Diagram of the slab assembly. The under-slab plumbing is buried in the ground beneath the entire assembly. Above that are two layers of two-inch EPS foam insulation, for a total of four inches. Above that is a sheet of 10 mil polyethylene plastic, which forms a vapor barrier that prevents moisture from diffusing into the porous concrete slab. On top of the plastic are the PEX plastic tubes for radiant in-floor heat. Additional rebar-reinforced footings are also poured along with the mesh-reinforced slab. Once the slab is poured it looks like one continuous flat concrete surface from above, but there’s a lot going on underneath!
One interesting issue I ran into with this assembly is that every building science resource says to put the poly above the insulation, directly in contact with the slab. However, the contractors I talked to told me they’d always seen the poly on the bottom, under the insulation. One told me “that’s how we do it in Alaska.” As an owner-builder, it’s difficult to know whether to follow building science guidelines or trust local knowledge. People with decades of experience might know things you don’t. Then again, just because “we’ve always done it this way” doesn’t mean you still should.
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