Both Ché Wall from Flux and Maria Atkinson from Atkinson Consultancy outlined some aggressive challenges to the building codes and rating tools. Their texts and responses from CSIRO on NatHERS and from the Green Building Council of Australia are extensive so are reproduced separately here for ease of presentation.
According to Ché Wall in terms of residential regulations “the only comprehensive assessment of actual building performance since NatHERS was regulated in 2003” showed that higher NatHERS ratings were associated with a “12 per cent increase in annual energy-related costs and emissions in the Brisbane climate”.
“Another study shows that higher NatHERS-rated homes in Sydney are more prone to overheating than those from the 1950s.
“The physics is simple: More insulation helps heating but can trap heat and increase cooling. We regulated a rating system that does not adequately address this tension, and increases maximum demand by over 50 per cent straining the electricity grids that hit peak demand in summer, which include most of Sydney’s residential areas.
“Thermal envelope regulations clearly fail their core objective.”
He said air tightnesswas another problem in the NCC with a 2024 CSIRO study finding that “15.6 per cent of houses and 39 per cent of apartments built since 2020 were already over-sealed” and that despite this, the same report recommended even stricter air-tightness controls across Australia and mandatory mechanical ventilation in every new home.
“A huge regulatory impost is recommended without any cost-benefit analysis or consideration of wider impacts.”
In 2024 a peer-reviewed study showed that high levels of airtightness raise lifecycle costs by 24 per cent in Australia, “while only moderate levels of air tightness are cost-effective, even in Melbourne”.
He also cited research by the University of Sydney that found changes in the 2019 NCC “significantly contributed to the increase in indoor mould cases in newly built homes.”
“So, we’ve over-regulated airtightness and thermal insulation, while natural ventilation has been systematically undermined in the NCC.”
There was therefore risk to public health, reduction of passive resilience, and higher costs, he said.
The trajectory of our energy efficiency regulations was ignoring the evidence, he said.
In housing sector modelling his company undertook to inform the Australian Sustainable Finance Taxonomy “found a 10 per cent increase in NCC energy efficiency levels in all new dwellings built from 2024 would deliver a cumulative benefit of less then (a negligible) 0.04 per cent sectoral emissions reductions in 2050.”
This was still a “highly optimistic scenario given the poor real-world results observed. We are focusing effort and capital on the wrong issues.”
Wall said, the grid was rapidly decarbonising so investing more energy efficiency in new construction was “increasingly irrelevant”.
“The real climate-mitigation levers are electrification, on-site solar,
refrigerants, and supply-chain emissions, and of course, repositioning our existing stock towards the same.”
Green Star, he said, showed a similar pattern of misalignment in using carbon offsets as primary mitigation response which he said contradicted IPCC and “every other credible mitigation framework”.
There was more focus on process than outcome, “with an increasing emphasis on narrative rather than measurable outcomes”.
PVC was called “responsible” and rewarded with credits despite the weight of peer-reviewed evidence against it.
“Most choices appear to favour industry convenience and maximum participation at the expense of sustainability goals.”
Wall also said NABERS algorithms were developed using data from only 226 buildings in 1996.
“Gaps were filled with judgment calls based on 1990s assumptions. It was effective when it launched, but we now have a substantial resource of mandatory disclosure data through the CBD scheme that shows these assumptions are outdated.
“Why haven’t we taken the opportunity to use this better data to improve the tool? Greenhouse gas accounting in NABERS is also very problematic.”
Wall also took aim at “double counting” of on-site renewables that was allowed, despite being a fundamental no-go in greenhouse gas accounting.
“We need modern benchmarking and transparency in energy and emissions reporting for the audiences that matter today: banks, institutional investors and Treasury, not the techniques developed for a fledgling consumer market in the late 1990s.”
(NABERS has been asked for comment)
Wall also said NatHERS was “not fit for purpose” in much of Australia and together with other tools, prevented better standards from emerging.
He disagreed with the use of star ratings as these might be useful for consumers but not for the targeted interventions needed to accelerate decarbonisation.
Maria Atkinson cited her deep involvement in the green building section including co-founding the GBCA, sitting on the board of the first World GBC and helping to establish ASBEC and board member of ABCB.
She pointed to the 2023 COP agreement that called for near-zero emission and climate-resilient buildings to be the “new normal by 2030”, just four years from now.
She called for innovation as a solution and a rejction of tools that were not doing the job they needed to do.
“Dr Nirmal Kishnani said a decade ago: ‘Green and sustainable are not the same thing’.”
Green was a brand for checklists and certificates while sustainability was highly localised and responsive to ecology and climate.
She said that “after 25 years and 1.5 million NatHERS ratings, researchers have found there is no statistically significant correlation between star ratings and actual energy consumption. NatHERS has not demonstrated it assesses energy efficiency features that reduce or avoid greenhouse gas emissions.”
“After 15 years and billions of dollars benchmarked through GRESB, we still cannot demonstrate that high scores correlate with buildings that are genuinely low-emission, because greenhouse gas emissions receive just 6 per cent of the GRESB weighting.”
“Green Star gives carbon offsets equal status with genuine emissions abatement and calls it Climate Positive.”
In addition, “6 Star – world best practice is awarded for using refrigerants with high global warming potential and forever chemicals.”
“And we are not understanding thermal safety.
“NatHERS weather files were updated, but only to 2015. Almost all of Australia’s most extreme heat events has occurred since then. We are rating buildings against a climate that no longer exists.”
The NCC was increasing airtightness. “But in a sealed, high-star-rated home in central NSW during a 45-degree heatwave, with no overnight recovery, means the airtight envelope traps the heat.”
“Health agencies say thermal safety is 24 to 28 degrees. Our code-compliant home by Day 3 has indoor temperatures exceeding outdoor temperatures.
“We built the standard now putting people’s lives at risk.”
Atkinson said this challenged the logic that NatHERS should be extended to existing homes at point of sale or lease, as it was not a “reliable consumer rating.”
In additional background notes supplied after the debate, Atkinson also said that traditional construction outperformed new builds in heat. “CSIRO research and multiple Australian studies show 6, 7, and 8-star homes experience more overheating than traditional 2-6 star homes. Traditional double brick construction has outperformed many high-star homes during summer periods.”
Atkinson said she was inviting the industry:
- To commit that innovation becomes code in less than three years
- That Green Star retires when the NCC mandates net zero operational and thermal resilient buildings
- That our planning regulations and NCC need to keep pace with what the science demands, pushing proven solutions into mandatory practice
- That we measure success by regulatory change achieved rather than certifications issued.
“The tools we built were built with the best intentions, but now even the climate has moved past them.
“Green Star, NatHERS, GRESB and the billion dollar real estate industry is built on ratings that don’t measure what we think they measure nor what we need them to rate.”
Several of the issues raised by Atkinson, Wall and during the QandA have been addressed below first. by Anthony Wright, research lead, building CSIRO energy simulation, assessment and communication, who has also referenced a detailed paper he has authored on NatHERS published here and separately, below, by GBCA
NatHERS weather files were updated, but only to 2015. Almost all of Australia’s most extreme heat events has occurred since then. We are rating buildings against a climate that no longer exists.
Most all building rating tools/schemes internationally use Typical Meteorological Year (TMY) or Reference Meteorological Year (RMY) weather files. NatHERS uses RMY files. These files are built by taking a 25 year period of real weather data (right now in NatHERS 1990-2015) and using a standard, recognised method, selecting the most typical January, February, March etcetera and combining these months into a 12 month weather file. This approach has a flattening effect because the most typical of each month, by definition, isn’t the most extreme.
For similar reasons, simply updating the period to (for example) 2000-2025 is unlikely to have a very great effect on the average temperatures in the files. Those who object to the NatHERS weather files usually want them to reflect extreme weather events or future climate projections. As far as I know, nowhere in the world has adopted this approach in minimum standards yet.
That said, NatHERS Australia is probably leading in this space in several respects. CSIRO developed projected climate files that can be used in NatHERS software in non-regulatory mode to give designers a sense of how their buildings might work under future climate change scenarios.
Further, the BOM (Bureau of Meteorology), CSIRO and the NESP (National Environmental Science Program) are working on extreme climate files. A prototype set covering six climate zones is being tested by industry at present. These will be made available to modellers in non-regulatory mode too.
- The NCC (National Construction Code) is increasing airtightness. But a sealed, high-star-rated home in central NSW during a 45-degree heatwave, with no overnight recovery, means the airtight envelope traps the heat.
My understanding is that the NCC air tightness is a voluntary requirement of 10ACH50 (10 air changes per hour at a pressure difference of 50 Pascals, a measure of air tightness).
Homes built below 5 ACH50 need mechanical ventilation. NatHERS (in one pathway through the code) uses a dynamic airflow approach which models the air movement through gaps and cracks in the dwelling individually, based on the internal/external pressure differential, so every room has a constantly changing airflow.
CSIRO and NatHERS are supporting ongoing work to support user-specified air-tightness values in NatHERS assessments, if verified by on-site testing.
In 2025, AccuRate was updated to calculate and display the modelled permeability, m3/h/m2 @ 50 Pa so designers can see the industry standard expression, while the model uses the dynamic approach.
This update is the completion of the first of three stages of work by NatHERS for improving airtightness modelling in NatHERS new homes assessments and allows stage 2 and 3 work to begin. Stage 2 and 3 work includes introducing options to directly model air leakage values into NatHERS for new homes software that correlate with a regulatory star rating improvement. In this instance, compliance could be verified through a blower door test.
In principle on site blower door test verification is feasible, but further work is required before it can be implemented. Once cloud-based tools are released, this could be possible through Application Programming Interface (API) integration with third parties such as the Air Tightness Testing & Measurement Association (ATTMA).
The NCC doesn’t force air tightness and neither does NatHERS. When we measure air tightness in houses and apartments, the actual air tightness is not far off what is predicted by the NatHERS rating and within what most building science folk would think was moderate. There are some outliers, with tighter, or leakier buildings than expected, and since our last study there is a trend toward better air tightness, but generally this is seen as a positive.
Health agencies say thermal safety is 24 to 28 degrees. Our code-compliant home by day 3 has indoor temperatures exceeding outdoor temperatures. We built the standard now putting people’s lives at risk.
Thermal safety is complicated. It depends on vulnerability, metabolism, clothing, humidity, temperature, activity etc. Early on in NatHERS, the only metric used was Mj/m2/year (megajoules per square metre per year). This meant that in order to reduce average annual Mj/M2 it was cheapest to increase insulation and decrease window size. This approach might push up cooling demand, but not by as much as it reduced heating demand, so on average there was an improvement – but with increased risk of overheating. In 2019 NatHERS introduced heating load limits to reduce this problem.
My team has been looking deeper at this issue for a long time. When we use the European CSIBE overheating standards it does look like a lot of our housing stock will overheat. But those standards are built for cold climates and do not include humidity or air speed. Mahsan Sadeghi is working on Australian Comfort Standards, which will define overheating better in Australian climate zones, using real Australian household data, and accounting for airspeed and humidity. The new standards should be published this year and have taken five years to develop (with five universities). You can see Mahsan’s scene setting paper here and the other papers for each climate zone are in the publication pipeline.
We have also embarked on a research program with universities to look at the health impacts of overheating. Just because a dwelling falls outside comfort does not mean it will have individual or population health impacts. There has been very little international research on indoor residential overheating. Heatwave impacts on population health usually do not distinguish between those who are indoors and outdoors during heatwaves or on the activity they are undertaking. We are trying to redress this gap in the literature and will continue to work on this problem until we have good data for policy makers to act on. This is a problem everywhere and I had a long conversation with a UK bureaucrat working on exactly the same issues there. No-one has nailed this problem yet and Australia is doing as much or more than any other jurisdiction I know of.
Traditional construction outperforms new builds in heat. CSIRO research and multiple Australian studies show 6, 7, and 8-star homes experience more overheating than traditional 2-6 star homes. Traditional double brick construction has outperformed many high-star homes during summer periods.
I don’t know what this claim is based on (although the second sentence refers to this paper based on the 5 star standard back pre 2013). My only response is that NatHERS has been shown to accurate model heavyweight, earth, and tropical buildings in the past. The 1st generation NatHERS tool, in use until 2003, did not account for natural ventilation. However, since 2003 and the second generation Chenath engine, NatHERS has accounted for the cooling effects of natural ventilation. If anything, NatHERS settings may overestimate the impact of natural ventilation, as it assumes occupants have almost perfect knowledge of indoor and outdoor temperatures, and operate them actively night and day. Ceiling fans (where they are specified,) and indoor air movement are modelled in all NatHERS simulations.
Overall comments on NatHERS rated houses:
Higher NatHERS rated houses probably deliver significant heating demand reductions, but cooling demand reductions are not clear. This is probably due to the wide range of occupant behaviours in cooling rather than the software or scheme per se.
Air tightness is improving over time. This can lead to mould issues if building assemblies and ventilation are not well designed – but NatHERS doesn’t tell people what assemblies to use or what ventilation to use, it just models the energy use of whatever they have chosen.
When a performance gap is found between a NatHERS rating and actual energy use it is often hard to separate the reason – is the rating wrong? Was the building built in accordance with the rating (ie. non-compliant)? Or do the occupants use it in a way not anticipated by the software? Where we correct for these factors the NatHERS ratings very closely predict actual indoor temperatures and energy use
From the GBCA
On refrigerants Between 2003 and 2019, Green Star rewarded the use low-GWP refrigerants. Effectively, building projects that used very low amounts of refrigerants, or refrigerants with a low-GWP, were rewarded with points. Those counted towards a rating. In 2019, our review found that the credit was seldom used, and not used in significant numbers to drive transformation. In Green Star Buildings in 2020, we changed the rule to require projects to consider the emissions impacts from refrigerants by putting a price on carbon through offsets as a mandatory requirement, and points were still given if you use low GWP refrigerants. This has led to project teams using better refrigerant equipment, as there is a cost to them not considering it. In fact, an often-asked question is a request for dispensation from the offsets, as people do not wish to address their refrigerant impacts as they perceived to be too small.
A second type of query comes from refrigerant manufacturers that are now being regularly asked for better equipment and are asking us why. In other words, the approach to get people considering the refrigerant impacts worked.
To continue with progress, in November 2025, after two years of work, GBCA launched a consultation paper with AIRAH called Freeze Frame making the case for stronger action in refrigerants. Green Star Buildings 1.1, launched in late 2025, and was revised soon after based on the findings of the report, includes mandatory requirements to meet an average lower emissions threshold for refrigerants (expressed as an average GWP number). It also has additional requirements for distinct star ratings that include more stringent requirements. These requirements become more stringent over time. This is intended to work to build a supply chain and continue the initial approach to creating enough market demand for change. All this work has led to significant criticism towards GBCA for our stance on the strict refrigerant limits in Green Star. At the same time, we have received strong support for moving industry forward.
Offsets
Green Star has specific emission reduction requirements in the areas of refrigerants and embodied carbon. It also has bans on natural gas in buildings for 5 and 6 Stars, and beginning in April, for all other new buildings. It also has strict energy demand reduction requirements in excess of the NCC.
Nature-based offsets are then required to zero out remaining emissions in limited cases, effectively acting as a carbon price. The cases are:
- Remaining refrigerant emissions (in addition to low-GWP requirements -see refrigerant commentary)
- Remaining embodied carbon emissions (once embodied carbon reductions are met without them from 2030).
Offsets are a strategy for addressing impact that cannot be reduced by other means. There can be, and have been, issues with offsetting programs. There are also valuable programs with carbon, economic, natural, and social benefits. These offsets are what we aim to encourage.
In addition, GBCA and the Property Council of Australia put out a paper on offsets called “Last but not later” describes why we should invest in these solutions as called for in IPCC report.
See other articles in this special report