Which option correctly completes: Vapour barriers are designed to ______ moisture in the building envelope according to the material?

Which option correctly completes: Vapour barriers are designed to ______ moisture in the building envelope according to the material?

• A. prevent moisture from leaving the building interior
• B. increase moisture within the wall cavity
• C. enhance air exchange
• D. reduce interior air temperature

Answer: (A)

Vapor barriers control the movement of water vapor through the building assembly. In a heated building, moisture from the interior tends to diffuse into the wall cavity, where it can condense on cold surfaces and cause damage like mold or wood rot. A barrier with low permeability on the warm side slows or stops that diffusion, so moisture is prevented from leaving the interior and entering the building envelope. This helps protect insulation, framing, and finishes behind the wall. It isn’t about increasing moisture, it doesn’t aim to improve air exchange, and it doesn’t change interior temperature; its job is to limit diffusion of vapor from the interior into the wall.

Vapor barriers control the movement of water vapor through the building assembly. In a heated building, moisture from the interior tends to diffuse into the wall cavity, where it can condense on cold surfaces and cause damage like mold or wood rot. A barrier with low permeability on the warm side slows or stops that diffusion, so moisture is prevented from leaving the interior and entering the building envelope. This helps protect insulation, framing, and finishes behind the wall. It isn’t about increasing moisture, it doesn’t aim to improve air exchange, and it doesn’t change interior temperature; its job is to limit diffusion of vapor from the interior into the wall.