Researchers from the University of Technology Sydney have developed a surface made of a combination of polyesters on a silver layer called a “coated polymer stack” that they say can remain cooler than ambient air.

According to an article in the journal Advanced Science, the surface may help reduce urban heat loads and thereby cutting greenhouse gas emissions and energy use.

An infrared image showing the temperature difference between the new surface (centre) and an existing cool roof used in testing.Image credit: UTSMost roofs heat up by absorbing sunlight. Although black roofs absorb heat more, even white roofs absorb enough sunlight to warm up by 9 C to 12 C. However, this new surface stayed 11 degrees cooler than existing white roofs.

It does this by absorbing around 3% of incident sunlight and simultaneously radiating heat at infrared wavelengths that are not absorbed by the atmosphere.

"We demonstrated how to make a roof colder than the air temperature around it, even under the most intense summer conditions," says Emeritus Professor Geoff Smith from the University of Technology Sydney, one of the study’s authors.

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

The research work has been supported by an Australian Research Council Discovery Project grant.