In an exterior wall, which side should the insulation with a vapor barrier face?

Insulation with a vapor barrier should face the warm side of the house to be effective in preventing moisture from condensing within the wall assembly. In colder climates, the warm side is typically the interior side of the wall, while the cold side is the exterior. When warm, moist air from inside the house comes into contact with the cooler exterior, condensation can occur if there is no vapor barrier in place to stop it.

The vapor barrier helps to keep moisture from the warm interior air from penetrating into the wall insulation and framing, where it could lead to mold growth or structural damage. Thus, proper placement is vital for maintaining energy efficiency and preventing moisture-related issues in the home. In contrast, positioning the vapor barrier towards the cold side would allow moisture to enter the wall assembly, increasing the risk of condensation and its associated problems.