Insulating using the 1/3 2/3 rule in zone 7
I would like to get some opinions on the 1/3 2/3 rule for applying insulation in walls in Quebec, Canada. Could one get by applying the rules for insulating the sheathing noted in the GBA article "Calculating the minimum thickness of rigid foam sheathing" (http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minim...)? For our region we would need to add R-15.
The proposed approach in our building is to insulate the 2x6 wall cavity with cellulose or roxul and insulating over our sheathing using cellulose or roxul filled larsen trusses. One other thought is to apply roxul ComfortBoard IS to the outside.
Asked by jessie pratt
Posted Tue, 02/19/2013 - 10:34
Posted Tue, 02/19/2013 - 10:34
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Jessie,
In most of the populated areas of Quebec, you will be quite safe using guidelines established for Climate Zones 7 and 8. In fact, Montreal, Sherbrooke, and the Eastern townships are probably all in Climate Zone 6.
However, the guidelines published in my blog, Calculating the Minimum Thickness of Rigid Foam Sheathing, apply to rigid foam sheathing, which has a very low vaor permeance. If you plan to use Roxul mineral wool rather than rigid foam on the exterior side of your wall sheathing, you have much more design flexibility. Mineral wool is vapor-permeable, so you can install as little or as much of it as you like. The one third/two thirds rule does not apply; nor do the guidelines in the table in my blog.
Posted Tue, 02/19/2013 - 13:06