Sustainable Cool Roofs: Innovative, Cost Effective, and Energy EfficientBen Jervey for Zondits, October 31, 2014
With tens of thousands of people regularly passing through Manhattan’s Jacob Javits Center, climate control is essential. Few visitors realize, however, that the four-block-long building hosts an amazingly innovative and unique cooling system—and that it’s right above their heads.
A massive green roof spreading more than six and a half acres is planted on the Javits Center roof. After the Ford Motor Company’s historic River Rouge facility, it is the largest living green roof in the country.
Such verdant roofs – and white, or reflective, roofs – are becoming more and more popular as building owners and managers attempt to reduce cooling costs and as city governments address both the urban heat island (UHI) effect and climate change.
The city of Chicago boasts more than 350 green roofs, which transform 125-plus acres of the skyline to living vegetation. This helps the city manage storm-water runoff and reduce local air temperatures. The buildings and their occupants themselves benefit as green roofs also keep indoor temperatures down, reducing the energy demands for summertime cooling.
In New York City, high-profile green roofs like those at the Javits Center and on the Empire State Building share the skyline with hundreds of acres of “cool roofs,” which have been coated with a reflective white paint. The white surfaces reflect the sun’s rays, similarly reducing cooling costs, energy demands, and greenhouse gas (GHG) emissions.
There is debate, however, over the net climate impact of such green and white roofs, with plenty of contradictory research arguing the benefits and consequences of each.
A Stanford University study from 2011 came to the surprising conclusion that white roofs could actually worsen global warming, largely because the local cooling means less hot air rising and therefore fewer clouds forming, and because some of the sunlight bouncing off the roofs would be caught by particles of soot, further heating the atmosphere.
Stanford University engineer Mark Jacobson, lead author of the study, concluded that there does not seem to be a benefit from investing in white roofs.
That conclusion has been taken to task by many scientists, including fellow Stanford climate modeler Ken Caldeira. Caldeira argued that local cooling has never been shown to create global warming. Caldeira asserted that while he did not deny the effect was possible, he was highly skeptical that it would be a common occurrence.
The Stanford paper also stood at odds with a 2010 study by researchers at the Lawrence Berkeley National Laboratory (LBNL), which found that painting white and cool roofs atop all eligible urban buildings in the tropics and temperate regions could offset some 44 billion metric tons of GHG emissions. That’s the rough equivalent of 300 million cars driving for 20 years.
A more recent study from LBNL, published this past January, also found that white roofs are the cheapest roofing option—considering all costs from purchase to installation to maintenance to heating and cooling.
Most recently, an Arizona State University study, published in the Proceeds of the National Academy of Sciences (PNAS) in February, claimed clear climate benefits from white and green roofs.
ASU’s Matei Georgescu, who led the research, said that a large enough number of white and green roofs could totally offset the warming effect caused by urban growth and cold potentially offset some future GHG effects.
Georgescu also wrote that the impacts on energy demands can vary greatly from city to city. The reason is that cool roofs lower building temperatures all year, meaning that those in northern cities benefit less because there is a greater part of the year that those buildings must be heated.
The study does find that green roofs don’t cool buildings in the winter as much as white roofs do – as they can actually offer some insulation and can even promote warming because of the water vapor present.
To extrapolate generally on those findings, green roofs may be a better bet in northern cities, whereas white roofs do a better job in the lower latitudes.
Ben Jervey covers energy, climate change, and the environment from his home in Vermont.