Where should a vapor retarder be installed in a building assembly?

A vapor retarder should be installed on the warm side of the assembly to effectively control moisture movement within the building structure. The primary purpose of a vapor retarder is to prevent warm, moist air from condensing within the wall or ceiling assembly when it encounters cooler surfaces. By placing it on the warm side – usually facing the interior of a heated space – the vapor retarder minimizes the risk of moisture accumulation, which can lead to mold growth, wood rot, and deterioration of building materials.

In colder climates, this strategy is particularly important, as the temperature difference between the interior and exterior can create conditions conducive to condensation if moisture-laden air penetrates through the insulation and into colder areas of the assembly. By properly locating the vapor retarder, it helps maintain the integrity of the building envelope and contributes to a healthier indoor environment.