Concrete
Vapor Barrier
A sheet of polyethylene film placed under a concrete slab-on-grade to block ground moisture from migrating up through the slab.
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In the Field
Vapor barriers are required under all interior slabs where moisture-sensitive flooring (hardwood, vinyl, carpet) will be installed. Use 10-mil or heavier poly; tape all seams. A torn or improperly lapped barrier is almost as bad as none.
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Rule of Thumb
Use a minimum 10-mil vapor retarder under interior slabs. Place it directly on the subgrade — not under the gravel base — so the slab dries upward, not downward.