I live in Central VT, the house has a cathedral ceiling, 2×10 rafters, 16″ OC, that are currently insulated with R-19 paper faced fiberglass, then plastic vapor barrier and drywall with no boxed off vent channel. There are soffit and ridge vents. My thoughts on a the best cost effective system are as follows:
1) Remove drywall and old insulation
2) Bring down cathedral ceiling by capping it at 8′ or 9′ with new collar ties
2) Construct 1 or 1.5″ air channel with thin panel board, or not, is this needed?
3) Install thick foil faced polyiso foam board over joists with strapping to hold in place, leaving gaps periodically to dense pack cellulose behind the foam board. After dense pack foil faced foam gets patched and taped seams.
4) Area above now flat ceiling portion of cathedral would just get loose fill with proper vent baffles maintained, also polyiso with strapping below.
4) Drywall over strapping

Other ideas that seem more expensive are adding proper air channels above roof, which also needs to be replaced, with 2×4 sleepers laid on side with new sheathing on top, then same as suggested above.

Any suggestions on this would be most helpful, thanks!