I am currently doing a remodel which is exposing about 1/2 of a cathedral space which is about 20′ x 22′ total, 12′ height at the peak. The vaulted space has 2×8 rafters and the roof currently has soffit vents + 3 box vents (only one of which will be accessible with the remodel) and no ridge vent. This is a late 1960s home, so the only insulation in the rafters is faced fiberglass insulation.

I’m planning to do the best I can with insulation (located in climate zone 5) but not planning to extend the depth of the rafters outside of possibly furring out 1/2″ to match drywall. My current plan for the remodel space is to use 1″ polyiso to make a 1″ baffle, add another 2″ of polyiso, and then mineral wool to fill the rest of the space. I’d be getting to about R34 which while it doesn’t meet full requirements is significantly better than what I have over the other half and in general around the old house.

I am realizing that my ventilation is likely inadequate since I don’t have a ridge vent. I am considering adding a ridge vent across that entire peak, but that means that I would also be venting the space that currently has the old fiberglass insulation in it that is up against the roof sheathing. I am not planning to open up that ceiling anytime soon, so I’m wondering if there would be any negative consequences by adding the ridge vent all the way across? Welcome any feedback or thoughts on this plan thanks!