Low slope roof options with I-Joists
Planning a garage/shop, looking for options if part of the roof is 2:12 monopitch with i-joists. CZ3a. Design phase; I have not boxed myself into any corners…yet. Metal roof, standing seam. I can abandon the 2:12 idea if needed, but it does solve some wall height problems.
Looking at the span tables, 11 7/8″ I-joists should be no problem. With R-38 high density fiberglass batts (10.25″?), that only leaves 1 5/8″ for venting (not including any vent chute thickness), which sounds risky for 2:12 pitch.
Bumping up to a 14″ I-joist is an option for additional venting (no structural need), would then be 3.5″ baffle with a R-38 fiberglass batt. Making/installing a deeper baffle (that attaches somewhere other than the 1.5 inch flange) seems like a lot of work. Probably not much price difference between 11 7/8 and 14 inch from the same series, but does weigh more when I’m wrangling them into position, and there will be a change in wall or ceiling height obviously.
Are either of those vented options feasible? The low-slope vent gap recommendations are all over the place for depth, and most don’t mention whether the recommendations include a baffle (some imply the insulation is open to the vent cavity).
I know I could do un-vented, but that seems like an expensive way to address a very simple roof plane if there’s a vented option.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Chris,
From Martin's article https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
"What’s a low-slope roof? It’s a roof that is flat or almost flat. While some sources define a low-slope roof as one with a pitch that is less than 3 in 12, ASTM (in Standard E 1918-97) defines a low-slope roof as one that has a pitch that is less than 2 in 12."
3/12 is more forgiving, but my take is that in most climates, with good interior air-barrier with few penetrations, you should have no problems going to 2/12 with the shallower I-joists - especially for a building where the interior humidity should be lower than in a house.
Sounds good, thanks. I may opt for the 14 inch if I can ignore the wall height, as it occurred to me that I could move to 24 inch OC if I did that (I can't go 24 OC with the 11 7/8, and I don't want to mess with insulating a 19.2 inch OC or whatever that actually is).
Chris,
A deeper vent space moves it into the roof assemblies I'd categorize as risk-free.