4 March 2024

Heat on black roofs as government study looks at the 'cool' alternatives

| Ian Bushnell
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New housing estate

Roof colours vary in the new suburb of Whitlam where energy rebates are having an impact. Photo: Suburban Land Agency.

Is the ACT Government going to ban black roofs in new residential areas?

It’s not saying, but it does want to know what the benefits of cool, or lighter coloured, roofs that reflect heat are and how much it would cost to mandate them.

The Environment, Planning and Sustainable Development Directorate has contracted UNSW to conduct a study into cool roofs as a first step towards potential requirements for new homes in the ACT.

Black or dark coloured roofs dominate the urban landscape in Australia despite their known contribution to the heat island effect.

A NSW Government bid in 2022 to ban black roofs faltered after a pushback from property developers complaining about the greater regulatory burden and cost.

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A directorate spokesperson said the study would look at what reductions in urban heat could be achieved with cool roofs, and the energy savings and costs for homeowners.

It would also assess any additional building costs that may be incurred or savings to be gained.

“Light-coloured materials themselves don’t typically have a higher cost,” the spokesperson said. “However, roof colour feeds into achieving a home energy rating for new homes. This is because cool roofs are beneficial for keeping a home cool in summer, but could potentially increase winter energy needs.”

The spokesperson said new suburbs could be much hotter than established ones, particularly while street trees and gardens were still growing.

“This can add to residents’ cooling costs, have health impacts for vulnerable people and make outdoor activities unpleasant in hot weather,” the spokesperson said.

According to the contract, UNSW must consider likely heating and cooling requirements under current and future climate scenarios, in both air-conditioned and non-air-conditioned homes, as well as the impact of urban heat in residential zones under a “do nothing” scenario and a scenario where cool roof provisions are put in place.

It has been tasked to develop cool roof rules that could be implemented through the planning or building system, and recommend the most effective pathway for implementing them.

The contract says UNSW should look at what barriers there may be to introducing requirements including physical disadvantages and cost, what exemptions there should be and what other ways could be used to reduce urban heat.

EPSDD said light-coloured roofing steel and tiles were readily available and high-tech options like green roofs and dark-looking materials that reflected heat were also available, though not widely used on homes yet.

“The study is unlikely to recommend mandating high-tech options,” the spokesperson said.

The Suburban Land Agency (SLA) offered energy rebates in new suburbs, including in Whitlam and Jacka, which provided homeowners with a financial carrot for meeting energy requirements, including for a cool roof.

This had had an excellent take-up in Whitlam where about 30 per cent of roofs were ‘cool’.

The spokesperson said the new planning system also included an urban heat assessment outcome for commercial zones and the Community Facility Zone that encouraged use of cool roofs for offices, schools and mixed-use apartment buildings.

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Master Builders ACT CEO Michael Hopkins said black roofs and black external materials impacted a building’s energy efficiency rating.

“The National Construction Code has recently increased the minimum energy rating to 7 stars, meaning black roofs are already considered when new buildings are being designed,” he said.

“The ACT Government has been working with industry on the implementation of the NCC, and we will continue to work with government on how we create more energy efficient buildings.”

A spokesperson for Sustainable Building and Construction Minister Rebecca Vassarotti said the government regularly investigated emerging technologies to understand their potential impact on its continuing work to mitigate the heat impact of climate change in the suburbs.

“The government is undertaking an exciting first step of investigation into cool roofs – to help us understand if cool roof requirements for residential areas would be beneficial,” the spokesperson said.

“The outcome of the study will determine if broader industry and community consultation is needed in future.”

The government spokesperson said the government was actively considering consultation with the building industry so that this initial investigation could incorporate its technical expertise and information.

UNSW is expected to deliver a final report in June.

The contract is worth almost $106,000.

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I’ve heard some people are having their existing rooves painted white. I wonder how efficient that is and how long the coating would iun Canberra’s hot and freezing weather.

New suburbs throughout Australia are nothing but grey deserts…..boring as…..

There are many, many stupid regulations in the world – but regulating dark roofs out of existence would not be a bad one.

Helen Cross, It can be 5-8C hotter under a dark roof than a light roof.

Nathan Burraston, it’s not always that the new blocks are too small, but that the houses have got too big. The average family home in 1950 was about 100sq.m (looking at a graph, about 3.7 people per average household); while today it’s over 200sq.m (with about 2.5 people per average household). So, houses getting bigger, but the average number of people living in them, gets less.

Helen Cross, No, I’m told by two different people who understand physics much better than me. They both said that a white roof works better in winter too, than a dark roof. I have tried to find information on this, and the best I could find was a short letter in a science magazine in response to someone about using a black roof for winter. The response reply said, “Physics says otherwise. In winter, a white surface will radiate less heat, helping to keep the warmth in.”
I put a pale roof on my energy efficient house. I say ‘pale’ because when I built my house, white roofs were not allowed. I chose a very pale grey.

Two real-world examples:
The SR-71 Blackbird was black not just for visual reasons, but because at the speeds it travelled it generated massive heat in the aircraft body through friction, and being black it could dump heat faster than a light-coloured airframe.
Desert-dwelling Bedouin who wear black. You would think that white would surely be a better colour for their robes, reflecting more heat. But black *radiates* more heat which means they can get cooler, faster.

For homes, in winter when you want to keep heat in, a black roof will radiate more heat and cool your house faster.
(in summer, the smaller temperature differential between the roof and the outside air means it doesn’t radiate as much, so the roof stays hotter)

“For homes, in winter when you want to keep heat in, a black roof will radiate more heat and cool your house faster.”
You don’t want a house to cool faster in winter. So have a pale roof.

Radiative efficiency is a big one, as mentioned in other replies. There are another couple of issues at play too. The first is that the sun is lower in the sky during winter, so less heat would be gathered by the roof, whatever colour it was. The second is that there are fewer hours of daylight in winter, so there is less time for any of this heat-gathering to have an effect. This where passive solar design comes in to play, for example, locating windows so they receive sun in winter but are shaded (e.g by the eaves of the house) in summer.

When I had my energy efficient house built, I naturally got a pale roof. The problem with dark roofs has been long known and I can’t understand why people still get them. Look at satellite views, fields of dark roofs, with an occasional pale roof from a thinking non-sheep person.
Even when I explained to someone the problem with dark roofs, and that their house will be hotter (in this case, not here, but for a house up north) they still got a dark roof. And they know how comfortable my house is, and how low (almost zero) my energy bills are. Madness; they ‘just had to have’ that dark roof like everyone else! Baa!

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