Effective R Value

Effective R Value represents the effective thermal resistance of the assembly or the total thermal resistance of the assembly accounting for continuous insulation layers, framing members, cavity insulation, sheathing, interior and exterior finishes, air spaces, surface air films, and any other material within the assembly.  The effective R value is a more accurate representation of the actual thermal resistance of the wall when compared to the nominal R value.  This is because the effective R value accounts for the heat loss/flow resulting from all the materials within the assembly.  The effective R value is calculated  by adding the RSI values of all the materials in the assembly together while considering the actual framing configuration since framing material has a lower thermal resistance than insulation and contributes to thermal bridging. 

References

National Research Council Canada. (2015). National Building Code of Canada 2015 s. A-9.36.2.1.(1)