Navigating unvented roof assemblies and building code
Hi everyone,
I’m looking to build a home in Chilliwack British Columbia Canada, not far from Vancouver. We’re lousy with rain and moisture and research documented here and elsewhere has shown problems with mold in unvented roof assemblies in our region that are not due to air moisture exfiltration from the inside, but the vent itself. All of which to say, there’s a reason to consider unvented roof assemblies in my area.
However, the BC Building Code is unclear on how to do this. Guidance looks like this (Article 9.19.1.1):
“Except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the transfer of moisture from the space to the exterior.”
A note adds:
”Controlling the flow of moisture by air leakage and vapour diffusion into attic or roof spaces is necessary to limit moisture-induced deterioration. Given that imperfections normally exist in the vapour barriers and air barrier systems, recent research indicates that venting of attic or roof spaces is generally still required. The exception provided in Article 9.19.1.1. recognizes that some specialized ceiling-roof assemblies, such as those used in some factory-built buildings, have, over time, demonstrated that their construction is sufficiently tight to prevent excessive moisture accumulation. In these cases, ventilation would not be required.”
Putting it together, an unvented conditioned attic/cathedral ceiling makes sense and I like Assembly #3 Martin has suggested here: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
I’m not sure if this will pass inspection in my area or not and I’m not sure what my alternatives would be.
Any ideas would be greatly appreciated.
Thanks,
Sean
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Replies
Sean,
I haven't tried to stick-handle an un-vented roof through an approval process, but this is my take on it:
As it does with similar prescriptive requirements (I'm thinking of things like the location of the interior vapour-barrier in walls) the BC code leaves the door open to alternatives if they can be demonstrated to meet the same results (in this case preventing moisture damage) by providing documentation that the assembly you are proposing works. There have been several successful appeals, so you can see what documentation worked: https://www2.gov.bc.ca/gov/content/industry/construction-industry/building-codes-standards/building-code-appeal-board/building-code-appeal-board-decisions/bcab-1762
That said, there is no guarantee that you will be similarly successful. What could be put against you is that although there is evidence in the research by RDH that code compliant vented roofs can experience mold problems in coastal BC, in the absence of other risk factors that damage is almost always cosmetic, and well executed vented roofs without other exacerbating problems perform just fine.
Hi Malcolm,
This is all helpful, thanks. Given my desire to avoid both closed cell spray foam and the building code appeal board I might just have to live with some mold exterior to the air barrier.
Sean
Sean,
I've seen a small amount of what I can only call discolouration on the underside of the sheathing in properly ventilated roofs here, but nothing more. On all the ones I've remediated that had moisture damage there was something more in play. Typically excess air leakage from the house below, or less frequently an inadequate vent path. The houses also almost uniformly had very high levels of interior humidity.
Good luck with your build!
I think our codes are similar, 9.19.1.1 only applies if all your insulation is bellow the roof deck.
With exterior rigid, the condensation concern is eliminated as long as you deal with condensation control. This is where 9.25.5.2. Position of Low Permeance Materials comes in. You need to match the exterior rigid ratio in the table there.
I had no problem with getting approval for an unvented roof with exterior rigid in Toronto.
Akos,
True - roofs with exterior foam, whether sloped or flat, are easy to make code compliant. I see them as a completely different type of roof, and didn't think to mention them based on the example Sean posted. Exterior foam does introduce a lot more complexity and cost compared to a conventionally built cathedral ceiling insulated with batts or cellulose.
Brilliant. That’s an excellent observation, thanks. It’s nice to know I can present a few options to the architect and builder as we try and sort out HVAC location and other challenges.