Paint it White: How New York City is getting cooler

New York vista by Tim Pearce.

It’s official: white roofs are cool. The declaration comes from the tastemakers at NASA, and, as always, New York City is at the vanguard of the new trend. In fact, the city’s recently brightened rooftops were found to be over 40 degrees Fahrenheit cooler than traditional dark roofs at the height of summer heat waves.

If you’ve weathered a NYC summer and were lucky enough to escape to the country for a weekend, you probably noticed that it feels several degrees cooler once outside the city – and consequently several degrees insufferably hotter upon reluctant reentry. Having myself spent over a decade in the city, I’d assumed this temperature gradient was caused by the fact the New York summers were literally Hell and the months of July and August a period of non-eternal damnation, but it turns out there’s a simpler explanation.

New York City is crowded, not just with people, but with buildings. Lots of buildings, built close together and typically topped with dark roofs that excel at absorbing the sun’s rays. Outside of the city, manmade structures are interspersed with these things called trees. The green of suburban and rural foliage reflects back some of that solar radiation, resulting in cooler ground temperatures. Meanwhile cities bake in their black asphalt casing. The phenomenon is called the “urban heat island” effect, and, scientifically speaking, it thoroughly and unequivocally sucks.

Higher temperatures not only make city dwellers miserable and irritable, they also result in greater energy usage and higher greenhouse gas emissions, which ultimately makes things even hotter. Hoping to reign in this unfortunate positive feedback loop, city planners have been working to replace traditional dark roofs – made of conveniently waterproof and durable asphalt and tar – with new fangled white materials (household staples like ethylene–propylene–diene monomer (EPDM) and a thermoplastic polyolefin (TPO)). Since not every building owner is willing and/or able to replace an otherwise functional black roof, the NYC CoolRoofs program is also promoting a less involved option – “retrofitting” existing dark roofs with old-fashioned white paint.

So is it working? A multi-year study recently published online in the journal Environmental Research Letters examined how the high-tech versus DIY brightened roofs fared against each other and as well as compared to standard dark roofs. Initially, both the two professionally-installed membranes and the civilian-applied white paint performed admirably – with white surfaces measuring an average of 43 degrees Fahrenheit cooler on hot summer days than dark samples. (Those asphalt rooftops can reach a disheartening 170 degrees Fahrenheit.) However, while the fancy whites were resistant to the ravages of time, the humble white paint lost some of its luster (and, more measurably, its reflectance) by the second year… not unlike the interior paintjobs in Manhattan apartments. Still, even the two-year old paint was an improvement over the dark roofs, and you can’t beat the price (about 50 cents per square foot, whereas the pro roofs ran between $15 to $28 per square foot).

So that’s all pretty impressive, but I seem to recall that New York has not just one but two problem seasons, in terms of both human suffering and energy usage. What about winter? Is white after Labor Day as gauche on rooftops as it is in ensembles?* The EPA, whose standards for roof reflectivity these white materials are striving to meet, acknowledges that brightening roofs in colder climates may come with a “winter heat penalty”. That is, in reducing unwanted summer heat, the more welcome heat that would have been generated by sunlight-absorbing dark roofs in winter is also forfeited, and gas or electric heaters must work harder to make up the deficit. However, since fewer hours of sunlight are available during the coldest months, it’s not a major loss (relative to the improved efficiency in summer) unless you’re dealing with frigid locations where heaters run 9 months out of the year. †

Additionally, the authors of the current study found that at least one of their measured materials (our friend EPDM) seemed to avoid the winter heat penalty entirely. They attribute this to the material’s emissivity level. Here’s the deal (in as much detail as I’m willing to tackle), white roof initiatives are looking only at a material’s reflectivity. This is the amount of sunlight reflected back instead of absorbed. To qualify for the EPA’s “energy star” rating, a material must have a solar reflectance of 0.65 or greater upon initial installation, and 0.50 or greater after three years. (It’s a zero to one scale, with 0.0 absorbing all light, and 1.0 reflecting all light.)

But materials also have an emissivity factor. Emissivity is the amount of heat something emits after absorbing solar radiation. Since close to half of that sunlight is being absorbed by the white surfaces (still far better than black, which is typically around 0.05 in reflectivity) how much of it they emit will also affect a building’s surface temperature. The white materials tested were assumed to have a high emissivity in addition to their high reflectivity (i.e., whatever sunlight got in would quickly be booted back out), but the EPDM seemed to cling a bit harder to its absorbed heat, thus keeping its surface toastier in the winter.

The authors therefore suggest that materials with high reflectivity but middling emissivity may be the best fit for colder climates. Got all that? Don’t worry, I’m not sure I understand it either. Emissivity is a tricky concept to get a handle on. Apparently even the folks manufacturing the EPDM couldn’t figure out how much heat their product would emit.‡

In any event, it’s nice to see that New York City is getting its environmental act together. When I left the place in July of 2008, it was a sweltering cesspool with insufficient bike lanes and a surfeit of Sex in the City-spawned shopping zombies. Less than four years later and, well, at least two out of those three problems are being addressed. While the world could do without some East Coast fads (I had to endure a second round of Brooklyn’s ironic mustaches when I moved to Austin), one can only hope that the trend of going green by way of white will soon fan out to other cities. Especially those in the south, where I currently reside. It's totally uncool here.

* Yes, I’m aware that no white after Labor Day is an outmoded fashion rule. Austin isn’t that behind the curve.

† And if you’re living somewhere truly freezing, isn’t your roof covered with snow most of the time anyway? Now if we could just get it to snow in summer instead of winter, then all our problems would be solved.

‡ The product was rated as having an emissivity of 0.90 (high), but the authors estimated it to be closer to 0.48 (not so high).