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Insulation below or above this slab?

user-6765831 | Posted in Green Building Techniques on

Hi everyone,
This is Asaf, building a house and in-law in Zone 3A.  In earthquake country, and house on poor soil.  Our foundation will be a slab suspended between grade beams (see attachment of in-law foundation- ignore wall detail, that is not accurate)  Plan had been to insulate above the slab to avoid thermal bridging through grade beams, as opposed to the usual insulation below and at slab edge.  However, insulation above limits our options- we otherwise would have likely simply finished the slab as our flooring.  Any idea how problematic thermal bridging through these grade beams would be?  Most people in our area don’t insulate above or below slab but at edge only, but if I do it I should do it right.

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  1. GBA Editor
    Martin Holladay | | #1

    You have two choices:
    1. Install vertical rigid foam at the perimeter of the foundation, without any horizontal rigid foam under the slab. In Climate Zone 3, which is relatively mild, this would be an acceptable solution (as long as there are no termite concerns).

    2. Install horizontal rigid foam above the slab, followed by a plywood or OSB subfloor, and the flooring of your choice. (In this case, the flooring probably wouldn't be concrete.)

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