Another suggestion was to box out the roof with 2 bys.
Insulating tongue and groove roof.
No insulation at all.
Such is the case with my house picture above it is a 1958 custom built a frame with 6 12 timbers and 2 6 tongue and groove roof boards.
Inside you see the boards and shingles are on the outside.
This is often a layer of tongue and groove boards nailed to the exterior side of the exposed rafters.
In this video i install the tongue and groove raw timber walls in the van with polyester insulation and reflective barrier for a more comfortable and homely.
The eps panels attach to the roof structure with long spikes.
Install a 6 mil polyethylene vapor retarder over the tongue and groove roof planks.
Styrofoam brand tongue and groove extruded polystyrene foam insulation xps features tongue and groove edges on all four sides of 2 x 8 boards 1 thick or less and on the long edges of all other boards.
Some have suggested using sips but others have advised that because this particular roof is cut up with valleys and dormers i would lose the labor savings associated with sips and end up with a very expensive roof.
The insulation for exposed rafters is installed on the exterior side of the finish ceiling.
R 10 tongue and groove rigid foam board insulation sheathing 24dd the home depot.
Ceilings such as this are usually insulated with six or eight inches of rigid insulation on top of the tongue and groove then a layer of plywood is screwed on thru the insulation to the tongue and.
Owens corning foamular tongue and groove r 10 insulation sheathing is designed for resisting moisture and to block mildew.
You can then roof over the osb surface with any conventional sloped roof application.