Which of the following is a barrier in the building envelope?

• A. Air barrier
• B. Carpet
• C. Paint
• D. Curtains

Answer: (A)

Controlling air leakage through the building envelope is what defines a barrier in that system. An air barrier is designed to form a continuous low-permeability layer that resists air flow between indoors and outdoors, reducing energy loss, drafts, and moisture transport carried by air. Among common materials listed, only an air barrier serves this purpose as part of the envelope’s barriers. Carpet is just a floor covering and doesn’t seal the shell. Paint is a surface finish and isn’t a continuous air seal. Curtains affect heat gain but don’t prevent air leakage through the envelope. Therefore, the air barrier is the barrier in the building envelope.