Can closed-cell spray foam be applied to the underside of the decking of a flat roof that has rigid insulation & EPDM above it?
I am from Winnipeg, Manitoba (Climate Zone 7a).
I’m planning to build a cottage with a low-slope roof (0.5:12).
I was planning the layers like this (top to bottom):
- EPDM (fully bonded)
- 1.5″ HD Isoboard
- 5.5″ rigid XPS insulation
- 3/4″ T&G plywood decking
- 12″ I-joists with 3″ closed-cell spray foam applied to underside of plywood
- drywall
I’m assuming that that the close-cell spray foam will be acceptable as a vapour retarder.
According to my designer, this would not be acceptable under s. 9.19.1 of the National Building Code which reads as follows:
Required Venting
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.
Because of this venting requirement, he says you can’t have any insulation under the roof decking if you plan to insulate above it. He says he cannot think of any example that fits within the opening words: “Except where it can be shown to be unnecessary”.
If he is right, my only options are to (i) insulate between the I-joists and provide venting above it, by adding purlins above the joists, or (ii) add more insulation between the roof deck and the EPDM.
Your comments and suggestions would be most welcome.
Jake
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Replies
Jake,
I'm not very familiar with the National Building Code of Canada. If you have any doubts about your designer's interpretation of the code, contact your local code official.
From a building science perspective (rather than a code perspective), there is no reason that you can't install insulation on the underside of roof sheathing as well as above the roof sheathing. There are a few rules, however: the rigid insulation that you install above the roof sheathing has to have a high enough R-value to keep the roof sheathing above the dew point during the winter; and the insulation that you install below the roof sheathing has to be in direct contact with the roof sheathing.
I would advise you to install vapor-permeable insulation on the underside of the roof sheathing -- fiberglass batts, mineral wool, or a blown-in insulation like cellulose or blown-in fiberglass. That way the roof sheathing can dry to the interior if it ever gets wet. (If you have your heart set on closed-cell spray foam, you can install it -- but it wouldn't be my first choice.)
Alternatively, you can just make the rigid foam above the roof sheathing thicker, to achieve any R-value you want.
Don't worry about a vapor barrier: your roof has several, including the EPDM roofing.
For more information on all of these issues, see Insulating Low-Slope Residential Roofs.
Jake, having looked at the section and appendix, I don't see much leeway beyond, as you say, trying for the "shown to be unnecessary " exemption - and like your designer I can't think of a good example you could use.
Would this help your cause? Lots of detailed info in the ruling. From Ontario:
Ruling No. 08-24-1198
http://www.mah.gov.on.ca/Asset7544.aspx?method=1
Issue: The construction in dispute involves whether the proposal to install an un-vented roof assembly, using a medium density, closed-cell spray polyurethane foam to insulate the roof, provides sufficiency of compliance with Sentence 9.19.1.1.(1) of Division B of the 2006 Building Code
Decision: