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Unit C is located on a portion of the site with a high water table, which dictated that we build the home on an insulated slab. After installing 12" of crushed stone, we installed 2" of extruded polystyrene (XPS) foam board insulation to go under the slab, which blocks the flow of vapor up to the concrete slab. Then, after the slab was poured, we installed 1.5" of expanded polystyrene (EPS) foam board insulation between pressure-treated sleepers with a subfloor screwed to the sleepers. EPS foam board differs from XPS foam board in that it allows vapor movement, which means that any moisture in the slab has a pathway to dry up through the foam and subfloor, which are both vapor permeable.
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