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Many architects, engineers, and builders use a vapor barrier under concrete slabs, but the location of the vapor barrier varies. Sometimes the vapor barrier is placed directly under the slab. In other cases, a layer of sand or crushed gravel is placed over the vapor barrier before the concrete is placed. Each location affects slab performance; is one better than the other? Why Use a Vapor Barrier? Anyone who's ever damp-proofed a foundation knows that concrete is permeable. Water draining down through backfill material or rising up with a high water table will eventually make its way through a concrete wall to the living space inside. The same is true of a slab. A concrete slab placed in direct