Carolyn Wood is building a house 80 miles north of Vancouver, British Columbia, and if nothing else she’d like to get all the details in the roof assembly right. The question is whether the house is too far along to let her reach that goal.
The roof, with a 2-in-12 pitch, is framed with I-joists, strapped with 2x4s, and sheathed with 1/2-in. plywood. Above the roof sheathing, the roofers plan to install NovaSeal roofing underlayment and standing-seam metal roofing.
Below the sheathing are two layers of Roxul mineral wood insulation, providing a total of R-36. Wood plans to finish the ceiling with 1×6 tongue-and-groove boards. Against her builder’s advice, there will be no polyethylene vapor barrier in the ceilings or in the walls, but Wood would like to know whether, as she has recently heard, there should be a layer of drywall between the T&G ceiling boards and the insulation.
As currently built, the roof assembly has 1-inch-high ventilation gap between the top of the insulation and the underside of the roof sheathing, Wood writes in Q&A post at GreenBuildingAdvisor, but the question is whether 1 inch will be adequate. That’s the topic for this Q&A Spotlight.
Yes, the ceiling needs an air barrier
There’s no question that an air barrier should be installed between the ceiling boards and the mineral wool insulation, write both GBA senior editor Martin Holladay and Dana Dorsett.
“Unless measures were taken to fully vent the 1 1/2-inch air space between the 2×4 purlins to the exterior in a manner compliant with [the International Residential Code], using poly would be a BAD idea,” Dorsett writes. “But you can use a ‘smart’ vapor retarder such as Certainteed MemBrain or Intello Plus, detailed as an air barrier.”
Either 1/2-inch plywood or…