Under slab insulation

Under slab insulation

Placing the under slab insulation. Insulation below the slab dramatically reduces heat loss into the ground, and also protects the slab from condensation. Most new homes in Anchorage have two inches of foam under the slab (though some have less and many older homes don’t have any). For my project I used two layers of two-inch 35psi EPS foam, for a total of four inches. This extra insulation will make the in-floor radiant heat more efficient and keep occupants’ toes happy year-round.

You might recall that I used XPS insulation for the foundation wall. I used EPS under the slab because it’s less expensive (so I can use more of it!) and it’s manufactured using gasses that are less harmful to the environment. EPS arguably has less resistance to water intrusion, but there should be no bulk water under the slab (which is above the level of the footings) and if it does somehow get wet, EPS can drain and dry out again just fine.

Under slab insulation Under slab insulation

Although I think that this was the right material for this step, I gotta admit I liked working with XPS more. EPS doesn’t cut very evenly, even with a sharp razor blade, and it produces millions of little white particles (aka “EPS snow”). For irregular shapes I used leftover scraps of XPS, which cuts smoothly and easily (see third photo).

Photos taken November 2-3, 2019. Posted December 30, 2019.

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