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How should I insulate cavities underneath a subfloor?

Shlomo Drexler | Posted in General Questions on

Part of our dining room is built over a small area that consists of 2×12 joists that sit directly on concrete. The reason this small area was built this way was because it was an extension to an old foundation.
What is the best way to insulate the space between the joists (i.e., the cavities)? This area becomes quite cold because it is on the outside edge of the house. My concern is that moisture will build up under the subfloor.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it's usually a bad idea to install 2x12 joists directly on a concrete slab. But you already know that.

    The solution to your problem depends on your access. If you are doing interior renovations that include new flooring, you could (in theory) remove the subfloor to allow access to the joist bays. That would allow you to install closed-cell spray foam.

    Presumably, that's not possible. If your only access is from the exterior, your best bet is to install rigid foam insulation vertically at the rim joist area, and to extend the rigid foam downward (burying the foam) into a trench, as deep as you are willing to excavate.

    At the top of the rim joist area, the vertical rigid foam should be flashed with Z-flashing, inserted under the WRB above. The above-ground portion of the rigid foam should be protected with metal flashing, pressure-treated plywood, or stucco.

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