Insulating a cathedral ceiling with self-adhesive roofing membrane installed
We are building a 640 sq ft cabin in CZ6. At the last minute we found a metal roof rated for down to a 1:12 pitch and made the change to a 2:12 pitch.
With that change to a flatter pitch came a recommendation from the roofing supplier to cover the roof sheathing with a self adhesive “Ice and Water Shield” type product.
So our roof (all complete) from outside to in:
24 gauge metal roof.
Self adhesive membrane. This is what I understand to be a re-branded product distributed by this tri-state steel roofing company. Similar to Ice and Water Shield. I asked for a perm rating on the product but haven’t heard back yet. Entire roof deck is covered with it.
OSB roof sheathing.
12″ tall I-joists on 16″ centers.
Roof is single sloping (aka shed style).
Currently there is no blocking between the I-joists and they are open into the soffits on both eaves. Soffit material is 3/4″ tongue and groove cedar.
This is a seasonal cabin and I do not have to follow any code for R value, vapor barriers, etc.
Cabin will be mostly un-conditioned in winter except for occasional weekends. One 8×12 area of the cabin will be heated to ~45 degrees in winter when we know we’ll be using the cabin on consecutive weekends. This area is where bathroom and all of our plumbing and appliances will be located.
Plan is for a drywall ceiling.
Spray foam is in the budget but I am concerned about spraying closed cell directly on the underside of the roof sheathing and not having a drying path. Original plan was to do flash and batt with closed cell foam but the I overlooked how the self adhesive roof membrane would prevent drying to the exterior.
My questions are:
What are my “safe” options for insulating this roof?
Should I leave an air space below the roof sheathing so there is a drying path for the sheathing? I’ve thought about taking advantage of the upper flange of the I-joists and putting in rigid foam (on the bottom of that upper flange) to create an air space.
Where to put a vapor retarder, if any?
Does the cabin being mostly unconditioned and un-used in winter affect anything?
Hope that’s enough background information. Thanks in advance.
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Replies
Chris,
If you are worried about have a drying path for your roof sheathing, you should proceed with the time-honored method of insulating this type of roof assembly -- the way people have been doing it for 50 years. Include a vent channel under the roof sheathing, making sure that the vent channel is connect to soffit vents and ridge vents. (You can, indeed, make site-built ventilation baffles out of rigid foam or plywood. The existing rafter flanges can be depth guides for 1.5-inch-deep ventilation channels.)
Underneath the ventilation baffles, you want to install R-49 of fluffy insulation if you can. You'll have only about 10" or 10.5" of room, however, which will only give you about R-37 or so of R-value. You can beef up the R-value by scabbing more framing members below your rafters, or by installing a continuous layer of interior rigid foam.
All of this is explained in my article, How to Build an Insulated Cathedral Ceiling.
You might also want to read this article: Site-Built Ventilation Baffles for Roofs.
Martin,
Thank you for your response and for editing the spelling error in my thread title (I'll assume that was you).
While I am concerned about a drying path, I'm also willing to be convinced I shouldn't be.
Doing a site built baffle won't be an issue but adding ventilation to an already installed cedar soffit might be, especially since the I-joists are on 16" centers. And there won't be a ridge vent since this is a single sloping roof.
I had one contractor suggest that the air space might be enough for drying and to not worry about air flow, in part because the space will primarily be un-conditioned in the winter.
Any thoughts on this?
Chris,
Lots of people in your situation install closed-cell spray foam on the underside of the roof sheathing. This approach works, as long as the roof sheathing is dry on the day that the spray foam is installed. You can do it, too, if you want -- but it is likely to cost more than the approach I suggested.
Any carpenter can help you install soffit vents. You can remove one or more boards to install an aluminum or vinyl venting strip, for example, or you can insert a circular vent in each rafter bay. It's also easy (and routine) to install a ridge vent for a shed roof like yours.
In zone 6 at least half the total R needs to be closed cell on the underside of the roof deck for dew point control if only latex paint is used as the interior vapor retarder, but there are safe ways to deal with it at somewhat lesser ratios. The prescriptive R is R25 foam (~4" at ~$4 per square foot, for R49 total, but if you meet code on a U-factor basis it doesn't have to be that much, since the thermal briding of the I-joist rafters is 1/3 that of milled lumber. On a U-factor basis the "whole assembly R" only needs to be R38.
In your case you could install 3" of closed cell foam (~R20), which for many products would have a vapor permeance of about 0.4 perms, which is the middle range of a class-II vapor retardency and is a very reasonable drying path. and ~R20 fiber below that (5.5" of damp sprayed cellulose- batts don't fit I-joists), and still meet code on a U-factor basis. If you're concerned that it's too close to the line on dew point control at the foam/fiber boundary, install 2 mil nylon (Certainteed MemBrain) as a smart vapor retarder.
That still leaves a 4.5" channel. If you want to fill that you could go with 4" of closed cell foam (~R26 and ~0.3 perms) filling the remaining 8" with cellulose, ~R30), and install the 2 mil poly. Cellulose can buffer quite a bit of moisture without damage, and even though the R-ratio is only 46%, between the buffering capacity and the class-II vapor retardency of the nylon when the cavity air is dry (which it will be, when the foam is cold enough to condense) the cellulose will never reach moisture levels that would support mold or lose performance. You could use rock wool or fiberglass too, you'd be leaning more heavily on the vapor retarder if you did, but it's still pretty safe. If the cavity ever reached mold-growth levels of humidity, the nylon becomes vapor open, and it can dry toward the interior.
The roof deck is completely protected by 2" or more of closed cell foam, and the fiber insulation is protected by the nylon membrane.
See also:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
I'd go with Martin's suggested approach. Install vents and fluffy insulation. But I wouldn't worry about meeting R-49. Such a tiny house will be easy to heat at anything close to R35 ish. Do a good job air sealing and insulating and maybe spend the money you'd spend on foam on nicer windows.
It's almost impossible to properly vent a 2:12 pitch roof in snow country. I'm normally not a big fan of closed cell polyurethane foam due to it's heavier environmental footprint (which would improve if all vendors switch to HFO1234_ _ blowing agents), but in this instance it's the better choice.
If the cabin sees only intermittent use in winter, 2" of closed cell foam (or even 1") and a lower overall R would be fine, and a lower foam-R ratio would also be fine, since it won't have interior moisture sources boosting the humidity to high levels. If you cheat the R-ratio, a smart vapor retarder on the interior is cheap insurance. But an all-fiber insulation solution is risky, even with with a 1" vent gap under a 2:12 roof. Read the whole document in that last link (it's short). The roof deck will be fine with even 1" and no vapor retarder unless the interior humidity levels are pretty high. But smart vapor retarder would improve that immensely.
If you're really going with a vented solution, give it fully 4" of vent space with BIG cross sectional vent openings. That will only leave you 8" for fluff (R30-ish), but you can still do pretty well if you cap-nail 2" of rigid polyiso (R12-R13) to the underside of the I-joists, tape the seams, and long-screw the gypsum through the polyiso.