t&.\
Vv/
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
Rural
Commercial
Suburban
Residential
Urban
Residential
Suburban
Residential
Downtown
Park
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
(evapotranspiration).
•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.
Compounds
Sunlight
AT
- Ozone (03)
tttttt
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
concrete-commonly
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
www.epa.gov/heatisland
EPA Global Warming Information
www.epa.gov/globalwarming
ENERGY STAR Qualified Cool Roof Products
www.energystar.gov/products
The Lawrence Berkeley National Laboratory's
Heat Island Group
http://heatisland.lbl.gov
International Council for Local Environmental Initiatives'
Hot Cities Information
www.hotcities.org
NASA's Global Hydrology and Climate Center (GHCC)
www.ghcc.msfc.nasa.gov
Cool Roof Rating Council
www.coolroofs.org
USDA Urban and Community Forestry Program
www.fs.fed.us/ucf
Green Roofs for Healthy Cities
www.greenroofs.ca/grhcc
U.S. Green Building Council
www.usgbc.org
Center for Green Roof Research
http://hortweb.cas.psu.edu/research/greenroofcenter
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
0.50-0.90
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
>1600to2000
> 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)
Miami/
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
Leaves:
Cool the Air through
Evapotranspiration
Leaves:
Filter Pollutants
from the Air
Roots:
Stabilize Soil,
Prevent Erosion
Leaves:
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
insulation
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.
-------
£
CO LJ_
5-0
CO J^
_Cp_ cl)
O ^"
Z..2 -10
co co
o
c
CD
O)
<
C
o
'•»-'
o
CD
-t-j
O
it
o
CD
O
CM
O
Q
Publication Number: 430-F-03-014
Printed with Vegetable Oil Based Inks on Recycled Paper (Minimum 50% Postconsumer)
Process Chlorine Free
^^^c
CD
C/)
8
CD
CO
=>
CO CD
CO >
CD -g
.E Q-
co ,_
DQ M-
T3
CD
0 ^ ~
III
m^->
CD £;
'o CD
^ c
X 0
O Q-
------- |