b'Roofing workers used to know how to spot a blister, repair a blister and how to avoid making them. But even that knowledge is fading.included. Warden goes into great detail and effort to establish the cause and behavior of blisters.Several interesting conclusions came from this work. The first was confirmationthatroofblistersarecomposedalmostexclusivelyof air and water vapor; no hydrocarbons or volatile gases were typically present. Second, Warden identified the gauge pressure inside blisters could be positive and negative compared with the surrounding atmo-sphere. Blisters under the effect of nighttime radiative cooling of roof membranes generally will cause a negative gauge pressure and deflate. Conversely, heat generated by solar loading and warmer daytime tem-peratures will cause positive gauge pressure, and a blister will inflate. Finally, Warden opined blisters had fissures that allowed them to ex-change a small amount of gases as these pressures changed.Several decades later, in 1987, Walt Rossiter, a technical researcher, and others examined the gases found in blisters on built-up roof sys-tems installed above polyurethane foam boards. (Polyurethane foam boards were the forerunner to modern polyisocyanurate foam boards.) The change from polyurethane to polyisocyanurate was necessitated by the need for more fire resistance provided by polyisocyanurate. In his research, Rossiter found a small amount of chlorofluorocarbons in the blisters. These CFC gases were the blowing agents present in the polyurethane. However, a majority of the gas found inside the studied blisters were still atmospheric gases and water vapor.Research on blistering seemed to be quiet until the early 1990s when blistering in polymer-modified bitumen sheets became a press-ing technical issue. In 1998, NRCA and National Research Council Canada published Investigating the cause of blistering in SBS poly-mer-modified bitumen roofing membranes. This research confirmed ORN THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ONTARIO ROOFING NEWSISSUE 3 202413'