Below grade insulation


Old 06-04-03, 07:14 PM
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Below grade insulation

I'm adding an attached garage to my house. Do I put styrofoam on the inside or the outside to help keep out frost from heaving the floor. I live in the north central Wi.
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Old 06-06-03, 07:27 AM
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Frost heaving is caused by water freezing because when it does freeze, water expands about 12% in volume. So insulating under, around and/or over the foundation and/or slab has no effect on heaving.

Moisture barriers are laid under slabs to prohibit moisture from leaving the cement. This is because the longer it takes for cement to cure, the stronger the cement will be. The less likely cracks will form in the cement. Gravel is laid under slabs to provide good drainage and prohibit heaving. This is because gravel wll not hold moisture like dirt will.

Styrofoam is applied to the outside of foundation walls to prohibit damage to the foundation when backfilling. It does provide some insulating value to the foundation but it is not its primary purpose. Will this prohibit frost heaving? The answer is no. What prohibits frost heaving is good drainage around the perimeter of the foundation (grading), downspouts for gutters that lead away from the foundation at least 8 feet and gravel under slabs (6 to 8 inches deep).
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