Under which material is polyethylene vapour barrier typically used?

• A. Under drywall
• B. Under roof shingles
• C. Under plywood
• D. Under concrete slab

Answer: (A)

The concept here is controlling moisture diffusion from the interior into wall cavities. In Manitoba, where indoor air can be humid and winters are cold, a polyethylene vapor barrier is placed on the warm-in-winter side of the insulation, basically against the inside surface of the wall before the finishing material is added. That means it sits under the drywall, protecting the wall cavity from moisture-laden indoor air. It isn’t typically placed under roof shingles or under plywood, and while a moisture barrier can be used beneath a concrete slab, that serves a different application than a wall vapor barrier.