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Community and Q&A

Rim Joist Insulation – REMOTE method

Joel Cheely | Posted in Building Code Questions on

Zone 5. I have insulated the conventionally framed walls and block foundation with two layers of 1.5″ XPS, all on the exterior. My air and water barrier is continuous on back side of insulation. I plan to fill wall stud cavities with fiberglass or cellulose. At rim joists, I sometimes replaced one layer of XPS with 2x framing in order to hang joists for decks. So at the decks I only have R-7.5 plus 3″ of wood (maybe R-4.2?), I’ll need some kind of insulation on interior side of rim joists at those locations to meet energy code. I don’t need much of a vapor retarder, so I could just cut up kraft faced fiberglass, but my experience is that I’d just be building homes for mice. I have leftover XPS. Is it OK to use that or should I get something a little more permeable like EPS? Thanks for the input.

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  1. GBA Editor
    Martin Holladay | | #1

    I think it's fine to install interior rigid foam in this case. Researchers have looked into whether this type of foam sandwich at the rim joist causes any moisture problems, and they haven't found any problems yet.

    Green builders always prefer EPS or polyiso to XPS, because XPS is manufactured with a blowing agent with a high global warming potential. For more information, see Choosing Rigid Foam.

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