July 11, 2012

Bright is the new black for cities

Construction Corner | Korky Koroluk

Drives through the countryside or a walk in the park are cooling activities enjoyed by many on hot days in the city. Even when it's hot in the country, it's hotter, somehow, in town.

That’s why people are trying to cool our cities down a bit, and a test of experimental coverings in New York City shows that it may be possible.

New York was a good place for the test. It gets about 14 days every summer above 31 C, a total that is likely to climb year by year.

So, some test roof coverings were installed and scientists measured how they performed.

One day during a heat wave last summer, dark, sunlight-absorbing roofs reached temperatures of about 77 C.

At the same time, a white roof covering was 24 degrees cooler, measuring 53 C.

Those readings weren’t just one-day variations.

Measured over the course of the summer, the temperature differences remained proportionately similar.

The measurements were taken as part of the city’s Cool Roofs program, which Mayor Michael Bloomberg is promoting as a way to reduce city temperatures in a warming climate while reducing energy use and the resulting emissions of greenhouse gases.

The results were published about three months ago and they should give a boost to the idea of reflective roofs to reduce the urban heat island phenomenon.

The urban landscape of asphalt, metal and dark roofs absorbs more energy from sunlight than forests, fields or snow-and ice-covered landscapes, all of which reflect more light.

Absorbing that extra energy makes cities warmer than the surrounding countryside and the effect is especially noticeable at night, when, even with nighttime cooling, cities remain three or four degrees warmer.

All that leads to spikes in electricity use and poorer air quality, which has had city planners throughout the industrialized world talking about reducing the effect by converting conventional dark roofs either to green roofs covered in plants, or to white roofs, which is a much less expensive alternative.

Three kinds of white roofs were tested in New York City — two membrane roofs, which require professional installation, and a white-paint coating that can be a simple do-it-yourself project. Cities have been progressively darkening the landscape for hundreds of years, said Stuart Gaffin, a Columbia University researcher and lead author on the paper detailing New York’s efforts.

“This was the first effort... to reverse that. City roofs are traditionally black because asphalt and tar are waterproof, tough, ductile and were easiest to apply to complex rooftop geometries. But, from a climate and urban-heat-island standpoint, it makes a lot of sense to install bright, white roofs. “That’s why we say, ‘bright is the new black’.”

The study measured the amount of energy reflected by the roofs, as well as the amount of infrared energy they radiated after absorbing solar radiation.

All three of the white-roof options performed about the same when first installed, but performance differences didn’t take long to become apparent.

Reflectivity and emissivity of the professionally installed white membrane coverings held up well, and continued to meet Energy Star standards after four years.

The effectiveness of the white acrylic paint-on coating was cut in half after two years and ended up falling below the Energy Star standard.

Even so, the do-it-yourself choice is attractive because of cost. The white membrane roofs cost anywhere from $135 to $250 per square metre; the paint-on alternative costs only about $5.50 per square metre.

The paint-on white acrylic “is the lowest hanging fruit,” said Gaffin. “It’s very cheap to do; it’s a retro-fit. You don’t need a skilled labour force and you don’t have to wait for a roof to be retired.”

So, if you are looking to brighten the cityscape, he said that this is the fastest and the cheapest way to do it.

Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments or questions to editor@journalofcommerce.com.

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