A.If the slab is well-insulated along the perimeter, no further insulation should be needed. Otherwise, any insulation between the sleepers will do. If your sleepers aren’t very deep, you may want to use rigid foam insulation to get the most R-value. The insulation should be installed around the perimeter of the room at least 2-feet in from the outside walls. It should not be necessary in the middle of the floor. A vapor retarder should be installed beneath the floor. The easiest way to install it is to lay a sheet of polyethylene over the sleepers before installing the subfloor.
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