Which barrier in the building envelope is designed to reduce outward moisture-laden air and inward cold air and must be 100% continuous?

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

Answer: (A)

Air movement through the building envelope is what drives heat loss and moisture transport. An air barrier is the layer or system specifically designed to resist that flow of air through joints, cracks, and around penetrations in walls, floors, and ceilings. By stopping outward leakage of moisture-laden indoor air and reducing inward infiltration of cold outdoor air, it helps keep the building dry, warm, and energy-efficient. For this to work, the air barrier must be continuous across the entire envelope—no gaps, holes, or gaps at joints or around openings. Even small discontinuities can allow air to bypass the barrier and undermine performance.

Vapour barriers, on the other hand, are about restricting the diffusion of water vapor through materials, not about blocking air movement. They don’t guarantee a continuous airtight seal and aren’t primarily responsible for preventing air leaks. The term moisture barrier is a broader, less precise label and isn’t the specific function described here. Weather or wind barriers relate to exterior protection from wind and rain but aren’t the same as an airtight air barrier designed to stop air flow through the envelope.