A GBA reader named BuildingFun is having no fun at all after spotting a potential moisture problem in a cathedral ceiling that’s virtually finished.
He writes in this recent Q&A post that his old go-to was an unvented cathedral ceiling (sometimes called a “hot roof” design). But he’s decided to switch gears and go with a vented roof assembly. Accordingly, he installed SmartBaffle ventilation channels made by DCI Products between the roof deck and the mineral wool batts in the rafter bays. The corrugated plastic baffles provide a 2-in. air gap for ventilation.
It all looked good, until some last minute wiring gave him an opportunity to look inside the assembly—and he didn’t like what he found.
“I built the entire ceiling, changed the attic venting to a ridge vent and correct soffit ventilation,” he says. “It was lots of work that I wouldn’t do with a ‘hot roof’ and I felt good about the end product . . . until I did a spot check and found lots of condensation on the underside of the smart baffles at night time.”
The moisture disappeared by midday as temperatures rose, but the idea of accumulating moisture in the roof of this San Diego (climate zone 3) home is troubling, to say the least.
BuildingFun is thinking about starting all over by replacing the baffles with perforated 1-1/4-in. SilveRboard graphite polystyrene. “The way I see it, this SilveRboard XS graphite foam board should eliminate or minimize condensation and hopefully allow any moisture that does accumulate to escape slowly through the board and up into the air channel,” he says.
The roof assembly includes tongue-and-groove ceiling boards followed by a layer of 5/8-in. drywall (a fire barrier), a layer of 3/4-in. plywood, and 7-1/4 in. of mineral wool insulation…
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