Where should the vapor barrier be installed in relation to insulation?

The vapor barrier should be installed on the warm side of the wall. This placement is crucial because it helps to prevent moisture from migrating into the insulation and the wall assembly from the warm interior of the building. When warm, moist air comes into contact with a cooler surface, it can condense, leading to potential moisture problems such as mold, mildew, and wood rot.

By installing the vapor barrier on the warm side, usually facing the interior of the building, it effectively blocks the movement of vapor from the inside air into the wall cavity where it can collect and cause damage. This is particularly important in climates where heating is used in the colder months, as it ensures that the insulation remains dry and maintains its effectiveness.

The other placements are not ideal for a vapor barrier. For instance, placing it on the cold side would allow moisture from the warm side to reach the cold wall surface, where it could condense. A center placement would not provide any benefit in managing moisture flow, and a base placement does not address the critical area where warm moist air interacts with cooler surfaces in the wall assembly.