RETA Breeze July-Aug 2024

PERMEABILITY & PERMEANCE DO YOU KNOW THE DIFFERENCE? By Ron King, Past President/Consultant, National Insulation Association

P HYSICAL PROPERTIES for mechanical insulation materials are normally found on the respective materials’ data sheet. In comparing materials, it is important to be sure the physical properties being com pared are tested to the same test method, procedure, and the values are expressed in the same units. If not, a true comparison is very difficult, if simply not impossible. All physical properties are important, and it is equally important to understand the effect each will have on the insulation systems performance. Two such properties that are extremely significant, and often misunderstood, in below ambient applica tions, are water vapor permeability and permeance.

A February 2013 article in Insulation Outlook, by Biji Thomas, (Perspectives on Water Vapor, Permeance Mechanical Insulation Systems) served as a great reminder as to the importance of under standing permeability and permeance. “Both are measures of a material’s ability to resist penetration of moisture from the air. The terms are defined in ASTM Standard C168-10, “Standard Terminology Relating to Thermal Insulation” as follows: Water vapor permeability - the time rate of water vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions. Water vapor permeance - the time rate of water vapor transmission through unit area of flat

material or construction induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions. Permeability is measured in units of perms-inch and is used to compare materials that are typically used in a variety of thicknesses (¼” or greater). Permeance is measured in units of perms and is used to describe thinner materials (e.g., jacketing products) that are used in the field in the exact thickness at which the material is tested.” Sound confusing? It is, but understanding those values, initially and over time, is imperative in below ambient applications. Most will tell you, as a general rule, the lower the perm the better. Equally impor tant is to understand how such things as the protective covering - jacketing being

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