A vapor retarder on blanket insulation must face what side of the wall?

The vapor retarder on blanket insulation should face the warm side of the wall. This placement is crucial because it helps prevent moisture in the warm, interior air from permeating into the cooler wall cavity, where it can condense and contribute to problems such as mold growth and wood rot. In colder climates, the warm side is typically the interior side of the building.

By having the vapor retarder on the warm side, it effectively minimizes the risk that moisture-laden air will come into contact with cooler surfaces, thereby protecting the structural integrity of the wall assembly. This is especially important in areas where humidity levels are high and heating systems are used, as the difference in temperature between the inside and the cold outside can lead to condensation issues if not properly managed.

The other options do not fulfill the primary function of a vapor retarder, which is to keep moisture from penetrating the insulation and wall assembly.