Typical roof coating dry film thickness vary from paint film thickness (plus or minus 3 dry mils) to more than 40 dry mils. This means a roof coating actually becomes the top layer of a composite roof membrane and underlying system. As such, the roof coating is the topmost layer of protection for the membrane, receiving the impact of sunlight (both infrared and ultraviolet (UV), rain, hail and physical damage.
Spray polyurethan foam does effectly prevent all six of these heat transfer methods from occurring. These should be the mechanisms by which you judge insulation, in addition to their R-Value. If you're uncertain about installing spray foam insulation, Energy Star has a map that recommends levels of insulation by R-value and cost-effectiveness in the United States.
I'm in the process of redoing my basement and read through the posts. I took out one inch styrofoam covered by 1/4 " beadboard and installed 2X4s and plan on putting in fiberglass batts between them(r-13) with 1/2 in drywall over top of that. Do you think I will see a significat increase in heat control? Should I have done more?... it's a little after the fact, but still curious to know if I will see a big difference based on your experience.