Custom windows substitution

After numerous discussions with the NatHERS Administrator over some time, the BDAV welcomes the announcement of a temporary solution authorised by the NatHERS Administrator in regard the substitution of custom windows.

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;

  • a default window with the specified opening type; or
  • an available custom window that meets all of the following parameters:
    • identical opening type (e.g. fixed, awning, casement, sliding.) to the window specified on the documentation and;
    • a total window system U-value equal to, or greater than, the window specified on the documentation (e.g if the U-value of the specified custom window is 3.0, the modelling window selected could be 3.5);
    • a total window system Solar Heat Gain Coefficient (SHGC) +/- 5% of the window specified on the documentation.

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.