A new interim rule in the NatHERS Technical Note (version June 2019) will allow NatHERS assessors to substitute custom windows as follows:
Clause 8.3: If a custom window is specified on the design documentation but not available in the NatHERS custom window library, assessors must model;
This interim rule is one of a number of enhancements to improve modelling of windows in NatHERS software. Assessors can apply this interim rule when the window specified is not available in the NatHERS custom window library. It can also cover substitution at the building stage when a window installed is different (but within the rule) from the one used in the assessment.
The updated NatHERS Certificate will include a note about this interim rule. Any additional changes to guidance on modelling windows will be reflected in future updates to the NatHERS Technical Note as appropriate.
BDAV looks forward to working with the NatHERS Administrator and other stakeholders to see the implementation of a permanent solution to the issue surrounding the default and custom windows within NatHERS software.