Building A-frame w/ unvented hot roof in northern Georgia. How to calculate the insulation?

Building an aframe in a mild climate (zone 3). Figuring out the details (I'm not the contractor). I have heard because the walls are the roof, the insulation factor would be calculated differently? As in if I put 2" of closed cell and some fiberglass for R-27 or 3" of foam alone for r-21 it is actually calculated as a higher equivalent? Can someone explain this to me?