In cold climates, where should air or vapor barriers be installed to control condensation in a building's exterior wall?

In cold climates, the primary goal of installing air or vapor barriers is to prevent moisture from condensing within the walls of a building. These barriers should be placed on the warm side of the building’s exterior wall because warm air holds more moisture than cold air. When warm, moist air migrates toward the cooler exterior wall, condensation can occur if it reaches the dew point, especially during colder months.

By placing the vapor barrier on the warm side, it effectively stops this moist air from penetrating into the cold exterior wall assembly, where it could condense and lead to potential issues such as mold growth and structural damage. This positioning is vital for maintaining optimal indoor air quality and protecting the integrity of the building's structure in colder climates.

Other locations, such as the cold side of the building, the basement, or the attic, do not provide the same level of protection from moisture infiltration and condensation issues that can arise in walls during colder weather.