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Perlite rigid board insulation is in wide use as a primary insulation board in built-up roofing systems. Perlite insulation board is also used as a cover or overlayment board with foam insulation boards such as polyisocyanurate. The standard methods of attaching the perlite boards are to either mechanically fasten them or attach with hot asphalt as an adhesive. Quite often the first layer is mechanically fastened and the second layer is attached with hot asphalt. Recent field problems have developed wherein the use of perlite boards attached with hot asphalt and then covered with a fiberglass felt roof membrane have led to blistering. The blistering is isolated in occurrence, but severe when it occurs. A testing program undertaken to examine this behavior has uncovered the fact that perlite has a precipitously fast moisture gain/release behavior, depending apparently on the cellulose filler. The current test methodology and resulting data point to a need to revise the ASTM standards for these boards. This paper will provide the data, background, and technical motivation for reaching a better understanding of this phenomena. The time sequence for short term moisture vapor migration was found to be in 60 seconds or less. Perlite recover boards with higher cellulose contents were found to have a faster rate of water vapor migration at temperatures in the 400° Fahrenheit range (204° C).
perlite rigid board insulation, perlite recover board, cellulose, moisture vapor migration, asphalt, fiberglass felt roof membrane, blistering, humidity
President, Structural Research, Inc., Madison, WI
Architectural Engineer, 3M Facilities Engineering and Real Estate, St. Paul, MN