What is an air barrier and how does it differ from a vapor barrier?

• A. An air barrier stops moisture diffusion.
• B. A vapor barrier stops air movement.
• C. Many materials act as both an air barrier and a vapor barrier, but they primarily stop different things.
• D. An air barrier stops air movement; a vapor barrier stops moisture diffusion; they serve different roles; many materials act as both.

Answer: (D)

The main idea is how these barriers control different ways moisture and air move through the building envelope. An air barrier is about stopping the movement of air through the assembly. Its job is to prevent air leakage in and out of the building, which reduces energy loss and also limits the amount of moisture and contaminants carried by that moving air. A vapor barrier, on the other hand, is designed to resist the diffusion of water vapor through materials, which helps control moisture buildup inside walls where condensation could occur due to vapor pressure differences.

These are two distinct processes: air movement is the bulk flow of air, while vapor diffusion is the slow, molecular movement of water vapor through materials. In practice, some products can act as both air and vapor barriers, but they have different emphasis depending on their permeability and how they’re installed. For example, a product might be excellent at blocking air leaks when sealed properly, while also providing good resistance to vapor diffusion, but another product might mainly slow vapor diffusion and not guarantee a tight air seal unless detailed with joints and edges.