Which moisture entry path is driven by diffusion through building materials?

• A. Bulk water
• B. Capillary water
• C. Vapour diffusion
• D. Air-transported moisture

Answer: (C)

Vapor diffusion is moisture moving through building materials as water vapor following a vapor-pressure gradient across the material’s pores. This diffusion is governed by the material’s diffusion properties and happens in the gas phase within the pore structure, not as liquid water or as air-driven transport. It’s different from bulk water intrusion (liquid water moving through cracks and openings), capillary water (liquid water rising in pores by capillary action), or moisture carried by moving air (advective transport). So diffusion through the material, i.e., vapor diffusion, is the mechanism at work.