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Community and Q&A

Insulate on top of rigid foam?

englishgirl12 | Posted in Green Building Techniques on

We insulated the concrete walls of our basement with 2″ rigid foam XPS insulation. We taped and caulked seams. Our insulation contractor says it meets code as is (R-10) and we studded it out and plan to drywall it. Some of the space is finished but most of it isn’t. One wall is partially exposed to air on the exterior. Should we add in some mineral wool or dense packed cellulose between the studs? We have mainly dense pack netted cellulose and mineral wool throughout the rest of the home. Thank you!

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  1. Expert Member
    Dana Dorsett | | #1

    The IRC 2012 and newer code minimum is R10 only in US climate zones 4A & 4B. In colder US climate zones it's R15"

    Using cellulose in below-grade applications is risky, since even a small flood or even seepage the bottom of the wall could result in having to gut the entire wall, since cellulose would wick the moisture far & wide. Unfaced batts (or even kraft faced, but not foil) works OK. In the event of minor flooding you'd have to cut the wallboard & insulation a foot or so above the high-tide mark and let it dry before making repairs, but the the small amount of moisture that would wick that far up the batts could still dry at a reasonable pace.

    Hopefully you either have an insulated slab, or put an inch of foam board under the bottom plate of the studwall as a capillary & thermal break(?).

  2. englishgirl12 | | #2

    Thank you for your response. We did install a capillary and thermal break.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with Dana's advice. If you are in Climate Zone 4 or anywhere warmer, you don't need any insulation between the studs (but you can install some if you want).

    If you are in Climate Zone 5, 6, 7, or 8, you need more insulation -- and either fiberglass or mineral wool is better than cellulose.

  4. englishgirl12 | | #4

    We do have cellulose in our framed walkout wall in the basement. Is that a bad idea? Our basement is exceptionally dry due to our location - everything slopes away from the house.

  5. GBA Editor
    Martin Holladay | | #5

    It's perfectly OK to use cellulose in your above-grade framed walls, even if those walls are part of your basement. You just don't want to use cellulose below grade (near a concrete foundation wall).

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