Vapor barrier
Also called: vapor retarder, moisture barrier, crawl space liner, poly sheeting
A sheet of material, usually polyethylene plastic, that stops water vapor from moving through it. In a crawl space it goes over the dirt floor to keep soil moisture from evaporating into the space. 6-mil poly is the minimum; 12–20-mil reinforced liners are used for encapsulation.
Bare soil under a house gives off a surprising amount of water — often several gallons a day into the crawl space air. That moisture condenses on cool framing and insulation and feeds mold and rot. A ground vapor barrier is the single most effective fix, and it’s cheap: about $0.15 per square foot for 6-mil poly, plus tape.
Installation matters more than thickness: cover every square foot of dirt, overlap seams by 12 inches and tape them, run the sheet 6–12 inches up the foundation walls and seal it, and cut and tape around piers. Any exposed dirt undoes the rest.
The term also applies to the paper or foil facing on batts. On a floor over a crawl space, that facing goes toward the warm side — up, against the subfloor — or you use unfaced batts and let the ground barrier do the job. Facing on the cold side traps moisture in the insulation.