It’s in the stars! How scientists figure out your home’s energy rating

By Kate CranneySeptember 30th, 2021

For nearly 70 years, CSIRO scientists have been working on the science behind the 10-star energy ratings for homes in Australia. We look at how CSIRO’s tools are used in the Nationwide House Energy Rating Scheme, and the science behind the stars.
Interior of a kitchen and living room

The award-winning, energy efficient renovation to Vasey House, by LightHouse. Image: Ben Wrigley

We use star ratings to make choices on which movies to watch, which hotels to stay at, and increasingly, which home we want to buy.

Australia has one key method of measuring a home’s energy efficiency. And it can help us to buy or build a more comfortable home that uses less energy.

NatHERS: A star (rating system) is born

The Nationwide House Energy Rating Scheme (NatHERS) measures energy efficiency and produces a performance-based star rating. It’s a scale of 0-10 stars, describing the ‘thermal performance’ of a home, based on a home’s design and materials. The higher the stars, the less energy that you need to keep a house comfortable through heating or cooling.

NatHERS began in 1993 and was adopted as a nationwide approach in 2004.

The rating system helps homeowners, architects and builders compare the energy efficiency of designs, and informs prospective buyers on the thermal performance of the property they’re interested in.

And thermal performance makes a difference.

Anthony Wright, Research Lead of Building Simulation, Assessment and Communication explains the difference between living in a house with a score at either end of the NatHERS scale.

“If you lived in a home with a 1-star rating, it would be a very poor performing house. You would feel cold in winter and hot in summer, it would be expensive to run and sometimes you might wonder if you’d be better off living in a cardboard box,” he said.

That could look something like a traditional breezy ‘Queenslander’-style home built in a cold climate.

“But if you lived in a home with a 10-star rating, you probably wouldn’t need any energy at all to keep your home comfortable all year round without ever turning on a heater or air conditioner. The house would use the sun and prevailing breezes to keep you comfortable.”

A 10-star house would be a carefully designed home that capitalises on passive heating, cooling and other sustainable, energy-efficient design principles.

Most parts of Australia require a star rating of 6 or above, for a new detached home.

Although new homes should rate at 6 or more stars, older homes are likely to perform much worse. Back in 2003, before star ratings were mandated, the average home was about 1.8 stars. (The 6-star minimum was mandated nationally in 2010; back in 2001, minimum star requirements were between 3 and 4 stars, depending on the state or territory.)

Fortunately, things have improved since then. New homes in Tasmania and Canberra rate at an average 6.5 and 6.9 stars respectively, quite a bit higher than the minimum standard.

A woman walking out of a sustainable house

The energy-efficient Whitfield renovation by POD (People Oriented Design) Image: Belinda Allwood

So how do you get a star rating through NatHERS?

With so many different types of homes and climates around Australia, the software needs to be smart.

NatHERS uses software based on 68 years of CSIRO scientific research.

There are approximately 600 professional NatHERS Assessors around Australia. They use one of the four software tools accredited for use under NatHERS, which are all based on the CSIRO physics engine, Chenath. One of the four options is the CSIRO-designed ‘AccuRate’ software.

Assessors input data on the house plans into the software. The data includes hundreds of factors like the local climate, the building’s orientation, construction and design – that includes information on the home’s zoning, floors, walls, windows, doors, ceiling and roof.

From all of this information, the software then estimates the amount of energy required to heat and cool the home, to keep it comfortable.

CSIRO’s software tool, AccuRate, can calculate a home’s annual heating and cooling energy requirements; the output we get is hourly data by zone for both energy and temperature. AccuRate can model up to 50 living spaces and 99 zones within a home. It takes into consideration the impact of variables such as insulation, natural ventilation, air leakage, thermal mass, roof spaces, and sub-floor spaces. Skylights, horizontal reflective air gaps, windows and external shading structures like trees, fences, and the neighbour’s house are also factored in.

Based on all of this data, the NatHERS tool then generates a star rating out of 10 and a Certificate.

(And since 2019, we have been collecting data on these Certificates through the Australian Housing Data Portal. It allows us to see trends in homes over time, and to understand our progress towards energy efficient, low emission dwellings and suburbs.)

NatHERS measures energy efficiency and produces a performance-based star rating. It’s a scale of 0-10 stars.

If you want more than the minimum (currently 6) star requirement, a NatHERS Assessor can provide you with advice on how to change your home to improve its energy use. (The software can help the designer to explore how different orientations and construction material can maximise energy and comfort.)

NatHERS remains the most popular pathway to demonstrate compliance with the National Construction Code energy efficiency requirements. Around 94 per cent of building approvals in 2020-2021 were assessed via NatHERS.

The science behind the software

Our scientists have been working on the science behind energy efficiency in buildings for almost 70 years.

For NatHERS, we’re working on three areas in particular: thermal physics, comfort studies and validation studies.

Thermal physics refers to how heat energy moves through a building. CSIRO’s software models how heat is transferred through all the elements of a building including concrete slab ground coupling effects, solar radiation, air movement, internal heat loads and shading effects.

The study of comfort is more complicated than you might imagine. People in Darwin and people in Tasmania do not usually report being comfortable in the same climatic conditions. Air temperature, radiant temperature, air flow, humidity and personal acclimatisation all influence when we feel comfortable. The software has to be able to estimate the comfort of an average user in each of Australia’s 69 NatHERS climate zones.

Validation studies look at whether CSIRO’s software can predict real-world outcomes. In our largest study we monitored more than 200 houses for 6 years and found a strong correlation between predicted and actual energy consumption when different occupants were accounted for. The largest discrepancies, which we continue to work on, were in air-conditioner energy consumption and the occupant behaviour profiles adopted in NatHERS.

The best thing about having a system like NatHERS using CSIRO’s software is that it’s flexible and homeowners and builders can decide how they want to meet the minimum standards. You can compare the cost and thermal performance impact of double glazing versus more insulation, or changing a window’s size versus adding a reflective coating to the glass. Any number of combinations can be compared to achieve the desired level of comfort, the aesthetic you’re looking for, and the cost.

Seeing stars: Where are minimum energy efficiency standards going?

The future is bright for star ratings: the minimum energy star rating for Australian homes is rising and they’re bringing appliances with them.

Right now, the Australian Building Codes Board is considering increasing the minimum standards in the National Construction Code for new homes in Australia. If approved these changes would come into effect sometime in 2022.

To support the proposed changes to minimum standards, NatHERS is expanding to include information on the energy performance of common household appliances and the relative impacts on a household’s energy bill. ‘Whole of Home’ assessments, as they are known, will include new energy performance information about heating, cooling and hot water systems, lighting, pool/spa pumps, and on-site energy generation and storage (e.g. solar panels and batteries). This move helps promote homes that use less energy, produce less greenhouse gas emissions and cost less to run.

So, the next time you’re looking at a home, consider its energy rating, and the science behind the stars! It will save you energy, money and help you create a comfortable home for you and your family.

Make sure you ask for a NatHERS accredited energy rating and you can be assured that you are getting a rating backed by national quality assurance processes, quality assessors, and best of all, 70 years of CSIRO’s science!

 

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