Air barriers have always been a big deal in flat roofs in cold miserable places like Canada even with black roofs. But back in the day you could get away without an air barrier in a flat roof in Washington, DC if the roof was black. Winters were not long enough or cold enough to cause problems and any moisture that did migrate upwards was driven downwards due to the extreme temperatures the black membranes would get to–above 180 degrees F. But with the advent of “greenie weenieism” and the preponderance of white roofs we began to experience roof moisture problems that we would once only see in Canada as far south as Atlanta. The solution was to add an air barrier directly over the top of the roof deck. The greenies got to keep their white roofs and the moisture problems went away.
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