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Community and Q&A

Insulating Rim Joists

mec231 | Posted in General Questions on

I’m getting ready to insulate my rim joists, but I’m not sure if my preferred approach–eps or xps sealed with spray foam–will work since with a atypical construction. The 2×4 rim joist does not run the full height of the 2×8 floor joists, and those joists have triangular blocking to shape the drip edge of the siding (see attached for illustration). Are there any issues with the air cavity formed if the rim joist is sealed behind foam?

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  1. Expert Member
    BILL WICHERS | | #1

    If that air space behind the siding is vented in some way to the outdoors, then you’re ok putting in blocks of rigid foam and sealing them with canned foam in the usual way.

    If that air space is NOT vented, then I’d use two part spray foam here. Spray foam is the best option for odd shaped things that need to be sealed and insulated.


  2. mec231 | | #2

    The air space behind is not vented, at least intentionally, though the seam between the sheathing/siding and base of the drip edge is not tight.

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