In cold climates, where is a vapor barrier typically placed in wall assemblies?

• A. On the exterior side of insulation
• B. In the middle of insulation
• C. On the warm interior side of insulation
• D. Not used

Answer: (C)

Moisture moves from warm, humid indoor air toward cooler exterior surfaces. In cold climates, placing a vapor barrier on the warm interior side of the insulation stops that diffusion at the boundary between living space and wall assembly. By blocking vapor early, you prevent interior moisture from migrating into the insulation and reaching cold layers where it could condense, which protects studs, sheathing, and insulation from moisture damage and mold.

If the barrier were on the exterior side, or not used at all, interior moist air could diffuse deeper into the wall and condense on cold surfaces, increasing the risk of rot and mold. Placing it in the middle doesn’t provide effective control over where condensation would form.