Vented vs. Unvented Roof in a Cold Climate
I was considering leaving the roof as a “hot roof” – unventilated – just removing the drywall and sealing the soffit and adding more spray foam insulation, but I was told (not by the renovator):
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new shingles will need to be replaced in 5-7 years – especially since the roof is black (everywhere else the roof is black so this section needs to be black)
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metal roofs also need ventilation
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“unventilated roofs in Ontario are rare and require specific approval and an engineer’s drawings”
Is that all true?
Thick-enough spray foam should provide a vapor barrier, so the roof decking should be okay, right? The framing would not have the complete depth of spray foam protecting it and would also be a thermal bridge, but if a strip of vapor barrier sheet was installed on the framing and sealed with acoustic sealant to the sides of the framing, should that not make vapor / humidity a non-issue?
I would get a rubber membrane above the roof deck, so if any water got under the shingles / around the nails, the roof deck should not get any moisture from above either.
So why would an unventilated roof be a problem?
Ice damming? Mold? No, if I can get the R-value to the Code minimum (for cathedral ceilings in Ontario) to R-31, those should not be issues, right? Getting R-31 in the 5.25 inch cavity will be difficult, especially since the 2″ of spray foam he used was only R-5/inch. (He used a DIY kit, 2-component. It is on the borderline as qualifying as closed cell, but it is not approved for filling bays, only edges (e.g. around windows).) Anyway, …
… if I can get it to R-31, why would an unventilated roof be a problem?
[Edit: code is R-50 for general attics and R-60 for general attics over electrically-heated spaces. This converted attic will be electrically-heated, but the code for cathedral ceilings is R-31 whatever the heat source. (Ontario)]
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You'll need to fix that "vent channel". Putting a vent channel UNDER the insulation in an application like this cancels out the insulating value of the insulation, since outdoor air can come in on what should be the "conditioned" side. You are correct that baffles should have been placed against the sheathing first if this were to be a vented assembly.
Good closed cell spray foam is around R6/inch. Some are rated to around R7/inch, but I don't really trust those to hold over the long term, so I consider them "R6/inch" too. If you completely fill your 5.25" cavity with closed cecll spray foam, you can just barely make your R31 target, but it will need to be done carefully and it will need trimming which isn't fun to do with closed cell spray foam.
A properly done hot roof shouldn't be any worse for ice damming, and is probably safer for mold. I've never heard of non-vented roof assemblies needing any special approvals, but I'm not familiar with Ontario building codes. I do think R31 is pretty low though, are you SURE that's your code minimum here?
Bill
Here’s a Fine Homebuilding article by Martin Holladay explaining four ways to insulate an unvented roof with climate-specific R-value requirements: Insulating Unvented Roof Assemblies .
Martin's options include "Rigid foam also can be installed above the roof sheathing in tandem with air-permeable insulation between the rafters". Can rigid foam be above the sheathing and spray foam (open? closed?) between the rafters?
Thanks
In some snow load areas, R31 in an unvented design is not adequate for ice dam prevention. A vented design with the same R value creates less ice (because some of the heat is vented away).
If you convert vented to unvented, do not leave an air space in the assembly anywhere where it might reach condensing temp.
Joe L estimates a 10% reduction in shingle life with unvented.
> a “ventilation channel” BELOW the foam
This is generally considered "crazy, then the foam accomplishes nothing". But I disagree somewhat. It does reduce solar heat gain in the Summer. And it reduces ice dams.
Most DIY SPF kits are R6.2/inch unless open cell foam. You can test by pushing on it, closed cell foam is very solid.
There is no special requirement for unvented roofs, as long as you meet the code min of R31 (I know it sounds low by US standards) you are fine. Strap out your rafters with 2x2 to increase the depth a bit and have the spray foam contractor top up to the correct thickness. Normally 2" of cc SPF is enough for condensation control which would let you use batts for the rest of the insulation but I doubt the DIY install would be consistent enough.
As for your questions:
1. SPF unvented roofs are very common in Toronto and been done for a long time. Shingles are definitely not failing at 5-7 years.
2. Metal roofs don't need any venting. My own home has a roof section with metal roof over SPF+batts without any venting.
3. There is no additional approval required. Unvented roof with closed cell SPF are a very standard assembly. For a while the inspectors were requiring poly to be installed bellow the SPF but luckily that has now been dropped.
Akos, what do you think of using polyiso strips instead of 2x2s to gain that extra depth? That could potentially help, since it would cut down on thermal bridging of these relatively shallow rafters. If ice damming is a concern, that little extra whole-roof R value could help.
Bill
Saying, "metal roofs don't need venting" is a potentially dangerous statement. All roofs need venting, unless properly designed to be unvented. In some cases they may work, but in many cases they won't, resulting in moisture buildup, rot and mold. The metal roofing itself will be fine, but the framing and indoor air quality can suffer.
Building science experts in the US, and the IRC building codes for cold climates recommend/require a lot more than 2-3" of spray foam for condensation control. I don't know how Ontario compares to US climate zones, but it's probably like zone 6, where we need at least 50% of the R-value to be foam for a flash-and-fill assembly. Edit: I see you are only required to have R-31. Because the insulation ratio is a health and safety issue, not just a code approval issue, I always use the aged value of spray foam, not the labeled value. (And not their reported long-term R-value, which estimates performance at 5 years.) Closed-cell foam will eventually reach about R-5.6/in, the value of still air, once the blowing agents are displaced by air.
Good point Michael, in my head I was referring to a proper unvented roof assembly, which case neither shingle or metal roof require any additional venting. There is nothing magical about metal roofs that it can only be installed over a vented roof.
The ratio local building department uses is 1/3 cc SPF bellow or rigid above deck combined with 2/3 fluffy insulation. That works out to 2" of cc SPF followed plus and R22 batt. This just about fits into a 2x8 rafter, so it makes for a pretty compact roof. Toronto falls just on the cold edge of zone 5, the ratio is a bit less than what BSC folks recommend but seems to work well enough.
Added to post:
[Edit: Code is R-50 for general attics and R-60 for general attics over electrically-heated spaces. This converted attic will be electrically-heated, but the code for cathedral ceilings is R-31 whatever the heat source. (Ontario)]