Green roof.

In Urban Heat Islands, Green Roof Systems Provide Cool Comfort

Photo courtesy of Jayson Trevino via Flickr.

A recent study found that “green roof” systems could offset excessive urban heat, which drives up energy costs and causes heat-related illnesses in many American cities.

The annual mean temperature in cities with populations over 1 million is between 1.8–5.4°F hotter than the surrounding areas, which can increase energy demand by up to 40%. And this difference can rise to as much as 22°F at night, according to the Environmental Protection Agency.

This is called the “urban heat island” effect, and it’s driving up energy demand in the summertime, placing a strain on electricity grids and causing increased emission of global greenhouse gases.

The following comparison of measured heat versus vegetated areas clearly shows that the coolest area of New York City are also the most vegetated:

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Image courtesy of Wikipedia. Modified by Curiousmatic.

A study sponsored by the Department of Energy found that as much as 5-10% of peak electricity demand for cooling was caused by this effect.

Heat islands also exacerbate heat waves, causing heat-related illnesses like heat strokes and and deaths, and reduce water quality by disrupting aquatic ecosystems. An annual average of 658 people die each year from heat-related causes.

With more than 80% of the U.S. population – and more than 50% of the world – living in expanding urban areas, this problem is only likely to continue.

However, a 2014 study published in the Proceedings of the National Academy of Sciences found that by using green roof systems, cities could all but negate the heat island effect.

What are green roof systems?

There are basically two kinds of green roof systems: living roofs and cool roofs.

Living roofs, a form of urban greening, are roofs that are covered in vegetation, planted in soil on top of a waterproof membrane (they can also be urban farms, which we covered here).

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A green roof atop Chicago City Hall. Photo courtesy of TonyTheTiger via Wikipedia.

The vegetation acts as a natural insulation, retaining cool air in the summer and warm air in the winter. This reduces the need for air conditioning and heating.

It also blocks sunlight from reaching the roof’s surface, reducing the rooftop temperature by as much as 50°F, according to one study.

Additionally, cooling is provided by the process of evapotranspiration, by which plants use heat from the sun to evaporate water.

The other kind of green roof is a cool roof – essentially just a roof painted silver or white to reflect sunlight.

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Photo courtesy of Tremco Green HQ.

It’s a simple, low-tech solution, but it can reduce roof temperatures by up to 50-60°F in peak summer weather, according to the EPA.

If cities were to adopt either of these methods, the study published in PNAS found, excessive urban heat would be completely offset.

On the horizon

Of course, that study is based on a hypothetical, future scenario. Will green roofs continue to be adapted in the future?

According to Michigan State University, the industry has been steadily growing over the last decade, although total square footage installed is less than in industry leading countries like Germany:

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Image courtesy of Michigan State University’s Green Roofs initiative.

But a series of local policy initiatives could encourage the growth of green infrastructure initiatives, such as New York City’s already-existent $6 million grant, or its plan to allocate as much as $2.4 billion in public and private funding for green projects, including roofs.

Chicago already completed the 24.5-acre Milennium Park a decade ago, and is considered an industry leader with almost 5 million square miles installed.

And it’s not just in the East: cities all over the U.S. are encouraging the adoption of green roofs, according to greenroofs.com, an industry news site.

So while a 100% green-roofed city might be somewhat futuristic, don’t be surprised to see a lot more green roofs in the near future.

Ole Skaar