What is a vapor barrier?

A vapor barrier (more accurately a vapor retarder) is a material that limits how much water vapor diffuses through a wall, floor, or ceiling assembly, helping control condensation.

Warm, humid air carries moisture that can condense when it reaches a cold surface inside an assembly, leading to mold or rot. A vapor retarder slows that moisture movement so it can't accumulate where it causes damage.

Where the retarder belongs — and how 'tight' it should be — depends on the climate, because moisture drives in different directions in cold versus hot-humid regions. It works alongside, not instead of, air sealing and insulation.