Originally published on April 18, 2016 by BNP Media through the Architectural Roofing & Waterproofing Blog. All information presented in imperial (I-P) units.
Water vapor transmission is a rather confusing issue. The difference in vapor pressure between two sides of a building envelope assembly is the driving force behind vapor transmission. (Although by comparison, gaps in the envelope account for vastly greater amounts moisture migration due to vapor-laden air infiltration.)
Vapor = Latent Heat; Air Temperature = Sensible Heat
The vapor pressure differential is to latent heat flow as the dry-bulb air temperature is to sensible heat flow. Just as sensible heat always wants to approach equilibrium in an environment, so too does latent heat. If there is less vapor pressure on the other side of a wall, vapor will slowly “drive” through the wall toward the other side. Very similar to how thermal energy (heat) will slowly drive through the wall if the other side is cooler.
Just as various building materials will resist the conductive transfer of heat at different rates, they will also resist the diffusion of water vapor at different rates as well.
There are two measures used to gauge the rate of water vapor transmission through building materials: permeance and permeability.