What Does “Perm” Mean? The Difference between Permeability and Water Vapor Transmission Rates in Air Barrier Materials

Abstract

Permeability is a normalized term that is calculated based on vapor flow, vapor drive, and saturation pressures. However, it does not take into account the total volume of water vapor that is actually transmitting across a wall assembly at a given temperature and relative humidity. It also does not take into account other factors, which may include chemical diffusion and moisture content concentration. This paper will present water vapor transmission data collected on a variety of air barrier membranes, both permeable and non-permeable, at different temperatures and relative humidity. These data show a variation in permeability as a function of temperature; however, this is not reflected in ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, Methods A and B, the standardized perm rating often used by professionals to design wall assemblies. Hygrothermal analysis tools primarily use a permeability input based on relative humidity and do not directly consider temperature outside of the general Schirmer equation. This paper will provide an initial look at the implications of this limitation based on the vapor drive data through the use of hygrothermal analysis and wall assembly testing. The aforementioned may have intriguing implications on how we assess materials selected for exterior wall assemblies in construction. These same implications potentially influence how the industry establishes performance metrics and how they may relate to a wall assembly’s actual performance. This paper will serve to initiate a candid discussion on the subject matter.