Improving the Heat Island Effect Through Green Roofs

Rachel Nurnberger- Environmental Science

David Pacheco- Building & Construction Technology 

Zac Wannamaker- Natural Resource Conservation

Green roofs add a beautiful shade of natural green to a dull urban environment.

New York City’s struggle with the urban heat island effect is no secret. The issue has seen a growing amount of concern in recent years due to the increase in hospitalizations and deaths caused by extreme city temperatures (Calma, 2018). This increase in inhabitant health issues has lead NYC to seek resolution to the issue through heat island mitigation programs.

Unfortunately, mitigation programs enacted by NYC officials have tended to take the “put a bandaid on it” approach more often than addressing the problem at the source. These efforts are exemplified by cooling centers and cool roofs. Cooling centers are air conditioned locations that allow city dwellers the chance to cool down and relax when overheated during heat waves (New York City Adapts To Deal with Projected Increase of Heat Waves, 2019; “Cooling Center”). Cool roofs are rooftop surfaces painted white or fitted with reflective materials to prevent the absorption of the sun’s rays into a structure. (New York City Adapts To Deal with Projected Increase of Heat Waves, 2019; “NYC CoolRoofs”). The fundamental flaw of both of these approaches is the small scale impact they have on mitigating the urban heat island effect. The greater focus should be placed on improving green infrastructure in New York City. More specifically, NYC should put greater focus into green roof initiatives, as rooftop greenery has been proven to help mitigate the urban heat island (UHI) effect. NYC should work towards improving existing incentive programs, thus encouraging owners to implement vegetated rooftops and minimize the heat island effect permanently.

The urban heat island effect is broadly defined as the abnormal rise in temperature in city areas due to light absorption in materials such as concrete and asphalt (“What is the Urban Heat Island”; McGrath, 2018). Urban heat islands cause city air temperature rises between 1.8˚F – 22˚F, depending on the population density of the city (“Heat Island Effect”, 2018). Heat islands form as a result of city vegetation being replaced by asphalt and concrete. Such materials absorb and retain more heat than vegetation (“Urban Heat Islands”, 2011; McGrath, 2018). A material’s ability to reflect light and absorb heat is measured by a factor known as surface albedo. Albedo is defined as a “measure of how much light that hits a surface is reflected without being absorbed (“Albedo”. Par. 1)”. A high albedo means that the surface reflects the majority of the light that hits it and absorbs very little heat, while a low albedo is the opposite; reflecting little and absorbing a lot. The scale on which albedo is measured goes from 0-100%, which defines what percentage of light is reflected by the material and how much is absorbed (“Albedo”). The albedo for concrete ranges from 6% – 16%, and asphalt falls in the range of 18% – 35%. Vegetation has a significant advantage over both, clocking in at an albedo of 70% – 85%. (Li, W. C., & Yeung, K. K. A., 2014; “Pavements”).

Urban environments now support over half of the world’s population and are expected to grow rapidly at rates as high as 2.6% per year in some areas (Sharma et al, 2016). The causes behind this heating effect in urban environments have much to do with the typical urban infrastructure makeup. Asphalt and concrete (needed for the expansion of cities), absorb huge amounts of heat, increasing the mean surface temperatures of urban areas. Tall buildings (often accompanying narrow streets), hinder the circulation of air and reduce wind speed thus reducing natural cooling effects (Balu, 2019). Ultimately it is the increased use of manmade materials and anthropogenic heat, which is heat released into the atmosphere due to human activity, that is causing the urban heat island effect. This leads to the fundamental understanding that increased temperature is the primary cause of the urban heat island. The urban heat island effect also leads to increased energy needs, contributes to the heating of our urban landscape, and to the associated environmental and public health consequences that accompany the increase in urban temperatures (Mohajerani et al, 2017). While many factors contribute to the formation of urban heat islands, urban planning aspects are related to both the intrinsic nature of the city, such as its size, population, building density, and land uses. External factors such as climate, weather, and seasons are also involved (Bhargava, 2017). City planning is, therefore, an important feature for the overall success of this proposal.

The urban heat island poses significant health risks to inhabitants of cities (“Heat Islands”, 2019). The increased demand for AC is driven by one major component of the urban heat island effect; increased temperatures. This increase in demand causes electrical companies to vamp up production to meet the excessive need for air conditioning. The electricity provided is created by the burning of fossil fuels, which results in the release of airborne toxins including sulfur dioxide, nitrogen dioxide, particulate matter, carbon monoxide, and mercury. All of these compounds are extremely harmful to the human respiratory system, can cause wheezing and coughing, and in long term exposure, permanent lung damage (Heat Island Impacts, 2019; “The Dangers of Smog: What You Need to Know About Air Pollution”).  The presence of heat islands also increases the risk of heatstroke, which can be fatal or non-fatal. This increase in heat-related illness is due to the prolonged duration of extreme daytime temperatures. This increased exposure causes strain on the bodies’ internal cooling system and causes heatstroke (“Heat Islands”, 2019). The likelihood of having a heat stroke in an urban area as opposed to a rural one is almost 4 times as great, as documented by Tan. et al. (2009). Findings show that in 1998 there was an average of 27.3/100,000 deaths due to heat stroke in urban areas, as opposed to only 7/100,000 in exurban areas. Researchers attributed this to the increasing issue of urban heat islands which, in turn, increased the severity of heat waves in city areas.

Green roofs combat the urban heat island effect through three fundamental functions. The first is evapotranspiration, which is the process of using heat in the air to evaporate water in plants and soil. This process cools green roofs as the moisture from the evapotranspiration lowers the temperature of the growing medium of the roof (Heat Island Compendium, 2008, p.2). This process can be compared to the body using the heat from the sun to sweat. When your body feels overheated it is triggered to start sweating thus moistening the skin and causing evaporation of the sweat to occur. This makes you feel cooler, much like how the moisture from within the plant cools the growing medium and in turn reduces overall city temperatures. The air temperature reduction from evapotranspiration is anywhere between 1˚C – 5˚C, making it a very effective process to cool the air (Qiu, et. al, 2013; “Using Trees and Vegetation to Reduce Heat Islands”, 2016).

The second benefit that green roofs provide is their contribution to increasing the surface albedo of buildings they are installed on. Although the primary focus of a green roof is not simply improving the surface albedo, like the intentions of cool roofs, it is still a significant improvement from a traditional black roof. Ultimately the albedo of a green roof is about twice higher than the albedos found for dark roofing surfaces.(Klein & Coffman. 2015). Using what’s known as a climatological model, one 50 year study undergone in New York City has evaluated the results of surface albedo of green roofs compared to black ones. The increase in surface albedo due to the substitution of one square meter of a black roof with one square meter of green roof results in approximately 38 kg of avoided carbon dioxide equivalent (CO2eq). (Susca et al. 2011). The improved surface albedos of green roofs simultaneously result in influencing of surface temperatures, and as a consequence, also impacts the energy use for heating and cooling of offices within the buildings.

Which transitions to the last discussed benefit that green roofs provide in helping to reduce the UHI, which is their ability to alter energy usage through the insulation of structures that green roofs are installed on. Green roofs in this scenario can be thought of as acting like a secondary means of insulation for buildings. The roof helps keep indoor temperature stable throughout the day and cuts down on ac energy consumption by 19-40%. (Jim, 2017; Saving Money by Going Green, 2017).  This reduction in air condition use from the green roof effectively helps mitigate the energy consumption impact air conditioning power draw has on the UHI (Rinkesh, 2017). During summer, according to analysis carried out in the Mediterranean city of Catania, consumption for cooling is reduced by between 31 and 35% for all the types of green roofs analyzed; while the annual energy reduction for air conditioning is between 20 and 24% for green roofs analyzed. Additionally, this study showed that even during the winter season, energy savings for heating are between 2 and 10%, compared to the same building without a green roof. (Cascone et al. 2018)

There are some drawbacks one should consider before investing in this infrastructure. Implementing a green roof into your building design is going to be more expensive than a traditional roof. Results from (Blackhurst, M. et al. 2010) suggests that green roofs are currently not cost effective on a private cost basis, but multifamily and commercial building green roofs are competitive when social benefits, or benefits that are payable under a social security system, are included. This is because of the incentives and benefits that already exist, and we propose continuing to improve on in relation to the installing of green roofs. There is no doubt that your upfront cost of green roof installation is going to be greater than a traditional roof. In a study done by the University of Michigan, there are comparisons done to the costs of the two types of roofs for a 21,000 square foot area. Results from the study concluded that the green roof would cost $464,000, whereas a conventional roof would only cost $335,000. However, researchers also determined that the green roof would save an additional $200,000 over its lifetime (University of Minnesota, 2017) from the combination of providing numerous environmental, economic, and social benefits. The EPA also released a cost estimate of installing and the maintaining of a green roof and compared those results to conventional roof costs. They also found that while green roofs are initially more expensive than conventional roofs, they provide significantly higher relative benefits per square foot over a 50-year lifecycle, in the form of energy cost savings, avoided emissions, and reduced stormwater fees. Compared to conventional roofs, the benefits of an extensive green roof is $14 more per square foot. (E.P.A., 2019) Another issue when designing green roof infrastructure in a city like New York is the height of buildings. When the green roof sits on a taller building, it is shown to be less effective than green roofs stationed on lower buildings (Jin, et. al, 2018). The most effective height range is 30-60 m vertically (Jin, et. al, 2018). This sentiment is supported by a 2018 study done by Lee et. al which measured the temperature of a green roof and a bare roof at the height of 15cm. As expected, the green roof was 32.98 C, while the bare roof was 29.2 C in the Summer. (Lee et. al, 2018). Evidence gathered from the literature draws to the conclusion that the higher the height of a green-roofed infrastructure the less effective it becomes and as an outdoor thermal heat reducer.

Attempts to help minimize the installation costs of Green roofs are government based incentive programs. These grant funds are awarded in a variety of ways including, Tax Breaks, Grant Programs, Fee Reductions, Expedited Building Permitting, Density and Zoning Leniency, and many others. (Achnitz, n.d.). These programs are intended to help reduce the large upfront costs associated with building green roofs. The problem that currently exists within these incentive programs is the lack of updating and funding that is supplied to them. New York City is a prime example of this issue and has let their program for tax incentives towards green roofs expire at this point in recognition of the need to propose a new plan (Welch, 2019). A lack of interest in these programs comes from the minute sums of money awarded to those who choose to install green roofs. In New York city’s case the city was awarded a tax right off, but only of a mere $5.23 per square foot of green roof for the first year after its installation. An amount that doesn’t come close to paying for the initial installation of the roof (Welch, 2019). Part of deliberations for the proposed new plan is to increase the payout per square foot to $15 and increase the length of the tax right off to a multi-year term, in hopes of enticing more to build green roofs atop their structures. Current programs are clearly proved ineffective through reporting that merely seven property owners have applied for the tax break in eight years from when the legislation was passed.  (Spiegel-Feld, & Sherman. 2018) On top of that, to further prove the failure of current programs, the state of New York is authorized to pay up to $1 million in green roof tax abatements each year, but a payout of the program has never reached anywhere near that total amount granted. (Spiegel-Feld, & Sherman. 2018) New York city poses a framed example of the failures of the green roof incentive programs in the US and shows us the work that needs to be put in for them to start to being utilized and implemented in future construction.

In 2009, a bylaw gave way to the construction of green roofs on new projects, making Toronto the first city in North America to do so (Stott, 2018). This bylaw started an extremely successful campaign for implementing green roofs in the Toronto area. It provides an excellent template for the US to follow in pursuit of more green roof installations in urban areas. This bylaw has seen phenomenal results since being implemented in Toronto, with 420 issued green roof permits as well as 500 additional application to build green roofs (Stott, 2018). These positive results stemming from this bylaw in Toronto are the direct result of carefully formulated laws and restrictions that mandate green roofs to be built into any “New commercial, institutional and residential development with a minimum Gross Floor Area of 2,000 m² (Stott, 2019)”. This bylaw shows the power a government can have over green roof installations and how involvement from politics officials can help translate the same results to buildings in the United States.

Our recommendation for increasing the presence of green roofs in New York City focuses on the adoption of improved incentivized government programs, as modeled in Toronto. This initiative is intended to mitigate the urban heat island effect in NYC. An understanding exists among policy administrators in New York City regarding the need to address urban heat islands, as well as the potential efficacy of green roofs. In fact, in 2008 New York City mayor Michael Bloomberg signed legislation agreeing that New York State will offer a property tax decrease to individuals in New York City who install green roofs on their buildings (Spiegel-Feld, & Sherman. 2018). This legislation was expected to bring upon a significant surge of new green roof development. However, due to insufficiently compensatory incentive programs, most property owners don’t consider green roof installation to be desirable. Although a small number of individuals have taken the initiative to install green roofs,  the vast majority of New York City’s rooftops remain covered in blacktop. With the current legislation set to expire in 2019 (Spiegel-Feld, & Sherman. 2018), the setting is more appropriate than ever to consider new successor programs designed to be more effective.



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