On which side of the wall assembly should the vapor barrier be placed?

The vapor barrier should be placed on the warm side of the wall assembly. This is because the purpose of a vapor barrier is to prevent moisture from the warm interior of a building from migrating into the cooler wall assembly, where it can condense into water. When warm, moist air comes into contact with cooler surfaces, it can lose its ability to hold moisture, leading to condensation. This moisture can contribute to issues such as mold growth, rot, and structural damage.

By positioning the vapor barrier on the warm side, you effectively reduce the chance of moisture penetrating the wall assembly and condensing. This is especially important in climates where there are significant temperature differences between indoor and outdoor environments. Ensuring that the vapor barrier is correctly installed is a key aspect of building durability and energy efficiency.