Cathedral ceiling insulation and synthetic roofing underlayment
I have a house under construction. The great room has a cathedral ceiling with 2 x 12 rafters. there is a ridge vent for most bays. One or two bays die into a chimney chase. There are soffit vents where possible, but several bays have none since there is an overlay roof (name ?) perpendicular to the cathedral with the associated valley. Those bays could have the sheathing opened at the bottom to allow some air from a conventional attic space to move into the bays (not ideal if worth anything?).
I was planning to leave a vent space in all the bays, load them with FG (dense stuff at R-38) leaving a pretty thin vent channel (1 inch or so), and then add 1 1/2 inches of ridge foam to the underside of the rafters. I am concerned that the vent channels will be thin and concerned that may bays without soffit vents might be very poorly vented. THOUGHTS?
Second thought is to go with spray foam directly on the underside of the roof OSB? EXPENSIVE and the kicker is, I have synthetic roofing underlayment which the manufactured state should not be used on unvented assemblies. This is more or less a DIY build so the OSB has been in place for about 18 months and the shingles on with no insulation. So I am wondering if the concern with synthetic underlyments and unvented assemblies is to trap moisture in wet OSB in the assembly. Since my assembly is old and dry as it will ever be would there still be a concern with spray foam and synthetic underlayments?
Recommendations for insulating this difficult assembly?
thank you.
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Replies
Bob,
In general, a roof with valleys and chimneys is not a good candidate for a vented cathedral ceiling, as I explained in my article, How to Build an Insulated Cathedral Ceiling.
If you've got valleys, you need to create an unvented roof assembly, and that means (at this point) that you need to install spray foam insulation.
Of course, it would have been better if you hadn't used synthetic roof underlayment. But frankly, I think that you can ignore the manufacturer's recommendations in this case, because your roof sheathing isn't going to dry outward in any case. (The asphalt shingles prevent outward drying.)
My advice to GBA readers: the time to make insulation decisions is at the design stage, not halfway through construction.
If you had considered your options earlier, Bob, you might have been able to install one or more layers of rigid foam above your roof sheathing, which would have been the best approach. But since you have already installed your roof shingles, it's too late for that.
You don't say where you're located, and that may affect some of the details, but...
The LAST thing you would ever want to do is to allow attic air to convect to the roof deck, where it would give up it's moisture to the cold roof deck in winter. A vented roof deck needs to be ventilated with OUTDOOR air to remove moisture, attic air would add moisture in winter.
In most US climate zones 1" of closed cell polyurethane foam is sufficient protection for the roof deck at about a buck a square foot, with the rest being blown fiberglass. An inch of closed cell polyurethane is on the boundary of class-II and class-III vapor retardency, and forms a non-wicking condensing surface. At ~1 perm foam the roof deck can dry seasonally, but won't allow rapid uptake of moisture into the OSB. Download a copy of BA-1001, and fast-forward to the summary tables near the end, and the column titled " 1" ccSPF + spray fiberglass" :
http://buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems/view
The green shading indicates a fairly safe moisture level for the roof deck, so with dark asphalt shingles (not "cool roof" shingles) you're pretty much good to go anywhere, as far as the roof deck integrity is concerned.
It doesn't address potential moisture build up in the fiberglass layer, but that too can be mitigated.
You can keep the fiberglass dry enough by installing a membrane type smart vapor retarder (eg Intello Plus, Certainteed MemBrain) on the interior side, between the fiberglass. When the interior air and cavity air is dry (as it will be in winter), the vapor retardency of the smart vapor retarder will be under 1 perm, but when the RH goes high (if too much moisture has been accumulated), it becomes more vapor-open than the latex ceiling paint, and won't impede drying.