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Insulating and Air-Sealing Gap at Rim Joist

MikeCell | Posted in General Questions on

Hi.   I am finishing my basement.  Using rigid foam followed by rock wool in the rim joists.   The rim joist running parallel to a 12 foot wall has a small (1-2 inch) gap and then another joist behind it.   I cannot get anything in this small 12 foot long space.   If I use rigid foam followed by rock wool in the larger joist in front will this lead to moisture accumulation in the space behind it?   If so should I just insulate and not air seal this space?   Mike

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  1. user-2310254 | | #1


    I will give your post a bump.

  2. walta100 | | #2

    Help me understand the problem better.

    It is not clear to me what the photo is of, I don’t see a floor joist, I don’t see any sill plate.


    1. MikeCell | | #3

      Hi Walta - Pictures turned when I uploaded them - trying again and trying a better explanation. I am finishing my basement and I have been attaching rigid foam to all of the rim joists and then to the band joists (the last parallel joist?). There is a joist space that is about 3 inches wide and running 12 feet deep as the last joist space in my family room and then another right in front of it that both sit on the top of the concrete basement wall. I cannot get any rigid foam or insulation in this space so can I put it on the inside of the other joist space that is right in front of it and also sits on the concrete foundation or will this lead to moisture accumulation in the 3 inch space? Thanks again, Mike

  3. user-2310254 | | #4


    Is either side of the 3-inch-wide space facing the exterior? If so, one of the small, two-part closed cell foaming kits might be the best way to seal up this awkward opening.

    1. MikeCell | | #5

      Hi Steve - The one side of the 3 inch wide space is facing the exterior but that space goes the length of my family room so about 12-14 feet deep so I could not insulate more than just closing off the gap where you see it in the picture. Would it cause a problem if I put rigid foam on the next joist over (the next one in from the 3 inch space that also sits on the concrete foundation wall) or could that lead to moisture issues in the 3 inch space behind it (that sits on the same concrete foundation). Thanks, Mike

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