3.2 VAPOR RETARDERS
The purpose of a vapor retarder is to retard, or slow down, the rate of water vapor diffusion through
the envelope. An effective vapor retarder decreases the potential for condensation within the
envelope by decreasing the amount of water vapor that diffuses to the colder portions of the
envelope. The diffusion of water vapor is analogous to heat conduction, with the vapor pressure
difference corresponding to the temperature difference and the resistance to diffusion
corresponding to the R-value. The rate of water vapor diffusion through a material is equal to the
vapor pressure difference across it divided by the material's resistance to water vapor diffusion.
The resistance of a material to water vapor diffusion is generally described by its permeance or
"perm" rating. The permeance is actually the inverse of the resistance, and therefore the lower the
permeance the higher the resistance to water vapor diffusion. While certain materials, with a
permeance below a specific value, are generally classified as vapor retarders, all envelope
materials have some resistance to water vapor diffusion. Therefore, when designing the thermal
envelope and considering the vapor retarder, one must do more than select a material with a
specific permeance. One must consider its resistance to water vapor diffusion in relation to that of
other envelope components.
In general, condensation control requires that envelope components increase in permeance in the
direction of vapor diffusion, whether or not a vapor retarder is specifically included in the system.
This approach will generally prevent the air diffusing through the envelope from reaching a
temperature below its dewpoint, the dewpoint being the temperature at which the water vapor will
condense. Locating a low permeance material on the low vapor pressure, and generally cold, side
of the envelope will slow the rate of diffusion and increase the water vapor content of the air at this
point. In this situation, sometimes referred to as a "moisture dam" or "vapor trap," the dewpoint is
increased, and water vapor may actually condense at this location and lead to various
condensation-related problems. Whether such condensation leads to problems depends on the
amount of condensation, the duration of the condensation episodes, the moisture absorptive
properties of the envelope materials and the durability of these materials over wetting and drying
cycles. These processes and material properties are currently being studied, and up-to-date and
time-tested guidance based on the consideration of these issues is not yet available.
Vapor retarders only control water vapor transport by diffusion and do not address the larger
amounts of water vapor transport caused by air leakage. It has been repeatedly pointed out that air
leakage can carry several hundred times more water vapor than diffusion. The dominance of air
leakage does not mean that diffusion can be ignored and vapor retarders eliminated. Rather, both
diffusion and air leakage need to be considered and effective means for their control included in the
thermal envelope design.