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Community and Q&A

Should we insulate our basement walls

Doelman | Posted in General Questions on

We’re building a house in zone 3 with a finished walkout basement with 8” poured concrete walls.  Our code requirements are to insulate the ceiling with R19, rim joists and stud walls with R13, and that’s it, no insulation requirement for the poured walls.  The walls are 10’ tall and 8’ below grade.  Should we insulate them?

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  1. Expert Member
    ARMANDO COBO | | #1

    I think you need to check 2021 IRC Table N1102.1.3:
    I highly recommend to install 1" R6 min. polyiso continuous insulation, then frame a 2x4 wall and install batt if you want, and open cell on the rim joists. The polyiso on the concrete wall serves as a moisture barrier.

    1. Doelman | | #2

      We're in the "warm and humid" area in zone 3 so we're excluded from needing basement wall insulation, I verified that with the permit office.

      How much difference does the 1" polyiso make in terms of basement moisture? Slight improvement or are we talking a world of difference?

    2. Expert Member
      ARMANDO COBO | | #3

      Concrete wicks moisture through capillary action, so it’s imperative to protect the wall on the inside, as well as the outside. Installing an organic material, like a wood wall, with R13 insulation is a recipe for failure, as in a mold factory.
      Just because the code doesn’t require CI in a hot, humid zone, it doesn’t mean is good building science, but hey, it’s your money.

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