A super cool roof solution to being hot in the city

May 28, 2015 by Terry Clinton, University of Technology, Sydney
An infrared image showing the temperature difference between the new surface (centre) and an existing cool roof used in testing.

Sydney materials scientists are claiming a breakthrough in cool roof technology with a surface they've developed that will stay cooler than the ambient air temperature, even under the mid-summer Australian sun.

The development, with major implications for reducing the heat load in urban areas and consequently cutting energy use and , is reported in the latest edition of the journal Advanced Science.

The study by Dr Angus Gentle and Emeritus Professor Geoff Smith from the University of Technology Sydney has reported results from a "coated polymer stack" – a combination of specially chosen polyesters on a silver layer.

"We demonstrate for the first time how to make a roof colder than the around it, even under the most intense summer conditions," Professor Smith said.

"Roofs heat up by absorbing sunlight, so darker roofs can get very hot. Even white roofs still absorb enough sunlight to warm up by 9 degrees Celsius to 12 degrees Celsius.

"This new surface, however, stayed 11 degrees or more colder than an existing state-of-the-art white roof nearby because it absorbs only 3 per cent of incident sunlight while simultaneously strongly radiating heat at infrared wavelengths that are not absorbed by the atmosphere.

"Furthermore the plastic materials used for the demonstration were available commercially and potentially suited to use on basic roofing."

Professor Smith said that cooling a roof below ambient air temperature had up until now been "an elusive target".

Dr Angus Gentle holding a piece of the special material over an existing cool roof used in testing.
"Cool roofing reduces the severity of the urban heat island problem in towns and cities and helps eliminate peak power demand problems from the operation of many air conditioners," he said.

"The added feedback benefits from cool roofs are not yet widely appreciated, but recent reports have shown they are substantial. Examples include ventilation with cooler air and higher performance of rooftop air-conditioning installations."

The test roof was placed above busy Harris St on top of the UTS Faculty of Science building with a clear view of the sky and no shadowing.

Data was collected for an unprotected new surface and one aged over several days in a polluted outdoor summer environment for assessment of the impact of the build-up of dust and grime.

Professor Smith said the surface maintained its high performance in all conditions. "Extensive dew formation is inevitable for a super cool roof and dew drops precipitate dirt. This roof site being 25 metres above a busy city transit road was a stern test. Results show that excellent thermal performance can be maintained.

"Much of the world's population lives in warm climates. Keeping a cool saves energy and makes building interiors comfortable in summer. If enough roofs in a precinct are kept cool then the local climate can also be beneficially influenced," he said.

Explore further: Cool roofs in China can save energy and reduce emissions

More information: "A Subambient Open Roof Surface under the Mid-Summer Sun." Advanced Science. doi: 10.1002/advs.201500119

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5 / 5 (1) May 28, 2015
If this material can retain it's properties for it's lifetime without having to be washed on a regular basis it could be quite a breakthrough.
1 / 5 (1) May 28, 2015
I don't think this material is safe; notice how the guy in the picture needs to where sunglasses just to demonstrate his product. If every house had these roofs, it would blind pedestrians, vehicles, and planes worse than laser pointers.
not rated yet May 28, 2015
Huuummm, that could be a valid point. The law of unintended consequences strikes again.
3 / 5 (2) May 28, 2015
Hi sbeare. :)
I don't think this material is safe; notice how the guy in the picture needs to where sunglasses just to demonstrate his product. If every house had these roofs, it would blind pedestrians, vehicles, and planes worse than laser pointers.
From the article:
strongly radiating heat at infrared wavelengths
This improved reflection/re-radiation component is not in the damaging wavelengths like UV. Nor would there be much more 'glare' in visible light than snow or white roofs. Anyway, it is mandatory "Health and Safety' guidelines/insurance requirement to wear relevant safety equipment at all times during such experiments/demonstrations involving any kind of glare/radiation which may damage eyes, irrespective. Especially for researchers/workers routinely and often exposed to same as part of daily professional duties. For example, Radiologists have to wear special Lead etc aprons etc to minimize CUMULATIVE exposure damage, even though each x-ray shot is safe.
3 / 5 (2) May 28, 2015
Wow, are you people over-thinking the sunglasses. He's on a roof on a sunny day.
not rated yet May 29, 2015
The man in the image is holding a mirror and deliberately aiming the reflective beam at the camera. Maybe he is trying to show you all that it is a sunny day.
Recently, in Adelaide, Aus, I changed the iron on my roof from grey unpainted zinc to Colorbond white. In previous summers I have needed to run the airconditioning for long periods. But now, even in very hot summer weather, I have to wear warm clothes inside. About 10degC cooler.
not rated yet May 30, 2015
Now if we could have cold roofs for Canadian summers and hot roofs for Canadian winters, that would be nice.
not rated yet May 31, 2015
If they could make it superhydrophilic thus getting the water to run off, this would be a version of the "dew collectors" in Frank Herbert's Dune.

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