Pursuing high NatHERS ratings results in major reductions in home energy use, a University of South Australia study has confirmed.
However, the results put the performance of builders in the spotlight and the need for stronger compliance checks around energy and thermal efficiency of residential construction, lead author Timothy O’Leary said.
Mr O’Leary, a PhD candidate and teacher in the School of the Natural and Built Environment, worked with Dr Martin Belusko, Dr David Whaley and professor Frank Bruno on the research project, which involved comparing the post-occupancy performance of homes in the low-carbon Lochiel Park precinct with homes built over a decade earlier in Mawson Lakes.
Lochiel Park homes were required to achieve a NatHERS rating of 7.5 Stars or higher. The homes also all have energy monitoring systems installed that give data on energy use by all systems, including for heating and cooling.
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The Mawson Lakes homes were constructed between 2000 and 2002, before the 2003 introduction of a mandatory four-star NatHERS minimum in the Building Code of Australia. The houses were rated at the time of construction as achieving between 3.4 and 4.7 stars, and a group was monitored post-occupancy for energy performance data, including for heating and cooling.
The study showed that homes generally performed as the NatHERS ratings predicted, with a few exceptions the researchers believed were the result of occupant behaviour. In one case a Lochiel Park home was using one-fifth of the predicted energy use for heating and cooling; in another it substantially exceeded it.
Overall, however, the higher-rated Lochiel Park houses are on average using 60 per cent of the energy Mawson Lakes homes consume.
On average, the benefits predicted by NatHERS have eventuated, measured over a big jump in stars from 4 to 7.5, Mr O’Leary said.
Heating an issue
While the cooling energy use was closely correlated, he said the heating was less so, with some homes using two to three times the NatHERS predicted energy use.
Whether this is because homes are leaky or whether some occupants simply crank up the heating more than modelling expects is uncertain. Mr O’Leary said it required more data, more monitoring of actual energy over more time, across more house types and more occupant conditions.
He said the results supported a case for a discrete study into the NatHERS settings for heating.
There is also a case for more occupant education regarding how to reduce energy use for heating, which in Adelaide’s climate is required for a longer season than cooling.
Mr O’Leary said energy monitoring technology that shows occupants at Lochiel Park live energy use data is helping to educate them.
“There is scope for the [federal] government and state governments to look at the existing housing stock and do research that educates consumers,” he said.
The report noted that the builders at Lochiel Park were required by the South Australian Land Development Corporation to have a proven track record of delivering energy-efficient homes.
Mr O’Leary said the correlation between the rating and actual performance in the majority of cases was in part due to the fact both developments were built to stringent design guidelines, rated by experienced Home Energy Rating assessors and that the builders “were scrutinised probably more than the average house builder would be”.
The study used only a statistically small sample, and highlights the need for extensive national energy consumption data collection and research, the researchers said.
A case for performance improvements
Mr O’Leary said there was a case for research to show what the improvements in performance have been when the BCA mandated an increase from five-star NatHERS as a minimum to six-star NatHERS.
The NCC 2016 has not mandated any higher performance than six stars, and Mr O’Leary said there was a case to build more stringent levels of energy efficiency.
“This could be achieved in NCC 2019 by either going to higher star bands, or by re-examining the existing bands. There could also be option of something like a concessional trade-off for homes that have solar PV installed.”
Lochiel Park showed that homes can be built to a 7.5-star standard “quite reasonably” in terms of cost. Going above minimum in terms of performance and aspiring to a higher star standard is also a reasonable proposition, as the building industry itself is brought along in terms of its focus on energy-efficient product.
Mr O’Leary said the idea of mandatory disclosure of energy performance, accepted in many parts of Europe and the US, is not gaining traction in the real estate sector in Australia because agents see it as more red tape that offers no benefit to them. This is despite COAG advocating for such a scheme since 2009, he said, as part of a suite of measures to reduce greenhouse gas emissions.
Two out of the four main actions COAG identified related to housing. One was the shift to a six-star NatHERS minimum in the BCA; the other was mandatory disclosure.
“But the thing about it is, a person buying an expensive piece of real estate should know something about it in terms of performance,” Mr O’Leary said.
Builder competence important
The research highlights the role of the builder in ensuring performance matches expectations.
“When you consider that Australia has a light approach to certification, you are relying on the skill and competence of the builder. It’s about having builders that can deliver the product.”
He said some of the reasons homes are delivered that do not actually perform as they should is because the industry is fragmented, cut-throat and has low barriers to entry.
“Energy efficiency is an easy target for poor performance of a builder.” This is because inspections focus on the structural aspects of a house – foundations, roof struts, drainage and systems such as plumbing – not on whether insulation was installed correctly, gaps sealed and glazing used as per plans.
“Nobody gets fined for energy-efficiency [faults]. There should be much tighter compliance and post-occupancy evaluation.”
Thermal imaging can now expose some of the faults that make homes perform worse than expected, he said.
In the absence of mandatory disclosure, buyers of existing properties might find it worthwhile investing in a comprehensive thermal evaluation including both thermal imaging and blower door testing to get an idea of how the house will perform for energy use, Mr O’Leary said.