Insulation under monolithic slab
I am located in climate zone 4a, but literally on the border with zone 3. Planning to build a 22’X22′ workshop on monolithic slab, with vented attic. For now the building will be insulated to code for a living space, with a double insulated garage door, and rigid insulation on the exterior of the slab foundation. I believe my local codes require a 2 inch termite inspection gap between the top of the slab insulation and the bottom of the siding. I plan to heat it only as needed with radiant electric panels for now
My question is whether it could be worthwhile to put a layer of foam insulation under the slab and poly vapor barrier. This would be to future proof the building in case i want to convert it to an efficient permenantly conditioned living space in the future. Would the 2 inch inspection gap overwhelm the benefit of underslab insulation?
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The inspection gap is often allowed to be insulated with removable strips of insulation. But even if the inspectors won't buy into that approach, a 2" gap is only 1 square foot for every 6' of perimeter, so you're looking at about 15 square feet of exposure. That may be a comfort issue near the slab edge during a Polar Vortex event, but in the grand scheme of things it's only adding up to the heat load of a couple of windows.
The big advantage of under-slab insulation is to keep the floor above the summertime outdoor dew point, which lowers the mold/mildew risk at the slab level, keeping rugs from growing crud on the bottom side due to the lower temperature. The deep subsoil temps are much lower than the summertime air's dew-point, and an insulating rug or cardboard box resting on a cool slab will take on moisture from the room air in summer. An inch of EPS is probably going to be enough, but 2" of reclaimed roofing EPS would give more margin, and would be cheaper than 1" of virgin-stock EPS.
Thanks for your answer Dana. Both good points.