I just finished the roof on my timberframed workshop. 4x6 common rafters 46" on center with 1x12" rough cut that I shiplapped. I decided to insulate with 2" polyiso then OSB, 15# felt and metalroofing. Went together well. Glad now I didnt cheap out and just put metal down. Winters are long here! What ever you do I would put felt under the metal.

BB