GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulating using the 1/3 2/3 rule in zone 7

415irwin | Posted in Energy Efficiency and Durability on

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” ( For our region we would need to add R-15.

The proposed approach in our building is to insulate the 2×6 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.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    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.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |