A typical city suburb - dark roofs, minimal shading and often baking hot.

Last year, CSIRO’s field evaluation of 5 Star homes reported some interesting findings. One big issue was a widespread lack of compliance, due to near-total failure of enforcement by governments and local councils. Another important finding was that, although the efficient homes had much lower heating energy use, their cooling energy use was not lower. The reasons for this outcome are complex, but it’s time we addressed them.

One reason may be that the default settings for cooling used in the NatHERS calculator seem to underestimate cooling. The thermostat temperatures and user behaviour patterns were set many years ago, based on quite limited information. Research has shown people typically use lower thermostat settings [see the report here]. A 2008 South Australian study proposed changes, but these have not yet been formally regulated.

When estimated cooling energy is too low, it has little impact on the energy rating. In climates that require both heating and cooling, designers are more likely to focus on building features that reduce heating.

This under-emphasis on hot weather performance in the energy rating scheme means features like dark-coloured roofs and absence of eaves have unrealistically low impact on rated summer performance. In cooler climates, the overall annual outcome can even improve the Star rating, as benefits from more winter solar gain outweigh worse summer comfort!

The energy rating is also averaged over the whole building. So some rooms may perform poorly without adversely affecting the overall energy rating. And the rating is for total annual heating and cooling. Separate heating and cooling ratings would ensure the home performs adequately all year.

The nature of modern building designs is having its impact, too. The upper storey of a two-storey house has no links to the stable temperature of the ground, and is exposed to higher solar radiation. If glazing is not very carefully designed and managed, it becomes a ‘solar oven’—although the amount of energy required to cool it is not very large if it is well insulated.

More broadly, we need to realise that a high thermal performance home – with good insulation, draught proofing and well-designed glazing – requires very little additional heat to raise its internal temperature above the outdoor temperature— in both winter and summer. Extra internal mass or phase-change materials can help to stabilise the temperature, but climate change is increasing overnight summer temperatures and the duration of hot spells, so thermal mass is becoming less effective.

Careful design is increasingly important, especially glazing and adjustable shading, so that summer sun can be screened out.

Air leakage and poor management of ventilation is another culprit. When an exhaust fan or rangehood is running in hot weather, it is actually bringing in a lot of hot outdoor air—equivalent to a cooling load of 2 kW or so. On a hot windy day, having a window on one side of a house open, even a little, can combine with exhaust fans, fixed ventilation in a laundry or another open window on the opposite side or upstairs to create heat input of up to 5 kW. So, leaky open-plan homes with doors to permanently vented laundries and bathrooms left open can have high cooling costs. Improved building quality and user education are needed.

Of course, this does not mean that 5 or 6 Star homes are a bad idea. If they are built properly and well managed, their peak cooling energy requirements are small, and hourly cooling cost is low, especially when combined with a high efficiency (5 to 7 Star) air conditioner.

We need to further strengthen regulations and enforcement, sort out the under-emphasis on summer performance in the rating scheme and educate home operators. We should also take advantage of the feature in rating tools that allows an energy rater to look at performance of each room or zone during weeks of hot and cold weather. Then designers could identify and address problem rooms.

Also, summer is the time when a typical rooftop PV owner may have excess free electricity. Using some of this for cooling to be comfortable – especially once we get batteries to store daytime generation for evening cooling – need not create load problems for the grid.

This article was originally published in ReNew magazine.

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  1. Hi Alan, nice article. Your photo shows a desert of roofs with barely a single canopy tree. A decent tree will reduce ambient temperatures below it by 3-4 degrees and Vision 202020 claim up to 8 degrees. I know this is a different issue to building ratings and how they are calculated but without knowing anything about the buildings I could look at that photo and tell you it is going to be a hot place.

    I have borrowed the desert theme from 5th Estate. We are creating an urban climate very like a desert, very hot, very cold, windy, sterile and vulnerable to flash flooding. We need green spaces and we need to remember the basics and your photo captures it beautifully.

  2. Because we are using R4 plus in ceilings the roof color has little effect. But if ducting is in the roof cavity with only R1.5 it is useless.It is much better to have the ducting underfloor or no ducting.Her in the cool climates homes that are 4star and more only use cooling a couple of times a year.

  3. Thanks Alan, you have raised some really valuable points for discussion and hopefully action. I agree that the star rating compliance should be separated to give a better bench mark for designers and energy raters to work towards and linking light coloured roofs with reduced aircon demand is very powerful. My particular interest is ensuring that these aspects that influence the home’s performance are considered upfront in the design and giving consumers the knowledge that key decisions in the design will influence the home’s performance. I am an advocate for solar passive design homes, where a 6 star passive solar designed home will out perform a standard 6 star home substantially with respect to reduced heating and cooling demands. There needs to be more consumer engagement around this topic.

  4. The other issue is that the rating report does not talk money! How many people know or care what a MJ/sqm/per annum means? But what about $/sqm/per annum? There is no reason why we cannot come up with something that couples energy output info from NatHERS with heating/cooling system options to determine what design choices influence more efficient system use (as mentioned by above commenter)and how much these choices influence ongoing bills for clients.
    Additionally, non-rating mode is extremely useful for providing hours of discomfort. It would be great if this was somehow incorporated into the report format using the adaptive thermal comfort model and the climate zone data to spit out hours too cold and hours too hot in each room. Comfort and money talk, but it comes back to NatHERS going beyond just a compliance scheme to being a design/decision making tool.

  5. The star rating compliance should split the total load for both summer and winter performance – like BASIX does – so that superior winter performance does not compensate for inferior summer performance. Also, while the color of the roof reflects on the passive thermal performance of the house, it should also in some way be linked to the performance of the AC ducting through the roof space. We conducted studies in a fairly recent project we worked on that the light coloured metal roof (“surfmist”)made the A/C about 20% more efficient. That reduction in energy load should be reflected in the rating tools.