In cold climates, on which side of the assembly should a vapor diffusion retarder be placed?

In cold climates, placing a vapor diffusion retarder on the inside of the assembly is crucial for preventing moisture from moving into the building envelope and condensing within the structure. As warm, moist indoor air rises, it has the potential to migrate through the building materials. If this moisture reaches a cooler section of the assembly—such as insulation or structural components—there is a risk of condensation, which can lead to issues like mold growth, rot, and material degradation.

A vapor diffusion retarder functions by reducing the amount of water vapor that can travel through the materials, effectively creating a barrier against this moisture. By positioning it on the interior side, just behind the drywall or interior finish, it ensures that the warmer, moisture-laden air is contained within the living space while minimizing the potential for moisture to reach colder exterior surfaces.

In contrast, placing the vapor retarder on the outside or at the midpoint does not effectively control the movement of moisture toward the colder outer regions where condensation can occur. As such, the best practice for maximizing building durability and energy efficiency in cold climates is to position the vapor diffusion retarder on the inside.