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Do I need to insulate the interior foundation wall if I already have 2 inches of rigid XPS on the exterior?

dae06 | Posted in General Questions on

My builder put up a 2×4 wall in my mechanical room against a concrete wall for my HVAC guy to mount the in-floor heating system. He wants me to put up 1/2″ plywood over the stud wall for the install. Like I said, I have 2″ rigid foam around the exterior of the foundation (This is a walkout basement, so foam is around 3 sides of the foundation), but this concrete wall is actually separating the mechanical room and 6′ of sand fill below the garage and below grade. There is no foam on this wall (it’s almost like an interior concrete wall???) If it matters, the garage concrete floor also has in-floor heat.

Is the any reason to insulate this wall? If so, what is the best method now that the stud wall is already there.


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is the any reason to insulate this wall?"

    A. Yes, since there is no insulation on the exterior of the wall -- just fill.

    Q. "If so, what is the best method now that the stud wall is already there?"

    A. Use closed-cell spray polyurethane foam. If you don't want to use spray foam, you can dismantle the stud wall and install rigid foam instead -- and then re-assemble the stud wall on the interior side of the continuous rigid foam. (Trust me -- taking down a stud wall and then re-assembling it isn't as hard as it sounds. It's fast work.)

  2. Expert Member
    Dana Dorsett | | #2

    Where is this house located? (Climate zone, that is.) IRC code-min for foundations in zones 5 & higher is R15. The 2" of XPS is labeled R10, but will drop to R8.4 over the lifecycle of a house.

    An inch of polyiso against the wall and a 2x4/R13 studwall meets code-min performance without having to take it to 3" of foam. In zones 6 & higher it would need to be 1.5" or 2" of foam for any above-grade section of wall to be able to use 2x4/fiberglass for the rest. If the wall in question is all below-grade, or with a heated conditioned garage on the other side of any above-grade section you can safely cheat that by quite a bit.

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