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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Replies
Rob,
Your suggestion doesn't meet code requirements (the code says that a hybrid roof like this will only work if the fluffy insulation is installed directly under the foam insulation), unless you install enough cellulose or fiberglass on the attic floor (above the ceiling drywall) to meet minimum code requirements (in your case, R-49).
Why not just insulate the attic floor (above the ceiling drywall) with R-49 cellulose and be done with it? If you do that, you won't need to install any spray foam in the attic.
For more information on all the different ways to create an insulated unvented cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.
Martin,
Thanks for your reply. My description of the situation was a bit confusing, my apologies. The flat ceiling only extends over the middle third of the space - on the edges of the room the ceiling follows the underside of the rafters (cathedral style). It's an old house, so the second floor in question is akin to a finished attic. I've attached a quick sketch (not to scale) illustrating the proposal in question. The red hatch represents the ccSPF on the pitched portions of the ceiling.
So the idea is to treat the pitched portions of the ceiling like an unvented cathedral ceiling and then vent the small triangle of "attic" space above the flat ceiling and cellulose. This venting would be via gable vents and/or ridge vents (no soffit vents or vent channels). My question is whether this venting is sufficient and also if I'm creating problems with this hybrid condition.
Obviously, a viable approach would be to simply foam the whole roof structure as an unvented, cathedral ceiling per your articles. However, I'm interested in this alternative as a way to reduce the amount of foam and not increase the amount of conditioned space. Thanks for your feedback on this.
R. Williams,
Your sketch corresponds to my suggestion, so you are all set. I'm sorry if I misunderstood at first.
Don't worry about the adequacy of the gable vents -- everything will be fine, as long as you pay attention to airtightness. For more information on this topic, see All About Attic Venting.