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.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Since it is new construction why not 18" I-joists with flat 2x4 cross strapping on the bottom with cellulous insulation. No need for the service channel. No foam
Is this a full cathedral or is it a scissor truss?
There is no problem putting up a layer of polyiso, and taping the seams -- if you also seal all the edges and also any penetrations -- will give you an air barrier. I just recently did this myself behind a T+G ceiling where I used 1/2" polyiso to act as an air barrier plus a little bit of additional R value for the assembly.
For new construction though, I like Tim's idea sinec it saves a step. I also agree that a service channel really isn't needed. Chances are you'll never be in that ceiling running wires again anyway, and you can bury the electrical stuff in cellulose without problems. If you think you might want to change your wiring around down the road, I'd run conduit instead of NM cable since you can reconfigure the wiring in conduit without entering the the ceiling space, and you don't need a service channel for conduit.
I have been working on my renovation, so I am stuck with my framing. 2x2 on top of 2x12. I made plywood baffles using 1 inch foam to hold the depth. R30 rockwool with r15 on top of that. Then intello. I will be putting 2x4 lsl perpendicular to that, with r15 rockwool - Then t&g vertical over that. Wish I could have had 18" of engineered lumber to stuff.
I really like Intello.