NatHERS ratings use historical weather: 7-star might perform like 5-star by 2050
Every new home in Australia needs a minimum 7-star NatHERS energy rating. But that rating is calculated using weather data from 1990–2015. Buildings are designed to last 50+ years. The climate they will operate in is not the climate they were rated for.
The performance gap over time
A 7-star rated home under current NatHERS weather files vs estimated actual thermal performance under SSP3-7.0 (regional rivalry) scenario
Indicative estimates based on CSIRO research into future weather files for NatHERS. Actual degradation varies by climate zone, orientation, and construction. These are not predictions of specific outcomes.
How NatHERS works
The Nationwide House Energy Rating Scheme simulates how much energy a home needs for heating and cooling to maintain thermal comfort. The simulation uses a Reference Meteorological Year (RMY) — a synthetic year of weather data assembled from historical observations for each of NatHERS' 69 climate zones across Australia.
The RMY represents “typical” weather conditions based on Bureau of Meteorology records from approximately 1990 to 2015. The higher the star rating, the less energy the building needs to heat and cool under those historical conditions. Since NCC 2022, the mandatory minimum for new residential buildings is 7 stars.
The problem: the RMY data describes a climate that no longer exists and will increasingly diverge from reality over the building's lifespan. A building rated under 1990–2015 weather conditions will experience hotter summers, more frequent heatwaves, higher humidity, and different seasonal patterns than the conditions it was designed for.
What the research shows
CSIRO has been developing future weather files for NatHERS since the mid-2010s, modelling how Reference Meteorological Year data would change under various warming scenarios. Their findings are consistent: buildings rated 7 stars under current weather files perform progressively worse as actual weather diverges from historical patterns.
Key findings from climate-adjusted NatHERS research
- Cooling energy demand increases by 20–80% by 2050 in most Australian climate zones under SSP3-7.0
- Western Sydney and tropical northern zones show the largest performance degradation due to extreme heat amplification
- Heating demand decreases in southern zones, partially offsetting cooling increases — but net energy demand still rises
- A 7-star home designed for Penrith's current RMY may require 40–60% more cooling energy by 2050 than the rating suggests
- Humidity projections (not well captured in current RMY files) could further degrade performance in coastal and tropical zones
The “5-star by 2050” headline is an approximation. The actual gap depends on climate zone, scenario, orientation, construction materials, and occupant behaviour. But the direction is clear: the gap between rated and actual performance widens every year.
Which climate zones are most affected
Western Sydney
Zone 28 (Penrith)
Projected +2-4°C summer maximums by 2070. Cooling load increase overwhelms insulation benefits. Urban heat island amplifies warming.
Tropical North
Zones 1-3 (Cairns, Darwin)
Year-round cooling demand increases. Humidity projections not captured in historical weather files. Condensation risk rises.
Coastal Sydney
Zone 56 (Sydney)
Ocean moderates temperature extremes but heatwave frequency is projected to double. Coastal humidity increases condensation risk.
Southern Highlands
Zone 24 (Canberra/Queanbeyan)
Reduced heating load partially offsets increased cooling. Net energy impact is smaller but direction is still negative.
What this means for buyers
If you are buying a new home with a 7-star NatHERS rating, the rating accurately describes the building's thermal performance under the historical weather conditions used for the simulation. It does not describe how the building will perform in 2040 or 2060.
The practical consequences include:
- Higher energy costs — cooling costs will likely exceed what the star rating implies, particularly in Western Sydney, North Queensland, and other heat-exposed areas.
- Comfort gaps — a building designed to maintain comfort under 35°C peak days may struggle with 40°C+ days that become more frequent.
- Retrofit costs — upgrading insulation, glazing, or shading after construction is significantly more expensive than designing for future climate upfront.
- Resale implications — as climate-adjusted ratings become available, homes rated under outdated weather files may need re-assessment.
What is being done about it
The gap is acknowledged. Several initiatives are in progress:
CSIRO future weather files
CSIRO has developed prototype future weather files for NatHERS that incorporate climate projections. These files model how the Reference Meteorological Year would change under SSP2-4.5 and SSP3-7.0 scenarios at 2030, 2050, and 2070 timeframes. They are not yet adopted into the NatHERS framework but represent the most likely path to addressing the gap.
NCC 2025 review
The National Construction Code 2025, adoptable from May 2026, includes updated energy efficiency provisions. However, the underlying climate data for NatHERS zones has not been updated to reflect future warming. The NCC 2025 retains the existing climate zone structure and historical weather files. A separate ABCB-led review of climate zone boundaries and weather data methodology is underway but has no published timeline for completion.
CC&NH SEPP (NSW)
The draft Climate Change and Natural Hazards SEPP for NSW would prescribe NARCliM 2.0 climate scenarios for development assessment. While this does not directly change NatHERS ratings, it creates a regulatory environment where forward-looking climate data is required for development approval — setting the stage for climate-adjusted building performance standards.
What buyers and builders can do now
You do not need to wait for regulatory reform. Several strategies help close the performance gap:
1. Design above minimum
A home rated 8 stars under current weather files will degrade to roughly 6–6.5 stars by 2050, still above today's mandatory minimum. Building to 8 or 9 stars now provides a buffer against future climate divergence.
2. Prioritise passive cooling
The biggest performance gap is in cooling, not heating. Cross-ventilation, eave depth, external shading, light-coloured roofing, and thermal mass positioning have a disproportionate impact on maintaining comfort during hotter-than-rated conditions.
3. Check your climate zone's trajectory
Some climate zones will shift categories under projected warming. Western Sydney, currently classified as a temperate zone, is projected to experience conditions more consistent with subtropical zones by mid-century. PlotDetect's climate risk assessment shows projected temperature and hazard changes for any NSW address.
4. Ask for future-adjusted modelling
Some energy assessors can run NatHERS simulations using CSIRO future weather files as an unofficial supplementary assessment. This is not a replacement for the mandatory rating (which uses standard RMY files) but gives a clearer picture of how the building will actually perform.
The broader picture
The NatHERS weather file gap is part of a wider pattern: Australian planning and building regulations are calibrated to historical climate data while the actual climate is changing. The AASB S2 mandatory climate reporting standard explicitly requires forward-looking scenario analysis for property portfolios. Building ratings, by contrast, remain backward-looking.
This creates an asymmetry: investors and lenders will soon be required to assess future climate risk on properties, while the building code rates those same properties using past weather data. The gap between regulatory intent and building performance standards is widening.
Frequently asked questions
Does this mean 7-star homes are poorly built?
No. A 7-star home performs well under the conditions it was designed for. The issue is that those conditions are based on historical weather data that is progressively diverging from actual and projected weather patterns.
Will my NatHERS rating be downgraded?
Existing ratings remain valid under the current framework. If future weather files are adopted into NatHERS, it is possible that re-assessment under updated conditions would produce a different rating. No retrospective downgrade mechanism exists or is proposed.
Which zones are least affected?
Southern highland and alpine zones are least affected because warming reduces their heating demand, partially offsetting increased cooling. Coastal zones with strong ocean moderation (such as parts of Sydney, Wollongong, and the Mid North Coast) are moderately affected. Inland and tropical zones face the largest gap.
See how climate projections affect your property
PlotDetect's climate risk assessment shows projected temperature changes and hazard exposure for any NSW address, using NARCliM 2.0 climate projections.
Check climate risk