Cathedral ceiling assembly
The “right” way to build an unvented cathedral ceiling is to use rigid foam above the deck and cavity insulation in the rafter bays (see Martin Holliday’s fine article on this). Ideally, you would also put a vapor-permeable air barrier (e.g., MemBrain or Intelloplus) on the underside of the assembly, to create an internal air barrier but still dry to the inside.
In rural southern Virginia, however, the local builders do not build this way. (They generally flash and batt, and call it good.) So I am trying to come up with an alternative cathedral ceiling assembly, one that will be more familiar to them but still give me a good air barrier and a thermal break on the rafters.
One alternative is to install the air barrier to the underside of the rafters, then separate the furring strip from the rafters with a rigid foam spacer (see attached sketch). The air gap between the air barrier and the drywall can then be filled with vapor-permeable insulation, and still dry to the inside.
This alternative is definitely a “poorboy” assembly. You wouldn’t build it in New England. But . . would it work in Zone 4?
Any thoughts?
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Replies
The cross-mounted 2" elements is often referred to as a "Mooney" assembly, after a person who popularized it. While it provides a decent amount of thermal break, it doesn't get rid of the need for an air-impermeable insulation layer (per R806.5 of the IRC) on the exterior side of the fiber for dew point control, or alternatively a 1" vent space to protect the roof deck. (MemBrain or Intello etc. isn't enough.) See:
https://up.codes/viewer/utah/int_residential_code_2015/chapter/8/roof-ceiling-construction#R806.5
Virginia is entirely within US climate zone 4A, so unless it's vented under the roof deck you'd need at least R10 of closed cell foam (out of a total R49) on the under side of the roof deck to keep the exterior side of the fiber insulation from accumulating mold-supporting levels of moisture over the course of a winter.
How about make it vented instead?
Roof Sheathing, 2" air gap, 3"+ rigid foam followed by ocSPF or whatever insulation type you need to meet latest code, smart vapor retarder, then drywall.
Code only demands a 1" air gap. High density "cathedral ceiling" R38C batts are manufactured at 10.25" of loft, to be able to provide that 1" gap using standard milled 2x12s, which are a nominal 11.25" deep.
With R38Cs you'd only need R11 in the Mooney-roof part of the assembly to meet code on an R-value basis, but with the much lower thermal bridging of the 2" polyiso + furring even R9 should be enough to get there on a U-factor basis. Assuming as-drawn there is 2" of polyiso and 1x furring (0.75" nominal thickness) to the cross elements, an R13 compressed to 2.75" would deliver the ~ R11, but a compressed R15 would be ~R12, more than enough to make it on an R-value basis alone.
Mark,
You've gotten good advice so far. If you want to install fluffy insulation (like fiberglass batts) between your rafters, the code requires (and building scientists advise) that you need either: (a) to include a ventilated air gap between the top of the fiberglass insulation and the underside of the roof sheathing, or (b) to include an adequately thick layer of rigid foam above the roof sheathing.
If you break these rules, you risk moisture accumulation and sheathing rot.
For more information on all of the different ways to insulate this type of roof assembly, see How to Build an Insulated Cathedral Ceiling.