In addition to adding solar panels or a cultivated green roof of plants to reduce a building’s carbon footprint,a building’s rooftop can be an effective tool to cut a building’s cooling costs by more than 20%.
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.
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.
Unlike green roofs, which can require considerable infrastructure to support and maintain, cool roofs can be achieved with a painted-on coating to an already existing roof. While white reflects the most visible light, coloured roofs can also be made cooler by coating with materials designed to reflect light in the infrared spectrum, which also contributes to how hot a roof gets. When combined with an elastomeric weatherproof coating, upgrading to a cool roof can add years to the life of an ageing roof, says Jensen.
The benefits of a cool roof extend beyond the building it sits upon. The heated air that sits above dark roofs is a large contributor to the urban heat island effect, which can put urban temperatures three to four degrees warmer than surrounding areas. Modelling of urban heating in Chicago estimated that the air temperature above cool roofs would be reduced by 7-8C compared with conventional roofs. Cool roofs even outperformed green roofs – by about a degree.
The energy savings are also large. Increasing a roof’s reflectivity from 10-20% to around 60% can cut a building’s cooling costs by more than 20%. But not all buildings benefit – a high-rise apartment or office block won’t reap anywhere near the same benefits as a sprawling low-rise warehouse, shopping centre or airport, or even a single-story dwelling. As an added bonus, work by Jensen and his colleagues has shown that solar panels installed on a cool roof produce almost 7% more electricity on a typical day than those installed on a conventional roof.
At a global level, the impact is nothing to be scoffed at. In 2009, researchers at the Lawrence Berkley National Laboratory in the US estimated that some 24 gigatonnes of carbon dioxide emissions per year could be offset if the world’s cities adopted cool roofs – not bad considering that urban areas constitute just 1% of Earth’s land area. The offset for a single house with a 100 sqm roof area is around 6 tonnes of carbon dioxide – a decent chunk of the 26 tonnes per capita of greenhouse gases that Australians are currently emitting.
One of the main challenges, says Smith, is confronting the aesthetics. Although lighter-coloured roofs are becoming more popular in the hotter climate of Queensland, he says, “a lot of people shy off the idea of a white roof.”