To control vapor diffusion within wall assemblies, vapor retarding materials are used. All building materials provide some resistance to vapor diffusion that varies depending on the properties of the material. These properties can change with the relative humidity and moisture content, age, temperature and other factors. Vapor resistance is commonly expressed using the inverse term "vapor permeance" which is the relative ease of vapor diffusion through a material. The metric units for vapor permeance are "ng/Pa∙s∙m²" or in IP units are "grains/inHg∙ft²∙hr", the latter of which is more commonly known as a "US perm." Both units are a measure of the mass flow over time per the vapor pressure difference and area of wall or other assembly. One US Perm is the same as 57.4 ng/Pa∙s∙m².

Building codes have grouped materials into classes (Classes I, II, III) depending on their vapor permeance values. Class I (<0.1 US perm), and Class II (0.1 to 1.0 US perm) vapor retarder materials are considered impermeable to near impermeable, respectively, and are known within the industry as "vapor barriers." Some materials that fall into this category include polyethylene sheet, sheet metal, aluminum foil, some foam plastic insulations (depending on thickness), self-adhered (peel-and-stick) bituminous membranes and many other construction materials. Class III (1.0 to 10 US perm) vapor retarder materials are considered semi-permeable and typical materials that fall into this category include latex paints, plywood, OSB and some foam plastic insulations (depending on thickness).

The Class of vapor retarder (I, II, or III) is used within many building codes and building enclosure design publications to provide guidance for the selection of appropriate vapor control layers within wall assemblies in North American climate zones. This guidance is also based on the expected indoor conditions for certain building types which is related to exterior climate, indoor moisture generation rates combined with ventilation rates. This guidance is not reiterated here, and should be consulted for the selection of appropriate vapor retarder materials along with consideration for other issues as covered within this bulletin.

The following graphics illustrate how a vapor retarder can be used in a cold or a hot climate to control the diffusion of vapor through the wall assembly.