Air movement as a moisture transport mechanism is typically far more important than vapor diffusion and bulk transport of moisture. As noted in the figure below the movement of water vapor through a 1-inch square hole via a typical air pressure differential is significantly greater than the movement of water vapor as a result of vapor diffusion and the transport of moisture via a rain event through a 32 sq.ft. sheet of gypsum sheathing.
In most climates, if the movement of moisture-laden air into a wall or building assembly is eliminated, movement of moisture by vapor diffusion is not likely to be significant. Keeping the building enclosure air tight is essential in keeping it dry.
Contact the air and vapor barrier experts at Carlisle Coatings & Waterproofing for project specific questions, details and applications guidance. They may be reached at 888-229-2199 or firstname.lastname@example.org .