Foam board under roof sheathing (but over rafters) for unvented cathedral?

I am in Zone 3 humid/coastal NC and want to build an unvented attic and cathedral ceiling without spray foam.  Why do you need roof sheathing, then R-5 foam, then… I am in Zone 3 humid/coastal NC and want to build an unvented attic and cathedral ceiling without spray foam.  Why do you need roof sheathing, then R-5 foam, then…Read MoreGreenBuildingAdvisor