Vapour Barrier vs. Air Barrier: Two Layers, Two Different Jobs

A vapour barrier controls the movement of water vapour through a wall assembly by diffusion, keeping humid indoor air from migrating into the wall cavity and condensing where it's cold. An air barrier does something different: it stops bulk air movement — actual moving air, not just vapour — through gaps, seams, and penetrations.

A single material like polyethylene sheeting can do both jobs at once if it's installed and sealed correctly, which is where the two concepts get blended in everyday conversation. But an air barrier has to be continuous — sealed at every joint, outlet box, and penetration — while a vapour barrier's job is about the material itself. Getting the air-sealing detail wrong is one of the most common sources of moisture problems in walls that otherwise look fine.