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Community and Q&A

Frostproofing an existing slab on grade

jeff hibbard | Posted in GBA Pro Help on

I am a residential designer working out of northern Wisconsin. I have a project I am working on that has an existing house with a full basement built about 10′-0″ from an existing slab-on-grade garage. I would like to attach the garage to the house with a breezway-type addition. Is there a way to frostproof the existing garage either by digging down 4′-0″ vertically & sliding rigid insulation into the trench, or I believe I have seen shallow frostproofing where rigid insulation is laid horizontally just below grade out to a point out 4′-0″ from the edge of the slab. Or a combination of both? If anybody has any solutions, or input I would greatly appreciate it.
Thank you,
Jeff Hibbard

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  1. GBA Editor
    Martin Holladay | | #1

    If the garage is unheated, I don't believe that it's possible to frost-proof the slab by adding foam at the perimeter. Think of it this way: when it's -20°F outdoors for three days in a row, it's going to be -15°F inside your garage. The slab will freeze, and so will the soil under the slab. Normally, assuming the slab is placed on a deep layer of crushed stone, such a "floating slab" can work fine for a residential garage. But once you try to tie it to a house on an independent foundation with footings below frost level, differential movement can lead to problems.

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