Where should a vapor barrier be positioned in a building?

The vapor barrier should be positioned on the warm side of the building's insulation. This is because the primary function of a vapor barrier is to prevent moisture in the warm, humid air from penetrating into the cooler part of the building structure where it can condense. When warm air comes into contact with cold surfaces, the temperature drop can lead to moisture accumulation, which can contribute to mold growth, wood rot, and other structural issues. By installing the vapor barrier on the warm side, it effectively limits the movement of moisture into areas where it could cause damage, thereby maintaining the integrity and health of the building envelope.

The positioning is critical and generally follows the principle of "keeping the warm and humid air inside," especially in colder climates. In warmer climates, the approach may vary, but typically, the vapor barrier is still placed according to the specific design conditions of the building.