Four Approaches to Reducing the Urban Heat Island Effect

What if it were possible to make cities cooler—that is, actually lower urban temperatures to improve the quality of life—with more-resilient development? Extreme heat is the number-one weather-related cause of death in the United States, according to the Centers for Disease Control and Prevention, and extreme heat can also harm infrastructure as well as the overall economy.

While several factors contribute to extreme heat, development and land use decisions greatly influence the temperature of cities, making those temperatures higher than those of the surrounding area, a phenomenon known as the urban heat island (UHI) effect. In fact, urban heat islands rather than climate change are responsible for the majority of temperature increase in U.S. cities.

In August 2019, ULI released a report exploring that dynamic—Scorched: Extreme Heat and Real Estate, available on Knowledge Finder. Although use of resilient development to “cancel out” UHI effects is certainly an optimistic notion, member and outside expert contributors to the Scorched report emphasized the observed success of numerous strategies that mitigate extreme heat.

Since the report was published, real estate experts and public officials have continued to prioritize resilience to extreme heat—efforts that have helped the field to progress rapidly. In June, ULI’s Urban Resilience program interviewed real estate and land use experts from around the United States to identify the latest progress in heat-resilient development, design, and policy.

Scorched is also available for purchase at the ULI Bookstore.

Why Is It So Hot in Cities?

Climate change has caused an increase in baseline temperatures across the United States and made heat waves hotter and more likely to occur. In addition, urban heat islands cause the majority of the temperature risk in urban areas. UHIs are the reason downtowns can be up to 22 degrees Fahrenheit warmer than surrounding areas and are warming up to 50 percent faster than the rest of the United States.

UHIs are the result of four factors related to building and urban design:

• Change in land use (the removal of trees and green space and the addition of heat-absorbing materials);
• Waste heat (mainly from energy use in buildings and transportation);
• Air pollution (UHIs create ideal conditions for formation of smog, which acts as a heat-trapping barrier); and
• Urban geometry (the pattern in which streets and buildings are arranged as well as the size and shape of a city are influential determinants of UHI intensity).

UHIs are not limited to warm environments. The relative change in temperature causes more damage—to people, infrastructure, and landscapes—in cooler places and where fewer heat-mitigation and adaptation strategies are in place. As a result, extreme heat is a growing risk factor and planning consideration across the United States.

Passive Design for Thermal Comfort

Architects and designers continue to use passive design strategies to maintain healthy indoor temperatures. Finch Cambridge, a new six-story, 98-unit affordable development in Cambridge, Massachusetts, is the state’s second certified Passive House building, certified by the Passive House Institute U.S.

“The whole point of a Passive House building is to have continuous air tightness and the right amount of thermal control,” says lead architect Michelle Apigian. “If you put those two together, you maintain temperatures inside no matter what is happening outside. If we have a crazy heat wave, residents can shelter in place, and that’s better than if they’re wandering out into hotter spaces.”

Features like continuous insulation and triple-glazed windows help the building maintain temperatures, and choices like underground parking (so cars do not heat up or give off heat when parked) and big shade trees keep the outside space comfortable. Even the building’s solar array is functional: the elements are mounted on a rooftop structure that looks like bird wings from the ground and provides shade for the building, minimizing solar heat gain.

Healthy and Heat-Resilient Development

In building to the latest U.S. Passive House design standard, the Finch Cambridge structure, developed and owned by HRI, will provide high air quality and potentially reduce chances of COVID-19 transmission. A Passive House building has intentional balanced ventilation. “That, in conjunction with air tightness and compartmentalization, helps reduce the odds of germs spreading,” says Apigian. “So my unit is really separate from your unit, and our air is not going to transmit, nor is the air from the hallway going to transmit into our units.”

Maceo May in San Francisco is another new development designed to prioritize resident health, including through stable indoor temperatures and high-quality ventilation, even during power outages. Maceo May will provide 105 units for formerly homeless veterans and their families when construction is completed in 2022. “All Maceo May’s resilient design strategies come back to clear health and quality-of-life benefits for residents,” says Joanna Ladd, senior project manager and development strategist for Chinatown Community Development Center, which is developing and will own Maceo May in partnership with nonprofit Swords to Plowshares.

Maceo May’s design includes passive strategies like shaded south- and west-facing sides, operable windows, and windows located at different heights within units to maximize airflow. The building is also all-electric: the developers reinvested about $250,000 in avoided natural gas infrastructure costs toward energy recovery ventilators for each unit and a community “resilience hub” providing critical amenities. Electricity will be provided by on-site solar photovoltaic arrays.

A new conversation guide created through a partnership between ULI and other membership organizations offers questions to consider when beginning projects for which managing extreme heat and ensuring resident health are priorities.

Cooling Cities with Zoning

At the city level, cities are continuing early work described in Scorched to develop heat-informed zoning policies. “We have a lot going on in terms of understanding the UHI effect and how we can modify our urban form to have less temperature exacerbation,” says John Bolduc, a climate resilience planner who is part of the Climate Resilience Zoning Task Force in Cambridge, Massachusetts.

The task force has developed a Cool Factor Rating System modeled after the Seattle Green Factor. Cambridge estimated the temperature-reduction effects of various cooling strategies—such as preserving mature trees and using light-colored paving and building materials—and drafted a site scoring system that is now being piloted at four sites.

In response to developer feedback, Bolduc says, the task force is also considering an innovation credit—“the idea being that if someone could give us evidence that a new strategy would work, they could get credit for it.” The task force is hoping to vote on whether to recommend adoption of the rating system at the end of this summer.

Neighborhood Solutions

Heat resilience efforts are also resulting in an increased focus on neighborhood-level heat solutions. Philadelphia is leading this approach with the publication of the city’s first Community Heat Relief Plan, developed with and by community members of Hunting Park, an area just north of Center City. With 9 percent tree coverage and a land cover of 85 percent buildings and hardscape—compared with 9 percent and 51 percent citywide averages, respectively—Hunting Park is one of Philadelphia’s hottest neighborhoods. It is also predominantly a community of color and a low-income neighborhood with a history of successful community-led advocacy of environmental education and environmental justice.

A neighborhood-level focus is critical, says Cheyenne Flores, a climate resilience fellow with Philadelphia’s Office of Sustainability who coordinated the project. “The community holds the most knowledge about the struggles they are facing with extreme heat and the tactics they’ve already been using to address those struggles,” Flores says.

Developed by a cross-departmental city working group, 30 community partners, five community heat ambassadors, and over 600 residents, the Hunting Park heat plan identifies where cooling assets and resources could be incorporated into the neighborhood and establishes a local heat relief network. The network will consist of air-conditioned community spaces that would be free and open daily during heat health emergencies.

With the network currently in a pilot phase, the Office of Sustainability and its key partners—Esperanza, a Hunting Park–based Hispanic nonprofit organization, and Pennsylvania Interfaith Power & Light, a faith-based organization focused on climate change—are working with other city departments to determine protocols for opening cooling centers during the coronavirus pandemic.

To otherwise connect or get involved with the ULI Urban Resilience program, email [email protected].

More from Urban Land on Sustainability:

Compounding Challenges: Communities Planning for Extreme Weather Events and COVID-19

From Droughts to Floods, Extreme Weather’s Increasing Impact

The New Normal of Extreme Weather Conditions