Nominal R Value

Nominal R Value represents the sum of all the thermal resistances associated with the insulation in an assembly.  This value does not consider any heat loss from thermal bridging or conductivity of the material in the wall assembly.  This value can be seen as a face value estimate for wall thermal performance.  It is important to note that the actual thermal resistance of a wall  assembly may differ significantly  from the nominal R value that is calculated due to the specific characteristics of the assembly including conductive materials that act as thermal bridges, air spaces and surface air films.

References

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