Insulating a Cathedral Ceiling
HowieD42
| Posted in Plans Review on
I have a newly constructed space with 2×8 16″ OC Vaulted (cathedral) ceiling. Roughly 10’x14′ (it’s small) The room also has 2×6 wall construction.. with lots of windows so i won’t be getting much insulation there as is.
I’m on the border of Zone 4/5
my current plan from outside to in
asphalt shingles /
15lb felt /
ice and shield /
3/4 plywood /
1″ pvc vents /
5.5 inch mineral wool /
.5 inch polyiso tape seals to rafters /
– edit (no longer using pine tongue and groove since it requires sheetrock)
Will take suggestions on how to cover the ceiling that makes it look like Shiplap.
Any advice would be helpful.
Thanks,
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Replies
The described stackup is a moisture trap (and a code violation). Half inch foil faced polyiso has a vapor permeance of less than 0.05 perms, as does the Ice & Water Shield. Without a path to drying there is all but guaranteed to be rot damage and wet insulation in at least some part of the roof deck over time.
It would be much safer to install rigid foam above the Ice & Water, enough foam to provide dew point protection for the roof deck layer, leaving the interior side relatively vapor open (either a smart vapor retarder or standard latex paint on gypsum board as the vapor retarder.)
Per the IRC prescriptives in Chapter 8, in zone 5 it takes about 40% of the total-R on the exterior to be sufficiently protective with just, so with R23 rock wool in the cavities it would take an absolute minimum of 2.5" foil faced polyiso (R15+) above the roof deck to work, and at that minimum level a smart vapor retarder would be advisable under the ceiling gypsum on shaded or north facing pitches. At 3" of foam (R18+) latex paint alone would be fine.
In zone 4 it only takes 30% of the total-R on the exterior to be protective, so with R23 rock wool that would be ~R10 minimum, so 2" of foil faced polyiso would suffice. On the boundary of zone 4/5 it's likely that it will become solidly zone 4 within the lifecycle of the roof, and it would still be pretty safe (though a gray area of the code) to go with 2" of above-deck polyiso + smart vapor retarder on the underside everywhere.
It would also be fine to use 2" of inexpensive reclaimed roofing polyiso + interior side vapor retarder (like MemBrain or Intello Plus, definitely NOT polyethylene sheeting) on the interior.
The roof foam can be held in place with a layer 5/8" of sheets of OSB or plywood through screwed to the structural roof deck with pancake head timber screws, with a #30 felt underlayment (not ICE & Water) for the shingles. If the roof leaks in 25 years it may require replacing sheets of damaged nailer deck at or below the leak point when re-roofing, but the structural roof deck would still be intact. A layup of #30 felt + asphalt shingles runs about 0.1 perms- it doesn't offer a huge drying path for any leak water, but it isn't a severe moisture trap the way a second layer of Ice & Water Shield would be.
With only 2" of foam the roof edge can be finished with commercial 3" metal drip edge (comes in various colors), no facia board detailing required (unless you want to.)
Dana, you might have missed or the OP might have added that they will have 1" pvc vents; if the vents connect to code-compliant soffit and ridge vents they should not have a moisture accumulation problem at the exterior, and in zone 4/5 I believe the 1/2" polyiso would provide enough condensation resistance against inward vapor drive.
But it's still only an R-25 assembly at best, less than half the code-required R-49. Even if they aren't required to follow the code, it's a good baseline for a minimum level of comfort and energy use.
With a vented roof..
9x 2" vents feeding 1" pvc vented roof
+
6" of Tuff-R gets R39
or
2" of Tuff-R R13
3" of mineral wool R10
1" of Tuff-R taped on to rafters? R7 Gets R30 (maybe?)
or
3" of Tuff-R
3" of mineral wool - Does this need a poly?
Is there something other than sheetrock to close up the ceiling that might look similar to shiplap that's code compliant?
Make sure you have at least one vent per rafter bay, and you need that at the top AND BOTTOM of the vent channel (an in and an out). You can't/shouldn't rely on "sideways" airflow paths from rafter bays to vents in adjacent rafter bays, for example.
Standard practice is to put up drywall as an air barrier, than put shiplap/T+G over the drywall. You only need to tape and mud the drywall in this case, you don't need to do a full finishing job.
Bill