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Community and Q&A

Insulating Rim Joists with Rigid Foam

leeken | Posted in GBA Pro Help on

I am in the process of insulating my rim joist areas with rigid foam. I have placed 2 inch rigid foam between the joists and sealed the perimeter with Great Stuff spray foam. Now I am in the process of removing the excess foam around the perimeter, using PL 300 to glue a second 2 inch piece of rigid foam to the first piece and also sealing the perimeter with Great Stuff spray foam. I have become aware that when I placed the first 2 inches, there are places where the spray foam pushed out a portion of the rigid foam leaving a space between the rigid foam and the rim board. Is this something to be concerned about?

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  1. Expert Member
    Michael Maines | | #1

    Leeken, the main issue with leaving gaps between layers of insulation or other materials is the risk of moisture accumulation--aka condensation. If there is not exterior insulation over your rim joist, the gap behind it will track closely with the exterior temperature and you shouldn't have a problem. The better the seal around your insulation, the lower the risk.

    1. leeken | | #2

      Michael Maines, thank you for your reply. There is no exterior insulation, so therefore it should not create a problem.

  2. Expert Member
    BILL WICHERS | | #3

    I've found it's easier to glue the two pieces of rigid foam together prior to installation on the rim joist. This lets you seal them both in place together and save the "trim the canned foam" step. If you cut the back piece a bit larger than the front piece, you can fake a wedge shape which helps to limit the amount the canned foam will push the rigid foam away from the rim joist. Ideally, you want to cut the rigid foam edges at a slight angle so that the force of the canned foam expanding will tend to push the rigid foam back tight against the rim joist.

    Small gaps shouldn't be a problem. If you have a big gap, I'd replace that piece. I've often used cut strips of scrap rigid foam to wedge between the floor or joists and the edge of the foundation wall to help keep the foam panels in place against the rim joist while the canned foam is curing. After the canned foam has cured, you can go back and pop out the wedged pieces to reuse them.


    1. leeken | | #4

      Zephyr 7, Thank you for your response. You provided several good ideas on how this process could be done to avoid the spray foam pushing out the rigid foam pieces. When I realized that the spray foam pushed the rigid foam piece away from the rim board after I had already done about 50 of the 80 rim joist areas, for the remaining 30, I have glued the first piece to the rim board with PL 300, and then glued the second piece to the first piece before applying the spray foam. For the first 50, at this point it will be difficult to tell for sure which ones have pushed out away from the rim board and how much of the piece has been pushed out. The only way I determined that some of that had happened was by measuring the face of the rigid foam to the edge of the sill plate and noticed that there was a difference from on piece to another. Then I cut a hole in a couple pieces to verify that there was a gap. At this point I feel like I would have to remove most if not all 50 pieces to really be sure there are no big gaps. Also, I'm not sure where to draw the line between small gaps and big gaps that would necessitate removal. Obviously I would hate to have to do that, but will if this really would potentialy cause a problem in the future.
      Zephyr 7 recommends that I replace any big gaps, but I don't understand why they might be a problem. With 4 inches of rigid foam sealed with spray foam around the edges I would think that the warm air from the house would not reach the rim board so why would the gaps be a problem? Previously the rim joist areas were insulated with approx. 8" of unfaced fiberglass batts. When I removed the fiberglass batts I did not find any evidence of rot or mold in the rim joist areas however there were some pieces of fiberglass that had some dark spots, which did not appear to be mold. I have read that dark spots on fiberglass insulation is from dust which comes from the outside through the rim joist areas, and mold could eventually form on the dust. The house was built in 2006. Additionally I will be finishing the basement with sheet rock walls and ceilings which I would think would also help to reduce transmission of warm air to the rim board area. The 8 " basement concrete walls were built with insulated concrete forms so there is 2" of rigid foam insulation on the inside and outside of the concrete walls. The joists and the rim board sits on a treated 2 X 8 sill plate which sits on the concrete wall. I will build 2 X 4 stud walls with 1/2" sheetrock around the perimeter of the basement. The stud walls will be approx. 1" clearance from the insulated concrete wall.

      1. Expert Member
        BILL WICHERS | | #5

        >"When I removed the fiberglass batts I did not find any evidence of rot or mold in the rim joist areas however there were some pieces of fiberglass that had some dark spots, which did not appear to be mold."

        It's very unlikely you'll have any problems with those gaps then. Mositure probably won't be an issue, your biggest potential problem would be the possibility of insects getting in and nesting there. If things are well sealed on the exterior, then you minimize the possibility of insect invasion too.


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