Vapor diffusion is the movement of water vapor molecules though porous materials (e.g. wood, insulation, drywall, concrete, etc.) as a result of vapor pressure differences.

Vapor pressure differences occur as the result of temperature and water vapor content differences in the air. These vapor pressures can be looked up on a psychometric chart or calculated.

Vapor diffusion flow always occurs through an assembly from the high to low vapor pressure side, which is often from the warm side to the cold side because warm air can hold more water than can cold air. In cold climates, this means that vapor diffuses primarily from the heated interior to the colder outdoors, whereas in hot climates, the vapor drive is reversed and instead is primarily from the warm humid exterior to the air conditioned interior. The direction of vapor diffusion can also be reversed when the sun heats up damp, absorptive wall claddings like masonry and drives water vapor inward.

The following schematics show the inward and outward vapor drives for hot and cold climates, respectively. The temperature through the wall is illustrated by the temperature gradient, and the relative humidity is indicated by the density of the dots which schematically represent the relative number of water vapor molecules in the air.

Overall, the direction of the vapor drive has important ramifications with respect to the placement of materials within a wall assembly. What works in Toronto or New York likely won't work in Miami! Improper use of vapor impermeable materials within a wall can lead to condensation on colder surfaces and lead to damaged materials and fungal growth. This bulletin provides guidance on the selection and placement of materials within wall assemblies to avoid issues related to vapor diffusion.