A.Howard Brickman responds: The best way I’ve found is to build a floating plywood floor over foam, then apply strip flooring over this (see "Laying Wood Floors Over Concrete Slabs," 10/94). Depending on how much foam you use, you will raise the floor level several inches.
Under any floor on a slab, I first lay down a double 6-mil poly vapor barrier, taping the seams at the overlaps and extending the edges up the walls. Next I lay down the foam, keeping it as flat and firmly supported as possible.
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