Detailing a Vented Cathedral Ceiling
We are in the design stage for a home, we plan to break ground summer 2020.
It’s a very simple 28’x36’ house with a 6/12 pitch vented roof. No valleys. The front 22ft are going to be cathedral ceilings with parallel chord trusses. The rear of the house will be conventional trusses.
Northwest Montana Climate Zone 6
The cathedral roof plan currently is:
Standing seam metal
Rain Gap Furring Strips
Ice & Water Membrane
Sheathing (Taped Seams)
Parallel chord Trusses w/ energy heel (not engineered yet, but estimate 20-24” depth due to span)
Cavity:
– Vented Channel (2” site build rigid foam) foam sealed
– 20+” of dense pack cellulose
Drywall air sealed
Questions:
Will the dense pack settle and leave uninsulated cavities at the ridge?
When the foam sealed vent channels meet at the ridge how do you seal them to each other?
Do I need some sort of air barrier material under the drywall to protect the cellulose from moisture?
I want to avoid exterior rigid foam but want to minimize thermal bridging.
My thought was if I dense pack the cavity and between the truss webbing I would minimize thermal bridging. Thoughts?
What’s the best way to transition the air barrier and insulation on the vertical wall between the conventional trusses to the cathedral ceiling?
Thanks for any help and advice.
– Nick
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Replies
Is that 2" of foam for the baffles, or a 2" air gap? Half inch OSB is cheaper and more rigid, less fragile, more vapor permeable than 2" foam. Even 2" foam might not hold up very well to the pressures need to install 3.5lb cellulose, but OSB would take it just fine. With OSB baffles OR foam baffles it's wise to use half-perm "vapor barrier latex" primer on the ceiling in your location.
Dense packed to 3.5lbs cellulose won't settle even walls, where gravity has even more effect. Even at 3.2lbs it won't settle in a 6:12 roof, particularly in a location with summertime dew point as low as US climate zone 6B, NW MT. Settling is a function of mechanical creepage from moisture cycling and gravity. With half-perm paint on the interior and low summertime humidity there would be very little moisture cycling in/out of the cellulose, even in an air conditioned house. It'll only cycle from "dry" to "drier".
Its a 2" air gap.
I'm not set on what to make my baffles out of 1/2" OSB sounds like a good option. What would I use to create the 2" of gap? Rip rigid foam then span with OSB?
Just rip some # 3 pine at 2" and nail on each side of the rafter top chord right below the roof sheathing. Fasten the OSB to the 2" strips, I like a 1/2" crown staple.
Nick,
Here is a link to an article that should answer your questions: "Site-Built Ventilation Baffles for Roofs."
Thanks!
Any thoughts on the performance, in regards to thermal bridging, of dense packing cellulose between the webbing of the parallel chord trusses?
Any suggestions on how to carry the insulation from the conventional trusses up the vertical wall to the cathedral ceiling?
Bumping this to see if OP had done this project and could report back on experience.
Will keep looking for similar posts.