Planning a new house in Climate Zone 6, Chad Kotlarz is reviewing his architect’s plans for the roof — and discovers he has a few misgivings.
The unvented roof will be framed with 2×12 rafters, sheathed with plywood and capped with standing-seam metal roofing. Closed-cell spray foam will insulate the rafter bays, and the interior of the cathedral ceiling will be finished with gypsum drywall. An exposed truss with a collar tie provides structural support.
“The main concern is with the thermal bridging from the 2x12s,” Kotlarz writes in Q&A post at Green Building Advisor. “The roof is a 10/12 pitch, with a dormer and two valleys. I was told that the thermal bridging created from the 2x12s would cause hot/cold spots in the roof and would lead to problems with the metal roof. I’m assuming this is from expansion/contraction or condensation, but I’m not certain.”
For reasons he doesn’t explain, Kotlarz doesn’t want to use rigid foam on top of the roof sheathing or on the interior, so those approaches to thermal bridge reduction won’t work.
Will thermal bridging through the rafters cause problems over time, such as rippling in the metal roofing or condensation in the roof framing?
Those are the questions for this Q&A Spotlight.
Consider using scissor trusses
Stephen Sheehy, who built a well-insulated house with a standing-seam roof in Maine, suggests that Kotlarz look at a different way of framing the roof. Sheehy used raised-heel scissor trusses that were deep enough to allow about 20 inches of cellulose plus a ventilation space.
Raised-heel, or energy, trusses are extra deep at the building perimeter to provide more room for insulation than a conventional truss. “A nice benefit to trusses,” Sheehy adds, “is the ability to put…