VAPOR BARRIERS
Many materials used as interior coverings for exposed walls, such as plastic dry wall, wood paneling and plywood, permit water vapor to slowly pass through them. When the relative humidity within the house at the surface of an unprotected wall is greater than that within the wall, water vapor will migrate through the plaster or other finish into the stud space, where it will condense if it comes into contact with surfaces colder than its dew point temperature. Vapor barriers are used to resist this movement of water vapor or moisture in various areas of the house.
All construction materials have some resistance to moisture flow, but only those materials highly resistant to vapor flow should be used as vapor barriers. The permeability of the surface to such vapor movements is usually expressed in perms, which are grains of water vapor passing through a square foot of material per hour, per inch of mercury difference in vapor pressure. A material with a low perm value (1.0 or less) is a barrier, while one with a high perm value (greater than 1.0) is a breather. Membranes which best serve this purpose include polyethylene film (four to six mil.), asphalt-coated or laminated papers and kraft-backed aluminum foil. Oil base or aluminum paints and /or vinyl wallpaper are often used in existing homes which did not have vapor barriers installed during their construction.