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Is there a frost risk with underslab inslation?

kyeser | Posted in General Questions on

I was talking with a fellow builder the other day and he asked me if I was worried about frost getting under my slab on grade houses because I use 2″ xps insulation under my slabs. I also install 2″ xps down four feet vertically along foundation walls. His thoughts were that heat loss from the house often keeps underslabs warmer and frost out.

I am in zone 6.

Your thoughts….

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  1. GBA Editor
    Martin Holladay | | #1

    Kye,
    The fact that there happens to be some frozen soil adjacent to a concrete foundation does not automatically mean that something is wrong. As long as there is adequate insulation between the frozen soil and the interior of the house, and as long as there is no chance of frost heaving, what's the harm?

    Here in Vermont, it's fairly common for the top one or two feet of soil around a concrete basement to be frozen. In the old days, before people insulated basement walls, it was common to see frost on the interior of a basement wall, especially on the upper one or two feet of the wall. Now people insulate these walls.

    I realize that you are talking about slabs, not walls, but I'm just pointing out that frozen soil happens; the insulation is intended to separate the interior from the frozen soil.

    To answer your question: in your climate zone, a foundation insulated as you have described will not get any frozen soil under the slab. The 2 inches of rigid foam insulation that you install on the interior side of your frost wall assures that the frost won't penetrate. Furthermore, the fact that you have footings below the frost level ensures that your foundation won't heave.

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