Vapor barrier yes/no?
I am looking to insulate a new, unvented,sealed, cathedral ceiling assembly on 2 x 10 joists. I plan on shooting 3 inches of closed cell urethane foam (r18 to r20 value) directly under the plywood or aspenite roof deck, after which, I will fill the balance of the rafters with 6 inches of Roxul batt (r22 value). I will also apply ice and water shield atop the plywood roof deck, then strap it, then lay down my metal roofing. My question is, do I need to apply or should I need to apply, a vapor barrier underneath the batt before installing my ceiling gypse? What type of vapor barrier, if need should go between the bottom of the Roxul and the gypse?
Thanks again
Jean
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What is your location/climate zone? See:
https://www.greenbuildingadvisor.com/sites/default/files/Climate%20Zone%20map%20including%20Canada.jpg
Does Roxul even manufacture batts that are a full 6"? (Most are designed for 5.5").
With 9.25" of rafter depth and 3" of closed cell foam you would have 6.25" left for fiber, and it has to be a snug fit to both the foam and the ceiling gypsum to be effective.
In US climate zone 5 or lower as long as at least 40% of the total R is the closed cell foam you don't need an interior side vapor retarder, so even assuming R4/inch fiber at 6.25" you'd be fine. The fiber would be R25, the foam R18, for at total of R43, and a ratio of R18/R43= 42%.
In zone 6 it needs to be at least 50% foam, in zone 7 at least 60% foam to skip the vapor retarder. The roof deck would be fine, but the fiber could accumulate moisture over the winter. Rather than a true vapor barrier, it's safer & better to use a "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) which becomes vapor open should the air in the rafter bays reach mold-growth levels. It's 2-3x the cost of 6 mil polyethylene, but still really cheap compared to another inch of closed cell foam.
Jean,
In your previous post, you said that you live in Gatineau, Quebec, near Ottawa. That puts you in Climate Zone 6.
Is this the same roof you began to write about in your previous post -- the low-slope roof?
Your planned roof insulation strategy is safe, but your total R-value (R-43) isn't particularly high. More R-value would be better.
This type of roof assembly needs an interior air barrier (for example, well-detailed gypusm drywall), but not a vapor barrier. In Canada, many building inspectors don't understand the building science behind this type of roof assembly -- one that is designed to dry to the interior -- and they still insist on interior polyethylene. If you have that kind of building inspector, you may need to install interior MemBrain (a "smart" vapor retarder). The MemBrain is unnecessary but harmless (unlike polyethylene, which traps moisture), and the MemBrain usually satisfies Canadian building inspectors.