Spray Foam Unvented Ceiling
Hello, new to this forum and I’ve seen a lot of good information in what I’ve read so far. Hoping to get some advice on how to insulate a ceiling in a detached office/accessory building on the Oregon Coast. I rebuilt an old garage and created a single pitch 2:12 unvented ceiling. Rafters are 2×10, uninsulated metal roof on top. I was planning to spray foam 2″ of closed cell in the ceiling cavity. My question is do I need to fill the remaining cavity with batt or something else or could I leave 7 1/2 inch gap between the foam and sheet rock?
It’s not going to be living space, simply a detached building (520 SF) used for hanging out, maybe some office work. Walls will have R-15. I have a few cadet heaters and a wood stove. Given that it’s on the coast and in a shaded area, I’m most worried about moisture. I have a dehumidifier I’ll run periodically. Temperature fluctuations aren’t great there. What I don’t know is will moist air potentially fill that gap between the foam and sheetrock and cause problems?
I don’t really want to add batt to the ceiling because of the time and that much more fiberglass I’ll have to use. Any thoughts? Thanks in advance.
Dan.
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It's fine to leave the rafter bays empty from a moisture point of view, but it would also be fine to fill them. A couple inches of closed cell foam is fairly pathetic performance, in part because the 2" thermal bridge path through the rafters is so conductive. Deeper cavity fill makes a longer, higher R path. With just 2" of foam the roof assembly would have significantly lower performance than what you have in the walls, despite the similar center-cavity R, due to the more severe thermal bridging.
In your climate zone (4C) the IRC prescribes R10 as the minimum amount of closed cell foam you would need on the underside of the roof deck for up to R49 (the prescribed minimum) of total R. With 2" of foam you're looking at about R12-R13, and you'd have enough space to drop in an unfaced R25 fiberglass batt (8" manufactured loft, performs at R24 when compressed to 7.25" in 2x8 framing), which would bring you up to R36-ish, still well below code min, but not bad at all.
If that's too much fiberglass for you, cheap contractor rolls of kraft faced R13s or R19s snugged up to the closed cell foam with the kraft facer facing the air gap/cavity would cut the heat transfer by more than half compared to 2" of closed cell on it's own.
Thanks for the response Dana, that's helpful. If this were a home I would undoubtedly take it the extra step. Since it's somewhere between a garage and a man cave, I was curious if I could save on materials without creating a future mess. Good advice on getting the R value up on the ceiling, I appreciate that.
Dan,
The only point Dana didn't address is the question of what type of sheathing you have.
If your roof has solid OSB or plywood sheathing, your plan will work.
If your roof has skip sheathing or purlins (for example, 1x4s or 2x4s, 24 inches on center), don't do it. You don't want to apply any spray foam on the underside of your metal roofing.