The Performance Requirements for purlins and rafters differ between residential buildings and non-residential buildings.
For residential dwellings, NCC requires adding R0.2 thermal break to be installed between the metal sheet roofing and its supporting metal purlins, metal rafters or metal battens. This is the case even if you are following the VURB Method instead of DTS method for energy compliance [1]
For non-residential building, you need to calculate the effective Total R-value that includes the negative impact of thermal bridging caused by metal purlins and rafters. The effective Total R-value must be not less than the required Total R-value.