Vapor barriers are materials used to slow or reduce the movement of water vapor through a material (water vapor is also transported by air leakage but this can be resolved by installing an air barrier). Vapor barrier materials are installed on the warm side of the insulation in a building assembly. The position of the vapor barrier in a building assembly will be determined based on the climatic conditions. In warm climates, it will be on the exterior and in cold climates, it will be on the interior.