A-Frame cathedral ceiling roof insulation – GreenBuildingAdvisor

I have a question: In the case of an A-Frame new construction house, Zone 5A, we’re planning to have glulam structure (60 degree slope) at 4-5′ O.C. with 2×6 T&G sheathing visible from inside spanning from A to A. That means that all of the insulation would be outside the sheathing, Current code calls for R-49. I expect we will have unvented rigid insulation layer on top of the board sheathing. I’m thinking that we’ll need 7.5″ of rigid insulation to get that R-49. Will I need another layer of plywood sheathing on top of the rigid insulation to attach the underlayment and metal roofing?