Where should a vapor barrier be installed?

A vapor barrier is a material used to prevent moisture from passing through walls, ceilings, and floors, which can lead to mold growth and structural damage. Installing a vapor barrier between insulation and interior walls is crucial because it helps to keep moisture from the interior of the building from reaching the insulation, which would otherwise reduce its effectiveness.

When warm, moist air from inside a building contacts cooler surfaces, especially in colder climates, it can condense and create water droplets. Placing the vapor barrier on the warm side of the insulation (the interior side) prevents this moisture-laden air from reaching the cooler surface of the wall where it can condense. This maintenance of a dry environment is essential for preserving the insulation's R-value and ensuring the longevity of the wall assembly.

Other placements, such as between insulation and exterior walls or in ceilings or floors, may not effectively address the primary concern of moisture migration from the interior of the building, leading to potential problems with condensation and energy efficiency. Therefore, the correct placement of the vapor barrier is vital for proper building performance and moisture management.