Painting the Town White

Studies show that white rooftops reduce carbon emissions and air-conditioning costs. But could they work throughout NYC? The roof at MoMA QNS might tell us.
February 22, 2011
The roof at MoMA QNS was painted white as part of PlaNYC.

About a year and a half ago, the city painted the roof of MoMA QNS white as part of PlaNYC, Mayor Bloomberg’s long-term strategy for New York. During the 2009 pilot project, volunteers coated a total of 100,000 square feet of rooftops in Long Island City, and last year the city exceeded its goal of painting another 1 million square feet of roofs, according to a spokesperson at the Department of Buildings.

By reflecting sunlight back into space, white roofs keep buildings cooler than black roofs do, limiting the need for air conditioning and reducing the city’s carbon emissions. But, like everything in the city, the roofs get dirty. “When they’re new, they’re white as snow,” said Stuart Gaffin, a research scientist at Columbia University, who is working to monitor the efficacy of the roofs. Today, the roof of MoMA QNS is closer to the color of dirty dishwater and is dotted with black puddles.

Gaffin had said that that white roofs can be unpleasant places to spend time. In the summer, they’re hot. “You really have to slather yourself with suntan lotion,” he warned. And, at least at first, they’re so bright as to be off-putting. “When you step on these white roofs, you start thinking, “Get me off of this,”” he said. “You’ll see what I mean.”

But on an unseasonably warm February afternoon, this white roof was a perfectly pleasant place to spend an hour. In the year and half since it was first painted, dirt and soot have coated the once-white surface, and the paint, which is smooth when first applied, had sunk into the roof’s bumpy surface, and the asphalt beneath was peaking through.

Scientist Stuart Gaffin measures the amount of sunlight reflected by the white roof at MoMA QNS.

Gaffin’s job is to measure those sort of changes. “This bumpiness, I’ve found, is not good for reflection,” he said. He considered the roof’s overall condition. “This is worse than I thought it would be,” he said.

There’s no question that white roofs do more to combat climate change than traditional black roofs. Black roofs absorb most of the heat and light that hits them, contributing to what’s called the urban heat island effect, the tendency of cities to trap and retain more heat than suburban or rural areas. (It’s why, for instance, in the summer, city streets are still sweltering hours after the sun has gone down.) On a federal level, energy secretary Steven Chu has endorsed white roofs, and President Obama’s recently released budget chalks up millions of dollars in savings to plans to paint roofs of government buildings white. In New York City, as part of PlaNYC, since 2008 the building code has required most new roofs to have most of their area covered with some sort of reflective white coating.

The data Gaffin collects will help the city construct a longer term white roof strategy. Last Friday, he was up on the MoMA QNS roof, installing pyranometers, devices that will measure the sunlight coming from the sky and the light that’s reflected off the roof. The difference will show how much light the roof is absorbing, or conversely, how much light it’s reflecting.

“Essentially, we’re measuring dirt and soot,” Gaffin said. One thing that’s not clear yet is how typical this roof might be. It’s down the block from an overland subway station, and right next to Queens Boulevard, which could make it particularly dirty.

The puddles are another problem; they’re scattered across the rooftop, and they’re black, which lessens the roof’s effectiveness. Gaffin also pointed out that the layer of paint on the roof doesn’t seem as thick as it was supposed to be, a quarter to a half inch thick. When Marty Odlin, a staffer helping with the installation, pulled up a wire that was painted over, exposing the asphalt roof underneath, it was clear that the paint was no more thick than a few sheets of paper.

The paint used for the roofs is a specific type, an elastomeric acrylic that has some elasticity to it, so it can stretch and contract with the roof as the temperature changes. It’s not the only option for coating a roof, but the alternatives are roofing membranes that require professional installation. The city’s white roofs program has recruited and used volunteers to meet its goals so far; painting roofs white could also turn into a low-skill green job.

While white roofs are relatively easy to install, one of the questions Gaffin’s research will help answer is how much maintenance they’ll need to retain their advantage. Will the city need to commit to cleaning them regularly, for instance? Or should the strategy be to invest in the roofing membranes, which, Gaffin says, tend to hold up better over time?

But the choice of rooftop materials isn’t black or white, so to speak. Green roofs — usually roofs paneled with shallow boxes of low-growing plants — cool the city, are more pleasing to the human eye, and help clean the air of pollution. In the past few months, as part of the NYC Green Infrastructure plan, the mayor’s office has also started touting blue roofs, which are designed to capture and store rainfall, reducing the burden on the city’s wastewater system. The city is putting money behind this plan, as well: last year the city’s Department of Environmental Protection awarded $2.6 million for green infrastructure projects, including green roofs, and the beginning of February, the department announced $3 million in grant money available for this year.

White roofs, though, are still the simplest and cheapest option. The instruments that Gaffin began installing last week will monitor the MoMA QNS roofs, recording data every 15 minutes, finding out how dirty the roof really gets over time — how long it takes for a white roofs to become brown.