Insulation and roof venting options for vaulted ceiling
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.
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One of our key architect advisors on GBA is Steve Baczek, and he always says, "Vent roofs until you can't." He means until something about the roof design and/or complexity prevents effective air movement.
One of my maxims is: you can't vent your way out of an air leak. Meaning, an airtight roof assembly is more important than an effective roof ventilation system.
Given that, your #3 would be my first choice.
Caveat: we don't know the pitch of your roof or its geometric complexity (hips, valleys, dormers, etc.). If your pitch is less than 3:12 or venting soffit to ridge is not possible or really serpentine because of roof complexity, than #1 would be my first choice.
#2 if you can - and I don't see anything that indicates that you can't.
Peter Yost: I've edited the question to include that the vault part is a simple gable roof with a 4:12 pitch. Does that change anything for you?