How the NatHERS Star Rating Works
Understand the 0 to 10 star rating scale, how heating and cooling loads are calculated, and what constitutes a good star rating under current Australian standards.
The 0 to 10 Star Scale
The NatHERS star rating scale runs from 0 to 10 stars, measured in half-star increments. The rating represents the predicted energy needed to heat and cool a home to maintain comfortable indoor temperatures throughout the year. This energy is expressed in megajoules per square metre per year (MJ/m2/year).
A lower MJ/m2 figure means the home requires less energy and therefore achieves a higher star rating. The relationship between energy load and stars is not linear — each additional star requires a proportionally greater improvement in building performance.
What Each Star Level Means
To put the scale into practical terms:
- 0-2 stars — Very poor performance. The home offers minimal thermal protection and will require significant heating and cooling year-round. Typical of uninsulated older homes.
- 3-4 stars — Below current standards. Some basic insulation and design considerations, but still reliant on mechanical heating and cooling for comfort.
- 5-6 stars — The previous minimum standard (6 stars was the national minimum from 2010 to 2022). Reasonable thermal performance with standard insulation and glazing.
- 7-8 stars — The current standard. Good thermal design with quality insulation, appropriate glazing, and considered orientation. A 7-star home typically reduces heating and cooling energy by 25-30% compared to a 6-star home.
- 9-10 stars — Exceptional performance. These homes require very little artificial heating or cooling. Achieving this level typically involves high-performance glazing, superior insulation, excellent airtightness, and optimised orientation and shading.
How the Rating Is Calculated
A NatHERS assessment uses accredited software to simulate the thermal performance of a home over an entire year. The software models the interaction between the home's design features and the local climate, taking into account:
- Building orientation — Which direction each wall and window faces
- Insulation levels — R-values for ceiling, wall, and floor insulation
- Glazing performance — Window size, type (single, double, or triple glazed), frame material, and any low-emissivity (low-E) coatings
- Thermal mass — The ability of building materials (concrete slabs, brick walls) to absorb and release heat
- Ventilation — Natural airflow through the home, including openable windows and ceiling fans
- Shading — Eaves, pergolas, external blinds, and neighbouring buildings or vegetation that shade windows
- Airtightness — How well-sealed the home is against draughts
The software uses climate data specific to the home's NatHERS climate zone to calculate how much energy would be needed to maintain the thermostat settings (typically 20-25 degrees Celsius) throughout the year.
Heating and Cooling Loads
The NatHERS assessment produces two key figures: the heating load and the cooling load, both measured in MJ/m2/year. These represent the energy required to heat the home in winter and cool it in summer, respectively.
The combined total of heating and cooling loads determines the star rating. In heating-dominated climates (such as Melbourne, Hobart, and the Australian Alps), the heating load is typically the larger figure. In cooling-dominated climates (such as Darwin, Cairns, and Broome), the cooling load dominates. Many parts of Australia have a mixed climate where both loads are significant.
Understanding whether your climate zone is heating-dominated or cooling-dominated helps you prioritise design strategies. For example, in a cold climate, investing in superior insulation and double glazing delivers the greatest improvement, while in a hot climate, shading, ventilation, and reflective roofing may be more cost-effective.
The 7-Star Minimum Under NCC 2022
Since the adoption of the National Construction Code 2022, new homes across most of Australia must achieve a minimum of 7 stars. This increase from 6 stars represents a significant step-up in thermal performance requirements.
Achieving 7 stars is readily achievable with good design practices including appropriate orientation, quality insulation, and performance glazing. Most builders and designers have adapted their standard specifications to meet the new requirements. If your home design is falling short of 7 stars, an experienced assessor can suggest targeted improvements to lift the rating.
What Is a Good Star Rating?
While 7 stars is the minimum legal requirement, many homeowners aim higher. An 8-star home provides noticeably greater comfort and lower running costs, often with only modest additional construction expense. The cost of reaching 8 stars from 7 varies by climate zone and design, but is typically in the range of $2,000 to $8,000 for a standard home.
Beyond 8 stars, the cost of each additional half-star increases significantly. Whether the investment is worthwhile depends on your priorities, budget, and how long you plan to live in the home. Your NatHERS assessor can model different scenarios to help you make an informed decision.