I wanted to get your opinion on this, as I've heard differing thoughts. As an energy auditor in Phoenix (8 inches of rain / year, relatively low outdoor RH), I see a number of "flat" roofs with two different thermal barriers (foam w/elastomeric atop roof deck, and unfaced batt (or even faced batt) in the attic space atop the ceiling).
The attic space is usually between 20 and 30 Pascal @ CFM50, and many times, the attic is vented.
It would seem that the vents render the foam relatively ineffective. In most cases, capping the roof vents would lift the air barrier to the roof deck, but then there are the batts, as well as the space between. Is capping the vents harmless (in the desert or otherwise) if the ceiling is sealed fairly tight? Also, if the vents are not capped, is there any point in spray-foaming the roof rather than simply painting it white with roof coating?