I will be remodeling a rental unit in the Eugene, Oregon area that has a shed style roof. There is  little rafter access as you might expect   Several years ago an  insulation company blew about 4 to  5" of fiberglass insulation between the rafters-thus allowing a few inches of airspace above the insulation for front to back venting but the r-value is pretty feeble.  My thought is to install a vapor barrier foam board to the inside ceiling of each room and then redrywalling.  Is polyisocyanurate the best application for this situation?  Does this seem the best solution to this type of situation? Any recommendations on thickness and type of foam as well as  attachment method.
Thanks, Tom

if you're going to remove drywall why not just "hot roof" it with ccSPF or ocSPF directly on the roof deck with no venting? You'll get better insulation value and air sealing as well as no venting drawing/exchanging cold for hot air in the roof cavity -whatever gets through the polyiso. I am not an expert so please wait for more advice.

The roof deck option would be expensive as this is 1 unit in a triplex and the $8,500 TPO roofing material is only 3 years old.
You wouldn't remove the ceiling drywall in the unit as there is about 5" of  blown fiberglass mixed with rat crap above it. In fact, the ceiling is 1/2" ply not drywall. Thus buy foaming over it and then installing drywall I would both insulate and have a proper fire rated material.
My thought is that 1.5" of polyisocyno is the best choice as it would moisture block and add about 11 to the r value. I could also attach the drywall with 3.5" screws.