I bought a home in Southern Vermont (Zone 6A) built in the 70’s which is in need of upgrading and remodeling.  The cathedral ceiling/roof needs a redo.  The current is 2x10s rafters with 2x4s lying flat and perpendicular to the rafters.  This framing is covered with ½” ply sheathing with standard metal panel roof.  There are fiberglass batts between the rafters.  The 1 ½ inch space between the 2×4’s is empty and I believe the purpose was to ventilate the roof as it opens to the rake edge with vent strips.  There is no vapor or infiltration barrier other than the paper backing.  Suffice it to say, other than to shed rain, this ceiling/roof does not work well.  I have ice dams and I am sure much heat loss.  The cathedral roof joins the rest of the house next to a side wall and a roof valley. 

I have thought of 4 ways to improve this ceiling/roof.

Option 1:  Remove the fiberglass and make for better ventilation by applying a solid barrier (1/4 ply or similar) between the rafters nailed from below to the 2x4s and sealed along the edge.  I could cut gaps into the 2x4s to make them interrupted.  Fill the rafter space with dense pack cellulose and apply 2” polyiso to the rafters under the drywall (total R50). This option allows the roof top to remain intact.   

Option 2:  Fill both the rafter space and the 2×4 space (11 3/4” total) with dense pack cellulose (R37) with an inch or 2 of foil polyiso under the drywall for a total of R50 or 57.  This also leaves the roof top intact.  Would this work without any ventilation?  

Option 3: as #2 above, but add ventilation above the ½” ply sheathing by using interrupted 3/4”purlins for drainage and ventilations to reattach the metal roofing to.  I could also place the purlins on top of ¾” battens running parallel to the slope.  I also have ready access to local rough cut full 1” material at very good prices. 

Option 4:  as #2 above, but instead of the polyiso below the rafters add the rigid foam (XPS) above the ply with the metal roof on ¾ interrupted batten strips on top of the XPS.  How thick would the XPS need to be to keep from having moisture problems in the ceiling?  I would have some issues with windows on the adjoining side wall with rising the finished roof level.   Two inches would bring me back to R50 and keep me under the window (I think), but would that be enough to keep the sheathing warm?  By math only 17% of the total R would be in the XPS.   Is it ok to sandwich the sheathing between the rigid top foam and closed cell spray foam from below.   

Thoughts? Problems? Better ideas?  Best option?  If technically these options work, cost and labor will determine. 

Thanks for your help and I look forward to the discussion.