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Community and Q&A

XPS vs. Polyiso Foam for Insulating Rim Joists

arthur_roberts | Posted in General Questions on

Hi everyone,

I live in Berkeley, CA and have a 1920s home. My crawlspace is vented, has concrete floor, and poured concrete foundation.  No insulation anywhere in the crawlspace.  I want to insulate the rim joists with rigid foam and caulk or spray foam around the edges of the foam.  After that, I also plan to put some batts in the floor joists, with foil facing up.

I have two questions.  First, does it make a difference if I use 2″ polyiso or XPS, other than the slight R-value difference?  Second, should it be foil-faced or unfaced?  I’ve read conflicting things on whether the rim joist should be able to dry inwards or not.  Most ppl seem to say it should be impermeable, but here it suggests you want some permeability:

Please help — thanks!

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  1. Expert Member
    BILL WICHERS | | #1

    I used EPS myself, to allow for a bit of inward drying. My main reason for doing this is the I have foil faced polyiso on the exterior, and foil faced polyiso allows for NO drying. What you want to do is make sure you can have some drying in at least one direction. If you can dry to the exterior, then it doesn't matter as much what you use to insulate on the interior, as long as it's not something completely vapor open like a fiberglass batt.

    You'll be much better off insulating the crawlspace walls than insulating the underside of the floor above though. Consider installing rigid foam against the crawlspace walls and not putting batts between the joists. I'd use polyiso for this unless you have issues with bulk water (puddles, etc.). I'd use EPS if there is a water problem.


  2. arthur_roberts | | #2

    Thanks Bill! I have a follow up re the joists versus walls. Can I insulate the walls even if I don't fully encapsulate? I don't have an easy way of getting conditioned air into the crawlspace (old boiler w/ radiant heat), so I don't want to go the encapsulation route. My thought was that if I still have vents down there it would be better to insulate joists rather than walls b/c cold air is still getting in. But if that's wrong I'd love to know that. Thanks again.

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