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Community and Q&A

Martin – Knee wall and foam

Bob Brown | Posted in Webinar Follow-up Q&A on

Martin, I recently asked question about air washing insulation in knee wall applications and you suggested foam backing and said it would not create a double moisture barrier. (can’t seem to add a follow-up question to the thread) I have since read several other posts on this site which suggested that foil faced foam in this application would create a moisture trap. Please clarify.

I was planning to use 3 1/2 fiberglass, as the application is in a truss with 2×4 “wall” chords, and then poly over the studs. Therefore, is there a particular type of foam to use on the back side for safest install, EPS, XPS, Foil or no foil.

any other suggestions.

Best practice for attaching foam? glue, nails and plastic washers?

Thanks again

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    You didn't mention your climate zone. Tell us where you live.

    If you are the kind of person who stays up at night worrying, you can size the thickness of the rigid foam according to the guidelines in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    If you do that, the assembly will be just fine -- even over-engineered.

    The fact is, moisture accumulation in kneewalls is almost unheard of, no matter what thickness of rigid foam you choose, because the attic behind the kneewall is not really outdoors. It is an intermediate space, and it isn't as cold as the outdoors. So you are worrying for naught. Thinner foam would work.

    When it comes to "moisture trap" worries, you have to look at all of the factors before pushing the worry button. For more information, see this article: Worries About Trapping Moisture.

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