I have vaulted shed roofs with 3 levels  (12/4 pitch).  The current construction is interior 1×6 T&G pine nailed to the roof rafters (not drywall) a poly vapor barrier, 2×10 rafters with R30 fiberglass batts, and 1/2 plywood roof sheathing.  There are several (17) intrusions in the ceiling for can lights which I have recently converted (removed the old can assemblies) and installed 4 inch Jboxes in the openings.  These are sealed.  I have an upcoming reroofing project, and as part of this project I would like to remove the exterior sheathing and upgrade the insulation.  The best method I think would be to add rigid foam on top of the exterior sheathing in the appropriate thickness ( I am at the top of zone 4 so I would use zone 5 specs to be safe).  This method creates a challenge as I have clear story windows installed at the deck level of one of the roofs so adding any height to this roof using rigid foam in not feasible.  A couple of things that have been discussed are a 1) reverse flash and batt. Spraying closed cell foam on the 1×6 interior Pine from above to 3 or 4 inches than adding rockwool and venting above that. 2) Spraying closed cell foam to about 8 inches with sent space between it and the roof deck (I know very expensive). 3)  Cut and cobble.  I have read the articles and know this is NOT recommended but if it is vented why would it be any worse than the current R30 fiberglass that is currently vented.  With the current insulation there is certainly are movement and moisture heading towards the underside of the roof decking.  Any thoughts on either of these methods or other options would be appreciated.