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Rigid foam insulation for basement

kyle8 | Posted in General Questions on

Hi, I recently built a house in southern Wisconsin where the winters get in the negatives and the summers get in the 80’s with very high humidity. I have a partially exposed basement with windows on most walls but I was not planning on finishing the basement off anytime soon. Because I am not finishing it my walls are half foundation and the other half studs. My exterior is Hardieboad on top of Tyvek on top of OSB. And on the inside I had batt insulation between the studs with poly sheeting as a vapor barrier. Because I didn’t put any drywall over the vapor barrier I had a lot of condensation build up on the inside of the wall and poly from the warm inside to the cold outide during winter. I removed all of the poly to prevent mildew (some had already begun) but I wanted to find a way to better insulate my basement than just the batt. I originally considered spray foaming but decided to go with rigid foam instead for a cheaper alternative. My plan is to put 4” rigid foam in between the stud cavities and spray foam around the edges of the foam. Should I be caulking gaps between studs when studs are doubled up next to each other? If I decide to put a layer of 1” or 1/2” rigid foam over the wall against the studs ( in place of drywall) is this acting as a vapor barrier trap because of the rigid foam between the studs? Or am I ok because the foam between the stud cavities has such a high r value?

any thoughts and input would be much appreciated.



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  1. patrick1 | | #1

    Kyle, when you say your walls are "half foundations and the other half studs". Do you mean that you have a full height foundation, with half of it covered with studs and batt? Or a half-height foundation with a stud wall sitting on top of it? ...does half your basement have a bare foundation and the other half has the foundation plus stud walls?

    For the condensation, is it only occurring in winter? The other possible cause is warm humid air coming in through your sheathing and condensing on the vapour barrier - this is a common cause if you air condition during the warmer months. Hardieboard is a reservoir cladding so solar vapor drive is an issue - when it rains, the hardieboard acts like a sponge and then when the sun comes out it drives water vapor inwards, causing condensation on the vapor barrier if you have air conditioning.

    If the stud wall is sitting on top of the foundation, a quick fix might be to swap the vapor barrier out for a smart vapor retarder like Certainteed Membrain or Intello Plus. These membranes become more permeable when conditions are humid (e.g., summer) and less permeable when conditions are dry (e.g., winter), reducing the risk of condensation. You'll also want to make sure they're detailed well to act as an air barrier (seal penetrations and joints with tape or acoustical sealant, etc.).

    If the stud wall is against the foundation then rigid foam the right solution because it won't have any moisture issues if your foundation allows a bit of moisture in as they often do. I would be weary about putting foam between the studs though. The studs will be a thermal bridge because there won't be any insulation behind them and that could be a spot for condensation, mildew, etc. to form. Also, usually building codes call for a fire barrier when using rigid foam since burning foam makes for pretty toxic smoke. So you would still have to cover your foam with drywall.

    If the stud walls are sitting in front of the foundation and they're not structural, you might consider removing them and replacing them with a fire approved foam like dow thermax (see GBA article on this approach here:
    DriCore smart wall might be another easy option:

    With both of these you'll need to think about how to insulate the rim joist area. Sprayfoam seems to be a preferred option but you can also cut foam to fit between the floor joists and used canned foam to seal the edges. See video here:

    If you decide to go all in and finish the basement, you could remove the existing stud walls then use any sort of foam attached directly to the foundation walls to create continuous insulation behind a new stud wall and drywall.

    Hope this helps.


    1. kyle8 | | #3

      Thanks Patrick, great info!
      To further explain and answer some of your questions...
      The stud wall is on top of the foundation wall, so the foundation is currently exposed with about a five foot wall on top. On the outside of the foundation I do have 2” rigid foam at R10 with grade coming to a few inches below the top, in case that is worth noting. I have not had any issue in the summer, the moisture build up seems to be just from the winter. I also do have 3”s of spray foam in between the joists on the rim board. I’m basically thinking of the rigid foam as a replacement for spray foam in the wall because it is cheaper, but if the foam isn’t tight against the osb is there a concern with having that air gab behind it? Or if I want to cover the wall in one inch foam after the stud cavities are insulated with the rigid foam am I creating a moisture block issue? In Wisconsin if you spray foam your walls, the rule is always to not put a vapor barrier on the wall after because you can trap moisture between the barrier and the spray foam. Am I at the same risk with rigid foam as a base under drywall when foam is also in the stud cavities?



  2. Expert Member
    Dana Dorsett | | #2

    >"I had a lot of condensation build up on the inside of the wall and poly from the warm inside to the cold outide during winter."

    In winter the condensation (if any) would be on the conditioned space side of the polyethylene, not inside the cavity, unless there was a very serious source of moisture leaking into the wall cavities. If outdoor air was leaking in and creating cold spots on the vapor barrier condensation could form on the ROOM side of the plastic, but on inside the cavity. In summer if humid air is leaking into the cavity
    and the room is cool or air conditioned THEN condensation can form on the poly inside the cavity.

    Please explain in more detail where & when the condensation was forming.

    Using polyurethane caulks/sealants to air seal the sheathing to the framing the full perimeter inside EVERY stud bay, and sealing any seams in the sheathing using appropriate tapes (reinforced with 1/8" of duct mastic troweled over the tape if it's not sticking well) goes a long way toward limiting air-transported moisture from entering the wall cavities. Yes, put a bead of polyurethane sealant on all doubled-up framing, as well as where the band-joist meets the subfloor and mudsill, and between the sill and the top of the foundation.

    Cut'n'cobbled foam between studs is a waste. You get a lot more performance out of using cheaper (and easier to fit well) fiber between the studs- save the foam budget for continuous layers. A half inch of foil faced polyisocyanurate with the seams taped with a high quality HVAC tape on the interior provides comparable vapor retardency to 6-mil polyethylene, and puts an R3 thermal break on the studs, and even with R20 batts would outperform a full cavity fill of cut'n'cobbled polyiso foam board. Going with a continuous 3/4" -1" is even better from a thermal break point of view. Do the math:

    1. kyle8 | | #4

      Thanks Dana,
      great info and article. The condensation was on the cavity side of the poly in winter if I rubbed my hand on it the surface was dry but I could see the condensation accumulating and dripping on the other side of the plastic.. I had the same thing happen to my garage area which is also insulated with fiberglass batt and then was covered with polyethylene as a vapor barrier. I did not run any heat in my garage for winter but being that it is enclosed and insulated the temperature remained warmer then it was outside.

      You are stealing me towards covering my walls in 1” foam and leaving the cavities with fiberglass. For the purpose of gaining more information and a better understanding, let’s say I had spray foamed the basement (and the expense and time of rigid foaming my cavities is out of consideration) am I receiving a benefit by still covering the wall in 1” foam? Is there any concerns of trapping moisture between the 1” foam and the foam inside the cavity?



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