Where must you install the vapor retarder?

The vapor retarder must be installed on the warm side in the winter, which is essential for effective moisture control in building structures. This positioning is critical because it prevents warm, moist indoor air from reaching the cooler surfaces of walls and roofs where condensation could occur. When warm air cools, it loses its ability to hold moisture, leading to potential issues like mold, mildew, and structural damage.

By placing the vapor retarder on the warm side during winter, it acts as a barrier to moist air, thereby protecting insulation and preventing condensation that could occur if warm air were to penetrate through to the colder surfaces. This approach aligns with best practices in building science for maintaining a healthy indoor environment and ensuring the longevity of building materials.

In contrast, installing the vapor retarder on the cold side would allow moisture to travel to the cold areas, increasing the risk of condensation. Similarly, positioning it outside the sheathing would not provide the intended moisture control, as it would not address the flow of warm air from the living space, rendering it ineffective in promoting healthy indoor conditions.