Cool Roofs in a Warm City

Today, around 200 million people living in over 350 cities around the world experience extreme heat conditions with average summer high temperatures climbing to over 35 °C (95 °F) (UCCRN, 2018). By 2050, roughly 45 percent of the global population will be living in cities and exposed to high summer temperatures, with more than 1.6 billion estimated city-dwellers in over 970 cities likely to experience regular extreme heatwaves (UCCRN, 2018). 

While climate change is driving higher global temperatures, air temperatures within cities and metropolitan areas are generally 1- 4 °C (33.8°-39.2° F) hotter than the surrounding areas, primarily due to the disruption of ecosystem services by the built environment (Stewart, 2011). This phenomenon is collectively referred to as the urban heat island effect or UHI.

 The New York City Panel on Climate Change (NPCC), an independent advisory body that provides scientific information to inform city policy, reported a temperature increase of  0.7 °F per decade between 1970 and 2013 (NPCC, 2019). This rise in temperature, projected to continue at an increasing rate, will expose vulnerable populations to major health risks. Elderly populations and people with preexisting conditions are particularly at risk of being affected by extreme heat events (Harlan et al., 2013), as are lower-income residents who disproportionately lack access to cooling systems in their homes (Anderson & McMinn, 2019).

Higher air temperatures from heat islands affect the local environment, compromise public health, increase energy use, and diminish the quality of life of communities. 

Temperatures higher than 35 °C combined with high humidity can cause serious health effects such as fatalities and other general health inequalities such as discomfort, respiratory difficulties, heat cramps, heat exhaustion, stress, and non-fatal heat stroke (put citations here). Additionally, the American Psychiatric Association notes that exposure to extreme heat has been associated with decreases in mental health, increased use of alcohol to cope with stress, increases in depression, anxiety, hospital and emergency room admissions, domestic violence, and suicide (Ursano et al., 2017). Health and productivity impacts are further exacerbated by major electric grid failures during extreme heat events – exposing large populations to severe heat stress inside and outside of buildings (Stone et al., 2021).

In response to UHI, energy consumption increases as residents attempt to keep cool. Without climate change mitigation, the energy requirement for building cooling is predicted to jump 300 percent by 2050, from 2,020 terawatt-hours (TWh) to 6,200 TWh (United Nations Environment Programme, 2021). The rising use of air conditions will require a more expansive electrical grid infrastructure as well as increased greenhouse gas emissions and expelled waste heat, creating a negative energy demand-climate change feedback loop, further exacerbating the UHI.

Currently peak electricity loads are largely dependent on fossil fuel-powered grids and the projected increase in future cooling needs will lead to an 18 percent increase in global carbon dioxide (CO2) emission between 2016 and 2050 (International Energy Agency, 2018). Based on our current trajectory, cooling emissions for residential buildings alone could result in up to an estimated 0.5 °C  of global warming by 2100, requiring further cooling (Sachar et al., 2018).

New York City has implemented a myriad of policies and programs to tackle urban heat. Among them, several vital initiatives seek to leverage a more sustainable roofscape. Cool roofs have a significant role here as both low-cost and low-tech passive cooling solutions. Due to the high reflectivity of light-colored surfaces, white roofs can reduce ambient air temperatures, thereby mitigating the UHI effect. However, to be a practical part of the city's overall urban heat resilience strategy, cool roofs must be implemented at scale.

Several local laws enacted by the city aid in the effort to cool New York City's rooftops.  Local Law 21 passed in 2011, set the ball rolling by updating the administrative and building codes to require that all roofs being entirely replaced install a reflective cool roof coating. Local Laws 92 and 94, passed subsequently as part of the 2019 Climate Mobilization Act (CMA), further strengthened these roof standards to require sustainable roofing zones with solar photovoltaic or green roof systems on any new construction or major renovation. 

Data collected by the New York City Council indicates that currently buildings account for a whopping   70% of the city’s emissions  . one of the most ambitious parts of the CMA, Local Law 97, targeted the improvement of overall energy efficiency by phasing in varying carbon caps for large individual buildings from 2024 onwards, with the goal of achieving carbon neutrality by 2050.

Given that cool roofs help    reduce the need for air-conditioning, significantly lowering the energy demand    of buildings and homes, these laws effectively incentivize roof conversions, which helps reduce the urban heat island effect and also contributes to New York City’s decarbonization efforts.

Launched in 2009, the NYC ºCoolRoofs program is a unique partnership between the NYC Department of Small Business Services, the  Mayor’s Office of Climate and Environmental Justice , and the NGO  The HOPE Program .

With grant funding from the Environmental Protection Agency, and other funding and in-kind assistance from corporations and community groups, NYC ºCoolRoofs works to install highly reflective coatings on roofs at little to no cost, if the building qualifies. The initiative provides New Yorkers with the opportunity to gain practical training and paid work experience while simultaneously supporting the city’s objective of becoming carbon neutral by 2050.

To date, over 10 million square feet of rooftops across the city have been successfully converted to highly reflective cool roofs. These conversions promote energy efficiency by cutting building cooling cost by 7-15%, which helps reduce GHG emissions across the city. They also help accelerate compliance with the beefed-up standards of the city's sustainable roof policies.

If you are interested in the NYC ºCoolRoofs program, use the link below to request or report a cool roof installation.

As New York City implements strategies and standards for cooling city rooftops, it is important to track how changes are occurring across the city through the years. Policymakers can then assess the effectiveness of these interventions, and design targeted incentives and outreach programs to encourage the adoption of cool roofs where they are most needed.

To aid in this tracking effort, researchers from the departments of Urban Policy and Planning, and Geography and Environmental Science at Hunter College collaborated with Senior Policy Advisors in the NYC Mayor’s Office of Climate & Environmental Justice to measure changes in rooftop reflectivity between 2010 and 2020.

The main aim of the NYC Roofscape project was to first and foremost identify cool roofs across the city (2010 -2020), and then quantify how recent regulatory changes to increase rooftop reflectivity have been effective. 

To detect cool roofs, we focused on the brightness of roofs in high-resolution orthoimages. These images offer four bands (Blue, Green, Red, and Infrared) at 0.5 feet organized in (5000 X 5000) pixel tiles. We downloaded and analyzed all tiles covering NYC for the years 2010, 2012, 2014, 2016, 2018, and 2020. We combined the values of the four bands using a linear regression model and rated the brightness of building rooftop pixels between a reflectivity value of 0 (lowest) and 100 (highest).

The following figure shows some examples of measured reflectivity values. 

We used the reflectivity value of 60 (on a scale of 0-100) as the cut-off threshold for a cool roof. This analysis, was conducted for each year, and every roof in the city then labelled to record if and when they were converted from a "dark" roof to a "cool" roof.

Use the dashboard below to explore the cool roof landscape of New York City. 

Pan to your area of interest in the city and click on any neighborhood block to view and compare rooftop reflectivity scores and cool roofs statistics at Neighborhood Tabulation Area (NTA) scale. Then zoom in and click on any individual building to view how the roof reflectivity value of an individual building (or selected groups of buildings) compares with the average for the city. 

Turn on additional data layers using the Layers icon, and use the Legend icon to help understand the color symbology. Use the Bookmarks icon to view and explore specific focus neighborhoods in the city.

Please allow a few moments for the data to load.

Our results show that average roof reflectivity for the city has increased from 2010 to 2020. 

This increase in roof reflectivity varies spatially and annually.

We observed a modest decrease in average roof reflectivity between 2016 and 2018, and an overall increase in average roof reflectivity after 2018.

This improvement, seen post 2018, is more evident in the Bronx and Brooklyn.

We found that average roof reflectivity in Brooklyn is significantly higher than in other boroughs, followed by the Bronx, Manhattan, and Queens. Staten Island has the lowest roof reflectivity among the five boroughs. 

The distribution of cool roofs in the city shows that neighborhoods in Brooklyn, central Queens, and the Bronx have a high percentage of roof reflectivity. Considering that some neighborhoods in Brooklyn, Queens, and the Bronx have a significant number of buildings with pitched roofs, the average roof reflectivity in these boroughs is considerably high.

The total roof area with cool roofs (reflectivity value ≥ 60 on a scale of 0-100) in New York City grew from 37% in 2010 to 39% in 2020. The conversion rate from dark to cool roofs has been relatively stable at around 2%.

The overall ratio of cool roofs has not exceeded 40% despite a robust rate of change, primarily because of the continuing negative changes (conversion from cool to dark roofs) detected.

In 2020, the area of cool roofs in New York City topped 675 million square feet, over 36% of roof surfaces across the city. However, with nearly 30% of flat roofs across the city still dark, there remains significant potential to increase cool roof surfaces.

Some neighborhood in Bronx, Brooklyn saw significantly more change than the rest of the city. The percentage change in cool roof area in these neighborhood of interest ranges between 6.2 % and 16.1%.

To dive deeper into the changes seen at zoning district level and more local scales in several key neighborhoods, explore our Cool Roof Stories Section.

The New York City Housing Authority (NYCHA) provides affordable housing for low and moderate-income New Yorkers. It is the largest public housing authority in the country with 177,569 apartments in 2,411 buildings across 335 conventional public housing and Permanent Affordability Commitment Together (PACT) programs.

These buildings together translate to nearly 18 million square feet of potential cool rooftop space, of which over 40% - approximately 7.6 million square feet - has been converted from dark to light cool roofs.

Located on a peninsula in the South Bronx, Hunts Point is a low-income neighborhood that is home to a vibrant residential community and one of the largest food distribution facilities in the world.

The Hunts Point Market consists of more than ten large buildings which together cover an area over 1.2 million square feet, coincident cool roof space potential. The area is also served by one NYCHA housing development.

The following figures show the distribution of cool and dark roofs in 2020 in different zoning districts across boroughs. Results show that the implementation of cool roofs is highly variable across different zoning types.

Despite this variability, key patterns emerge.