Hybrid unvented cathedral ceiling ratio
HH_august
| Posted in General Questions on
Trying to determine the correct ratio of closed cell spray foam and blown in cellulose.
Roof is an unvented cathedral with 24 inch cavity. Climate zone 5A (2021).
1. Closed cell spray foam directly to roof sheathing = 6 inches (about R-40)
2. Blow cellulose = 18 inches (about R-60)
Does this ratio seem correct? No chance to change ceiling design, just need to work with the ratio to make sure I correct moisture control. Yes this will be an overly insulated roof!
Thank you so much – H
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Replies
Seems to me your goal of a R100 does not make economic sense in that the high cost to build this assembly seems unlikely to survive long enough to save enough energy to recover what was spent building it. Given the very high cost for spray foam the ROI caps out about R20 and low cost fluffy insulation blown cheaply on a flat ceiling ROI tops out about R60.
From a green point of view, you are forcing a lot of nasty chemicals to be mined, manufactured and transported in an attempt to save a very small amount of fuel.
Have you read this article?
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
I'm not sure offhand what ratio is needed in your location, but you should do the calculation based on the long-term R-value of the foam. The R-value will diminish over time as the blowing agents leak out and are replaced by air, and may be as low as R-5 per inch after a decade or so.
https://www.greenbuildingadvisor.com/article/the-ratio-rule-for-hybrid-roof-insulation
Because this is about long-term resistance to moisture accumulation, and not an arbitrary thermal goal, I go with conservative insulation values. CCSF will age to around R-5.5/in, and dense-packed cellulose can be as high as R-3.6/in. That suggests 8" of foam and 16" of cellulose.
If I had that much space to work with, as did a client I visited today, I'd use a vented roof, unless there was a good reason not to. 8" of foam represents a lot of embodied carbon emissions and a lot of cost. I've used as much as 36" of cellulose at client request, but the ROI drops off after about R-40 and after R-60 you might save a few dollars a year, literally, while spending thousands more than necessary.
Michael et al--
Thank you all very much for your comments--just what I was looking for.
I agree the approach above represents a design choice failure (home owner here, also doing some of the design) both from a ROI point of view and environmental one. Though we are using Nu-Seal product Spray Foam which lessons the impact abit theoretically. next time (if ever!), it will be vented.
As Michael suggested--at this point in the game, I wanted to make sure the design was going to at the very least perform well and as intended rather than have moisture issues down the line.
Everything I was ready was suggesting 40% ratio but wanted to make sure there was no different when considering spray foam vs rigid exterior.