2025 Jan-Feb RETA Breeze

There is some inconsistency in the nomenclature used for these materials. The terms jacketing, lagging and facings are sometimes used interchangeably to describe the outer protective covering of an insulation system. Adding to the confusion, the term vapor retarder has evolved. Historically, the term vapor barrier was used, but this has been generally replaced with the term vapor retarder in recognition of the fact that an absolute barrier to water vapor flow is difficult, if not impossible, to achieve. Water Vapor Permeance is defined by ASTM C 168 as 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. For protective covering or facing materials, water vapor permeance is commonly expressed in units of perms. In below ambient applications, it is important to minimize the rate of water vapor flow to the cold surface. This is normally accomplished by using vapor retarders with low permeance, insulation materials with low permeability, or both in combination. In above ambient

applications, it is often desirable to have a breather facing that allows water vapor to escape without condensing. In either case, it is important to know the perme ance of the facing materials. ASTM Test Method E 96 is used to measure the water vapor transmission properties of insulation materials. Especially in below ambient service applications, it is important to under stand all aspects of an insulation system. The exterior finish of your insulation system is your first line of defense – clari fy its purpose and investigate if you need one or more finish to protect your insulation system investment. It is not unusual, and in many cases preferred, to have a vapor retarder to minimize the rate of water vapor flow to the cold surface and one to protect the vapor retarder. Specific applications such as ammonia refrigeration have very specific specifica tions which must be followed. This article is provided by the National Insulation Association (NIA) as a RETA member service. Please visit www. insulation.org for more information related to all aspects of mechanical insulation.

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