Cool roofs: beating the midday sun with a slap of white paint

There’s no shortage of measures that homeowners and businesses can take to lessen their environmental footprint – from better insulation and double glazing, to easing up on air-con usage and swapping in LED lights.

One part of the house that seems to be attracting more than its fair share of attention is the roof. Replacing a standard roof with a lovingly cultivated green roof of plants or adorning it with increasingly trendy solar panels can do wonders for reducing a building’s carbon footprint.

Rooftop solar panels are an obvious option for many, and increasingly affordable. The average price of a residential solar photovoltaic (PV) system has been steadily dropping, from $2.40 per watt in 2012 to $1.58 per watt in March this year, according to Solar Choice’s PV price index. On the commercial front, too, prices are at an all-time low of just $1.23 per watt.

Add to this the advent of affordable battery storage systems, and Tesla’s plans to roll out roof tiles that dispense with the need for separate solar panels over the coming months, and rooftop solar has never been more attractive.

But a building’s rooftop can be used for more than just harvesting the sun’s rays. Indeed, cool roofs aim to do exactly the opposite, reflecting as much of the sun’s energy as possible. A flat roof in the midday sun receives about 1,000 watts of sunlight per square metre. A dark roof will absorb most of this energy, heating the roof and underlying building, as well as the surrounding air. Air conditioners that suck in this hot air can further exacerbate a building’s cooling requirements.

“If you have a cool roof, that problem can be eliminated,” says Geoff Smith from the University of Technology Sydney, a specialist in green roofing technologies.

The easiest way to reflect the sun’s rays is to paint a roof white – something the Greeks have been doing for centuries. A white roof reflects around 85% of the sunlight that hits it – at least when it’s clean – and heats to just a few degrees warmer than the outside air temperature. A black roof, by contrast, can heat to more than 80C, according to sustainable construction expert Chris Jensen from the University of Melbourne.

“On a black tile roof, you could fry an egg, and on a cool roof you could walk on it in your bare feet,” says Jensen, “that’s the magnitude of the difference.”

Recently, Smith’s group and others have produced roof coatings that keep roof temperatures even lower than the ambient temperature. They achieve this by reflecting sunlight using thin plastic sheets – akin to a plastic food wrap – often combined with layers of silver and other reflective nanoparticles.

But there’s a catch. “These experiments might look marvellous in the scientific literature,” says Smith, but in most cases, they “just won’t work in practice.” That’s because most of these super-reflective coverings look essentially like a mirror. 

Read more https://www.theguardian.com/sustainable-business/2017/apr/13/cool-roofs-beating-the-midday-sun-with-a-slap-of-white-paint

Courtesy of Guardian News & Media Ltd