Without an effective moisture barrier, water can penetrate the pores of a concrete foundation slab, which can potentially lead to moisture-related issues. To prevent moisture from wicking up into the concrete slab and foundation walls, a water-proof layer is installed under the concrete consisting of 6-mil plastic sheeting overlapped and taped at the seams or soil-contact-rated rigid foam taped at all seams. Under this barrier is a 4-inch layer of gravel to help moisture drain away. In crawlspaces, plastic sheeting should installed over the dirt floor.