In the interior of the wall build-up, which component serves as the vapor barrier?

• A. Building paper
• B. Polyethylene vapor barrier
• C. Plywood sheathing
• D. Exterior siding

Answer: (B)

This item tests where moisture control is placed in a wall assembly. In cold climates like Manitoba, the vapor barrier is positioned on the interior (warm) side of the insulation to prevent indoor moisture from diffusing into the wall cavity and condensing. A polyethylene vapor barrier is a continuous, low-permeability sheet that effectively blocks this moisture flow, making it the best choice for the interior side. Building paper can act as a vapor retarder but is not as reliable as a dedicated interior polyethylene barrier. Plywood sheathing is a structural layer, and exterior siding is exterior cladding, neither of which serves as the interior vapor barrier.