Which statement about moisture movement is true?

• A. Vapor diffusion moves more moisture into building structures
• B. They move moisture equally
• C. Moisture does not move
• D. Airflow has the potential to move significantly more moisture into building structures than vapor diffusion

Answer: (D)

Moisture moves in two main ways: by diffusion of water vapor through materials and by bulk air movement through openings and cracks. Diffusion depends on a vapor pressure gradient and the material’s permeability, so it tends to be slow and limited by the barrier it must pass through. In contrast, airflow driven by pressure differences from wind, stack effects, or mechanical systems can carry large amounts of moisture quickly, especially through leaks and cavities in the building envelope. Because bulk air movement can transport moisture without the same barriers as diffusion, it has the potential to move significantly more moisture into building structures than diffusion. This is why infiltration or exfiltration of moist air can lead to substantial moisture transport and potential condensation risks inside assemblies.