As the nation’s building ministers announce they will finally support the increase in energy efficiency in new buildings to a 7 Star NatHERS rating, it could be a good time to see what we understand – as an industry – about what exactly this means. 

The present situation with building star ratings is confusing and potentially misleading for consumers.

The NatHERS building star rating scheme is not linear. This means that a one-star rating improvement from 1 to 2 Stars achieves a much larger energy saving than an improvement from 5 to 6 Stars. 

For example, for Melbourne, a shift from 1 to 2 stars saves five times as much energy as a shift from 5 to 6 Star NatHERS: 175 MJ a square metre saving compared with a shift from 5 to 6 stars which saves 35 MJ a square metre. The graph shows this relativity is the case for all capital cities except Darwin, where the humid, tropical climate complicates things.

There is no doubt that there is enormous long-term value in improving new residential energy performance from the present 6 Stars (in most states and territories) to 8 Stars. This achieves significant improvements in building tightness, optimisation of ventilation, comfort, health and noise. But most existing homes rate just 1 to 3 Star. 

If we want to maximise community benefit through improving health, cutting energy costs and reducing peak heating and cooling demand to reduce costs and stress on energy supply infrastructure, we should focus much more on existing home upgrades, especially poor performing rental properties, not just new homes.

An important issue with the use of NatHERS 10 Star ratings is that consumers generally still think 5 Stars is very good. Melbourne university research has shown that home builders have misled consumers into thinking that the mandatory 6-Star building rating reflects outstanding energy efficiency. Yet this is well below best practice, with the equivalent 8 Star homes becoming increasingly common, and being mandated in some countries.

Builders should be required to clearly show that a 6 Star rating just meets the mandatory minimum. Maybe the NatHERS label could be redesigned to include markings to show the mandated star ratings for 5 Stars (2006), 6 Stars (2010) and 7 Stars (2022), and formal requirements should be introduced for where and how building ratings must be displayed. 

Governments must invest far more resources in consumer education, as the effectiveness of energy rating depends on well-informed consumer pressure.  

Yet another challenge is to achieve high thermal performance in both summer and winter in a changing, more extreme climate, for both new and existing homes. 

The 2019 National Construction Code followed the NSW BASIX sustainability rating scheme by introducing separate summer and winter thermal energy ratings in some climate zones in addition to the annual rating. But these don’t have a high profile in consumer information. And NatHERS may not adequately reflect the real world benefits of some summer options such as adjustable external shading and vegetation. 

We have a 1990s rating scheme that needs serious review to make it fit for the 2020s and beyond. The recent consultation and related developments are certainly worthwhile and important, but we need more.

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  1. Should be noted 6 to 7 stars reduces heating & cooling loads 300 to 900 kW PA. $90 – $200 PA saving.
    1kW PV in NSW will reduce consumption 1400kW PA.
    7 stars cost range $10k – $80k, majority of homes about $20k
    at $200 PA, 50 to 100 years to realise the saving, at what point will it be knocked down and rebuilt, less than 50 years?
    1 kW PV range $500 -$1k
    Which is the better to environment, which is better for budget, which did we pick?

    1. Brad, I think we need to pick both, and there’s value in increased comfort!
      Also, what solar panels do the heating job in the winter, both night time and cloudy day times?