Even before adding solar panels, your roof can help you keep cool and lower your energy bills
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With an El Niño under way in the Pacific, Australia could face record temperatures this summer. For those in hot homes, reflective roof coatings can be a cost-effective way to beat the heat.
The upstairs bedrooms and office in Nic Jacobsen’s two storey home in inner-suburban Adelaide used to get unbearably hot, even when his large air-conditioning system was running flat out.
“The insulation was not great, so I did some work upgrading that,” says Jacobsen, an engineer who has worked in renewable energy for two decades. But after his changes, Jacobsen could still feel intense heat radiating through the flat metal roof and blue board and plaster stud walls.
“The western wall in particular copped a lot of sun; because it was the second storey there was no shading from trees or adjacent buildings,” he says.
After Jacobsen painted the roof and wall with a heat-reflective coating, the change was immediate.
“It definitely lowered the heat penetrating the house upstairs,” he says. Although he can’t quantify the savings, he reckons he cut his air conditioning bill. “I’m very happy with the decision I made,” he says. “I think it’s a good product and look for opportunities to recommend it.”
Most Australian house roofs are either metal or tiled, though many apartments have concrete roofs. The majority are medium to dark in colour, according to CSIRO’s Australian Housing Data, even though lighter shades generally reflect more heat.
Shane Strudwick says Australia has a large blind spot when it comes to the role roof surfaces and dark colours play in heating homes. Strudwick distributes Super Therm, the coating Jacobsen applied to his home, which is more commonly used on commercial and industrial properties.
A water-based ceramic paint developed by Nasa for the space shuttle, Super Therm repels infrared radiation, preventing roofs from heating up and transferring heat inside the house.
In a basic trial on a 22C day in Adelaide, Strudwick coated some concrete roof tiles and left others untreated. By 1pm, he says, the surface temperature of the original tiles had hit 60C, while those with a single coat of Super Therm were just 26C.
He says the cost of coating a roof will generally pay for itself in lower energy bills in three to five years, though he cautions that payback times vary depending on the original surface and previous household energy use.
A cost-benefit analysis by researchers at the University of New South Wales concluded that the cost of coating an otherwise functional roof is recovered not just through energy savings, but also by prolonging the life of the roof itself (which also reduces waste going to landfill).
Leading cool roof researcher Prof Sebastian Pfautsch says people who’ve unwittingly bought homes with dark roofs find themselves “in a real schemozzle” when summer hits, especially if their roof is covered in cement tiles rather than metal.
“Cement has a much greater thermal mass, so it takes much longer to cool down,” explains Pfautsch, who is based at Western Sydney University.
Pfautsch says retrofitting a roof with a reflective coating should be a first step, ahead of other options such as installing solar panels or better insulation.
Solar panels can help cool a roof by converting radiant energy into electricity. Yet photovoltaic systems become less efficient if they get too hot, so installing panels on a dark, heat-absorbent roof is a false economy.
“The first and most cost-effective thing to do is coat it,” says Pfautsch. “After that, put solar on. Once you have a light-coloured roof your solar panels work more effectively.”
Combining panels with plants can create the triple win of a “bio-solar” roof – saving energy through insulation, boosting solar power output and increasing biodiversity.
But planting a garden on top of your home can be challenging and expensive. In most cases, recoating a roof is cheaper and simpler.
“There are many ways to retrofit a house to keep it cool and save energy but applying a coat of paint on the roof is one of the most efficient and low cost”, says Prof Chiara Neto, a physical chemist at the University of Sydney.
“It could save up to 40% of your energy bill, especially if you have an older building with poor insulation and a dark roof.”
Neto developed a film that reflects 96% of heat from the sun and also collects water.
In 2022, Neto and her former PhD student founded a startup called Dewpoint to commercialise their invention. It hopes to retail the coating in Australia at a price competitive with existing premium paints. The product is undergoing trials in the Middle East, Asia and Australia. “People want to see it in action in a real building and measure energy efficiency over time,” Neto says.
“The paint is designed to let go of droplets so they roll off the roof readily and can be easily collected before they evaporate,” she explains. And because the roof stays cooler for longer, it’s more efficient at condensing moisture from the atmosphere. Plus, it’s self-cleaning, which maintains its reflective properties.
Pfautsch says Super Therm is one product already on the market that is worth considering.
“I have no shares and I’m not a sales agent for them,” he stresses. But some products don’t deliver what they promise, he warns, noting that Dulux paid a $400,000 penalty in 2016 for making unfounded claims about the temperature-reducing properties of two paints.
A heat-reflecting roof will generally be light-coloured but doesn’t have to be white, Pfautsch says, so it’s possible to avoid an excessively bright roof that reflects glare into nearby buildings. Neto notes that heat-reflective roof coatings mostly have a positive impact on nearby residents.
“If all roofs were treated, that could reduce the air temperature in a suburb by two or three degrees,” she says. “By preventing surfaces from getting hot, removing heat and passively cooling the surface, neighbouring buildings will benefit.”
If you’re suffering under a hot roof, Pfautsch encourages you to do yourself, your neighbours and the planet a favour.
“It’s an ethical question of how you want to live in your community and make your contribution,” he says.