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State and Local Climate and Energy Program

Heat Islands

Heat islands are characterized by urban air and surface temperatures that are higher than nearby rural areas. Many concentrated areas have air temperatures up to 10°F (5.6°C) warmer than surrounding areas with open land and vegetation. Heat islands form as dense built-up areas replace natural land cover with pavement, buildings, and other infrastructure.

Urban Heat Island Profile
Urban Heat Island Profile

Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas (GHG) emissions, heat-related illness and mortality, and water quality.

Heat Island Mitigation Options

Communities can take action to reduce urban heat islands, save energy, and achieve multiple benefits using these key mitigation strategies:

  • Increasing tree and vegetative cover
  • Installing green roofs (also called "rooftop gardens" or "eco-roofs")
  • Installing cool–mainly reflective–roofs
  • Using cool pavements

Employing these strategies in combination can enhance their effectiveness. For example, installing a permeable pavement parking lot that includes shade trees can extend the longevity of the pavement and vegetation.

Local Government Actions

Learn What Governments
are Doing

EPA's Urban Heat Island Community Actions Database includes information on local government heat island initiatives.

Local governments can reduce heat island conditions through voluntary efforts, as well as policies and regulations.

Voluntary Actions

Voluntary efforts can generally be grouped into the following categories:

  • Demonstration projects – Demonstrations can be most effective when they target high-visibility projects, measure benefits, and convey lessons learned. The City of Tucson documented that installation of a cool roof reduced the building temperature and saved more than 400 million British thermal units (Btus) annually in energy.
  • Incentives – Governments, utilities, and other organizations can offer below-market loans, tax breaks, product rebates, grants, and giveaways to motivate investments in heat island mitigation strategies. In 2007, as part of an innovative energy saving grant program run by Pennsylvania's Department of Environmental Protection, over $500,000 was awarded to construct eight green roofs across the state.
  • Urban forestry – Programs generally have broad goals that emphasize the multiple benefits trees can provide and unite diverse stakeholders. Groundwork Elizabeth, a nonprofit group in New Jersey, works to involve neighborhood residents in community revitalization projects in partnership with New Jersey's Cool Cities Initiative. Joint projects included tree planting at schools and parks to increase tree canopy in Elizabeth, NJ.
  • Weatherization – Programs usually involve making the homes of qualifying residents, generally low-income families, more energy efficient at no or low cost to the residents. The Energy Coordinating Agency of Philadelphia, the city's weatherization entity, has applied cool roof coatings as part of its package of energy efficiency treatments. Through its Cool Homes Program, more than 550 residences in the Philadelphia area have had their roofs coated.
  • Outreach and education – Campaigns can focus specifically on students or include the larger community. The Utah State government developed the Utah Kool Kids program to teach students about urban heat islands, their impacts on energy and air quality, and heat island reduction strategies. The program supports teachers to engage students.
  • Awards – States, local governments, community organizations, or corporations can reward exemplary work as a way to highlight innovation and promote solutions to mitigate heat islands across the public and private sectors. Since 2003, Green Roofs for Healthy Cities' has annually recognized a variety of green roof projects for integrated design and implementation. The program rewards extensive and intensive green roof projects, as well as research teams and citizens who have advanced the implementation of green roofs though public policy.

Policy Actions

Policy efforts can include:

  • Procurement – Local governments can procure cool technologies, such as cool roof coatings and roofs for government buildings and revise bid specifications to include these products.
  • Resolutions – A resolution stating a group's awareness of and interest in a heat island mitigation project can be the first step in getting an initiative started. Organizations such as a city or county council, air quality boards or planning commissions have issued such resolutions.
  • Tree and landscape ordinances – Many local governments have enacted tree and landscape ordinances, which can ensure public safety, protect trees or views, and provide shade.
  • Comprehensive plans and design guidelines – Comprehensive plans are adopted by a legislative body of a local government, and set forth policies, goals, and objectives to direct development and conservation that occurs within its planning jurisdiction. Design guidelines provide a connection between general planning policies and implementing regulations, such as zoning codes and subdivision regulations.
  • Zoning codes – These regulations generally dictate function for an area, building height and bulk, population density, and parking requirements. Zoning codes can promote heat island mitigation in various ways, such as through parking lot shading requirements.
  • Green building standards – Green building initiatives can capture heat island reduction strategies.
  • Building codes – Building codes establish standards for construction, modification, and repair of buildings and other structures. For example, local governments can include cool roofing in their building codes as an energy saving measure.
  • Air quality standards – Ground-level ozone forms more readily when air temperatures rise. Strategies to mitigate the urban heat island reduce air temperatures and therefore decrease concentrations of ground-level ozone. EPA has developed three policies that may help states include heat island reduction strategies in their State Implementation Plans (SIPs) as required under the Clean Air Act. The policies are listed on page 18 of the Heat Island Reduction Activities Chapter (PDF) (23 pp, 2.71M) of the Compendium.

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Tools and Resources

ENERGY STAR Roof Products

ENERGY STAR qualified roof products reflect more of the sun's rays, which can lower roof surface temperature by up to 100°F and decrease the amount of heat transferred into a building. ENERGY STAR qualified roof products can help reduce the amount of air conditioning needed in buildings, and can reduce peak cooling demand by 10–15 percent.

ENERGY STAR Roofing Comparison Calculator

The ENERGY STAR Roofing Comparison Calculator Exit EPA disclaimer helps consumers estimate how much energy and money they can save by installing an ENERGY STAR labeled roof product on your home or building.

Heat Island Compendium

EPA's Reducing Urban Heat Islands: Compendium of Strategies describes mitigation measures that communities can take to address the negative impacts of urban heat islands. The compendium includes six sections: Urban Heat Island Basics, Trees and Vegetation, Green Roofs, Cool Roofs, Cool Pavements, and Heat Island Reduction Activities.

Roof Calculators

The U.S. Department of Energy's Oak Ridge National Laboratory supports two roofing calculators. The steep slope calculator estimates cooling and heating savings for residential roofs with non-black surfaces. The flat roof calculator estimates savings for flat roofs with non-black surfaces.

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