In a cold climate, where should the vapor diffusion retarder be facing?

In a cold climate, the vapor diffusion retarder should face the inside of the building. This configuration is essential for controlling moisture accumulation within the building envelope. The primary purpose of a vapor diffusion retarder is to prevent warm, moist indoor air from moving through the walls and condensing on cold surfaces, which can lead to mold growth, water damage, and other issues.

When the vapor retarder is installed with the facing towards the interior, it effectively blocks moisture from the warm interior spaces of the home from reaching the colder surfaces of the external wall assembly. This is particularly crucial in cold climates where significant temperature differences between indoor and outdoor air exist, causing the potential for condensation.

In contrast, if the vapor retarder were placed on the exterior, it would allow moisture generated indoors to reach the colder exterior walls, increasing the risk of condensation and related problems. Similarly, horizontal or downward orientations do not address the fundamental issue of moisture migration that the vapor retarder is designed to mitigate. Thus, installing the vapor diffusion retarder with the facing towards the inside is the proper and most effective practice in cold climates.