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Old 10-11-2017, 04:04 PM   #1
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Ontario Building Code min R-value for Vented Cathedral Ceiling


I want to condition my attic and knee wall in my 1-1/2 story home, to create more space for storage and new HVAC.

I'm having a hard time figuring out the requisite minimum r-value for the rafter cavities.

It looks like the Ontario code was updated post-Dec 31/16.

http://insulation.owenscorning.ca/pr..._Jan_01_17.pdf

It requires an r-value of 31 for "Ceiling Without Attic Space," and an r-value of 60 for "Ceiling with Attic Space."

My plan for 8" rafters is to do:
  • 2" site-built baffles from vented soffits to ridge vent.
  • 5 1/2" of mineral wool insulation (r-value of 22)
  • 2" of rigid polyiso foam (r-value 10), which will also act as vapour barrier.

Will then add strapping and sheetrock on top of the rigid foam. This should give me a nominal r-value of 32.

But - because this will be vented - does it count as "ceiling with attic space" (with the attic space being the 2" baffle), or can I get away with the r-31 minimum?

Before you ask, I know that closed-cell foam will give me more options, but I'm trying to be cost-conscious.

Thanks all!
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Old 10-11-2017, 04:15 PM   #2
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Re: Ontario Building Code min R-value for Vented Cathedral Ceiling


It sure sounds like ceiling with out attic space.
The venting alone would not be called an attic.
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