1. A material with a high resistance to vapor movement, such as foil, plastic film, or specially coated paper, that is used in combination with insulation to control condensation. 2. Any material that has a water vapor permeance (perm) rating of one or less. 3. A type of plastic sheeting that both eliminates drafts and keeps moisture from damaging a building or structure. 4. A waterproof membrane placed under concrete floor slabs that are on grade. 5. Material used to retard the movement of water vapor into walls, and prevent condensation in them; applied separately over the warm side of exposed walls or as a part of batt or blanket insulation. 6. Thin plastic or metal foil sheet used in air-conditioned structures to prevent water vapor from penetrating insulating material.