Vent channel size in cathedral ceiling?
Greetings all – I’m getting ready to tear the roof off my 1944 Cape Cod in zone 4C. The house has a finished attic and I would prefer to not disturb the interior, so (since the roof is coming off anyway) I plan to do any work from the outside.
This may all sound familiar – I just learned, quite accidentally, that my house was the star of a GBA blog post last month!
Presently the plan is to add 2x4s to the existing 2×4 rafters in order to provide more space for insulation. My contractor is proposing adding 4.5″ of rigid foam (to meet code req of R30), but assuming a 7″ cavity, this will leave 2.5″ of space.
I assume I’d be better off adding more rigid foam, but I don’t want to compromise the ventilation in any way as minimizing heat gain in the attic room is one of my primary concerns.
2.5″ seems like more than enough (too much?) for the ventilation channel. I’ve seen different numbers – on a roof with a fairly steep pitch, how much space is considered ideal? Is there such thing as too much? Is there a formua for vent channel size to soffit/ridge vent area?
Thanks as always.
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