Cooling Summertime Temperatures
Strategies to Reduce Urban Heat Islands
                                          September 2003

For millions of Americans living in and around cities, elevated summertime temperatures are of growing concern.
Commonly referred to as urban heat islands, this phenomenon can impact communities by increasing peak energy demand,
air conditioning costs, air pollution levels, and heat-related illness and mortality.

Fortunately, there are common-sense measures that communities can take to reduce the negative effects of heat islands.
What Is a Heat Island?
Heat islands are characterized by urban air and surface temperatures
that are higher than nearby rural areas. Many U.S. cities and suburbs
have air temperatures up to 10° F (5.6° C) warmer than surrounding
natural land cover.
The heat island sketch below shows a  city's heat island profile. It
demonstrates how temperatures typically rise from the urban-rural
border, and that the warmest temperatures are in dense downtown areas.
              Urban Heat Island Profile
Heat islands are often largest over dense development but may be broken
up by vegetated sections within an urban area.
What Causes Heat Islands?
Heat islands form as cities replace natural land cover with pavement,
buildings, and other infrastructure. These changes contribute to
higher urban temperatures in the following ways:
  • Displacing trees and vegetation minimizes the natural cooling
   effects of shading and evaporation of water from soil and leaves
  •Tall buildings and narrow streets can heat air that is trapped
   between them and reduce wind flow.
  •Waste heat from vehicles, factories, and air conditioners may
   add warmth to the air, further increasing temperatures.
Heat islands are also influenced by a city's geography and prevailing
weather conditions. For example, strong winds and rain can flush out
hot, stagnant air from city centers, while sunny, windless conditions
can exacerbate heat islands.
                                      When Do Heat Islands Form?
                                      Heat islands can occuryear-round during the day or night. Urban-
                                      rural temperature differences are often largest during calm, clear
                                      evenings. This is because rural areas cool off faster at night than
                                      cities, which retain much of the heat stored in  roads, buildings, and
                                      other structures.
                                       How Do Heat Islands Affect  Us?
                                       Increased urban temperatures can affect public health, the environment,
                                       and the amount of energy that consumers use for summertime cooling.
                                       Public Health: Heat islands can amplify extreme  hot weather events,
                                       which can cause heat stroke and may lead to physiological disruption,
                                       organ damage, and even death - especially in vulnerable populations
                                       such as the elderly.
                                                                                   - Ozone (03)
                                          Heat from City Surface
                              The Environment: Summertime
                              heat islands increase energy
                              demand for air conditioning, raising
                              power plant emissions of harmful
                              pollutants. Higher temperatures
                              also  accelerate the chemical
                              reaction that produces ground-
                              level ozone, or smog. This threatens
                              public health, the environment,
                              and, for some communities, may
                              have implications for federal air
                              quality  goals.

                              Energy Use: Because homes and
                              buildings absorb the sun's energy,
                              heat islands can increase the
                              demand for summertime cooling,
                              raising  energy expenditures. For
                              every 1°F (0.6° C) increase in
summertime temperature, peak utility loads in medium and large cities
increase by an estimated 1.5-2.0 percent.

Cities in cold climates may actually benefit from the wintertime warming
effect of heat islands. Warmer temperatures can reduce heating
energy needs and may help melt ice and snow on roads.

In the summertime, however, the same city may experience the
negative effects of heat islands.
                                      Ozone forms when precursor compounds
                                      react in the presence of sunlight and
                                      high temperatures.
    Cool Roofs in Action
               The Energy Coordinating Agency (EGA) in Philadelphia initiated the Cool Homes Program to help elderly residents
               escape extreme summertime heat. EGA installs cool roofs and uses other measures to reduce indoor temperatures to
               promote comfort and minimize health risks. As of April 2003, the Cool Homes Program had installed over 450 roofs.

High-albedo pervious pavement supports light
traffic while mitigating the heat island effect
and allowing stormwater to pass through.
Both types of green roofs can be used on residences, industrial facilities,
offices, and other commercial  property. Green roofs are widespread in
Europe and Asia, and are becoming more common in the United States.

Cool  Pavement
                                      Pavements with low
                                      solar reflectance absorb
                                      large amounts of heat and
                                      can be up to 70° F (40° C)
                                      hotter in the sun than
                                      cooler alternatives.

                                      Portland cement
                                      concrete and  asphalt
                                      called  "concrete" and
                                      "asphalt," respectively-
                                      are the most common
paving  materials for sidewalks and streets. Most new concrete has a
solar reflectance, or albedo, of 35-40 percent; the solar reflectance of
fresh asphalt is typically 5-10 percent.

Over time, the albedo of these pavements change. Concrete  darkens
from the build-up of tire residue, dirt, and oil, and asphalt lightens as the
asphalt binder wears away to expose the underlying rock aggregate.

To maximize the albedo of both types of pavement, lighter-colored
aggregate can be used in the pavement mix. Alternatively,  asphalt
pavements can be covered with high-albedo sealcoats, small rocks
set in binder, or a thin layer of  concrete. For concrete applications,
using lighter-colored sand and cement can increase reflectivity.

Permeable, or porous, pavements allow water to percolate and evaporate,
cooling the pavement surface and surrounding air. Permeable pavements
can be constructed from a number of materials including concrete,
asphalt, and plastic lattice structures filled with soil, gravel, and grass.

Although there is no official standard or labeling program to designate
cool paving materials, communities interested in  reducing  the heat
island effect may consider surface reflectivity and  permeability-along
with other costs and benefits - when selecting a paving product.
   The Difference between Heat Islands and
   Global Warming
   Heat islands describe local-scale temperature
   differences between urban and rural areas. In contrast, global
   warming refers to the gradual rise of worldwide average
   surface temperatures.
                                                              What is EPA Doing to Reduce
                                                              Heat Islands?
                                                              Through its Heat Island Reduction Initiative (HIRI), EPA works with
                                                              community groups, public officials, industry representatives, researchers,
                                                              and other stakeholders to identify opportunities to implement heat
                                                              island reduction strategies and evaluate their impacts on energy
                                                              demand, local meteorology, air quality, health, and other factors.
                                                              For More Information

EPA's Heat Island Reduction Initiative

EPA Global Warming Information

ENERGY STAR Qualified Cool Roof Products

The Lawrence Berkeley National Laboratory's
Heat Island Group

International Council for Local Environmental Initiatives'
Hot Cities Information

NASA's Global Hydrology and Climate Center (GHCC)

Cool Roof Rating Council

USDA Urban and Community Forestry Program

Green Roofs for Healthy Cities

U.S. Green Building Council
                                                              Center for Green Roof Research

   Cool Pavement in Action
                The village of Fair Oaks in Sacramento, California installed a permeable portland cement concrete
                parking lot at a local park. It avoids the cost of a stormwater drainage system and helps reduce the heat
                island effect.

What Can Communities  Do to Reduce
the Heat Island Effect?
Communities interested in reducing heat islands have several options.
Strategies to lower urban temperatures and achieve related benefits
include installing reflective cool roofs on  residential and commercial
buildings; planting trees and vegetation, including green roofs; and
using cool paving materials for roads, sidewalks, and parking lots.
Additional heat mitigation options include modifying urban design
and layout, and choosing  efficient heating and cooling systems.
                                     Widespread implementation
                                     across a community can
                                     reduce urban temperatures,
                                     energy use, air pollution,
                                     and heat-related health
                                     impacts. Heat island
                                     reduction strategies also
                                     benefit individual home and
                                     building owners directly.
                                     Cool roofs and shade trees,
                                     for example, can save
                                     money on summertime
                                     cooling bills.
 Highly Reflective
 Reel 0.60- 0.70
        Colored Paint
 Corrugated  0.15-0.35
 Roof 0.10-0.15
                          Red/Brown Tile
                          Roof 0.10-0.35
White Paint
Various urban environmental albedos.
Cool Roofs
The term "cool roof" describes roofing materials that have a high
solar reflectance. This characteristic reduces heat transfer to the
indoors and can enhance roof durability. Cool roofs may also have high
emittance, releasing a large percentage of the solar energy they absorb.

On a hot, sunny, summer day, traditional roofing materials can reach
peak temperatures of 190° F (88° C). By comparison, cool roofs reach
maximum temperatures  of 120° F (49° C).

In buildings with air conditioning (AC), cool roofs can save money on
energy bills, lower peak  energy demand, and reduce air pollution and
greenhouse gas emissions. In buildings without AC, cool roofs can
increase indoor occupant comfort by lowering top-floor temperatures.
In both cases, cool roofs can help reduce urban heat islands.
                                                                Types of Cool Roofs

                                                                 • Commercial (low slope): Most cool roof applications for low-
                                                                   slope, primarily commercial, buildings have a smooth, bright white
                                                                   surface to reflect solar radiation and achieve related benefits.

                                                                 • Residential (steep slope): Most cool roof applications for sloped,
                                                                   primarily residential, buildings come in various colors and may use
                                                                   special  pigments to reflect the sun's energy.
Albedo, Solar Reflectance, and Emittance
The albedo, or solar reflectance, of a surface is the percentage
of incoming solar radiation that is reflected by that surface.
Albedo is measured on a scale of 0 to 1, where a value of 0
indicates that a surface absorbs all solar radiation and a
value of 1 represents total reflectivity.
Light-colored surfaces typically have higher albedos than
darker surfaces. While a traditional black shingle has an
average albedo of 0.05, or 5 percent, the average  albedo for
a white roof coating is 0.75, or 75 percent.
The emittance of a material refers to  its ability to release
absorbed heat. Scientists use a number between  0 and 1 to
express emittance. With the exception of metals,  most
construction materials have emittances above 0.85, or 85 percent.
                                                                EPA's ENERGY STAR® program has voluntary product specifications
                                                                for both commercial and residential roofs. Low-slope roofs must have
                                                                an initial solar reflectance of at least 65 percent, and steep-slope
                                                                roofs must have an initial solar reflectance of 25 percent or more.
                                                                Emittance is not a qualifying criterion for the ENERGY STAR label,
                                                                but a high rating can further reduce energy costs.

                                                                Community-Level Benefits from Cool Hoofs

                                                                Installing cool roofs across a city can provide substantial energy
                                                                savings. The figure on the next page illustrates this potential for 11
                                                                U.S. cities according to research conducted at the Department of
                                                                Energy's Lawrence Berkeley National Laboratory (LBNL).
    The Utah Olympic Oval used cool roof technology.
                                                                    Factors Affecting Building-Level Energy Savings
                                                                    from a Cool Roof
                                                                      • Air conditioning: Cool roofs can reduce summertime
                                                                       energy use in air-conditioned buildings. In buildings
                                                                       without air conditioning, cool roofs can improve comfort
                                                                       by reducing top-floor temperatures.

                                                                      • Local climate: Cooling energy savings are typically
                                                                       greatest in areas with long, sunny, and hot summers.

                                                                      • Building height: Cool roofs are generally most effective on
                                                                       one-or two-story buildings with large roof areas. They provide
                                                                       less energy savings for multi-story buildings with small roofs.
    Trees and Vegetation in Action
                  The City of Austin, Texas's NeighborWoods program uses aerial photos to identify neighborhoods with insufficient
                  tree coverage. Austin Energy, the city-owned utility, then provides residents with free saplings that will ultimately
                  provide shade, beauty, and  energy savings.

                                                                                                           400 cooling hours
                                                                                                           400 to 800
                                                                                                           800 to 1200
                                                                                                           1200 to 1600
                                                                             > 2000 to 2400
                                                                             >2400 to 2800
                                                                             > 2800      173
                        New Orleans

Metropolitan-scale savings per 1000 ft2 of roof area of air-conditioned buildings ($)
                                                         Metropolitan-scale savings ($ millions)
                                                                                    Ft. Lauderdale

                                                  Metropolitan-scale savings per 1000 ft2 of roof area of air-conditioned buildings (kWh)
Metropolitan-scale potential savings from cool roofs in 11 U.S. cities. Results are stated in net energy savings and factor in any increased heating costs
from the cool roof "wintertime penalty."
Trees and Vegetation

Increasing a  city's vegetative cover by planting trees, shrubs, and
vines is a simple and effective way to reduce the heat island effect.
Scientists at  LBNL estimate that planting trees and vegetation for
shade can reduce a building's cooling energy consumption by up to
25 percent annually.
In addition to direct shading, trees and vegetation cool the air through
evapotranspiration. Urban vegetation also provides economic,
environmental, and social benefits such as enhanced storm water
management and reduced air pollution.
       Leaves, Branches:
       Absorb Sound,
       Block Rainfall
                                       Cool the Air through
                                               Filter Pollutants
                                               from the Air
            Stabilize Soil,
            Prevent Erosion
Provide Shade,
Reduce Wind Speed
            Roots, Leaves, Trunk:
            Provide Habitatfor Birds, Mammals, and Insects

Trees provide a variety of benefits, from cooling the air to stabilizing the soil.

Where to Plant

Strategically placed shade trees and vegetation block the sun's rays,
minimizing heat transfer to building interiors, and reducing the need for
air conditioning. In most U.S. cities, trees should shade the east, and
especially west, walls to maximize cooling savings. Planting trees
directly to the south may provide little shade in the summertime and
block desired sun in the wintertime.

What to Plant

Deciduous trees work well as they balance energy requirements over
the course of a year. In summer, foliage cools buildings by blocking
solar radiation. In winter, after the leaves have fallen, the sun's energy
passes through the trees and helps to warm buildings.
Green Roofs
Another alternative to traditional roofing materials is a rooftop garden
or "green roof." Installed widely in a city, green roofs contribute to
heat island reduction by replacing heat-absorbing surfaces with plants,
shrubs, and small trees that cool the air through evapotranspiration.
Planted rooftops remain significantly cooler than a rooftop constructed
from traditional heat-absorbing materials. In addition, green roofs
reduce summertime air conditioning demand by lowering heat gain
to the building.

Green  roofs consist of soil and vegetation planted over a waterproofing
layer. They can be intensive or extensive depending on the amount of
soil and plant cover, and whether the roof is accessible.

  * Intensive green roofs require a minimum of one  foot of soil. Trees
    and shrubs are usually planted, adding 80-150 pounds per square
    foot of load to the building. These roofs need complex irrigation
    and drainage systems, and significant maintenance. Intensive
    roofs are often accessible to the public.

  • Extensive green roofs require only 1-5 inches  of soil. Low lying
    plants and grasses are usually planted, and 12-50 pounds per
    square foot of load may be added. These roofs use simple irrigation
    and drainage systems, and require little maintenance. Extensive
    green roofs usually are  not accessible to the public.
                                             Gravel-ballested Roof
                                          gravel -
                                     protection layer-
                                     waterproofing -
                                     moisture barrier/x
                                     separation layer
                                             Green Roof
                               Green roofs remain significantly cooler than rooftops made of traditional
                               heat-absorbing material.
    Green Roofs in Action
                   The City of Chicago installed a 20,300 square foot green roof on its City Hall. The city expects the roof to reduce
                   annual air conditioning costs by $4,000. Businesses such as the Gap and Ford Motor Company have also
                   installed green roofs  on their corporate headquarters buildings.

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