With the building industry moving more toward light colored roofing it has become increasingly important to consider the application of an air and vapor barrier (AVB) in the roof assembly.  It is a false assumption to consider that the roofing membrane acts as an AVB in the roof application.  The reason is that moisture is a nasty little bugger that can get into most any assembly.  The goal is not to just prevent moisture from entering the assembly but to get it out when it does get into the assembly.