Compare the air barrier vs vapor barrier in interior wall assemblies and specify when each is used.

• A. Vapor barriers control moisture diffusion, typically on the warm side of walls
• B. Air barriers control air leakage and are used in exterior or high-performance assemblies
• C. Some walls use both in a coordinated system
• D. All of the above

Answer: (D)

Air barriers and vapor barriers address different ways moisture and air move through wall assemblies. An air barrier stops air leakage through the wall, which reduces drafts, heat loss, and the convective transport of moisture—this is especially important in exterior or high-performance assemblies where you want to control infiltration and exfiltration. A vapor barrier, on the other hand, slows diffusion of water vapor through the wall, and is typically placed on the warm side of the assembly to keep interior moisture from migrating into colder parts of the wall where it could condense and cause damage. Some walls use both in a coordinated system, providing continuous airtightness while also managing vapor diffusion. Because each statement describes a valid aspect of how these barriers are used, all of the above is correct.