Vapor barriers are an important part of moisture control for interiors.  A vapor barrier is a material, typically a plastic or foil sheet, which resists the diffusion of moisture through ceiling, wall and floor assemblies of a building. Vapor-diffusion retarders are also effective for controlling moisture in basements, crawlspaces, and slab-on-grade foundations.

The capability of a given material to resist the diffusion of water vapor is measured by units called “perms,” which quantify its permeability.  A perm at 73.4° F (23° C) is a measure of the number of grains of water vapor passing through a square foot of material per hour at a differential vapor pressure equal to 1 inch of mercury (1-inch water column or WC).  Any material with a perm rating of less than 1 is considered a vapor retarder.