Which barrier is used to deter moisture traveling through exterior assemblies by capillary action and is often applied on foundations and roofs?

• A. Vapour Barrier
• B. Weather/Wind Barrier
• C. Moisture barrier
• D. Thermal Barrier

Answer: (C)

Moisture can move through porous building materials by capillary action, pulling water upward into the structure from the ground or through joints. A moisture barrier is designed to stop that liquid water movement at exterior interfaces, preventing capillary rise from foundations and along roof assemblies. This barrier is focused on blocking the liquid phase of moisture rather than hopping into the realm of vapor diffusion or heat flow.

A vapor barrier, by contrast, targets vapor diffusion driven by vapor pressure differences and is not primarily about stopping capillary liquid water. A weather/wind barrier deals with liquid water intrusion from rain and air infiltration while still allowing drying, and a thermal barrier is about reducing heat transfer. So for deterring capillary moisture traveling through exterior assemblies, the moisture barrier is the best fit.