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.
What are the driving forces that move moisture out of an assembly? Vapor pressure and heat are the primary drivers for moving water from an assembly. However, light colored roofing membranes, such as TPO or PVC, are purposefully provided to reduce the amount of heat on the building assembly. With less heat we now have less of a driving force to remove water from the roof. This can allow for condensation to collect in the roofing assembly potentially damaging insulation and allowing for leaks.
Many TPO membranes are not fully-adhered to the roof decking, this combined with a light color roof and without air barrier will potentially allow for warm, moist air from the building to be trapped under the roof membrane. Here it could condense and we are left with the same condensation problem.
An AVB barrier at the roof deck will prevent vapor penetration from the building and allow for what could be considered a secondary roof membrane giving us more protection. Ideally this AVB would be fully applied to the roof deck sheathing, with two layers of rigid insulation with staggered seams to prevent thermal breaks. Coverboard and the fully-adhered membrane would complete the assembly making it as full proof as a roof can be.
Good installation practice and consideration to the details will help ensure a roof installation that will last the entirety of the warranty and give quality performance.