J,
The roof sounds interesting. Are you going to use real standing seam (crimping every 12") or one of the more modern 36" wide strips which you attach with stainless screws where the humps overlap ? In any case recommend you dont forget the ice dams on the lower 3' of every roof section. Also use 3/8" closed cell foam with a mylar skin facing up under the standing seam to both insulate and reflect the sun heat back out. Don't forget to let the main roof breathe to get rid of moisture. A vented ridge cap works well with vented soffits. The insulation in the ceilings should stay in place to give you an enormous thermal envelope on the top.
What sheathing is underneath the exterior wall shakes when you remove them ?
Do you think it is necessary to remove the interior drywall to place the rigid panels on top of the existing studs ? If you plan to remove the existing paper backed fiber glass batts I would recommend you replace them with sprayed in cellulose. Much tighter seal and so much easier to get it airtight. Check the R values of the sprayed in stuff with your vendor and add about R3 for the new sheetrock. If you can get over R20 for the net of the 2x4 walls you will be doing well. All of which is for naught if you don't use double pane windows and spray around those frames with low expansion foam.
Will chat about the exterior sheathing treatment in the next chapter.
Deralte