Battling Heat Islands: Why and How

If you walk into the city on a sunny, summer day and feel like it is warmer than the suburban home you left earlier, it isn’t your imagination. The “Heat Island Effect” is a term that refers to a localized rise in ambient, outdoor temperature due to dark materials absorbing light from the sun. While asphalt roads contribute meaningfully, roofscapes are the most abundant culprit with a large number of existing flat roofs in this county being black—either tar, asphalt or a rubberized membrane. In full summer sun, a black roof can reach temperatures of 170 degrees. When the number of these roofs located in close proximity rises so does the amount of heat captured and radiated back into the air.

Though it is occurring at some scale on any dark-colored, man-made surface, the effects are most strongly felt in urban conditions where streets and the tops of buildings make up most of the net surface area facing the sky. In a city like New York, the result can be a local air temperature 5 to 10 degrees higher than surrounding townships in the depths of summer. Keep in mind, Manhattan is also on the coast which means that this temperature rise is happening despite the cooler normally found from coastal winds.

So it’s Hotter, Who Cares?

A common first question is: why is this an issue? There are a number of negative effects of heat islands, each causing problems at different scales. The easiest reason to change is for convenience. Adding five degrees to the air temperature when it is 70 degrees outside could be negligible, but adding it to 90 degrees is downright uncomfortable. When it comes to New York, there are plenty of days every summer where everyone would appreciate walking out into a day that was 5 degrees cooler. Studies show that the effect can actually be more pronounced at night when the thermal mass of materials that have been gaining heat all day still radiate warmth after the sun has set. While this may be the easiest reason to change, it is not the most important.

Roof Heat island Diagram

The repercussions go beyond the level of comfort on a sidewalk stroll. As exterior temperatures rise, the cooling systems of our buildings need to work harder to keep the indoor environment in the manner that we have grown accustomed. Summer months bring the peak daytime electricity load of New York City to its highest points of the year, at times pushing the grid to capacity. In these stretched situations, emergency power at the fringes of installed capacity can come from the power plants that are used the least—which often tend to be the oldest, dirtiest and least efficient–and brought online when we are out of other options to pull from. We consume needless amounts of energy getting rid of the heat that our buildings are designed to inherently attract. To add insult to injury, in order to produce more power we need burn more coal or natural gas, only releasing more heat (and tons of other great stuff) into the local atmosphere. According to the Department of Energy, space cooling uses an average of 9% of energy consumption in the average American household.

The final layer is the effect that higher temperatures have on natural ecosystems. Hotter air and building surfaces can burn past the range that is palatable of plants and animals. Though most cities are not teeming with natural life, there are opportunities to increase levels of biodiversity through the creation of microclimates. These seemingly small ecosystems help clean our air, retain our stormwater and pick up our waste. Cities may never be the capitol of the animal kingdom, but there is plenty of potential for them to support a complex ecology of species that extends far beyond humanity.

How Can We Fix It?

There are plenty of our societal norms that require large and complex changes in order to render a sustainable replacement, but this is not one of them. The problem of heat islands is extremely easy to address. Quite simply: light colored roofs. There is no longer any inherent reason of functionality or constructability that forces roofs to be black. We have evolved past the era where hot tar and asphalt have to be the go-to solutions for facilitating drainage.

white roof membrane

the installation of a white EPDM roof at a residential project in the northeast

When it comes to low-sloped roofs, a white EPDM (ethylene propylene diene monomer) membrane offers a great solution with no extra work compared to its black counterpart. EPDM is basically a rubber roof membrane adhered either to insulation or a roofing substrate. We have recently specified a new Eco-White product on three separate projects, the first of which I got to go see installed last week. This is a product yielding no compromise in drainage performance at virtually the same price while saving the customer money on cooling costs. There is no reason this should not be the industry standard.

A recent study of temperature performance for roofing material conducted at Columbia University yielded powerful results to the dialogue of prompting more building owners and developers to make the switch. According to the results of the test, the dark, sunlight-absorbing surfaces of some New York City roofs reached 170 degrees Fahrenheit on July 22, 2011, a day that set a city record for electricity usage during the peak of a heat wave (which means that the air was full of the emissions of metro area coal plants). But in the largest discrepancy of that day, a white roofing material was measured at about 42 degrees cooler!

new york roof temps

Temperature variations reported for black and white roofs under the same conditions for a study conducted at Columbia University

Green roofs go a step further. Instead of reflecting that energy upward, plants harness it to purify the air and consume carbon dioxide. The combination of  roots and soil not only provide an insulating barrier for the building below, but the process of transpiration (plants releasing water into the air in the form of vapor) removes heat from the soil to leave the roof surface even cooler in the summer months. Green roofs may be a more expensive option, but the results yield more dividends including the retention of storm water as well as fostering the very urban microclimates that heat islands deteriorate in the first place.

More on green and blue roofs: One Roof, Two Roofs, Green Roofs, Blue Roofs

For weekend warriors or the cash-strapped population, there are still easy options to be deployed on a budget. Roofing grade white or reflective paints can turn an existing black roof into one with a high albedo. Though the effectiveness of a painted roof deteriorates by 50% after 2 years, it is still much better than nothing. “It’s the lowest hanging fruit. It’s very cheap to do; it’s a retro-fit. You don’t need a skilled labor force. And you don’t have to wait for a roof to be retired,” said Stuart Gaffin, leading scientist of the Columbia University study. “So if you really talk about ways in which you brighten urban albedo, this is the fastest, cheapest way to do it.”

There is no reason why this should not be the new standard for how we construct and renovate our buildings. The challenge is straightforward, the problems are clear and the solutions are plentiful.

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