Vapor Barrier

Built into a home's floor, wall, or slab, a vapor barrier (sometimes called a vapor retarder) is a moisture-blocking layer, usually a tough polyethylene sheet, whose job is to keep water vapor on one side from migrating to the other. Under a concrete slab, it lies between the soil and the concrete so that ground moisture cannot rise through the slab and into the finished floor and air above.

The barrier matters because concrete is porous and soil holds water, even in the desert after monsoon rains or under landscape irrigation. Without a barrier, that moisture wicks up and can cup wood floors, lift tile, blister coatings, and feed mold under cabinets. A continuous, sealed sheet stops it. The International Residential Code and the Department of Energy both call for a vapor retarder under slabs and in moisture-prone assemblies, with the seams lapped and taped so there are no gaps.

For example, a custom home in Casa Grande on irrigated farmland soil will get a heavy under-slab vapor barrier before the concrete is poured, because that ground stays damp enough to ruin wood and luxury vinyl flooring over time. Skipping it to save a small material cost is a common, expensive mistake.

A vapor barrier controls moisture, while insulation rated by R-value controls heat, and the two are separate layers doing different jobs, though both can sit in the same wall or floor. The barrier also has to be placed on the correct side and kept continuous, or it traps moisture instead of stopping it. We install a sealed under-slab vapor barrier on every foundation, because protecting against ground moisture is far cheaper than tearing out a damaged floor later.