Approaching R-60 in a vented cathedral ceiling?
I’m trying to achieve “close” to R-60 (CZ5) insulation in a ventilated cathedral ceiling (new construction) and have been reading Martin’s “Five Cathedral Ceilings That Work” article for pointers (thank you, Martin!). The very first illustration shows a 2×4 scabbed onto the bottom of a rafter to increase the depth. Using a 2×12 rafter with a 1-1/2″ ventilation channel and a 2×4 gets me close to R-49 with dense-packed cellulose (using approximately R-3.6 per inch). I could use a 2×6 instead and that gets me close to R-55.
Would it be better to stick with the 2×4 and add 2″ of rigid polyiso below? That gets me close to R-60 and provides a thermal break. In either case I’d have 2x4s strapping installed on the flat to provide a service channel for 1-1/2″ deep electrical boxes for lighting.
If the polyiso seams are carefully taped would that allow me to skip an air barrier under the strapping? Or should I plan on Intello or MemBrain or some similar product?
I’m hoping to avoid use of spray foam where possible both for environmental and expense reasons.
Of course, I’m obsessing over the idea of the “scabbed” 2×4 or 2×6 separating from the 2×12 and dragging the 5/8″ drywall ceiling down with it. Is it going to the extreme to use 6″ or 8″ strips of plywood along both sides of the 2×12 to 2×4 or 2×6 joint to ensure nothing falls down?
Appreciate any thoughts you might have. By the way, going with insulation above the roof deck is probably a non-starter in this case, so I’m looking for the best result using below-the-deck methods.
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