Materials

Researchers in California believe that making city roofs and pavements more reflective could counter global warming by reducing the amount of solar energy absorbed by the earth – equivalent to massive CO₂ emission savings.

Hashem Akbari and co-workers at Lawrence Berkeley National Laboratory and California Energy Commission have calculated that the solar reflectance of typical dark-coloured flat roof can be increased from 10-20% to as much as 70-80% by using white roofing materials. Over a typical roof area of 1000 ft², this could result in a CO₂ emission offset of 10 tonnes.

And of course, white roofs also save about 20% on a building’s air conditioning costs. If adopted across the US, simply installing white roofs could save over $1 billion a year in annual energy bills.

Since 2005, California has required that flat roofs are white but from July 2009 will also require that sloped roofs are cool-coloured.

However, Akbari argues that as well as roofs, the solar reflectance of pavements can also be increased by using low-carbon, light-coloured concrete, light-coloured aggregates or novel materials like grasscrete to offset CO₂ emissions.

“This is such a low level technology that developing countries can find their own technology solution,” says Akbari.

Taken together, the researchers’ projections for increasing the solar reflectance of roofs and pavements in the world’s growing urban regions could offset 44 Gt (billion metric tonnes) of CO₂. This is more than the total annual CO₂ emissions projected for 2025.

Installing white roofs doesn’t have to cost more either, says Akbari. Depending on the material used, white roofs cost the same or sometimes less than conventional roofs. There is even some evidence, he adds, that white and light-coloured roofs actually last longer because they are less prone to diurnal cycles of expansion and contraction.

Akbari and his colleagues are calling for an international campaign to encourage cities in tropical and temperate regions to install white roofs and cool pavements.

“An international ‘cool cities’ programme is a win, win, win case,” says Akbari. “Cool roofs save cooling energy in air conditioned buildings and improve comfort in buildings that are not (win #1). Cool roofs and pavements reduce summer heat islands, leading to better air quality (win #2). And now [we] show that cool roofs and pavements cool the entire globe (win #3).”

Discussions are currently underway with the Clinton Foundation and other organisations to take the idea forward, Akbari told Energy Efficiency News. “We are making qualitative progress and I am optimistic,” he says, “but it won’t happen overnight.”

For further information:
heatisland.lbl.gov/
Hashem Akbari, Surabi Menon, Arthur Rosenfeld. Global Cooling: World-wide Urban Albedos to Offset CO₂. Climate Change (2008), to be published.