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Vapor retarder materials of extremely low permeance are specified and used in many insulation applications. For mechanical systems operating at below ambient conditions, such materials are required to minimize intrusion of water vapor into the insulation; the lower the operating temperature, the more critical the vapor retarder function becomes. Some of these materials exhibit water vapor permeance of under 0.01 perm, and some are literally impermeable. The question of how reliable test method E96 is for testing such materials is often asked. In 2010, the task group under Committee C16 for thermal insulation that is responsible for E96, “Standard Test Methods for Water Vapor Transmission of Materials,” undertook an Inter-laboratory Study (ILS), or “round robin,” in which four materials of extremely low permeance were tested. The objective of this ILS was to develop a precision and bias (P&B) statement to characterize the robustness of the test for evaluating such materials, and to see what problems the labs experienced in the course of testing at this low level of water vapor transmission. The data obtained by the six participating labs and the statistical analysis of it would suggest that good precision can be obtained and it can serve the need for testing at these low levels. If performed with careful sample preparation, good control of conditions and close attention paid to data generated during the test, reliable results can be obtained. However, erroneously high results are not uncommon, and a critical take-away is the need for recognition of those outliers and the cause of them. As a related matter, within ASTM and the insulation industry there has been discussion of what defines, and how to classify, so-called “zero perm” vapor barriers. An overview of this topic is presented herein.
vapor retarder, vapor barrier, zero perm, dry cup, permeance, water vapor transmission, below ambient, partial vapor pressure
Lamtec Corporation, Mount Bethel, PA