Thermal Performance Assessment

What Is a Thermal Performance Assessment?

A thermal performance assessment is a detailed computational analysis of a building's thermal behaviour — how it gains, loses, and stores heat throughout the year. Using specialised simulation software accredited under the Nationwide House Energy Rating Scheme (NatHERS), an accredited assessor creates a three-dimensional model of the building and simulates its thermal performance across an entire year of weather data for the specific climate zone.

The result is a star rating from 0 to 10 under the NatHERS framework, which quantifies the building's annual heating and cooling energy requirements in megajoules per square metre (MJ/m²). A higher star rating means the building needs less energy to maintain comfortable indoor temperatures. Under the current National Construction Code (NCC 2022), all new residential buildings must achieve a minimum of 7 stars.

The assessment considers every element of the building envelope: walls, floors, ceilings, roofs, windows, doors, and their interactions. It accounts for insulation levels, thermal mass, glazing properties, shading devices, building orientation, air leakage, and internal heat gains from occupants and appliances. The result tells you exactly how much energy the building will need for heating and cooling to maintain comfortable indoor temperatures throughout the year.

NatHERS-Accredited Software Tools

Thermal performance assessments under the NatHERS framework must be conducted using software accredited by the Administrator of the Nationwide House Energy Rating Scheme. The three primary NatHERS-accredited software tools are:

• FirstRate5 — Developed by the Victorian Government, FirstRate5 is the most widely used NatHERS software in Australia. It provides a graphical interface for building the thermal model and generates star ratings along with detailed energy breakdowns by building element. FirstRate5 is particularly well-suited to standard residential designs and is the tool most assessors are trained on.
• AccuRate — Developed by CSIRO, AccuRate is a detailed simulation engine particularly suited to complex building designs, split-level homes, and non-standard geometries. It uses the same underlying calculation methodology as FirstRate5 but offers additional modelling flexibility for unusual designs.
• BERS Pro — Developed specifically for Australian conditions, BERS Pro is widely used in Queensland and other states. It includes integrated tools for whole-of-home assessments under the NCC 2022 provisions and is particularly suited to tropical and subtropical climate zones.

All three tools produce NatHERS star ratings using the same calculation methodology, so the choice of software does not affect the validity of the assessment. The accredited assessors we connect you with will use the most appropriate tool for your project type and location.

What Is Measured in a Thermal Assessment?

A thermal performance assessment analyses every component of the building envelope and its interaction with the local climate. Key factors assessed include:

Building Envelope Insulation

The R-values (thermal resistance) of all insulation in walls, ceilings, floors, and roofs. Higher R-values mean better insulation performance. The assessment considers not just the insulation product but the total system R-value, including air films, framing members, and other layers in the construction assembly.

Glazing Performance

Windows and glazed doors are typically the most significant factor in a building's thermal performance. The assessment analyses the U-value (thermal transmittance) and Solar Heat Gain Coefficient (SHGC) of each window assembly, including frame type, glass type, number of panes, gas fills, and low-E coatings. External shading devices such as eaves, pergolas, louvres, and vegetation are also modelled to determine their effectiveness in reducing unwanted solar heat gain.

For projects struggling to achieve the 7-star minimum, glazing optimisation is often the most effective strategy. Upgrading from standard aluminium frames to thermally broken aluminium or timber frames, or switching from single to double glazing, can significantly improve the star rating. An accredited assessor can model different glazing scenarios to identify the most cost-effective combination.

Orientation and Solar Access

The building's orientation relative to the sun's path is a critical factor. North-facing living areas with appropriate shading perform best in most Australian climate zones, allowing passive solar gain in winter while preventing overheating in summer. The assessment models solar angles throughout the year to determine the effectiveness of the building's orientation and shading design.

Thermal Mass

Heavy materials like concrete slabs, brick walls, and rammed earth absorb and release heat slowly, moderating indoor temperature fluctuations. The assessment models the thermal mass properties of all materials in the building and their interaction with insulation, ventilation, and solar access.

Air Infiltration

Uncontrolled air leakage through gaps, cracks, and openings in the building envelope can significantly increase heating and cooling loads. While the NCC does not currently mandate airtightness testing (unlike the Passive House standard), the thermal modelling software accounts for estimated air infiltration rates based on the construction type and detailing.

Whole-of-Home Energy Budget

Under the NCC 2022, residential energy compliance now includes a whole-of-home energy budget in addition to the building envelope star rating. This means that the energy used by fixed appliances — heating and cooling systems, hot water systems, lighting, and pool and spa equipment — must also be assessed and must not exceed a prescribed energy budget for the climate zone.

The whole-of-home assessment is conducted using the NatHERS Whole-of-Home tool and expressed in megajoules per square metre per year (MJ/m²/yr). This requirement drives the selection of efficient appliances alongside good building envelope design, encouraging an integrated approach to residential energy performance.

When Do You Need a Thermal Performance Assessment?

A thermal performance assessment is required in the following circumstances:

• All new residential buildings — Every new Class 1 (house) and Class 2 (apartment) building requires a NatHERS assessment before a building permit can be issued.
• Renovations and extensions — Renovations that significantly alter the building envelope or add conditioned floor area typically require an assessment of the new or altered portion. The specific triggers vary by state.
• Building classification changes — Converting a non-habitable space (garage, shed) to a habitable room triggers energy compliance for the converted space.
• Voluntary performance verification — Property buyers, homeowners planning renovations, or architects seeking to optimise designs may commission assessments outside the regulatory context.

Thermal Assessment Costs

The cost of a thermal performance assessment depends on the project size and complexity:

• Standard new home: $400–$800
• Large or complex home: $600–$1,200
• Extension or renovation: $400–$600
• Multi-unit (per dwelling): $300–$500
• Design advice iterations (included or additional): $100–$300 per round

For detailed pricing information, see our energy rating cost guide. You can also use our energy rating calculator for an initial estimate of your project's likely performance before engaging an assessor.

Getting Started

To obtain a thermal performance assessment, you need architectural plans that show floor plans, elevations, sections, and a site plan with north point. Specifications for insulation, glazing, and construction materials should also be provided where available. If specifications are not yet finalised, the assessor can advise on what is needed to achieve compliance.

We connect you with accredited NatHERS assessors who can review your plans, conduct the thermal modelling, and provide a certified assessment for your building permit application. Submit your details through the enquiry form for a free, obligation-free quote, or use our find a consultant tool to search for an assessor in your area.