The ways in which liquid and vapor move through our building envelopes are complex, and even today not completely understood; but the fact that lots of water can (and does) move through porous building materials is a phenomenon that rules over so much of the way we build.

Two major modes of moisture travel—gravity and capillary action—are related to bulk water management, which is essential to the longevity of any building envelope assembly consisting of porous materials.

Two other major modes address how water vapor can work its way through a building envelope assembly. Managing vapor drive is a critical damage function of a building envelope assembly. Building science professionals, designers, and engineers have long debased solutions for managing how vapor may diffuse through an assembly and pose a condensation risk. The bottom line with regard to diffusion is to understand seasonal vapor drive and afford an assembly the opportunity dry. Trapping moisture in an assembly may lead to major problems.

Air infiltration poses a greater condensation risk than diffusion—and it's not even close. 

Project teams will pore over issues related to permeance, condensation risks, and strategic placements of vapor retarders in hopes of managing vapor-related moisture issues. However, controlling the flow of air infiltration is far more important than controlling vapor diffusion.

According to a classic analysis by Building Science Corporation, as offered in their Builder's Guide for Cold Climates, during a the heating season in a cold climate region, vapor diffusion through a solid a 4-foot by 8-foot sheet of gypsum board may result in about 0.3 L (1/3 quart) of water being transmitted to the interior. By comparison, the air leakage through a 1-square-inch hole in the middle of the gypsum board over the same period of time may result in approximately 28.4 L (30 quarts) of water being transmitted to the interior—90 times more water than through diffusion.

Figure: Quantity of water transmitted through a sheet of gypsum board during a heating season in a cold climate via vapor diffusion and air leakage through a 1-square-inch hole. This figure is adapted from an illustration created by Building Science Corporation. 
 

Infiltration can account for over half of the annual heat exchange through a poorly sealed building envelope; and with that air potentially comes a great amount of vapor. A leaky envelope can undermine even the most optimized vapor diffusion strategy. This underscore the critical need for tight building envelopes as standard practice.