Perm ratings are assigned to many of the materials intended for use as vapor retarders. Materials intended for use as vapor retarders are constantly being improved and tested to establish their effectiveness. The perm ratings assigned to such materials are usually established by a reputable testing laboratory. Such ratings may be considered reliable when the data are published, or certified, by the testing laboratory.
Water vapor transmission is the rate of water vapor flow expressed in grains per hour per square foot. A pound contains 7000 gr, and a gram contains 15.43 gr. Permeance is the water vapor transmission of a material under unit vapor pressure difference between two specific surfaces. The concentration of water vapor also may be stated by giving its pressure, commonly expressed in inches of mercury (inHg). The unit of permeance is the perm, which is equal to grains per hour per square foot per inch of mercury. The ability of a material to retard the diffusion of water vapor is measured in units known as perms, short for permeance. A perm measures, at 73.4°F (23°C), the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury. Technically, a vapor diffusion retarder is a material with a perm value of 1.0 or less. A material with a perm value greater than 1.0 is not a vapor diffusion retarder because it allows too much water vapor to pass through it. Knowledgeable professionals typically use vapor diffusion retarders with ratings of 0.1 or less to guard against possible future changes in building science. One square foot of a material with a rating of 0.1 perm will transmit approximately 2 fluid ounces of water in 1 year. Polyethylene films, which have perm ratings in the 0.02 to 0.08 range, are the most popular. Several new products that are more resistant to tearing than polyethylene are also available.
There is a common rule of thumb, as reported by the Department of Energy and the Southface Energy Institute, to prevent trapping any moisture that may enter a perimeter structural assembly. The structure’s cold-side material permeance should be at least 5 times greater than the perm value of the warm-side vapor retarder.