What Is a Vapor Retarder (Vapor Barrier)?
A vapor retarder is a sheet of low-permeability material, usually polyethylene, laid over crawl space soil to slow water vapor rising from the ground into the space under your house.
Explanation
Bare soil gives off moisture all the time, even when it looks dry. In a crawl space, that moisture raises the humidity around joists, subfloor and ductwork. A vapor retarder blocks most of that evaporation.
"Vapor barrier" is the everyday term. Building codes say "vapor retarder" because almost no material stops vapor completely. Codes sort vapor retarders by permeance, a measure of how much vapor passes through:
- Class I: 0.1 perm or less. Polyethylene sheeting falls here.
- Class II: over 0.1 up to 1.0 perm, for example kraft-faced insulation
- Class III: over 1.0 up to 10 perms, for example some latex paints
In North Carolina, the Residential Code's closed crawl space section (R409.2.1) calls for at least a 6-mil polyethylene ground vapor retarder or equivalent, with joints overlapped at least 12 inches. That ground cover can be carried up the foundation walls as a sealed liner. It must stop short of the wood sill to leave a termite inspection gap.
Thicker, reinforced liners, often 10 to 20 mil, cost more but hold up better to foot traffic and service visits. A ground cover on its own is not full encapsulation. A closed crawl space also seals vents and adds mechanical drying.
When to seek help
- The plastic is torn, bunched up, missing in sections or has standing water on top of it.
- You see mold, wet insulation or musty odors even though a liner is in place.
- You want to convert to a closed crawl space, which requires a permit in NC.
Compare vapor barrier installation with full encapsulation, or read vapor barrier vs. encapsulation.