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.
The perfect wall is an environmental separator—it has to keep the outside out and the inside in. In order to do this the wall assembly has to control rain, air, vapor and heat. In the old days we had one material to do this: rocks. We would pile a bunch or rocks up and have the rocks do it all. But over time rocks lost their appeal. They were heavy and fell down a lot. Heavy means expensive and falling down is annoying. So construction evolved. Today walls need four principal control layers—especially if we don’t build out of rocks. They are presented in order of importance: