Vented Cathedral Ceiling Assembly
Planning a kitchen remodel in Lincoln, NE (climate zone 5) which currently has vented cathedral ceilings on the lower 1/2 of the roof with a traditional vented attic on the upper half.
I am not a fan of unvented spray foam roof assemblies, so I am looking for a vented roof assembly. See attached PDF which shows my current plan for the cathedral ceilings. I intend to add 2×4’s attached via 1/2″ OSB gusset plates to extend the rafter bays down an extra 7″. This allows room for true R-60 insulation plus a 1.5″ air gap which should net an R-56 total assembly R-value. I am considering dense-pack cellulose or a combo of mineral wool and fiberglass batts.
1) If my exterior layer of insulation is mineral wool (MW) batt, is it necessary to extend the vertical wind baffles horizontally into the rafter bays? MW is rigid enough to fit tightly within the rafter bays. A couple foam spacers every few feet would be all I need to maintain a 1.5″ air gap. Wind washing is a problem with fiberglass, but the nature and density of mineral wool makes me wonder if continuous wind baffles are necessary with mineral wool?
2) If continuous wind baffles are necessary, thoughts on using asphalt fiberboard for the baffles? I generally despise fiber board when it’s used as wall sheathing, but I need something cheap that is vapor open. At 23 perms and $13/sheet, fiberboard checks both boxes.
3) I’m considering a combo of mineral wool & fiberglass batts (R-15 FG is half the cost of MW). I like MW for the outer layer because it’s easier to fill odd-sized crevices & fit tightly above the top plate of my walls. I envisioned using MW for the 3rd layer thinking the rigidity of MW would make it easier to install continuously between the existing rafters and my new 2×4’s, but I could also rip XPS strips to infill the rafter/2×4 gaps then use FG for all the interior insulation layers. Any drawbacks to switching back & forth between MW and FG?
4) If anyone has an easier idea for the gussets, wind baffle, or the assembly in general, I’m always open to new ideas.
FYI, the Acoustibilt panels noted on the inside of the assembly will serve as our finished surface. Acoustibilt is an acoustic panel made by Armstrong. Seems are finished like drywall then a special spray finish is applied over the top. It’s expensive, but the acoustics in this space are awful and Acoustibilt solves that.
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Replies
Wind baffles aren't really necessary except right at the eaves, especially with higher-density batts. The main reason for using baffles is to prevent the batts from being pushed in too far and blocking the vent space, which is more likely to happen with narrow vent spaces. If you want to have a baffle, asphalt fiberboard is a good choice.
I would fill the gusset space gaps first using strips of EPS or mineral wool board. Then install two layers of thicker batts instead of four thinner ones. The thicker and denser the batt, the more readily it will stay in place (thin FG batts often just end up falling out). Plus, it will be less work.
Yes, I could infill the area between the rafters and new 2x4’s with rigid polyiso then do the batts in 3 layers instead of 4. I still want the outer insulation layer to be mineral wool, but I suppose I may be able to use R-38 fiberglass then get by with 2 layers of batts instead of 3 and still achieve R-61 insulation.
If I’m buying polyiso anyway I would probably just get an extra sheet and use 2x2 blocks of polyiso glued to the roof sheathing to serve as my insulation spacers if I do not install a continuous wind baffle.