Water Vapor Permeability
The average home produces six buckets of water vapor a day. If this is not released, it can condense on and between wall studs, and as a result can cause both structural and health related problems – moistened or wet drywall is transformed into a perfect growing medium for molds, fungi and bacteria. These organisms multiply quickly; the molds and fungi release spores into the home, promoting a host of respiratory related problems. The result is what has become referred to as the "sick home syndrome," an issue which is gaining momentum as a topic of concern with consumers.
The effectiveness of a vapor retarder is measured by its perm rating, which is the ratio of porosity of material to the passage of water vapor. A material having a vapor-transmission rate of 1 perm or less is considered a vapor barrier. Accordingly, a material with a perm rating of more than the 1 perm is considered a vapor retarder.
Water Vapor Transmission
Perm ratings are tested in accordance with ASTM E96. A perm rating for a material is the number of grains of water vapor (7000 grains equal to 1 lb.) that will pass through 1 sq. ft. of the material in 1 hour when the vapor-pressure differential between two sides of the material equals 1 inch of mercury (0.49 psi).