While vapor diffusion is a relatively slow process which can take extended periods to accumulate enough water to be of concern, air leakage in wall assemblies can act much more quickly and deposit large amounts of water in a short period of time. Air leakage is driven by air pressure differences between the interior and exterior of a building, which can be created by a number of different factors including ventilation systems, wind and stack effect (created by indoor to outdoor temperature differences). These pressures drive air through wall assemblies from the high pressure side to the low pressure side. Importantly, air pressure differences are not the same as the vapor pressure differences which drive vapor diffusion.

To control air leakage in building enclosure assemblies like walls, an air barrier is installed. Unlike diffusion, air leakage is not typically governed by material properties of this air barrier. Instead, air leakage most often occurs at discontinuities (i.e. holes) in the air barrier. Consequently, prevention of air leakage depends primarily on detailing, material compatibilities and quality control during both the design and construction process. Notably, air leakage can bypass the vapor barrier and consequently is an independent phenomenon from vapor diffusion.

When air leakage does occur, it can carry moisture with it, and if this air then comes in contact with a surface that is below the dew point temperature of the air, condensation can occur. The dew point temperature is the temperature at which the air is 100% saturated with moisture, and consequently if the air is cooled below this temperature, it can no longer hold all of the moisture it contains and it will deposit the moisture on these colder surfaces. Condensation from air leakage can deposit significant amounts of moisture within a wall and potentially lead to fungal growth and/or degradation.