Building Green Cities: Mitigating the Urban Heat Island Effect with Green Roofs

Authors: Jill Banach (Environmental Science), Michael Mason (Building Construction Technology), Mitchell Good (Urban Forestry, Natural Resource Conservation), Sydney McGrath (Horticulture)

A short film, Brooklyn Farmer documents a group of urban farmers growing food on the rooftops of New York City. The head farmer, Ben Flanner, kneels in the dirt cutting fresh salad greens to send to restaurants later that day. As he glances up, the earthy green plants and brown soil contrasts starkly with the concrete skyscrapers on the horizon. He acknowledges that “the city itself has made it possible to do this by being so overbuilt and having all these impermeable surfaces that need sponges on them” (Cherrie & Tyburski, 2013, 6:24). Ben and his team set out to build the world’s largest rooftop farm. With success, two rooftops in the city are now abundant with tomatoes, herbs, root vegetables, and even beehives. Qwen Schantz, another essential person of the operation, describes the potential for future innovation: “When I look out at New York City rooftops and I see thousands of acres of empty space, I truly am moved to cover them with vegetation. And I think that this is something that has to happen. And I think it’s something that will happen” (Cherrie & Tyburski, 2013, 24:47). As the sun sets behind the New York skyline, Ben knows that this farm is making a difference in people’s lives. He is bringing the people of Brooklyn closer to their food, increasing vegetation in a way that is “flashy and weird and interesting” (Cherrie & Tyburski, 2013, 6:57), and contributing to the greater movement of green roofs to reduce the impacts of urbanization.

Across the world, studies estimate that 80% of the human population live in cities that contain one million or more residents (“Human Population: Urbanization,” n.d.). If current trends continue, this number will continue to grow and more infrastructure will be built to accommodate the increasing population. The increasing amount of impermeable surfaces that characterize current urban environments will steadily eliminate vegetated land. The term impermeable describes materials that prevent, or do not easily allow, the passage of water and other liquids (Office of Ground Water and Drinking Water, 2009). These impermeable surfaces also typically have low albedos, a trait that is represented by a ratio between 0 and 1. Low albedo means that the material reflects very little of the sun’s radiation, high albedo means that the material reflects a lot of the sun’s radiation (Li & Yeung, 2014). Conventional urban materials therefore reflect little, and absorb a lot of the sun’s radiation, which causes an increase in temperature that dissipates into the surrounding air (Razzaghmanesh, Beecham, & Salemi, 2016). Temperatures in cities become much warmer than surrounding rural environments, creating “islands” of heat that occur on surfaces and in the atmosphere. This is known as the Urban Heat Island effect (UHI). Roof and pavement temperatures can be 50-90℉ warmer than the ambient air temperature on a hot, summer day. As a result, urban air temperatures can be as much as 22℉ warmer than surrounding, less developed environments (United States Environmental Protection Agency [EPA], 2017b). UHI can create serious health problems for people living in cities. Those who are affected the worst are children, older adults, and those with existing health conditions. Effects can harm the human body by contributing to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality (EPA, 2017b). Although all kinds of urban materials contribute to UHI, roofs are one of the most heat absorbent and problematic. On average, roofs make up approximately 20-25% of urban surfaces (Susca, Gaffin, & Dell’Osso, 2011), a fairly significant portion of cities across the globe.

In addition to the replacement of vegetation with impermeable surfaces and a low urban albedo, the UHI also largely depends on urban canyons and thermal properties of building materials (Susca et al., 2011). The space above the pavement and between the walls of tall city buildings are referred to as urban canyons. Radiant energy, or energy produced by the sun, is easily trapped in these spaces. A study of European and American cities compared urban canyons among several cities and determined that urban areas with taller and denser buildings develop heat islands more quickly (University Corporation for Atmospheric Research, 2011). Furthermore, thermal properties of building materials refers to the effect of the sun’s radiation on the material and the material’s release of stored heat (Montavez, Rodriguez, & Jimenez, 2000). The UHI can be intensified depending on the time of the day, weather conditions such as humidity, and anthropogenic heat (Montavez et al., 2000; University Corporation for Atmospheric Research, 2011). Although green roofs cannot reverse the UHI, they are important in decreasing the intensity. They effectively increase the amount of vegetated, or permeable, surfaces in urban areas and increase the albedo, issues that results from using conventional roofing materials.

As mentioned above, the low albedo of conventional roof materials contributes significantly to the UHI effect. The typical albedo of conventional roof materials is about 0.066 (on a scale from 0 to 1). Green roofs, however, have a much higher albedo, thus helping to reduce the warming of surrounding air by reflecting much more of the sun’s radiation. Green roofs typically have an albedo of 0.7 to 0.85, an albedo comparable to the 0.8 of white roofs (Gaffin et al., 2006). Another study found that, at minimum, green roofs have a 200% increase in albedo over conventional roofs (MacIvor & Lundholm, 2011). A minimum albedo increase of 200% provides a huge potential for decreased absorption of the sun’s radiation, and thus decreased warming effects on the surrounding air.

In addition to the increased albedo of green roofs, the materials used to construct them are permeable, meaning they allow for the passage of water. Green roofs have the ability to cool the surrounding air because of this characteristic. The trapped moisture in the plants and planting medium evaporates as temperatures increase. This state change of the water, from liquid to gaseous vapor, allows for an exchange of energy, or heat. The heat is transferred from the plant, or planting medium, to the water, and remains in the water as it floats away in the form of vapor (MacIvor, Margolis, Perotto, & Drake, 2016). The resulting cooler surface temperatures of the plants and planting media help to cool the surrounding air. This can cause a large air temperature change if it occurs on many buildings across an entire city. This effect is clearly visible in the statistics presented by the EPA, stating that temperatures between urban and rural areas can differ by 1.8 °F to 5.4 °F during the day, and up to 22 °F at night (EPA, 2017a). A study conducted in New York City found that there was an average of a 3.6 °F temperature difference among the least and most vegetated city areas (Susca et al., 2011). Another study suggests that an additional 50% of green roofs in New York would result in a decrease of 1.4 °F, a very significant decrease in average temperature (Bianchini & Hewage, 2012). Though the ability of green roofs to mitigate the UHI effect is significant and important, they certainly come at a cost.

A large amount of skepticism still exists regarding green roofs, especially in North America, where they are a relatively new concept (Snodgrass & McIntyre, 2010). Much of this skepticism is concerned with whether or not the costs of a green roof are worthwhile. The upfront cost of a green roof is higher than that of a conventional roof. The construction of a green roof costs approximately $22 per square foot, compared to $16 per square foot for a conventional roof (EPA, 2016). The maintenance costs of a green roof are also higher than that of a conventional roof. A green roof requires more upkeep than that of a conventional roof, typically $0.75–$1.50 per square foot annually (EPA, 2016). These higher construction and maintenance costs are worrisome to the public. However, the money that a green roof can save in energy savings, far outweighs the investment costs. The layer of planting medium and plants on the green roof act as an extra layer of thermal insulation, reducing indoor and outdoor temperature fluctuation more so than conventional roofs (City of Portland, 2008). This allows for lower energy requirements for heating and cooling the building because more of the energy input is retained. A study testing the energy savings effects of green roofs states that the cooling load reductions throughout the 120 day summer cooling period (from June to September) in Hong Kong, China for extensive and intensive roofs are 9.3% and 13.2%, respectively, of the total summer cooling energy consumption. While seemingly not very large, these energy saving levels can actually save thousands of dollars over the course of just a single year (Peng, & Jim, 2015). The energy savings through the roof’s lifetime are even more substantial, being estimated at approximately $200,000  (EPA, 2016). While there are current cost challenges affecting the implementation of green roofs, the money that will be saved in energy consumption far outweighs the higher initial investment.

In addition to the financial concerns, there are also practicality concerns about green roofs. Green roof buildings require substantial construction, given the weight load they must support. An extensive green roof is estimated to add an additional 15 – 30 pounds per square foot to a building and an intensive roof is estimated to add 25 – 50 pounds per square foot (DC Greenworks, n.d). For this reason, green roofs are often more feasible when designed for new building projects. This added construction also makes green roofs more costly than other environmentally friendly roof options, such as white roofs. White roofs reduce UHI by reflecting heat back into the atmosphere, and they are slightly less expensive to construct than green roofs. A green roof is estimated to be $8.90/ft² more costly than white roofs (Sproul, Wan, Mandel, & Rosenfeld, 2014). However, although white roofs are marginally more cost effective, they also lack the ability to regulate temperature fluctuations, reduce runoff, and provide aesthetically appealing environments (Sproul et al., 2014). Although green roofs require additional construction, there is substantial evidence that proves that their benefits set them apart from other roofing options.

As discussed, the ability of green roofs to mitigate the UHI has both environmental benefits and individual benefits. These benefits come at a higher cost and it’s understandable that many business owners and homeowners are deterred by the initial cost of green roofs. To fix this problem, legislation should be crafted to offset the initial cost of investment through incentives. City governments should encourage and regulate the implementation of green roofs to mitigate the effects of the UHI that results from ever-expanding urban development. Our proposed legislation is modeled after legislation from three cities: Washington, D.C., Chicago, Illinois, and Toronto, Canada. Washington, D.C. and Chicago, Illinois are the leaders in green roof implementation in the United States. The city of Chicago was the first city to successfully pass legislation to encourage green roof construction with strong incentives, which resulted in the city having the most green roofs in the United States until 2010 (Stutz, 2010). For this reason, Chicago is known as “City in a Garden” (“Green Roofs in Cities,” n.d.). Surprisingly, this was not simply a push for more sustainable initiatives. It was actually the result of an intensely hot summer in 1995, which caused over 700 heat related deaths (“Green Roofs in Cities,” n.d.). Recently, Washington, D.C. surpassed Chicago to lead the U.S. in green roof installations (Stand & Peck, 2015). And lastly, Toronto, Canada has the greatest area of green roofs in Canada and was the first city in North America to implement a green roof bylaw regulation (Stand & Peck, 2015).

To help encourage green roof construction, city governments should create an expedited permit program. Permit programs exist in Chicago indicating their potential for success in cities such as Phoenix, Arizona, Houston, Texas, and other cities that currently have no existing legislation. An expedited permit program allows building projects with proposed green elements (green roofs, rainwater harvesting systems, solar panels, and others) to be prioritized in the building review process (Department of Buildings, n.d.). A New York Times article acknowledges that the permit process for green buildings is currently lengthy, so it would be beneficial to remove this obstacle of the process (Galbraith, 2009). In cities like Phoenix, green roof projects are reviewed with all of the other building permits; however, the city of Chicago reviews these permits before all other building permits (Department of Buildings, n.d.). This is important for developers to have guaranteed priority, especially if the city experiences an influx of permits. This incentive would be even more effective if city governments guaranteed developers that they would receive a permit in half the amount of time of the normal process. An actual number of days for the process will differ as cities vary greatly in size and have different staffing compositions.

In addition to this time saving incentive, rebate programs can be used to offset construction costs. Washington, D.C. has current legislation to provide rebates for green roofs depending on their size and property location (Anacostia Watershed Society, 2016b). The motivation for this program is to create a city that is more RiverSmart to reduce stormwater runoff that pollutes the local watershed (Anacostia Watershed Society, 2016a). However, it could also be possible to implement a rebate program based on roof albedo value. As mentioned, conventional roof materials typically have an albedo around 0.066, green roofs are usually between 0.7 to 0.85 depending on composition, and white roofs are typically 0.8 (Gaffin et al., 2006). Since albedo values range from 0 to 1, green building projects with materials close to 1 would receive a rebate. Rebates would be $2 to $4 per square foot depending on albedo and roof size; this would make green roofs more competitive with conventional roofs. This is based on the approximate construction costs of green roofs, $22/ft2, and conventional roofs, $16/ft2 (EPA, 2016). A rebate is important because it provides an actual monetary return rather than only guaranteed savings. Additionally, incentives are regarded as an effective way to modify human behavior. Monetary incentives can be used to effectively encourage people to move in a specific direction (Jensen, 1994), or in this case, towards the more sustainable construction choice.

In addition to encouraging the construction of green roofs through incentives and accelerated permit programs, cities need to implement minimum green roof requirement regulations. This means that the city in question should pass a law requiring a certain percentage of a roof to be covered in green roof material. Toronto, Canada already implemented such a policy, requiring green roofs on new commercial, institutional and residential developments with a minimum total floor area of 2,000 square meters as of January 31, 2010. The amount of roof coverage required ranges from 20% to 60% depending on available roof space. A roof with surface area ranging from 21,500-54,000 square feet is required to have a 20% green roof coverage, and a roof with surface area 215,000 square feet or greater is required to have a 60% green roof coverage (“Green Roof Bylaw,” n.d.). Toronto observed complete compliance with these bylaws and in just five years, the number of green roofs in the city increased by 300, introducing over 2,700,000 square feet of green roof surfaces (“Green Roofs,” n.d.). Laws such as this could greatly improve the environmental conditions of cities across the world in just a few short years if implemented immediately.

The effects of continuous urban expansion are a major concern across cities both domestically and internationally. Implementing green roofs is a sustainable choice for mitigating the effects of UHI as they increase the amount of permeable surfaces and have a significantly higher albedo than conventional roofs. Green roofs cost more to install initially than conventional roofs; however, they generate a significant amount of energy savings and proposed legislation like rebate programs can further offset the cost. Coupled with an expedited permit process, green roof installation will be attractive and rewarding. Lastly, minimum green roof requirements for newly proposed infrastructure should be implemented by cities to guarantee improved environmental health and quality of life. Although heat islands and global warming are two distinct phenomena, strategies to reduce heat islands may also contribute to reduced carbon emissions that contribute to climate change. Installing green roofs will reduce air conditioning demands in the summer resulting in fewer emissions of greenhouse gases (University Corporation for Atmospheric Research, 2011). This is not only important for environmentalists and climate scientists, but also for the average person that experiences the effects of UHI daily. Ultimately, a shift in vegetated urban environments will contribute to a more sustainable world. Green roofs have significant potential to increase this vegetation and create a canvas for innovation. Rooftops farms, like the Brooklyn Grange, have the potential to spark a revolution in green roof technology.

A photo of the Brooklyn Grange Farm located atop Building No. 3 of the historic Brooklyn Navy Yard. Photo taken from Brooklyn Farmer.



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