Hi–I am working on renovating an old barn into a community space. Part of this renovation will be insulating the walls and cathedral ceiling. I want to keep the framing and original exterior board and batten siding exposed from the inside (the interior walls and ceiling will not be finished) and so I want to insulate exclusively from the exterior with rigid foam sheathing installed over the original siding and roof decking.

My plan is attach foam panels to board and batten siding and the roof sheathing, then install a rainscreen, lap siding and a new roof. On past projects I have used a layer or 2″ foam sheathing in additional to fiberglass insulation but I have never insulated just with foam. If I am doing the math correctly, my walls will need to have 6″ foam to reach R15 and the roof would need 15″ to get R38 (climate zone 4). That seems like overkill, especially on the roof. Am I missing something to consider? Does anyone have any other insulation suggestions that allow me to keep framing exposed? Thanks