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Community and Q&A

Insulating rim joist

Randall Kingery | Posted in Green Building Techniques on

I’m in the middle of finishing the basement. One wall has a half wall that’s poured concrete and the other half is traditional framed. The other walls are poured concrete. The exterior has a r3 rigid foam sheathing and currently the rim joists are insulated with r10 fiberglass. My question is should I pull out the fiberglass and either use the cut and cobble method with r-10 rigid foam or spray foam or just leave the fiberglass in place. Thanks for the help in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it's always a good idea to tell us your geographical location or climate zone. We can't give a good answer to this question unless we know where the house is located.

    You also haven't told us where this R-3 rigid foam is located. Is it located (a) on the exterior side of the concrete wall, or (b) on the exterior side of your plywood or OSB wall sheathing (presumably including the rim joist area), or (c) both a and b?

    -- Martin Holladay

  2. Randall Kingery | | #2

    Sorry. I'm located in Indiana zone 5 I believe. The r-3 is on the osb wall sheathing. Concrete is not insulated on the exterior. I have put 2" xps on the interior of the basement attached to the concrete.

  3. GBA Editor
    Martin Holladay | | #3

    You didn't really tell us whether the R-3 rigid foam covers the exterior of the rim joist area, but I'm going to assume that it does.

    In general, it's a good idea to insulate rim joists to the same R-value as above-grade walls. Most building codes require R-20 insulation for above-grade walls in Zone 5.

    When combining exterior rigid foam with fluffy insulation in Zone 5, at least 27% of the wall's total R-value must come from the rigid foam layer. (To learn why, see Combining Exterior Rigid Foam With Fluffy Insulation.)

    If you are aiming for R-20, the rigid foam layer needs a minimum R-value of R-5.4. So your R-3 rigid foam isn't quite enough. The best approach would be to cut-and-cobble a bit more rigid foam on the interior side of the rim joist.

    That said, you can probably get away with simply installing fiberglass batts here, as long as your basement isn't too wet. If you take this approach -- basically, cutting a few corners -- it wouldn't hurt to check the rim joist occasionally to see if it is getting damp or moldy.

    If you just want to sleep well at night without worrying, stick with the cut-and-cobble approach.

    For more information on this issue, see Insulating rim joists.

    I hate to be the bearer of bad tidings, but your house may have another problem: The rigid foam that was installed on the exterior side of your wall sheathing isn't thick enough to keep your wall safe from condensation or moisture accumulation. In your climate zone, you want your rigid foam layer to have a minimum R-value of R-5 for 2x4 walls or R-7.5 for 2x6 walls. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    -- Martin Holladay

  4. Randall Kingery | | #4

    Thanks for your help. Yes the rim joist is covered with rigid foam. My main concern was that if I put rigid foam on the interior and sandwich the joist between foam that I would create moisture issues.

  5. GBA Editor
    Martin Holladay | | #5

    Researchers have looked into the question about this type of rim-joist sandwich. When it comes to rim joists, the researchers haven't noticed any down side to making a sandwich like this. Rim joists will stay dry when they have rigid foam on both sides.

    Obviously, you need to use common sense. Don't install the interior rigid foam on a day when the rim joists has frost on it or appears damp. Wait until the rim joist is warm and dry.

    -- Martin Holladay

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