Outboard to Inboard Ratio

Materials that are installed on the outside of the building assembly which are of a low vapour and air permeance must be checked to ensure they do not significantly contribute to cavity moisture problems.  The ratio of R-values between all materials outboard of the plane of low air and vapour resistance compared to the R-value of all the materials inboard of that plane must be checked.  This only applies when a material installed on the outside of the building assembly has a low vapour permeance (less than 60 ng/Pa^.s^.m²) and is considered an air barrier material (less than 0.2 L/(s^.m²).  The acceptable minimum ratio is dependent on the heating degree days for the location of the construction.  See the National Building Code article 9.25.5.2 for additional details.  The Effective R Tool represents the outboard to inboard compliance with pass or fail icons for that assembly, as shown below.  For an example of how an assembly that requires an outboard to inboard ratio, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 14985.    

References

National Research Council Canada. (2015). National Building Code of Canada 2015 s. 9.25.5.2.(1)

National Research Council Canada. (2015). National Building Code of Canada 2015 s. Table 9.25.5.2 Ratio of Outboard to Inboard Thermal Resistance