Vaulted / Cathedral Ceiling — Unvented
Hi, I’m in the process of building a cabin near Saskatoon, Saskatchewan, Canada. I can’t tell for sure since the climate zone map is for the US only, but I’m guessing this would be equivalent to Zone 6B or 7B.
Part of the cabin has a vaulted or cathedral style ceiling. I’ve read the ‘How to Build an Insulated Cathedral Ceiling’ article and am concerned the recommendations of all the contractors I’ve talked to up there suggest to vent this ceiling structure. No-one has heard of doing an unvented ceiling. The response I get is the roof will get too hot if you don’t vent it or you’ll trap moisture in the ceiling without air movement to dry it out. After reading the article mentioned above I realize this is inaccurate.
The cathedral ceiling area was built with engineered rafters/trusses which are ~18 inches deep. I’ve attached a few pictures. The part of the cabin that has the cathedral ceiling is in the right side of the cabin when looking at the ‘cabin exterior’ picture. The roof-line is perpendicular to the roof-line of the 2nd story section.
Part of my concern is one side of the cathedral ceiling butts into a second story of the cabin, which means there is no soffit system at the lower edge of the roofline to provide an entrance for air to travel up vent channels that would then exit at a ridge vent. The other side of the cathedral ceiling has a soffit that runs the full length of the roofline so this side could draw air into the soffits, up the vent channels, and out the ridge vent.
My general contractor suggests doing standard insulation similar to a wall. Vapor barrier (poly-ethylene), batts of fiberglass insulation, vented air channel, building sheathing, asphalt shingles. A spray foam contractor suggests installing some Rockwool (Roxul) batt insulation 5″ away from the building sheathing then spray foaming against this on the interior side. Essentially using the Rockwool insulation as a backer fill because its cheaper than the spray foam and is denser than fiberglass insulation so it won’t likely move/sag over time. The spray foam would provide a vapor barrier. The ceiling would then be finished below the spray foam. It’s likely there would be some amount of air gap between the spray foam and the finished ceiling since the rafters/trusses are 18″ deep and I doubt it would all be filled in. The 5″ gap mentioned earlier between the Rockwool batt and the roof sheathing would create the vented channel.
After reading the GBA article mentioned above I think the safer option is to build an unvented ceiling and I’d like to confirm how the option for spray foaming on the underside of the roof sheathing (“flash and batt” I believe) would work since asphalt shingles have already been installed so I can’t install rigid foam insulation above the roof sheathing.
If I assume my climate zone is 7B does that mean the spray foam applied to the underside of the roof sheathing must be R-30 by itself?
Then I’d need to install any type of batt insulation that would provide a minimum R-value of R-19 ex. fiberglass, rockwool (roxul), cellulose?
In total this would provide the minimum R-value of R-49 mentioned in the article above. What would you recommend for a total R-Value in zone 7B?
After the insulation I understand no additional vapor barrier (ex. poly-ethylene) is installed because we want the batt insulation to dry to the interior if necessary. Given the rafters/trusses provide 18″ of depth to the roof sheathing and I’m not likely to fill that in order to achieve R-49 to R-60 can I just leave an air space between the batt insulation and the finished ceiling?
Now here’s a part I’m not quite sure about. If I’m planning to install a T&G fir (or pine) ceiling do I need to install a substrate on the interior side of the batt insulation ex. dry-wall that’s taped/mud/primed?
If I do need a substrate of some kind what’s the most economical option above T&G fir?
In other related articles I saw it was mentioned the substrate acted as an additional air barrier, but in this case is it needed since R-30 spray foam was applied?
I appreciate your help,
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