A Cool Roof Innovation to Make Heat Waves More Bearable
Recent deaths in India show that during extreme heat waves, access to air conditioning can be a matter of life and death.
With heat waves increasing in frequency owing to the impacts of climate change, that’s a problem for the billion-plus people without access to reliable electricity—or those anywhere who can’t pay for the power it takes to run an air conditioner.
Cool roofs—roofing materials that reflect sunlight away from their surface—are one way to lower indoor temperatures and cooling costs. But they still pass on some heat to the spaces below.
To solve that problem, two Australian researchers have developed a material that’s the first, they say, to cool a roof to a temperature lower than the air around it.
The material works by mimicking what happens on a roof after the sun goes down, said Geoff Smith, a professor emeritus at University of Technology in Sydney, who collaborated with UTS researcher Angus Gentle on the project.
Once the sun has set, roofs made from heat-absorbing substances not only release heat back into the atmosphere but also become cooler than the ambient temperature. “Our surface achieves this in the daytime by reflecting almost all of the incident sunlight, thus allowing the surface to remain cooler than the atmosphere,” Smith said.
The material can be easily applied like a coat of paint, according to Gentle and Smith, and could save a household 25 to 50 percent of its energy costs for cooling. Tests performed in Australia’s summer heat showed that the material kept a roof cooler by about 20 degrees Fahrenheit compared with a white roof nearby.
For people sweltering in summer heat waves, the material could help lessen the effects of extreme high temperatures by reducing the heat island effect: the tendency of built-up areas to become hotter than rural areas nearby during the same weather event. It could also reduce peak demand for power, lowering energy costs as well as the chances of a power grid failure or brownout.
The material would likely be effective in both industrialized and less developed communities, Smith said, as there are examples of similar coatings working on weathered or otherwise imperfect surfaces, such as rusting or partially degraded roofs.
The results were published in the journal Advanced Science in May.
Smith said he and Gentle were inspired to start the project after they observed some limitations in the cool roof material used in another researcher’s experiment. They felt that the other scientist’s coating material wasn’t sturdy enough to be used as a surface on a real rooftop.
“Something was needed for normal everyday roofing,” Smith said. “Either a paint or special flexible polymer.”
The material the pair devised is composed of layers of commercially available plastics that have slightly different refractive properties and are stacked together with a silver film that creates a mirroring effect.
“The layering structure is designed to reflect strongly across the visible and some but not all non-visible, solar wavelengths,” Smith said.
When Smith and Gentle applied the knowledge gathered during their previous work in nighttime coating material, development was quick. “In less than three weeks, we had the new working surface deployed on our rooftop test rig,” Smith said.
But don’t pull the plugs on your fans just yet. Before their material can move beyond the working prototype stage, the researchers must meet a key challenge: how to reduce the glare that results from the material’s highly reflective qualities, which could be distracting to drivers and others in a densely built environment.
They also have to figure out how much the material would cost on the market.
Said Smith, “[That] is one focus for the next stage of research, as variations on our plastics are only used in niche markets at present—such as small displays and mirrors for solar thermal energy.”