One of the most common misconceptions I come across is the idea that all windows in energy compliance must fall within 5% of the SHGC and U-values listed in the energy report. This simply isn’t true.
That rule belongs to NatHERS simulations only.
When a home is modelled in NatHERS, the software assigns U-values and SHGC values to each window. To make sure the actual house matches the modelled performance, NatHERS requires that the installed windows be within 5% of the values used. If not, the report must be amended.
But under the Deemed-to-Satisfy (DTS) elemental provisions in the NCC, glazing is assessed differently. Using the ABCB glazing calculator, compliance is achieved when the total glazing allowance is 100% or less. There is no “5% tolerance” rule. Windows can perform better than the DTS requirements, and that’s fine. If they perform worse, the calculation must be re-run, but there’s no arbitrary 5% band to worry about.
Why this matters
Builders, designers, window manufaturers, and building surveyors sometimes insist on a report update when window specifications change, thinking the 5% rule applies across the board. In fact:
- NatHERS pathway → 5% tolerance applies
- DTS elemental pathway → compliance is based on ≤100% allowance, no 5% rule
So next time you hear someone say “but the SHGC is outside 5% of the report”, first check: Which compliance pathway was used? The answer determines whether that rule applies at all.