Hi.. the situation referred to is in a single story rancher zone 4a northeastern Maryland. I currently have non vented roof made of  4×6 exposed beams 4’ o.c. through out entire house except for to small “vaulted” sections on 1/2 of the width of each end of the ranch where beams are not readily accessible. The sheathing/insulation currently on my roof is 2” homasote panels that are original so I believe from 1972. A lot of the panels are sagging and water damaged. The panels are r-5 with vinyl barrier attached to underside of panels which is the finished surface you see inside the house. I have been researching and studying this topic of vaulted /cathedral non vented roof assemblies and the right way to re roof my structure to no relief .i don’t want to stick with homasote panels after seeing how bad they have failed due to mutiple reasons. I paid a local structural engineer to give me an awnser and he wants to demo out homasote (agreed is a must)seat cut and drop in 2×12’s 24”o.c. Then 5/8 cdx sheathing with 1-2  “ ccSF sprayed under the roof sheathing then r-38 fiberglass batts in order to reach current code of r-49 with Kraft faced seal to structure .. then smart vapor retarder before drywall . Some areas inside the house arent readily accessible to execute the insulation to underside of decking. I understand his approach but 1,700 sq ft of this style installation plus demo of multiple areas of fine wood work and bringing in multiple installers for multiple bids..loss of exposed beams and just the astronomical cost to do the extensive job( I’m on a budget and don’t think I can afford this method) I’m searching still for another awnser. One recommendation from a contractor the other day was to use t&g then insulate from outside with rigid foam panel  insulation . I contacted my local code office they stated 2×6 t&g will suffice or 3/4 cdx using h clips for the 4’ span between rafters .Also they stated no r value met Is necessary just” get in” what I can  since it’s existing structure and built before r -values were regulated in Maryland. So my question is that does anyone think the ladder idea of t&g and the rigid foam outside will be ok opposed to what the engineer wants? And for the sandwich would t&g or 3/4 cdx directly to beams- grace water and ice (or some other smart vapor barrier) – then 4 “ of polyiso then 5/8 cdx then roofing g materials . At edges of the sandwich use spray foam to seal for air barrier. Then inside the house between rafters at top plate cut 2” polyiso to fit and spray foam to seal edges. Any help or suggestions is needed direly. Thanks