How does the International Residential Code (IRC) define vapor retarders?

The International Residential Code (IRC) defines vapor retarders primarily based on their permeability. This is a crucial factor because it pertains to the material's ability to allow moisture vapor to pass through it. The permeability of a vapor retarder is typically measured in perms, which quantifies the rate at which moisture vapor can move through the material. Understanding permeability is essential for ensuring effective moisture control within building assemblies, particularly in areas where condensation may occur, such as attics and insulated walls.

While factors like thickness, material, and color can play roles in the overall performance and application of vapor retarders, the IRC emphasizes permeability as the defining characteristic. Measuring and categorizing vapor retarders by their permeability helps determine their effectiveness in specific scenarios and environments, aiding in the correct application of building practices. This focus on the functional aspect ensures that moisture management strategies are appropriately aligned with the construction specifications and climate considerations within a home.