Paint me cool: scientists reveal roof coating that can reduce surface temperatures up to 6C on hot days | Extreme heat

Australian scientists have developed roof coverings that can passively cool surfaces up to 6C below ambient temperature, as well as extract water from the atmosphere, which they say could reduce indoor temperatures during extreme heat events.

Heat waves are becoming more intense, more frequent and more deadly due to human-caused global warming.

A coating made from a porous film, which can be painted over existing roofs, works by reflecting 96% of incoming solar radiation, rather than absorbing the sun’s energy. It also has high thermal emissivity, which means it efficiently dissipates heat to space when the sky is clear. Its properties are known as passive radiative cooling.

“Painting, even during the day when the sun is out, can be colder than the air around it,” said the study’s lead author, Professor Chiara Neto of the University of Sydney.

The coolness of the painted surface also means that vapor from the atmosphere easily condenses on its surface – much like dew that forms overnight on a car.

“This material is capable of extending the period during which dew can form by at least a few hours,” Neto said. “Instead of having dew only form… say four to six hours on a good night, you can extend that to eight or ten hours.”

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In a study published in the journal Advanced Functional Materials, researchers tested a prototype for six months on the roof of the Sydney Nanoscience Hub, combining the cold paint with a UV-resistant topcoat that encouraged dew droplets to roll into a container.

According to scientists, up to 390 milliliters per square meter per day could be collected for around a third of the year. Based on this water collection rate, an average Australian roof – around 200 square meters – could provide up to 70 liters on good dew-collecting days, they estimate.

“This would supplement existing water supplies, whether mains or rainwater,” Neto said, suggesting the system could be used in buildings in remote locations or where there is little access to groundwater.

“The cooling aspect and water go hand in hand when we talk about impact,” she said.

In well-insulated buildings, a 6C decrease in roof temperature “could result in a smaller fraction of that cooling being reflected into the upper level of the house”, Neto said, but greater temperature reductions would be expected in most Australian homes, “where insulation is quite poor”.

She said the cladding could also help reduce the urban heat island effect, in which hard surfaces absorb more heat than natural surfaces, resulting in urban centers being 1-13C warmer than rural areas.

The prototype coating studied in the paper was made of poly(vinylidene fluoride-co-hexafluoropropene), which is used in the building industry but is “not a scalable technology in the future,” Neto said, due to environmental concerns with perfluorinated materials.

However, researchers are now commercializing a water-based paint with similar properties, which they say is suitable for widespread use. Neto said the paint would cost about the same as standard high-end paints.

Sebastian Pfautsch, a professor of urban management and planning at Western Sydney University, who was not involved in the research, said cold coatings had been in development for a decade but had not yet achieved widespread commercialization. “I expect this to happen before 2030,” he said.

“The big application of these coatings is that you can retrofit [existing buildings].

“Three or four watering cans that you get from your roof for free – that’s a fantastic principle,” Pfautsch said of dew collection.

However, the real test would come during times of water restriction, he said.

“In drought conditions, your relative humidity would also be very low,” leading to less water collected.