In tight houses, fresh air intakes are needed in areas where fuel-burning appliances are located. Some appliances may allow intakes to be coupled directly to them. In houses equipped with exhaust fans, adequate intake vents are needed to replace air that is exhausted. Ideally, a supply air fan is used to provide the correct quantity of makeup air. Materials that resist the passage of water vapor are called vapor- retarder materials.
Low perm means low passage of water vapor.
Plastic films, foils, sealers, or paints with a perm rating of less than 1.0 are generally considered adequate as vapor retarders. A combination of materials may be used in the vapor-retarder system. An Air-Barrier System Vapor retarders are not to be confused with air barriers. However, construction practices currently in use in the United States generally lack the quality control required for vapor retarders to also serve as air barriers. Considerably more moisture is lost in a home through convection than through diffusion.