Best Practice for Flat-Roof Venting
Hi All,
I’m building a home in the spring.
40×30′.
Due to height restrictions, two and a half stories, (and for simplicity’s sake) I’d like to go a single slope, 1:12, roof with 2″ mechanical seam metal roofing, with a high temp Ice and water shield membrane underneath it.
I’m in the British Columbia interior, with lots of snow, but this will be a non-sliding roof using some S5 snogards.
I don’t believe our codes allow for unvented roofs, unless proven by a Building Envelope Engineer that it isn’t needed (which sounds costly)
Link to BC building Code(http://free.bcpublications.ca/civix/document/id/public/bcbc2018/bcbc_2018dbp9s919#:~:text=1)%20Except%20as%20provided%20in,of%20the%20insulated%20ceiling%20area.)
Will spray foaming the underside of roof sheathing, and having a water/vapor barrier on top of the sheathing, create rot issues?
Or is it a non issue, as if the roof fails to the point sheathing is getting wet, and you need a new roof anyways?
What’s the current best practice for flat roof venting?
Thanks!
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Replies
There is a lot of information here including in the discussion bellow:
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
My $0.02, peel and stick over the roof deck, enough rigid foam for condensation control, underlayment, strap out with 1x4 and install standing seam over that. You might even want to bump that 1x4 up to a 2x4 on flat and have some edge venting to bring some cold air under the metal to keep the snow from melting.
The SPF approach does work just as well but more expensive. Provided the spray foam is installed under a dry deck, there is no issues with the double vapor barrier.
I finally bit the bullet and got the GBA subscription to read that article.
They don't go into much detail about spraying the underside of the sheathing.
Is there a minimum R-value you need to spray in with CC?
Or just the minimum 2" for it to be a vapour barrier, then add vapour permeable insulation above your drywall and you're good to go?
For information on whether or not flat roofs should be vented, see this article, Insulating Low-Slope Residential Roofs, by Martin Holladay.
The amount of SPF under your roof deck depends on the whole assembly R value and the climate you are in. See bellow for ratios:
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
In my area (zone 5) with R31 cathedral ceilings, this means that you need 40% (R12.5) as either SPF bellow or rigid insulation above. Realistic long term R value of SPF is around R5.8/inch, use that instead of the R7 most installers quote.
Keep in mind that this is a ratio, not absolute value, the more insulation you have in your assembly, the more SPF you'll need for condensation control.
No matter how you insulate, in snow country, you should still add some venting bellow your roofing. Snow is a good insulator and on a flat roof you'll have a lot of snow sitting. Without silly levels of roof insulation, the underside of a thick level of snow will get warm enough for it to melt and cause ice issues. Venting under the roofing ensures that the air there is closer to outdoor and prevents this.
https://www.buildingscience.com/documents/building-science-insights/bsi-097-de-icing-ice-dams