Which barrier is intended to slow the diffusion of interior moisture into insulated exterior assemblies and does not need to be completely continuous?

• A. Air Barrier
• B. Moisture barrier
• C. Thermal Barrier
• D. Vapour Barrier

Answer: (D)

The thing being tested is how moisture moves as vapor through building assemblies. A vapour barrier is designed to slow that diffusion of water vapour from the warm interior into the cooler exterior insulation and wall components. It provides a high resistance to vapour flow, so even though vapour diffusion isn’t halted completely, it is greatly reduced, protecting the insulation and exterior layers from condensation and wetting.

This is why it’s the best choice: an air barrier focuses on stopping air movement (which can carry moisture with it), a thermal barrier reduces heat transfer, and a general moisture barrier is about resisting liquid water or moisture in general. The vapour barrier specifically targets vapour diffusion. In practice, you aim for a continuous layer, but even with small gaps, the overall vapour resistance remains the main protective function of the barrier.