Unvented Cathedral, Spray Foam, Standing Seam Roof Questions – GreenBuildingAdvisor

Hi All,

Thank you for the great info on this site. What a valuable place! After a deep dive into the articles and Q&A’s, I’m looking for some more input on remodeling my home after water intrusion. 

House details:

Climate Zone 3b (Los Angeles) 

Unvented vaulted ceiling, discovered extensive water intrusion and dry rot along the main ridge beam and rafter. Had all the water damage remediated, main beam replaced, and rafters replaced and/ or sistered in. Prior to the demo, there was 5 inch thick rigid foam board insulation, pressure fit into 24 inch on canter 2×6 rafters. As best as I can te, the foam board was not airtight or foamed around the edges for a seal. There are two main sections to the home (the house layout is shaped like the letter “T”), one with unvented cathedral assembly, which butts up against/ meets a section with a flat ceiling and vented attic (no apparent issues in that area). 4/12 slop roof.

My current game plan for remodeling the unvented section is, from the bottom up:

-New drywall, taped and painted, no recessed can lights

-Small air gap (not for ventilation, just so spray foam doesn’t peak over the bottom of the 2×6 rafters) 

-4-5 inches of closed cell spray foam, sprayed on underside of new roof decking. 

-Roof decking (seems plywood would be slightly preferred to OSB?)

Peel and stick underlayment

-New standing seam metal roof, unvented metal ridge over cathedral, no soffit vents, etc.

Questions:

-Is it a bad idea to install a standing seam roof directly over this sort of system? Is is safer to go with asphalt or cement shingles so the decking can have a chance to breath? Adding a batten grid for an air gap above the roof may not be feasible with the home’s geometry. Any other considerations?

-Any concerns with having this unvented cathedra section get more ‘properly’ insulated with spray foam, vs. how it has been with the not-full-sealed-in-place rigid foam board?

-Any advantages of doing ‘flash and batt’ vs. filling most of the rafter bays with ccSPF? The newly framed rafters are now 12 inches on center, so wondering if the batt is just not worth the extra labor.

-Any other funny business to anticipate with the vented assembly butting up against the unvented cathedral in the “T” shaped house?

Thank you very much!