Many U.S. cities are often disproportionately warmer than their surroundings. According to NASA, the temperature difference can be as high as 10°F because of a phenomenon known as the “urban heat island effect.”  To combat higher temperatures, more cities are turning to rooftop gardens.

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A heat island forms when an area’s natural vegetation is replaced by heat-absorbing materials like asphalt and concrete. These surfaces trap the sun’s heat, resulting in higher overall surface and ambient temperatures. 

Heat islands are most apparent in cities, where dense population growth leads to rapid changes in the composition of the land. Buildings and narrow streets in cities block the flow of air, thereby increasing ambient heat. Waste heat from air conditioners, vehicles, factories and other human-generated activity also contributes to local temperature differences. Other factors like air pollution and minimal plant cover further exacerbate this effect.

To counter the heat island effect, one solution is gaining prominence: rooftop gardens, or so-called “green roofs.” Vegetated or green roofs are “any type of green infrastructure that can reduce building energy use and urban heat,” according to the journal Sustainable Cities and Society. Green roofs can take many forms but are generally categorized as “intensive” or “extensive.” Intensive roofs are characterized by a medley of diverse vegetation coupled with thick soil and irrigation, similar to a traditional garden. Extensive roofs are typified by shallow soil and low-lying, low-maintenance plants such as shrubs and grasses. Other types of green infrastructure found in urban spaces include greenspaces, street trees, and “vertical forests.”

While green roofs have been around since at least the 1960s, little research has been done on which types of green roofs are most effective at curbing heat in urban areas. Now, researchers at NASA’s Goddard Institute for Space Studies in New York have concluded that rooftop gardens and green infrastructure can help alleviate some of the extreme heat in cities.

“As cities grow and develop, they need to make good decisions about their infrastructure, because these decisions often last for 30 or 50 years or longer,” said Christian Braneon, one of the principal NASA climate scientists in the study. “In the context of more frequent heat waves and more extreme heat, it’s important to understand how these urban design interventions can be effective.”

The study researched three sites in Chicago that had installed green roofs in the early 2000s: Millennium Park, City Hall, and a Walmart shopping center. Using satellite imagery data from 1990 to 2011, the research team analyzed changes in land surface temperatures and vegetation density at the study sites versus nearby control sites. They found that the sites with larger “intensive” rooftops and greater plant diversity saw the greatest cooling benefits and reduced temperatures. In contrast, the sites with extensive, monoculture green roofs exhibited the lowest cooling capacities. 

“Our research moves beyond the general policy directive that green roofs are an effective urban heat mitigation strategy by demonstrating a methodology that can assess which types of green roofs cool most effectively and whether various green roof types can fully counteract background warming trends. Results from the three sites studied suggest that, in temperate regions such as the City of Chicago, intensive green roofs with heterogeneous plant types may have a superior capacity to mitigate [urban heat],” the researchers concluded. 

Companies like Apex Green Roofs are harnessing this knowledge as demand for sustainable building design solutions skyrockets. The company’s portfolio features over 100 eco-roof installations ranging from intricate residential rooftop gardens to extensive “living roof” systems in commercial areas. Apex’s work has been featured in various universities across the country, including Harvard where they led green roof projects in the Chao Center, Shad Hall, and the Rowland Institute. 

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Other key players in the global green roof market include Axter Limited, Bauder Ltd, and Green Roof Blocks. The latter builds “self-contained, portable units that can be arranged and rearranged as the building owner sees fit,” allowing for maximum flexibility at a low cost.  

The national average cost of installing a green roof starts at $15 per square foot but can get as high as $50, depending on the type of green roof selected (intensive roofs run on the higher end). For a standard American home, a green roof would cost between $13,000 and $50,000. Although green roofs can be hefty upfront, the real benefits are evident in the long-run. A 2012 report from the University of Michigan found that installing a 20,000-square foot green roof on the campus would save the institution about $200,000 in energy costs over the course of its lifetime. This is because green roofs “become self-sustainable” systems over decades, although the economic paybacks from increased longevity, health benefits, and stormwater tax credits are not apparent in the beginning. 

Additionally, while green roofs are not currently subsidized on a federal basis, there are many programs nationwide that offer local incentives for green roof installations. As with many renewable energy products, the price of green roofs is expected to come down as competition in the industry grows.

As urban centers continue to grow, so will the likelihood of urban heat islands exacerbating extreme heat waves. Green roofs are a proven heat mitigation strategy that are not only scalable but offer a host of environmental benefits including improved air quality, reduced noise pollution, and space for wildlife. Exposure to green spaces has also been tied to reduced feelings of anxiety and depression in residents, according to a long-term study from the journal Environmental Research. 

In an industry that’s predicted to hit $4.2 billion by 2027, the private sector is at the frontlines of the green rooftop revolution.

Nathalie Voit is a freelance content creator and a graduate of the University of Florida. She is an alumni of The Heritage Foundation’s Young Leaders Program. 

The views and opinions expressed are those of the author’s and do not necessarily reflect the official policy or position of C3.