Hybrid vented and unvented roof/attic
I’m working on a small renovation to the second floor/attic of old house in climate zone 5. A gable roof with basic dormers on each side. In short, the finished ceiling will follow the pitched roof (cathedral style) for about half the length of the rafters and then transition to a flat ceiling beneath horizontal framing akin to rafter ties. This leaves a small triangular attic space above (enough room to crawl around, but not enough room to stand).
Given the depth of the rafters, ccSPF is almost a must in the pitched portions of the roof in order to meet code (we can’t afford to lose any headroom in the occupied space). However, I’m hesitant to use ccSPF throughout the entire roof/attic; because of both cost and environmental impact I’d like to minimize the use of foam.
Would it work to apply ccSPF (3″-4″ min. to limit condensation) directly to the underside of the sheathing for the pitched portion of the ceiling and then use a thick layer of cellulose above the flat portion of the ceiling? So there would be no soffit vents and no ventilation channels, but the triangular attic space above the flat portion of the ceiling would be vented via appropriately sized gable vents. We would use air-tight drywall methods at the ceiling so that both the air barrier and the thermal barrier follows the ceiling line. There will be no ductwork in the remaining attic space. Does this make sense?
By my thinking, this seems like it would work. Condensation is not an issue at the pitched portions of the ceiling because of the ccSPF. And if any moisture moves through the flat ceiling and cellulose, then it can dry to the outside via the gable venting. However, I’ve only seen this detail done once before and I never hear anyone talking about anything like this. Any thoughts? Where am I going wrong in my thinking?
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