Insulating a trussed cathedral ceiling
I’m renovating a house in Rochester, NY (5A) and have an insulation question about a vented cathedral ceiling of a mid-1980s addition.
When I bought the house, the ceiling had sheetrock with significant sags between rafter bays, including one much wider sag in a single bay. I removed the sheetrock and found the ceiling cavities packed tightly with cellulose, which was weighing down very stretched-out poly. I also found a 42″ span with no framing from old skylights. I’ve since had all the cellulose removed and am starting fresh.
The roof is a vented assembly with a ridge vent and several soffit vents on each side, but there were no baffles before so I suspect it wasn’t functioning well for the past 40 years.
The existing roof sheathing isn’t great looking from below, it’s a hodgepodge of old/new with bad gaps. There aren’t active leaks, but I expect to replace it when I redo the roof down the road. Therefore, I don’t feel that spraying foam on the underside of the roof deck is an option.
The geometry of the trusses also feels like an issue, as there’s not an 8-10″ rafter surface to work with, but rather the 2×4″ bottom cord of each truss. As each truss terminates into the exterior wall, I’m left with ~5″ of framing depth, not nearly enough to achieve proper R values. The wall height at the lowest point is about 7ft, so extending the framing depth downward into the room isn’t ideal. Most of the rafter bays are different widths as well, and only two of them are actually 24″ OC.
It feels like by maintaining the vented assembly and avoiding spray foam, my options are limited. I’ve read several of Martin’s articles on this site. While understanding the potential drawbacks to the cut and cobble approach, I do wonder if that must be part of my solution in conjunction with some mineral wool. While I don’t want to drop my ceiling height much lower, I understand that it may be necessary given the truss/room size constraints.
I’d love any thoughts on how to properly insulate this space – thank you very much in advance!
My background – paint contractor by day (obsessed with why so many building materials/methods, paint included, regularly trap moisture and cause rot), reasonably capable carpenter/renovator on my own homes. I’ve insulated rim joist cavities and packed Rockwool into wall bays before, but this is a more advanced problem than I’ve handled!
First time posting so wondering if this link will work for photos…:
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Replies
Oh, scissor trusses... that's the part that was missing. Pictures work fine, by the way.
Assuming there's no structural issues with the existing trusses, you can strap across the bottom chords to better support the drywall from insulation sagging. Without modifying the existing ceiling profile there's not a lot you can do to add insulation depth, and that 5 inches at the top plate isn't growing very quickly with the truss pitch obviously.
Have you considered adding insulation above the roof deck (if you're already going to be replacing the roof deck and re-roofing) and moving to an unvented cathedral ceiling assembly? Not sure how much un-needed cavity depth will be left past the necessary insulation depth at the ridge area though.
It's probably a good time to remove those can lights, for reasons of air leakage.
Something else I noticed from the pictures: You've got a gable wall hinge (flat bottom gable truss with scissor trusses) but may not be an issue if that partition wall is providing lateral bracing to the gable wall, or if you're not in a high wind area. Does make for a rather floppy gable wall though.
Noah,
Consider sheathing the underside of the roof trusses with 1" of polyiso foam board, before strapping the ceiling with 1"x3"s as Chris suggests. That will both reduce thermal bridging, and add about R-6 to the roof insulation, while only losing 1 3/4" ceiling height.
The low slope on the lower truss chord means that if you ran baffles continuously from eaves to peak, you could fill the entire attic with loose fill cellulose, and it wouldn't slide.