How much is too much when designing a roof for a cold climate where lots of snow is the norm?
Writing from climate zone 7, Michael Sterner says he’s worked with an architect to design a high-performing “Pretty Good House” where snow loads may reach 60 lb. per sq. ft.
As it stands, the roof system will consist of 16-in. I-joists. The assembly will include a 1-1/2-in. deep vent chute below the roof deck—a detail created by nailing plywood to the top flange of the I-joist. Then, dense-pack cellulose will be added to the 14-in.- deep roof cavity. Below that will be an Intello air barrier and a service cavity on the ceiling below.
“In working with an energy consultant on mechanical design, heat load calculations, and ‘redlining’ the building for performance improvement opportunities, they expressed concern over the roof system,” Sterner writes in this recent Q&A post. “The R-value was not high enough (they wanted ‘whole wall’ R-50 with less thermal bridging), and they wanted to see a 2-in. vent chute.”
Sterner’s consultants say they are concerned about the possibility of ice dams along the eaves, and that there are better options available.
“The energy consultants would like me to change to either a lumber or I-joist rafter with plywood and 6 in. of rigid foam on the roof deck, then do 2x rainscreen strapping and a second layer of roof sheathing with underlayment and roofing on top of that,” Sterner says. “I would have to apply the overhangs as outriggers using the rainscreen strapping.”
Should he be worried? Are there, in fact, better roof assemblies than what he has planned? Can he use something less than 6 in. of foam? In short, is it worth the extra effort and hassle?
Those are the questions for…
Get building science and energy efficiency advice, plus special offers, in your inbox.