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Insulating concrete wall that is both above and below grade

bossestruck | Posted in General Questions on

I am insulating the first floor of a 1970’s home in the mountains of Vermont, zone 6. Two of the walls atop the concrete slab are above grade, 2X4 framing and only one layer of T1-11 siding. The other two walls are concrete and they are slightly above grade at their opposite corners then meet about 6′ below grade in the corner. Working from the interior I have installed 2″ foil face polyiso against the concrete and built a 2×4 stud wall (one at 16″ OC and one at 24″ OC.”) right up against the foil foam. We would prefer to avoid spray foam due to costs, however the consensus on this site seems to be that no batts should be used in a basement. What should I insulate in the stud bays in front of the polyiso against the concrete? Should I fill it with more rigid foam? What should go in the stud bays of the walls entirely above grade and no concrete? We will likely do some exterior insulating in the future.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"We would prefer to avoid spray foam due to costs, however the consensus on this site seems to be that no batts should be used in a basement. "

    As long as there is no history of flooding and there is enough rigid foam for dew point control at the above grade section it's fine to use batts. (I recommend that solution fairly often.) In US climate zone 6 it only takes R7.5 on the exterior for dew point control on 2x4 framing, per the IRC chapter 7 prescriptives, in R702.7.1 . With 2" of polyiso you have plenty of margin.

    Kraft faced or unfaced is fine, but avoid foil-faced goods.

    >"What should go in the stud bays of the walls entirely above grade and no concrete? We will likely do some exterior insulating in the future."

    R15 batts and an interior side "smart" vapor retarder works.

    To get closer to current code-min performance with interior side foam without making it too vapor-tight would take R13 batts + 2.5" of interior side unfaced Type-II EPS, which ends up at about 1-perm, enough for seasonal drying, but still pretty vapor-tight. Air tightness would be absolutely critical in that stackup.

  2. user-1072251 | | #2

    Rockwool batts are a great insulation for basement walls like yours, as well as anywhere else, and a higher insulation value than fiberglass.

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