How do I model default double clear skylights in FirstRate5 for BASIX compliance?

  • BASIX,
  • Skylight
  • NSW
Last Updated: February 20, 2020


We have a NSW project that has been assessed for BASIX at DA stage and has been modelled with the FirstRate5 default double clear skylights (U=4.22 & SHGC=0.72). We are now reviewing the design for CC and the client would like to use the Velux FCM product (U=2.9; SHGC=0.28). There are no other changes in the design between DA and CC that would warrant re-modelling so we are trying to determine if re-modelling would be required due to this skylight product change.

The BASIX Thermal Comfort Protocol (refer Section 6, Table 3) states the following for skylights, in particular clause (d), which would imply that re-modelling would not be required due to a change in the skylight product, as long as an approved skylight was incorporated into the model.



NatHERS protocol provides the understanding that window substitutions are acceptable if U-value is equal or better and SHGC is within plus or minus 5%.

Without seeing the detailed design it is not possible to provide a categorical response.

It might be effective to run the calculation and verify the effect. Making the change could potentially have 0.1 star change, and will change heating and cooling loads, which might impact on BASIX (or the new ABCB) caps.