A roof assembly's primary purpose is to keep exterior water from entering a building through the roof. However, water vapor inside a building may enter a roof assembly and condense, causing problems such as corrosion of steel decks and fasteners, microorganism growth and reduction of roof insulation's thermal resistance.
One method to prevent moisture from entering a roof assembly is to incorporate a vapor retarder. In low-slope roof assemblies, vapor retarders typically are roll materials or sheet materials with relatively low permeance values, expressed as "perms." Materials with low perm values are more resistant to water vapor diffusion than materials with high perm values. NRCA considers appropriate materials with perm ratings less than 0.5 perms to be effective vapor retarders.
There are two categories of vapor retarders: bituminous vapor retarder membranes and nonbituminous vapor retarder materials.