by Nick Gromicko, CMI® Uses and Characteristics 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 term “vapor barrier” is commonly used, but “vapor-diffusion reta