Posted By edith on 09 Oct 2014 09:40 PM
Dana,
I am getting from your advice that in a slightly warmer climate, such as Ottawa, there is no need for a vapor barrier if you have sufficient EPS on the exterior to keep the sheathing warm. We are planning on 4" and already have 8" of dense pack cellulose in our stud bays. Simple tyvek though. Sound safe? And painted gyprock should do the trick as a vapor retarder?
It's possible to work from the 25 year weather history to come up with an exterior-R that would be protective in all but the high-arctic/antarctic locations.
With 4" of EPS (~R16.8) and 8" of dense packed cellulose (~R29.5) you're looking at about R46.3 total, with about 36% of the R exterior to the sheathing. The January mean temperature in Ottawa is about -10C, with
total heating degree-days and mid-winter temps comparable to the warm edge of US climate zone 7 or the cool edge of climate zone 6. In a colder than average year you might have about 4900C HDD (8820F HDD), which would make it more comparable the cold edge of US zone 6 (still less than 9000F HDD). The average heating season is more like 4600C HDD/ 8200F HDD, which is still would be the colder half of a zone 6 climate.
If you refer to the
IRC prescriptives for going with exterior foam and no interior side vapor barrier those climates you'll see that in US climate zone 6 you need about 33-35% of the R to be on the exterior of the sheathing to work, and in zone 7 it's about 40-42%. For Ottawa you probably don't need 40% of the total on the exterior, but you surely need more than 30%. At 36% you're in the right range, and adding another half-inch to inch of EPS would give you margin.
In Canada the national building code doesn't absolutely require interior vapor retarders (news to some people).
This document seems to imply that 20% would be OK for Ottawa, (per Table 9.25.1.2 in the NBC 1995), though I'm skeptical that would be sufficient during a cooler than average season. Some inspectors may not sign off on it unless you put the printed page in front of them. The
National Building Code website has been hacked, and the code isn't currently searchable, but Table 9.25.1.2 may have been updated since 1995 with more conservative ratios.