IECC insulation requirements
I am currently in the early stages of designing a home and currently looking at Quonset huts as an alternative to stick framing. After looking over IECC for the region that I would like to build in it seems I would need an R value of 49 on ceilings and R15ish if I remember correctly on the walls. The question that I have is how, in a dome shaped Quonset hut (or other round “dome home” for that matter) would code delineate where the wall stops being a wall and starts being a ceiling. I would imagine that it would not be required to have R49 on the whole structure and that at some vertical point the R15 would need to transition to R49 to meet the ceiling insulation requirements, but I haven’t found a concrete answer on this subject. My plan is to use closed cell spray foam for its water tight properties and would like to get rough estimates on cost, but there would obviously be a huge difference in applying a R49 layer throughout as appose to a R15 R49 hybrid. Any insight is appreciated.
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Replies
Mitch,
Here's my opinion (as a former roofer): 100% of the steel exterior of a Quonset hut is roof. There is no exterior wall (except at the "gable" ends, of course).
But my opinion is irrelevant. The only opinion that matters here is the opinion of your local code official. So call up your local code authority and ask.
If local code allowed it, you would be better off with a more equal R values (or even one R value for the entire surface).
If your local code official doesn't deem the building to be 100% roof like Martin would, but you'd like to have equal R-values over the whole surface like Jon R is suggesting, one avenue to explore would be to see if you can use ResCheck UA tradeoffs to demonstrate energy code compliance with a uniform R value across your dome structure: https://www.energycodes.gov/rescheck
The new and much more environmentally friendly HFO-blown spray foams can hit around R-38 in a single lift, for what it's worth.
It also might make sense to do a hybrid of closed cell foam to an adequate thickness for moisture protection, and open cell foam for the remainder of the desired R-value.