What does a vapour barrier do?

• A. Prevent Moisture From Passing Through
• B. Increase Airflow
• C. Reflect Heat
• D. Strengthen The Structure

Answer: (A)

A vapour barrier is used to control moisture by blocking the diffusion of water vapor through building components. It slows or stops moisture moving from the warm indoor air into cooler parts of the wall or roof, which helps prevent condensation, mold growth, and damp insulation. It isn’t about increasing airflow, reflecting heat, or strengthening the structure. While other barriers address air movement, the vapour barrier’s job is specifically to limit moisture diffusion, so it best fits the idea of preventing moisture from passing through.