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Interior insulation for rim joist covered with rigid exterior foam

jamestimmerberg | Posted in Green Building Techniques on

I plan to use R-20 exterior rigid foam. My foundation will be Superior Walls, with R-20 insulation. Should I add insulation to the interior of the rim joist in Zone 5? If so, what’s the best option? I understand that having rigid foam on both sides of an assembly can be problematic, and that fiberglass can be problematic in a rim joist. Would the exterior rigid foam make fiberglass or mineral wool viable? If not, what would be my best option? Open cell spray foam?

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  1. GBA Editor
    Martin Holladay | | #1

    If you want, you can call your R-20 rim joist good enough. However, if you want more interior insulation, you can use almost anything. Fiberglass would be fine, because the exterior rigid foam will keep the rim joist warm and dry. You can also use rigid foam or spray foam if you want, as long as the rim joist is dry on the day these materials are installed.

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