2″ drywall, some plywood and OSB products and even some solid wood siding products.

      • Doug says:

        Thank you. I will be checking with building inspector. I realize the XPS needs to be enclosed. But I assume you mean to install the 1x t&g pine ceiling boards and 1/2″ drywall on the stud walls as usual. In otherwords I don’t need to install any drywall/wood/etc directly ON the rigid foam board in the rim joist itself? Correct? Thank you again, this is a GREAT website.

        • Todd says:

          Not exactly…what I meant is if you don’t have a ceiling that protects that area you may have to install some drywall or wood over the rim joist foam in order to provide sufficient protection. This can sometimes be avoided with a ceiling that meets the criteria.

      • Jr says:

        Would puting that bluejean insulation against the foam count as a fire protection as it not flammble. Thanks,

  33. Fernando says:

    I have insulated my joist space using the, “Foam Board Rim Joist Insulation” method (using 2″ ISO), and My basement walls using XPS (using your method). What do you recommend insulating the piece of wood (that sits on top of the foundation walls), where the joist’s rest on?

  34. Tony says:

    First of all let me tell you that you have a great site. I too am running into a difficult situation when it comes to insulating my rim joist. Let me try and paint a picture for you. I have a 20 year old house with open-web floor joists apx 20″ tall. My foundation is half basement and half garage. Obviously the garage must be sealed from the rest of the house but I have a problem here. The sheathing which my aluminum siding is nailed to is 2″ foam board which can and has been easily chewed through by chipmunks. They nested in the insulation above my garage until this summer when I started to tear everything out. I have since plugged all of the holes and am preparing to reinsulate the garage this coming week. I am planning to use 1/4″ chicken wire to rodent proof the inside and then completely seal the rim joists with your above mentioned method (ie foam board/great stuff expanding foam). From there I will insulate the ceiling and reinstall drywall. I have taped all of the seams of the duct work and insulated all of the pipes already, but when I seal up the ceiling with 5/8″ drywall is their any risk of condensation leading to mold, being that the space is unconditioned, unventilated and completely sealed? Should I leave an air gap between the garage drywall/insulation and the 1st floor sub-flooring, or fill all 20″ with insulation (Cellulose or batts or a combination)? Any thoughts would be greatly appreciated. Thanks for your time.


    • Todd says:

      Tony – Thanks for the kind words. I hope you’ll consider signing up for my FREE weekly newsletter as well.

      If I’m understanding your question correctly, you want to seal the rimjoist, then insulate the ceiling between the garage and house, then sheetrock? If so that’s a very common issue and not a big deal. The trick is getting a vapor barrier on the warm side (floor) or using foam. You don’t want the vapor barrier on the cold (ceiling) side.

      • Tony says:

        Thanks for the quick reply. There is a vapor barrier under the hardwood floors on the first floor. From my understanding that should be sufficient. My main concern is the air gap. The cavity that will be formed by the sheetrock and subfloor will be 20″ deep BUT 12″ of blown in fiberglass will give me R-38, so I will be left with a substantial gap. Should the air gap be between the top of the insulation and the subfloor or the bottom of the insulation and the sheetrock?


        P.S. I have ductwork and plumbing stacks all over the place so batts dont seem very efficient.

    • Bert Burdick says:

      Use steel wool to fill the voids. The chipmunks will chew thru the steel wool and the steel wool fibers cut up their insides from the steel wool fibers swallowed. Causes them to bleed to death.

  35. Jr says:

    Can ultratouch blue jean insulation be used as a fire block on the foam insulation as it is class A rated. So if you put the foam on the rim joist and then place some blue jean insulation over that. Would that be ok for thermal protection.

  36. Steve says:


    A few questions regarding insulating the walls & rim joists.

    With the 2″ foil faced polyiso, do i put that right up against the face of the rim joist? In the you tube video you have, it was kind of hard to tell if is was placed forward or further back.

    My sill plate is anywhere from 0 inches to 1 1/2 inches from the inside face of my foundation wall, bad construction job there. I was planning on placing a thin strip of XPS from face of foundation to edge of sill plate to cover the top of the foundation wall and then placing the XPS from floor to bottom of joists, is that good, or is there another way to do it?


  37. tyler says:

    my question is – when installing 1 1/2 inch xps against the block foundation wall. from basement floor to joists?
    are form basement floor to ceiling (subfloor)?

    you have a very good website

  38. Bert Burdick says:

    Should the rim joist be waterproofed before placing the foam against the wood. My rim joists are currently wet from heat in basement going thru fiberglass insulation. I just thought it might be best to seal before installing foam.

    Any suggestions- and if you would seal the rim joists what product would you use. Project is on hold until rim joist dry.

  39. John says:

    In your second method using foam board, how are you achieving proper R-value?

  40. Jim says:

    i have tyvek and 1/2″ blue tuff foam board under my siding which all covers my sill plate /rim joists etc. i am now in process of finishing the basement. i have sprayed already great stuff along the sill plate/block foundation to seal the gap and all cracks within the rim joist and then placed my left over r-19 faced fiberglass in the rim joists. i plan on using the 2″ blue tuff foam board against the block foundation from the slab to the joists and build my walls in front of it. i get the idea of the foam board against the block foundation however why would the insulation be bad for the rim joists? im confused – the rim joists are above grade – isnt it no different than the insulation used in the main level walls? (again under the siding is blue foam/ tyvek sheathing studs r/13 and sheet rock in main level.) i know after i hit the rim joists n sill plate with gr8 stuff and closed all gaps i had no draft and my basement was more comfortable. i also noticed a difference with the r-19 in the rim joists. i havent noticed any mold or water/condensation on any surfaces in my basement. is it just the possiblity of getting wet and possibly getting mold that u recommend the foam in that void over the fiber? but wouldn’t that apply than to the r-13 in my main level walls to than. is fiber just a bad idea anywhere then? thanks

    • Todd says:

      Jim – I completely understand where you’re coming from. The rim joist area “seems” like the walls above but that’s just not the case. Mostly due to the environment it’s near (CONCRETE WALLS :)). I can’t tell you how many rim joist areas I’ve pulled away the fiberglass to find mold, mildew etc. Every insulation contractor we deal with here in our area now uses foam only for rim joists. It’s just a far better approach and one that we know works every time.

      Good luck.

  41. Jim says:

    thanks todd. just trying to wrap my head around the idea. before i start replacing everything i did already.

    • Todd says:

      It’s certainly a tough situation. Honestly, I think it’s drastically different because it’s an area thats typically not sealed well, it’s certainly not an area that’s adjacent to conditioned air like an interior wall is, there are usually lots of penetrations through it, just an all around tough area to deal with.

      • Jim says:

        Thanks again. i just finished up my basement with the tuff r stuff that steve mentioned below. it has a r value of 13. should i bother putting in any additional insulation in wall cavities?
        also fyi i found it vry ez to cut the boards with a 3″ spackle knife after scoring with a utility knife.

        • Todd says:

          Jim – Glad to hear things went well. The answer really depends on what is required by code and what is required for your comfort level. The reality is energy will never get cheaper only more expensive. I always like to insulate as much as I can afford. Your local energy code may require more as well.

  42. Steve says:


    I can not find any stores that carry 2″ foil faced polyisocyanurate, but have found places that carry a product from DOW described on there website as “Super TUFF-R Commercial Insulation features a high-performance polyisocyanurate foam core with three-ply poly/aluminum foil facers laminated to the core, resulting in exceptional durability. One facer is blue; the other facer is reflective foil”, can be viewed here


    Can this be used instead of the 2″ foil face that you refer to from Johns Manville in the article above for rim joist insulation? If this product is OK to use, would I put the blue face against the rim joist, and the foil face sticking out towards the room?

    The super tuff-r has the reflective foil on one side, and blue facer on the other, which i believe is still a foil face described as “polymer/foil super Tri-Plex non-reflective facer”. The regular tuff-r from Dow has foil facers on both sides.

    Thanks in advance,

  43. Ian says:

    I’m having a home rebuilt after losing it in a fire. The builder says they will be using spray foam insulation to seal the joist spaces. I have a couple questions about this topic. Se background first:

    There are a number of cantelevers too, which have just been covered with the sub floor. They’ve placed vapor barrier netween the subfloor and floor joists wherever there are cants.

    1st question: does this plastic vapor barrier need to be installed if they will be sealing the voids with spray foam?

    I ask for two reasons. It seems to me that the plastic would prevent the foam from bonding with the joists and sub floor. Whether that’s a problem or not, I don’t know. Also, the home’s far from being closed up and we’ve had a lot of rain. I just discovered that the vapour barrier’s been trapping a lot of water – which is sitting there, keeping the wood wet. I’d prefer the plastic to be removed if it doesn’t need to be there (because I the spray foam).

    2nd question: i the plastic needs to stat, can I cut slits in the plastic to allow the water to drain? Then seal with Tuck Tape before they close the cants in.


    • Todd says:

      Ian – The first real question is what type of foam are they using? Some contractors use an open cell foam because it is really cheap, but it doesn’t stop moisture and it can absorb moisture. I NEVER recommend using open cell foam. If they are using closed-cell foam then the plastic is not needed in my opinion. Having that water sit there is not acceptable, it should be removed before any insulation is installed.

      I would be sure you demand closed cell, get rid of the plastic, and let things dry out properly.

      Good luck.

  44. Ian says:

    I saw many typos in my first post! Sorry – I’m on a mobile and tend to hit the wrong keys – and then there’s auto correct, that can make a mess of things.

    Anyway, to clarify my last question, I’m asking “If the plastic needs to stay, can I cut slits in it?”.

    Thanks again!

  45. Ian says:

    Tod. Thank you for your offering your opinion, and confirming my own thoughts. The framer won’t be working on the house for another 4 days and the builder told me to leave it alone until he has a chance to look at it. I bet the OSB would soak it all up by then and then take forever to dry! Maybe I’m wrong. Would you be interested in seeing a photo of the trapped water? If so, where do I send it?

    • Todd says:

      Sure…you can send it to:

      todd “at” frontstepsmedia “dot” com

      I’ll be away all weekend so my response might be quite slow.

  46. Connie says:


    I live in the Milwaukee, WI area and haven’t been able to find 2″ foil faced polyiso unless I special order 24 sheets- about twice as much as I need – but can get it in up to 1″ thickness. Would it work to just double it up in each rim joist? Or do you have any ideas about how to get the right stuff?

    If we do 2 layers do we foam in each layer as we put them in or just place them in together and foam around the outer edge?

    Thanks so much for all this great info. We have a large basement that we are working on insulating before the cold winds blow and your site has given us so much great info.

  47. Mary Maslanka says:

    Hello, Todd!
    I would like to have the rim joists sealed in the basement of my 45-year-old home. If the rigid 2″ foam board with polyisocyanurate is used, how is it fitted over wires, conduit, pipes, etc.? How does the fire rating for this compare to the closed cell spray foam?

    For the closed cell spray foam, I am concerned about minute particles that may be emitted into the air over time, that could cause allergies, etc. Has long-term exposure to this been studied?

    Since the basement will be tightly sealed, do I need to worry about paint cans, cleaning products that are stored there–could there possibly be more of a combustion problem? The contractor will be doing a combustion test after the installation is completed.

    Thank you!

    • Todd says:

      Mary – Insulating an older basement with foam board vs spray foam can be challenging when you consider all the wires and pipes. Ultimately it comes down to cost. Foam board can be cut to fit around those things and they cans of spray foam are used to seal around the holes. Both spray foam and foam board typically require a fire barrier of some sorts depending on your local code. Many of the codes require at a very minimum a 20 minute thermal barrier. This typically consists of a layer of 1/2″ drywall or wood sheathing. Again, it really depends on your local code.

      Both products do have differing off gas issues. Again it depends on brand, type, ventilation, and many other issues.

      Neither product is going to change the combustion characteristics of the materials you store down there.

      I hope this helps.

      • AJ says:

        This is what’s been getting me. My house has wires all along the rim joist in the unfinished half of my basement. Due to the offgas issues with spray foam, it’s not an option in my situation. Small quantities like great stuff should be fine, but there’s too much toxicity potential for a 2″ spray throughout. How should I approach foam board insulation? Spray onto the back of the foam board piece and set it in place, letting the foam fill the gap?

        • Todd Fratzel says:

          AJ – Renovation work is rough especially with old plumbing and wiring in the way. The best results will happen either with spray foam that seals the bay, or foam board placed directly against the rim joist and sealed to the framing. So….I can only recommend spraying it, or moving the wires and then installing the foam board. I will say we’ve been using a spray product that uses a soy based component that you might want to look into: http://www.demilec.com/Products/Closed-Cell/Heatlok-Soy-200-Plus.aspx

          Good luck.

  48. Katie says:

    I have spray foamed (great stuff) the gaps in my rim joists which were large and numerous. now i want to put in rigid foam. does the foam need to touch the rim board (header board) or can i have a gap. the spray foam expanded so i will either have to cut the rigid foam boards smaller, or cut them to fit the rim joist space, but then have a 1/2 inc gap becasue the spray foam juts out, or i will have to go to each rim joist and smooth down the spray foam.
    what is your advice.

  49. Greg says:


    Again, want to say your website is great.

    Question on the topic of Rim Joists:

    I’ve read alot of the posts here on this, but not specific to what I’d like to do.

    I can’t get 2 inch foil faced, so I;ll use R10 2inch owens corning foamular to fill the rim joists with and use the great stuff to surround the foam. – My question is:

    After I do this, can I fill the rest of the remaining space of the rim joist area with fiberglass, or is that still a bad idea?

    How about if I put in multiple 2inch foamboard pieces per rim joist?

    thanks for options here.

    Minot, ND

  50. Mark says:


    We are having Spray Foam insulation application tomorrow (Tues).

    In prepping our basement we started taking down the batting the builder installed. There is a 15′ section in the rim joists on the north side of the house that has evidence of mold. Black spots on OSB and fiberglass. Some of the OSB is still damp to the touch. We’ve been running fans on it since Sunday night.

    So, couple of questions…
    1) Should this be completely dry before we allow spray foam insulation application?
    2) Will spray foam kill mold that is currently there? Is it a hot application?

    Still waiting to hear from spray foam installer. Can’t be the first time they’ve come across this. I just don’t want him telling me it’s fine to spray over it when it isn’t, because he doesn’t want to throw off his schedule.

    Any other guidance?

    • Todd says:

      Mark – Ideally in a perfect world you’d get it as dry as possible. Also, you’d want to get ride of the mold. I wouldn’t be as worried about the moisture (as most wood has a high moisture content in new construction and we spray foam that) as I would be the mold. If it were me, I’d use a bleach/water mixture and scrub the area today to kill off the mold. Keep the fans running to dry things out as best you can. Then I’d just keep the schedule and keep moving forward.

      Good luck.

      • Mark says:

        Thanks. Just wanted prepped in case SprayFoam installer was gonna try and push us to spray today. We postponed until we could clean up.

        House build was 2010. Called the builder just before posting my original message to you and upon receiving my voice mail, sent someone out from their warranty dept. They then brought in a disaster team with fans to blow out the egress (in case it was airborne) and dehumids to get humidity down.

        In the evening a rep came out to educate us on what’s going on, which we kind of knew. First he pointed out our 77 HERS rating the house scored prior to purchase and told us how well wrapped and tight the house is. Downside, the house needs to breathe more. He provided us with humidistats for all three floors and told us to monitor to keep humidity in the 35-45 range. Had a reading of 55 in the basement earlier in the day. He suggested the furnace fan to be on at all times, not auto, and that it would tack on appx $25/yr extra on the electric bill but would help circulate the air in the house and make it healthier living space. Also informed us to run a dehumidifier in the basement to help keep in the 35-45 range.

        Air Quality test done today/results tomorrow. Disaster team cleaning up the mold tomorrow. Fans/dehumid thru Friday. Builder spraying foam in rim joists after another air quality test Friday. Hopefully getting back on track with original spray next week and drywall soon after.

        The rep that came out informed me that they built 300 houses in the past 3 years and they are going back to all of them and checking their houses because they are all wrapped/insulated so tight.

        Thanks for your help.

        • Todd says:

          Mark – Glad you got answers that are meaningful. Today’s home are definitely tight that air quality is an issue. I also recommend that all bath exhaust fans have a timer on them so you can let them run longer. This pulls more fresh air into the home and also does a better job exhausting damp air.

          Best of luck.

  51. Bob says:

    Wow Todd, what a great website!

    I am finishing a unfinished house that I purchased. 3″ ICF foundation, 10 inch concrete, additional 1″ of foil faced on both sides. 8 1/2 ft tall “crawlspace”. Had to rip out non code wood basement floor and interior walls from previous Owner/Builder. Not wanting to use a wood floor again, and not wanting to pump concrete thru window wells, I decided to cover dirt floor with concrete pavers flush with the top of the foundation. I first used road base, then 6 mil vapor barrier, then more road base, then 2″ foil faced foam (R-Max), another 6 mil layer, paver sand,
    and finally, pavers. This totals 18″ thick. Other than the two window wells and the 3 ft door, it is unventelated. I completely sprayed great stuff foam around the rim joists, (and have also sprayed great stuff around ALL of the framing of the two wood floors and ceiling)
    2 x 6 Wood wall framing insulated with Roxul R-23, cathedral ceiling (top floor) with kraft faced
    Fiberglass R-38
    Now to my question!
    Since I have a building envelope (insulation), what should I do, if anything, regarding insulating the ceiling of the basement (8 1/2 ft tall crawlspace) ? Ceiling is I-joists 16″ OC.
    The inspector has signed off the “crawlspace”
    Thanks Again for your time

    • Todd says:

      Bob – That’s a tough situation. I’d recommend you check out the buildingscience.com page for details of unvented crawl spaces. Good luck.

  52. Steve says:


    Great website, have used quite a few of the sites regarding insulation and I sure am glad I found these sites. I just wanted to say thanks for all the info you have provided.

    I am in the proccess of finishing my basement, house is 4 years old, live in Chicago area. I have already put up 2″ XPS on the walls, and now I am using the 2″ foil face in the rim joists. Last night as I was taking down the old fiberglass and putting up the foil face, I couldn’t believe how much condensation was actually built up on the inside face of the rim joist. The nails that protruded through were 100% rusted. The backs of the old fiberglass was damp. Everything you have stated in your articles was 100% spot on. I just cant believe that the houses are built that way, when its obviously the wrong way. I know its cheaper for the builder, but all its going to do is lead to mold, and give the homeowner problems.

    Thanks again for all the info.

    • Todd says:

      Steve – My pleasure. As you can imagine many folks don’t believe how bad it can be. Glad you found the information helpful and informative. Please be sure to share our site with others, Google Plus the articles, or share on Facebook. It’s the only way our little site will rise to the top above all the corporate junk that floats around the Internet.

  53. JAH says:

    Your recommended approach says to put the rigid insulation board flush against the inside face of the rim joist in the basement. In my 1930 house, many of the spaces between the joists have 2×4 blocking or studs coming down from above, sometimes with narrow openings between the 2x4s and the joists. Can I install the insulation board in front of these studs so I can use a single piece in between each joist? This would leave a 3″-4″ gap between the insulation and the rim joist. Would this direct cold air up into the first floor above these gaps?

    Also, you talk about problems with exposed nail ends on the inside of the rim joist. Where I am able to put the insulation board flush against the rim joist, is it a problem if the nail ends puncture the insulation board (but not extend all the way through the 2″ board)?

    Thank you very much.

    • Todd says:

      JAH – In many cases it’s not feasible to get the insulation tight to the rim joist. While this isn’t ideal it is often the best solution. The important detail is making sure whatever you install is tight and sealed so cold air cannot get into the conditioned space. The nails are fine like that.

  54. Wayne says:

    Great site. I learned a lot. Am in the middle of my project. I was suprised at the air movement coming in from the rim joist area and the dampness behind the fiberglass. I am using foil faced. I started by removing the fiberglass and cleaning the areas. I caulked the top and bottom with silicone, cut the foil faced with a circular saw and miter saw. This worked well. I started with spray foam. What a mess. Then I got an idea. 1/4″ plastic tubing ($.16 per foot at the big box) can be put on the end of the spray foam tube and attached to a paint stick with tape. Much less mess and much easier to reach around pipe, wiring and heat vents etc. A lot less waste from the cans too. I order a pro kit for the rest of the spray foam and I am sure i can extend my reach using the same method. Again thanks for a great site.

  55. Taco Mike says:

    Hi Todd. Thanks for the informative article. I live in a townhouse where the builder had covered the oriented strand board rim joists with standard fiberglass batts. Also, recently, water was being soaked up by the OSB due to the perimeter of the front door needing to be resealed. This problem has been taken care of and the OSB has remained dry.

    Needless to say, since this incident occurred, I now worry about moisture especially since I am planning to finish the basement. Is there any added benefit at all by using a water seal paint on the OSB before insulating with foam board, or is it best to keep it as is? Thanks.

  56. John Leite says:

    I have a question concering the foam board you used. I went to the Johns Manville site and it specifically states: “Do not leave exposed. AP sheathing requires an interior finish of a minimum 1/2” (13 mm) gypsum board or equivalent 15-minute fire barrier. It doesn’t look like you covered the foam board with anything. Are you at risk in the event of a fire or am I missing something?



    • Todd says:

      John – You are absolutely correct. What you can’t see is the fact that I’m finishing the ceiling in that area with 1/2″ drywall, which prevents the foam from being exposed. The code provision does NOT require the fire rated covering to be in direct contact, it just has to keep the area from being exposed. Make sense?

  57. sheila says:

    there was a problem with the trim work done by siding guys that resulted with water infiltration along rim board. I would take it this rim board has to be dried out before we would do the 2 inch rigid foam and spray? otherwise it would rot behind? no?

  58. Don says:

    Hi Todd: I am about to insulate my Rim Joists with 3″ foam with the foil face. My question is do I push the foam all the way back to the end of the joist or do I line it up evenly with the front of the cinder block?

  59. Todd,

    Thanks for your website. I have found info here that I have been unable to find elsewhere.

    When insulating rim joists with foil-faced foam board, do you leave the foam board exposed or cover it with sheetrock? We are finishing our basement and I think our building code requires foam board be covered with a fire resistant barrier. So I was going to use foam board where we have drywall on the walls and ceiling and regular fiberglass insulation in the unfinished areas.


    • Todd says:

      My pleasure Paul.

      In most jurisdictions the insulation (foam or fiberglass) must be protected against fire. The interpretation varies from location to location but it’s good practice to protect insulation with some sort of wall/ceiling barrier. The easiest way is to drywall the walls and ceiling sealing off the insulation from a fire hazard.

      Best of luck.

  60. Mike says:

    I think my situation is similar to others so I apologize in advance if this is repetitive: I want to insulate the rim joist areas and was planning to use foam board with caulk or Great Stuff.

    The difficult part will be the build outs for the gas fireplace and a bay window. Both are about 2 feet out and currently stuffed with fiberglass. The bay window has both a HVAC register and a electrical outlet in the wall, which will make the job more difficult. I can access both through the unfinished basement.

    Because the bay window has the HVAC duct should I insulate the bottom of this space? The bottom of the space appears to consist of just the other side of the vinyl siding. Should some pressed board be placed there and then foam board?

    Also, most of the foam board / Great Stuff insulation jobs show the area recessed but along one side of the basement the joist actually sticks out over the sill plate. Should I just affix foam board to that area? Thanks.

    • Todd says:

      Mike – Bay windows and bumpouts for fireplaces are very difficult areas. In an ideal world they would be spray foamed. Having said that there are ways to improve the situation. You need to consider R value and just as important is sealing drafts. Often times drafts from below are worse than the lack of R value in the insulation.

      The best approach is installing 2″ of foam to the bottom of the joists, then plywood. Then filling the joist bays with fiberglass or foam. If you can get to the underside from outside I’d recommend adding some foam, then being sure it’s sealed really well. On the inside, then I’d try to seal off that area with more foam as if there were a rim joist above the sill plate.

      Good luck.

  61. Justin says:

    I recently did some of my header spaces with foamboard and sprayfoam, the sprayfoam pushed some of the boards out a bit so there is most likely a 1/4 inch gap btwn board and rim joist in some areas.
    I am also concerned with the sprayfoam itself. I used great stuff in cans, is this open or closed cell, what do you recommend.
    Also I have huge drafts in the basement headers and areas where poured foundation meets sill plate, should I seal with caulking before finishing bsmt walls with foamboard?
    Thank you for your reply

    • Todd says:

      Justin – The best way to do the headers and the rim joist is cutting the foam board so there’s a small 1/4″ to 1/2″ around the foam and framing. Then use a product like Great Stuff to seal the edges and lock the foam in place. This also helps keep it from moving away from the surface. Great stuff is fine for this application. I’d also use it to seal any gaps in that sill.

      • Wayne says:

        I still recommend a small bead of caulk where the rim joist meets the foundation Smooth it out with your finger and then install the foil faced with a slight gap around it and follow up with spray foam. That will eliminate any air gaps. Cauld should also be used around any thing that passes through the sill plate.

  62. Tory says:

    Hi Todd,

    My crawlspace (Chugiak, AK – zone 7) is currently vented and has a poorly installed vapor barrier over the dirt floor. I haven’t had bulk water issues, and have improved exterior grading and gutters in recent years. I don’t have an interior drain and/or sump basin. Sealing and insulating (floor, rim joist, walls?)are next on my list.

    I’m asking this in the rim joist article because I can’t quite get over my concern for moisture transfer in the sill and/or rim joist. There is evidence of sill gasket (white foam) that should stop capillary action, and the walls were damp-roofed and insulated (2″ blue xps) on the exterior. I’m unsure of exterior footing drains. There is no evidence of a capillary break between the footer and first course of block.

    Will using rigid foam (I’m thinking 4″ – two 2″ layers) and leaving the inner edge of the sill exposed do anything for me, or should I call the spray foam guy and seal the whole works? What about insulation on the inner wall? I read a semi-recent article that quoted Dr Joe saying he isn’t concerned with basement/crawlspace walls being able to dry to the inside, as they tend to dry at the top of the foundation wall. I’m afraid to fully seal the edge of the sill and top course of bock – doesn’t the moisture have to go somewhere?

    • Todd Fratzel says:

      Tory – As with all basements the solutions are seldom perfect. Block and concrete foundation walls contain moisture forever, that we know for sure. Over time moisture comes and goes as the surrounding conditions change. My theory is that so little moisture actually escapes out of the top narrow portion of the wall that it’s likely negligible, at least in the grand scheme of things. More importantly, that small amount that might affect the sill plate, is of less concern to me than moisture getting into the basement as a whole. Make sense?

      • Tory says:

        Thanks Todd,

        Since bulk water isn’t a huge concern (except that my uphill neighbor just paved about 2/3 of his lot) I’m leaning toward complete encapsulation with insulated walls and spray-foamed sill. I’ll probably have to add an HRV.

  63. Sean says:


    I am planning on insulating rim joists with foil-faced foam board and am reading that it is a good idea for foam board be covered with a fire resistant barrier. I am not finishing my basement per say, just looking to tighten up insulation of my basement. Would you recommend not using the foil-faced/spray foam method since there will not be a ceiling or walls to cover the insulation?

    Thanks for the help, and all of the great information on your site!

    • Todd Fratzel says:

      Sean – This is a code issue that many face and struggle with. The insulation will make a huge difference in the performance of your home. In some locations, the code requires that the foam be protected against fire. In some locations, the foam manufacturers have received code compliance based on testing which allows the product to be used without a barrier. I can’t tell you what the code is in your location, just that you should be aware of the issue. There are thousands, possibly hundreds of thousands of applications without the barrier, but it’s really a personal decision for you to make. Good luck.

  64. Eric says:


    Do you know if local code allows foil-faced rigid insulation in basement sills without a fire barrier in New Hampshire?


  65. Debrah says:

    I need some guidance please!
    We bought a house with the basement finished except for the large utility/laundry room. We discovered last winter that when the basement was partially finished a year before we bought the house, that no insulation was installed…anywhere. We are planning on remodeling the basement, spray foam is not in our budget and rewiring has to be done for a few different reasons, one being our house was built in 1947. What my main question is is: does the framing have to come down with the drywall because there is no vapor barrier? My second question: is a vapor barrier always necessary for the floor (this was not done either)? My third question: In a house this old, we have not had any issues with wetness in the basement (surprising) but what is the best way to prevent moisture in the rim joists without using spray foam insulation?

    • Todd Fratzel says:

      Debrah – I’ll try my best to touch on your questions. The first question about moving the studs really depends on the approach used to insulate those walls. I’d recommend either foam board insulation or spray foam. With spray foam you can likely leave them in place. With foam board you’ll need to at least slide the wall forward to make room for the foam board so that it’s continuous behind the studs. You can read more about using foam board here: http://www.homeconstructionimprovement.com/how-to-insulate-basement-walls/

      Most concrete slabs are poured today with a vapor barrier below them. Being built in 1947 that may not be the case. So this issue is best dealt with depending on the flooring type you use. I’d work closely with your flooring supplier to figure out what products will work best for your situation.

      For the rim joist I’d recommend the process of foam board described in this article.

      Good luck.

  66. Dennis says:

    Is the rim joist foam board insulation method appropriate for houses without sill plates? I am concerned the natural moisture in the joists will be condensed by the contact with the top of the cold concrete blocks. Then, with foam board encapsulating the joists, the joists may not dry causing rot to develop. Instead of the joists being able to “breathe”, will the foam board tend to accelerate, rather than prevent, any potential rotting?

    • Todd Fratzel says:

      Dennis – Very interesting question and quite honestly not a situation I’ve had to deal with. It’s really hard to say if the foam will accelerate any natural tendency to decay. The joists that are in direct contact with the foundation most certainly will decay over time. One might argue that the insulation makes it less likely for the joists to have moisture condensate on them, but one might also argue that any moisture will get trapped and therefore accelerate the problem. From my perspective those joists have a problem regardless of which approach you take, and my bet would be to save on energy and deal with them at some point down the road..which is likely regardless of whether you insulate or not. Good luck.

  67. Edwing says:

    Hi, it may be a silly question or questions. Is Spray foam 100% effective preventing mold and vapor issues? same question for Board foam, which I am more likely to use depending on your answers.


    • Todd Fratzel says:

      Mold needs oxygen, water, and a food source. Spray foam, and foam board do not provide a food source, and used correctly, prevent water from getting into a space through vapor transmission. It’s far more effective than other insulating materials.

  68. Fred says:

    I am installing polyiso on the rim joists as per your articles.

    My problem is the wall where the joist beams (engineered) run parallel to the rim board. I have a 20ft run which also contains a 6″ heat duct in between the rim joist and the floor joist. This area is currently insulated with fiberglass insulation ( typical to your home). I don’t think I can get the poly in-place without removing the 6″ pipe.

    Would it be OK to leave the current insulation in place, and perhaps stuff more fiberglass in place?

    The pipe is about 3-4″ from the rim joist. If I use the poly, will the heat from the run affect the polyiso insulation?

    Please advise

    • Todd Fratzel says:

      Fred – I would try really hard to get that polyiso in there. Having only 3″ of fiberglass between a heat pipe and the rim joist is not a good detail. The contractor who did that really didn’t do you any favors. The improved efficiency of your heat system that you’re realize with better insulation in there will be significant. Maybe take down the duct, insulate, then replace? The heat won’t bother the polyiso (assuming this is duct work and not a flu pipe).

  69. Fred says:


    I have all the foam board against the poured walls in my basement and the rim joist insulated and foamed per your suggestions.

    Ready to do the electrical My questions are:
    Is it necessary to install fiberglass insulation between the metal studs /drywall and foam board?
    and if so would I use foil or craft faced insulation and which way would it face?

    I assume I would not need any additional vapor barrier between the dry wall as the foam board is a vapor barrier itself?

    Also what type of electrical box would you suggest I use to attach to the metal studs. Does the wiring need to be fastened to the studs…could I use metal stand offs?

    I appreciate your help as always


    • Todd Fratzel says:

      With metal stud framing I would only use unfaced fiberglass. You don’t need it unless you want the extra R value. No additional vapor barrier. You need boxes that are designed specifically for metal studs.

  70. ted says:

    Maybe this is discussed somewhere on here, but I didnt see it.. why exactly foil faced polyiso instead of just XPS for the rim joist insulation?

    • Todd Fratzel says:

      Ted – Just a higher R value. I’d use it on the foundation walls as well but the foil doesn’t do well in contact with concrete.

  71. My rim joist have spray foam in them (had fiberglass installed before and a mold problem) my question is can I put rigid insulation over the spray for a better R value. Wally

  72. Diana J. says:

    Phenomenal site. Thanks.

  73. Clara says:

    Hi Todd,

    I have received a suggestion to use Roxul Comfortboard 80 instead of XPS due to environmental considerations and indoor air quality. Would this be acceptable in your view?

    Thanks, Clara.

  74. Jeffery Swietlik says:

    The only polyiso foam board I found is foil faced on BOTH sides… Is that a problem?

  75. Matt K says:

    Hi Todd,
    Great site. Thanks for sharing all of this information.

    I have a brick faced bungalow in SE Michigan, built in 1954. My basement walls are cinder block, and access to the interior side of the rim joist is closed off by wood blocking installed between each floor joist. I’ve cut open or removed a couple of these to run electric or gas lines to the exterior, but I do not want to remove all of them just to insulate. I have several drafty spots along the perimeter of the first floor, so I want to seal and insulate the actual rim joist, and not just the backing of the wood block.

    If I drilled a hole in the blocking board, can a closed cell foam (such as Froth Pak) be sprayed into the cavity to effectively to fill, seal and insulate that space?

    Thanks for any feedback!

  76. Mark says:


    Excellent site and data! I already have numerous sheets of 1 inch foil faced rigid insulation (foil on both sides). Is it okay to simply put two 1 inch foil faced boards together in the rim to equal the 2 inch suggested depth, or should I go buy full 2 inch sheets?

    Also, I am curious to get your feedback on using the same 1 inch foil faced insulation to go in each full joist bay (up against the sub floor) to act as a vapor barrier, and then pad the remainder with fiberglass or other batt insulation to hit the desired R-value?

    This work is all in a 4 foot high unheated poured concrete basement/crawl space with finished space above.


  77. DKP says:

    No question, just wanted to chime in: Thanks to this article I’ve bitten the bullet and begun doing this in my new house, an early 50s Cape Cod. One shortcut the builders took was no sill plate – the joists sit directly on top of the concrete block walls, with messy and uneven cement to fill in the blocks on the tops. This makes cutting and fitting the pieces of foam board trial and error. Oh well. I bought (had to special order) 2″ foil-faced and use Loctite foam to seal around. First I caulk the crap out of the bays before I put the foam in. I went an extra mile and use Loctite foam board adhesive and fasteners to set each piece in place. Very helpful post!

  78. Matt says:

    Todd- Regarding my basement, would insulating my rim joists alone without insulating my below grade walls help improve temperature and comfort in the basement? And if so do you recommend 2 In polyiso foiI faced closed cell? I live in upstate NY. Thank you!

    • Todd Fratzel says:

      Any insulation you do will help. I think you’ll find that insulated the rim only will help on the main floors more than the basement, but it certainly will make a difference. 2″ at a minimum.

  79. Gilly says:

    Todd, your articles are always a wealth of information, thanks for always sharing these hard to find answers. Even when it’s a topic that I thought I had no use for, after reading up…I always walk away with something! Awesome…
    In my basement, they ran the 4″ septic pipes along the foundation walls…literally pinned to the wall. This leaves no room for any foam-board, and very minimal space for spray foam…like I said, it’s pinned to the wall. Because of how it was installed, I can’t pull it away or move it. I want to always have access to the clean outs, so I’m struggling with how I’d insulate around it? Do I just insulate right up to it, maybe bevel the edge on the foam-board so it gets really tight to the pipe? Or do I box around it with foam board and just mark up where the clean outs are in case I ever need to access them?

    • Todd Fratzel says:

      Gilly – Glad you find the site useful. This is VERY common and not the end of the world. You have two options as you pointed out, and either will be just fine. Typically we’ll box around them when it’s a finished space, and install an access door for the clean-out.

  80. Gilly says:

    Another question, my basement has various sized walls (in height), they range from an 8′ wall, to a 6′ wall to a 4′ wall to a 2′ wall (it’s a full walkout). Starting with the 6′ height and down, they insulated with fiberglass batt from the rim-joist upward, making the difference to 8′. So for example, a 2′ high wall of foundation, would get handled how? Foam-board from the slab up 2′, then cover the top of the foundation wall to where it meets the rim joist, correct? The remaining wall above the 2′ foundation wall has the fiberglass batt, covered in plastic. Because that wall isn’t concrete, I can just leave it alone? If I were to decide to run the foam-board the height to the ceiling, should I remove the plastic from the fiberglass batt not to trap it in plastic or is it ok to leave as is…with the foam board on top of it? I hope that’s not too confusing. I was hoping to upload some photos, but I don’t see that option… Thanks again!!

    • Todd Fratzel says:

      Gilly – This is also very common. Couple options here. You can having regular fiberglass and vapor barrier but often in a basement that doesn’t work well, and we see mold issues (too much moisture on both sides of that assembly). In my cases we spray foam that upper wall cavity, or install rigid foam in that cavity and can foam it in tight.

  81. Jim says:

    I’m looking to properly insulate my rim joists, but I believe I have a unique setup. My 1st floor wall studs (2×4) run directly down to the foundation sill plate, so that the stud bays are continuous from the first floor to the basement. In other words, the subfloor doesn’t create a separation between the floors. Currently the stud bays are filled with fiberglass insulation from the ceiling of the first floor down to the sill plate in the basement. Also, my foundation walls are roughly 20″ thick, so the cavity between the floor joists and the top of the foundation, back to the rim joist, is roughly 18″ deep. What would you recommend? Thanks so much.

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