Where are vapour barriers typically placed relative to insulation in the material?

Where are vapour barriers typically placed relative to insulation in the material?

• A. on the cold side of insulation
• B. on the warm side of insulation
• C. in the middle of insulation
• D. outside the insulation

Answer: (A)

Moisture control through the wall is achieved by placing a vapor barrier at the boundary where diffusion would most likely cause condensation. Because vapor moves from warm, humid spaces toward cooler ones, putting the barrier on the cold side of the insulation places it at the outer face of the insulation. This position stops vapor that has diffused toward the exterior from passing further into the exterior fungi- or sheathing layers, reducing the chance of condensation forming inside the insulation or at the exterior surface. If the barrier were on the warm side, it would stop diffusion earlier, but the scenario described here emphasizes intercepting vapor at the cold boundary. Placing it outside the insulation or in the middle wouldn’t effectively interrupt the diffusion path toward the cold exterior, and would not provide the intended protection against condensation inside the wall cavity.

Moisture control through the wall is achieved by placing a vapor barrier at the boundary where diffusion would most likely cause condensation. Because vapor moves from warm, humid spaces toward cooler ones, putting the barrier on the cold side of the insulation places it at the outer face of the insulation. This position stops vapor that has diffused toward the exterior from passing further into the exterior fungi- or sheathing layers, reducing the chance of condensation forming inside the insulation or at the exterior surface. If the barrier were on the warm side, it would stop diffusion earlier, but the scenario described here emphasizes intercepting vapor at the cold boundary. Placing it outside the insulation or in the middle wouldn’t effectively interrupt the diffusion path toward the cold exterior, and would not provide the intended protection against condensation inside the wall cavity.