Vapor Permeable or Impermeable, Does It Matter?
The building envelope (or enclosure) is the part of the building that physically separates an interior conditioned space from the exterior environment. Its role has evolved from basic protection to providing a well-controlled and comfortable indoor space and consists of many components that must work together. The control functions include thermal, air, water, and vapor barriers for heat, air and moisture management. The thermal barriers are materials with high resistance to heat flow known as thermal insulation, the air barriers are materials with high resistance to airflow, the water barriers (Water-Resistive Barriers, WRBs) are materials with high resistance to bulk water infiltration, and the vapor barriers (retarders) are materials with high resistance to moisture vapor diffusion.
While each individual barrier is designed for a primary function, it is not uncommon for a material to perform multiple functions. Unfortunately there are often unintended consequences that are not always understood when a material performs functions beyond its primary intended uses. The most common unintended function is that of a vapor barrier: any vapor impermeable building envelope material provides an unintentional vapor barrier which could be located at the “wrong side” of the building enclosure or could introduce multiple vapor barrier(s) in the building assemblies. Such practices could have significant consequences on moisture management and long term durability.
This whitepaper describes the potential consequences of unintentional vapor barriers and why vapor permeance of building envelope layers is relevant to moisture management and long term durability.
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