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Flash and Batt with XPS on interior side of garage wall.

jfoxjfox | Posted in General Questions on

I’m replacing the mouse-compromised insulation in my 2 car under garage/workshop.  I plan to get R19 garage doors.  One wall is all concrete, the opposite wall is 3’6″ of concrete topped with 2×6 framing.   I intend to use XPS on the interior of the concrete walls.  But for the framed walls I would like to use closed cell spray foam 2″ thick, fiberglass for the remainder of the wall thickness and then 1″ of XPS on the interior.  Then strapping and drywall.  Is this a bad idea?  My understanding is that 1″ of XPS is considered permeable and so the interior side of the closed cell foam should theoretically be able to dry to the inside.

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  1. user-2310254 | | #1


    Where are you located?

    1. jfoxjfox | | #2

      Central Massachusetts - Zone 5.

  2. Expert Member
    PETER G ENGLE PE | | #3

    Your plan is generally OK. If you are using the interior foam as a thermal break, you might want to consider Bonfiglioli strips, where you cut just strips of foam to place over the studs. You then fill the larger cavity with batts. The advantage is less foam (carbon and $), better permeability. You do need to figure out exactly where your air barrier is going to be and make it as airtight as possible.

  3. jfoxjfox | | #4

    Thanks for bringing the B-strips to my attention. Wasn't aware but it makes perfect sense. My biggest concern was the thermal break and generally wanting to do more than the bare minimum. I was also concerned about dealing with a spray foam kit. I think I'm going to follow this:

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