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The Emperor’s new Speedos: how home energy ratings really work in the tropics

Does Australia's energy rating scheme for houses actually encourage the use of air-conditioning?
In the 2014 AS Hook Address, Adrian Welke and Phil Harris of Troppo, well deserved 2014 Australian Institute of Architects Gold Medallists, had this to say about minimum energy performance regulations: “…we pat ourselves on the back for tackling greenhouse gas emissions with approval frameworks that are predicated on the inclusion of energy-heavy air-conditioning. A sealed, insulated box – an esky – becomes the ultimate energy efficient building paradigm. Come on… the emperor’s turned up in the nude again!”
As an examination of the submissions to the Queensland Parliamentary Inquiry into Energy Efficiency reveals, Welke and Harris were voicing concerns held by many tropical housing design practitioners in relation to the Nationwide House Energy Rating Scheme (NatHERS) tools
1. NatHERS tools simulate the temperatures inside a house using a full year of hourly weather data and the heat needed to be added or extracted to maintain comfortable temperatures i.e. heating and cooling. There are 69 Australian climate zones with this data available. To rate a house, the tools extract or add heat in the simulated house until a comfortable temperature is achieved. The amount of energy required to do this will determine the house’s energy rating.
Contrary to what many practitioners believe, though, new research has shown that Nationwide House Energy Rating Scheme (NatHERS) tools do not assume a sealed box, nor do they assume heavy use of cooling. A report I completed in 2012 for what is now the Department of Industry examined how NatHERS rating tools work in hot climates
2. The analysis showed the NatHERS tools assume that ventilation is used to make the house feel cooler by opening windows much more than they close windows and artificially cool the house. Second generation tools, introduced in 2006, simulate the internal wind speed in a house and the additional comfort this provides. Before extracting heat to make the house cooler, these newer NatHERS tools will first open windows to see if this makes the house comfortable. They only extract heat if ventilation is not adequate to maintain comfort. NatHERS tools assume that heating and cooling is available from 7am to midnight in living areas and from 4pm to 9am in bedroom areas. While overnight heating thermostats are lower than daytime thermostats, overnight cooling thermostats are the same as daytime thermostats. 
Typical tropical house design strategies like deep verandas, louvre windows and light colours all led to significant improvements in NatHERS star ratings in climates where cooling loads are greater than heating loads.
In houses designed for the tropics I evaluated in Darwin, the tools assume that windows are opened annually for around 66% of hours (i.e. average 16 hours a day) and the house is artificially cooled to achieve comfort 16% of the time (4 hours), which is comparable with reported hours of cooling use in Darwin. In a Brisbane house that I evaluated, windows were opened for, on average, 11 hours a day and it was artificially cooled for half an hour. This report also showed that the comfort benefit due to air movement predicted by NatHERS tools was 50% higher in a house designed to promote cross ventilation (elevated, windows on opposite sides of all rooms) than in a typical volume market house. Maybe it should be even better, but it is showing a significant benefit. Further, typical tropical house design strategies like deep verandas, louvre windows and light colours all led to significant improvements in NatHERS star ratings in climates where cooling loads are greater than heating loads. This shows NatHERS tools do not assume a sealed box. With this level of use of ventilation, the emperor must at least be wearing his speedos.
So if all this is working correctly, why are experts so dissatisfied with the rating? One contributing factor may be that the better performance of bedrooms in traditional tropical houses isn’t being rewarded by the rating. In NatHERS tools, bedroom cooling loads are much lower than living room loads because they mainly cool overnight when outdoor temperatures are cooler and there is no sun. Bedroom cooling loads can be as low as only 12% of the total and are generally around 25%. As a result, their performance doesn’t have much impact on the star rating. NatHERS tools assume that the overnight cooling thermostat temperature in bedrooms is the same as the daytime thermostat. At night the body is insulated from heat loss by the bed, so comfort theory would support a lower thermostat temperature in bedrooms. Lowering the overnight bedroom cooling thermostat increases predicted energy loads in bedrooms. This would ensure that bedroom performance is better valued by NatHERS tools. The rating of a traditional tropical design with well-ventilated bedrooms would be at least a star higher compared to volume market houses if the NatHERS tools used a three degree lower overnight thermostat.
In reality, it is the relationship between architect and client and their design process that NatHERS tools can’t address. 
While further research is needed into how much lower the thermostat should be, the report I prepared for the Department of Industry recommended the use of lower overnight cooling thermostats. It may not be the only solution, but it will help. I would agree with the general sentiment of Welke and Harris’ comments, though because there is a limit to what can be assessed by NatHERS tools. Even if NatHERS is modified to better reward traditional tropical design techniques, there will be some specialist tropical houses that will never receive a good rating. Houses that are essentially no more than a set of permeable screens used to enclose spaces for living will never do well under NatHERS. Predicted cooling loads from NatHERS tools will always be high in such buildings because they let too much hot and moist air in. The temperatures in these houses are easy to simulate: outdoor air temperature minus an allowance for air movement. These houses are designed to match the comfort needs of the occupants – people who, as Phil and Adrian suggested, will “get out a beer” on those 60 uncomfortable days each year.
In reality, it is the relationship between architect and client and their design process that NatHERS tools can’t address. If occupants of lightly enclosed houses never use cooling appliances, regardless of their NatHERS rating, then they are meeting the policy objectives of the regulations. There is a reasonable argument that we need a formalised alternative solution compliance path for such houses in the tropics within the National Construction Code (NCC). My only concerns would be to ensure that:
  1. This doesn’t create a loophole for volume market houses – which will be air-conditioned – to get away with lower standards.
  2. The alternative solution is suitably robust – such houses should never be allowed to be air-conditioned without bringing the fabric up to an appropriate standard, although this would be hard to enforce.
  3. The alternative solution should be rigorous – it needs to have quantifiable criteria to ensure that the design will actually deliver the comfort it promises.
  4. Occupants need to understand what level of comfort they are getting – monitoring of naturally cooled houses shows that they can spend considerable time outside the conventional comfort zone in the hottest climates3. They are not for everyone, particularly those more sensitive to heat stress in the hottest climates.
For most Australians, however, a beer is not all they need in hot weather. Over 70% of Australian houses have air-conditioning installed. In the Northern Territory, over 90% have air conditioning and half of all houses have three or more air conditioners. I’m sure that air conditioning ownership would be less if there were more Troppo houses – but it wouldn’t be 0% either. Furthermore, continued global warming will only see increased use of air conditioning, so we need a regulatory approach like NatHERS that minimises heat gain and facilitates heat loss for conventional houses in hot climates.
NatHERS tools do not simulate a sealed box, and in fact pursue the use of ventilation aggressively as a solution to comfort. As I have said, the emperor is at least wearing speedos. And maybe, in a hot climate, that is all he needs – particularly if the NCC is modified to develop an alternative solution to deal with lightly enclosed houses, a condition that NatHERS was not designed to deal with.
CSIRO recently investigated whether a higher NatHERS star rating does in fact lead to lower energy use in hundreds of real houses located in Adelaide, Melbourne and Brisbane. While heating savings were even higher than NatHERS predicted, there were virtually no cooling savings associated with higher ratings. CSIRO threw some new light on these findings at a meeting I attended recently. It seems that the way in which higher ratings were achieved across the sample was to implement design changes that had a much greater impact on reducing heating loads than cooling loads. As a result the variation in cooling energy loads predicted by NatHERS across the sample was small and it is therefore not surprising that no cooling savings were associated with higher ratings. This points to another possible deficiency with the NatHERS scheme: it adds together heating and cooling loads and assigns a rating to the total. This means that in mixed climates, which have significant heating and cooling loads, you are not guaranteed to get lower cooling energy with a higher rating. The BASIX system in NSW sets a cap for both heating and cooling. Perhaps this is the way forward for NatHERS as well.

1. Queensland Parliament, 2010.“Energy Efficiency: Queensland’s First Energy Resource”, Report on the economic and environmental potential provided by energy efficiency improvements for households, communities, industry, and government, Report No.2 of the Environment and Resources Committee, 53rd Parliament, February 2010.
2. “Tropical validation study: an investigation into the impact of energy ratings on house design in hot climates for the department of climate change and energy efficiency prepared by tony isaacs consulting, with assistance from Michael Plunkett, Smartrate, and Ray Fogolyan”, Home Star Rating Australia, Canberra, 2012.
3. Soebarto, V. et al, “The performance of award winning houses”, PLEA2006 - The 23rd Conference on Passive and Low Energy Architecture, Geneva, Switzerland, 6-8 September 2006

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