I’ve read Martin Holladay’s article about insulating a cathedral ceiling (https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling).  I’ve also read a TON of other stuff about this and talked to a couple local roofing contractors.

We’re building a new house and the trusses over the vaulted part of the house are basically going to be “truss rafters” that are 2’3″ deep…. way more than something like a 2 X 10 rafter.

So, we’ve considered the following, and are looking for guidance for cost effectiveness, quality insulation, roof/ceiling design……

1.) Flash and batt with an unvented roof system.  We’d use closed cell spray foam for the “flash” side of things to act as a vapor barrier.
2.) Deep batt insulation with a vented roof system.  We probably wouldn’t need baffles given the depth of our “truss rafters”.
3.) Use closed cell spray foam directly under the OSB roof sheathing, then build a “cold roof” on top of that (think a “cold roof” over SIPS concept).  We’d lay 2 X 4’s flat from eaves to peak, and then add a 2nd layer of OSB on top of the 2 X 4’s in order to create an air channel between the two layers of OSB.  The point would be to have a “hot roof” below and a “cold roof” above separated by the air channel.

Note #1: We’re open to other options (including employing rigid foam somehow).  Lastly, we’re near Spokane, WA which puts us on the border of climate zones 5 and 6.

Note #2: It’s a 4:12 pitch and a regular gable roof.  No hips or valleys over the vault part.

Thanks for reading and your consideration.