What is the primary purpose of a vapor barrier in cold climates?

• A. To slow moisture diffusion from interior to cavity
• B. To speed up interior moisture diffusion
• C. To block heat transfer entirely
• D. To act as a radiant barrier

Answer: (A)

Moisture management in cold climates hinges on restricting vapor diffusion from the warm interior into the wall assembly. Water vapor moves with indoor air, and if it reaches cold areas inside walls, it can condense on cold surfaces, leading to mold, wood rot, and decreased insulation performance. Placing a vapor barrier on the interior side of the insulation slows that diffusion, keeping most moisture from entering the cavity where it could cause problems. It isn’t meant to block heat transfer entirely—that’s the role of insulation and air barriers—and it isn’t a radiant barrier. It also doesn’t accelerate moisture movement. So, the primary purpose is to slow moisture diffusion from interior to cavity.