Fluorescent roofs may keep homes cool in summer and warm in winter: study

Choosing the colour of the roof on your home can impact your electricity bills and your comfort as well as the planet at large.

From up in the sky looking down, rooftops are a large part of the 2–3 per cent of the Earth’s surface covered by urban areas.

And the way that big built surface absorbs and reflects light and solar radiation from the Sun can affect not only ambient temperatures in our suburbs, but also global warming.

Black roofs barely reflect any light and absorb a lot of solar radiation, meaning they trap a lot of heat during the day — making your house and neighbourhood hotter.

This heat is then emitted at night, when the ambient temperature cools down, and gets stuck in the Earth’s atmosphere.

Lighter colours, on the other hand, can reflect about 80 per cent of sunlight during the day and absorb less radiation.

In commercial development, there’s an even higher class of “super cool” materials used on roofs, ranging from synthetic polymers to paints.

These products have upwards of 95 per cent reflectivity, and one group of US scientists has even made a paint so white it reflects about 98 per cent of sunlight.

At that rate, so much solar radiation is emitted back into space, out past the atmosphere, it actually has a cooling effect, and makes the treated surface several degrees cooler than the surrounding air temperature.

This is known as passive daytime radiative cooling (PDRC) and can result in a surface being 7 to 8 degrees Celsius under the ambient temperature.

There’s a lot of research going into PDRC materials because it offers a way of cooling our buildings and the planet, without using energy on things like air conditioning, which can lead to a warmer climate.

If you could cover just one per cent of the world’s surface in these reflective materials, it’s been theorised it would offset a lot of the heating caused by humans.

So why aren’t we doing just that?

Cool in summer, warm in winter

So far, PDRC materials have mostly been used on high-rise commercial buildings, but there are drawbacks when adopting them for low-rise homes.

Cost and scalability is one issue. Another is the fact PDRC materials are white or silver, and their high reflectivity can create glare.

But the main problem is that PDRC materials are so effective at cooling they can make your house colder in winter, meaning you’re then more likely to use heaters.

Architectural engineer Hassan Khan and his colleagues at UNSW have been working on a different approach to the problem by adding a fluorescent coloured film on top of PDRC materials.

These materials are known as “passive-coloured radiative coolers”.

In a recent study led by Dr Khan, several colour configurations (using orange, red and green) were found to have similar cooling effects to white PDRC materials in hot and dry conditions, while also retaining some warmth in winter.

The results of the study were published in the peer-reviewed journal Solar Energy Materials and Solar Cells.

One of the orange cooler tests in Alice Springs was found to be on par with a PDRC reference sample.

Meanwhile, testing in Sydney during winter saw orange, green and red configurations result in temperatures up to 3.8C, 3.6C, and 5.5C hotter than a PDRC.

Dr Khan said fluorescent dyes were added to a synthetic film for the study, but that researchers could develop other forms of the “super-cool” material.

“Now we are going to bring [coloured super-coolers] into a paint form, and those paints can be applied on any surface,” he said.

Since the study was undertaken, the range of super-cool colours has expanded to include purple and different shades of red.

Dr Khan said the team was also working on a potential black option.

“These black surfaces are not going to absorb the radiation the way that conventional black surfaces absorb. They are going to provide much better results,” he said.

Using fluorescent dyes was also cheap, Dr Khan added, making the technology more scalable.

The researchers are now working on improving durability and extending the life of the fluorescent materials from about five years to upwards of 15.

But beyond the technical hurdles still to come, cool roofs face another challenge.

No Australian standard for a ‘cool roof’

While Europe and the US have cool roof councils that certify the claimed performance of commercial products, there’s currently no standard in Australia for what constitutes a cool roof.

Mat Santamouris, a professor of high performance architecture at UNSW, said he tried to push for an Australian version to no avail.

“We need to have certified products. This is very simple,” Professor Santamouris said.

“Once the materials are certified, the clients have guaranteed results.”

While there isn’t a standard on roof products in Australia, that doesn’t necessarily mean roof materials aren’t taken into account with new builds.

Most of Australia has now adopted updated national building code standards that require homes to have a 7-out-of-10-star energy rating.

And some states such as South Australia and Victoria are considering wider bans on dark roofs.

So what should you do if you want a cooler roof?

Elham Monavari, a sustainability expert at Green Building Council Australia, said if you want to choose a regular cool roof, colour was still an easy starting point.

Tiles and steel are the most common roof materials for house-builds in Australia. Ms Monavari said for steel products, there was usually information about its solar reflectivity level.

“The higher that value, the better that roof is going to be at reflecting the heat,” she said.

Ms Monavari said ultimately, getting a cool roof was “low-hanging fruit” from a home sustainability point of view.

“When I say low-hanging fruit, there’s not usually an extra cost for the materials,” she said.

“It does become a bit of a no-brainer. Why wouldn’t you do something that makes you more comfortable inside [your home]?”