MITIGATING THE HEAT ISLAND EFFECT:
GREEN ROOFS AT DUKE UNIVERSITY
by
Monica Varona
Dr. Dalia Patiño-Echeverri, Advisor
December 2010
Masters project submitted in partial fulfillment of the requirements for the Master of
Environmental Management degree in the Nicholas School of the Environment of Duke
University
2010
ABSTRACT
In exchange for flourishing cities built upwards, urban communities have
sacrificed permeable and moist surface areas for impervious surfaces (buildings,
pavements) and as a result, cities experience warmer climates than surrounding rural
areas. This phenomenon is referred to as the Urban Heat Island Effect.
Both domestically and internationally, cities have utilized cool pavements,
increased tree cover and vegetation, green and heat-deflecting roofs to counteract the heat
island effect. Planting vegetated cover on the roofs of city buildings is the
ideal method because it simultaneously mitigates the heat island effect, reduces energy
bills and improves building aesthetics.
This study focuses on the potential application of green roofs in two older
academic buildings on Duke University’s campus in Durham, North Carolina. Dynamic
modeling results through eQuest show green roofs have much higher net present values
(NPVs) for both buildings by significantly reducing both operating and energy costs.
Moreover, the difference in NPVs is large enough to warrant installing them before the
old conventional roof requires replacement.
ACKNOWLEDGEMENTS
I would like to thank my advisor, Dr. Dalia Patiño-Echeverri, for being a great mentor. I
would also like to thank the individuals at Duke University who offered valuable
suggestions, insight and resources throughout various stages of this research: Adem
Gusa, Steve Palumbo and Tim Pennigar. To my family, Patrick McNamara, and close
friends, thank you for supporting my endeavors and for listening to me babble about
green roofs.
TABLE OF CONTENTS
ABSTRACT……………………………………………………………………………………………2
ACKNOWLEDGEMENTS…………………………………………………………………………...3
INTRODUCTION……………………………………………………………………………………..5
BACKGROUND……………………………………………………………………………………….5
HEAT ISLAND EFFECT………………………………………………………………………..5
GREEN ROOFS………………………………………………………………………………...9
OBJECTIVES………………………………………………………………………………………...13
METHODS……………………………………………………………………………………………13
ESTIMATING ENERGY USE………………………………………………………………….14
ESTIMATING ENERGY USE FOR A GREEN ROOF INSTALLATION ………………………..16
DATA………………………………………………………………………………………………….16
BUILDING CHARACERSTICS………………………………………………………………..18
DATA ON CURRENT ENERGY USE………………………………………………………….18
RESULTS……………………………………………………………………………………………..21
NET PRESENT VALUE OF INVESTMENTS…………………………………………………..24
DISCUSSION…………………………………………………………………………………………25
REFERENCES……………………………………………………………………………………….26
1. INTRODUCTION
The Urban Heat Island Effect, which plagues urban areas, can be reduced through
the implementation of mitigation strategies (such as: reflective roofs and pavement,
planting more trees and increasing green space). Planting more vegetated cover is the
ideal goal and it is often brought to fruition through the installation of green roofs. A
primary driver in choosing green roofs as the preferred mitigation strategy is the benefit
of energy consumption reduction in the retrofitted buildings.
The goal of this study is to quantify the potential energy reductions that may be
achieved by installing green roofs on two buildings at Duke University in Durham, North
Carolina. In section two an overview of the heat island effect is presented and in section
three an overview of green roofs, their costs and benefits, and their relationship with
building energy consumption is provided. Section four details the objectives of this study,
section five provides detailed methods, section six presents the study data, and sections
seven and eight present the results and recommendations
2. BACKGROUND
2.1 Heat Island Effect
Automobiles, buildings, roads and various infrastructures have slowly replaced
open land and vegetation throughout time in the pursuit of developing urban areas. In
exchange for flourishing cities built upwards, urban communities have sacrificed
permeable and moist surface areas and as a result experience warmer climates than
surrounding rural areas. Darker man-made surfaces absorb more of the Sun’s heat than
areas with natural vegetation that provide shade and aid in cooling the air; urban regions
become warmer than the rural surroundings. An urban ‘island’ of higher temperatures is
consequently observed when temperatures of the landscape are graphed. This
phenomenon is known as the ‘heat island effect.’
The vegetation of rural areas has a direct role in reducing air temperatures through
the process of evapotranspiration. The plants will absorb water through their roots and
release it through their leaves into the air, through the act of transpiration and
evaporation. This in turn helps disperse the ambient heat (Burba 2008). The impervious
surfaces of urban areas, such as conventional roofs, parking lots, sidewalks and roads,
replace vegetation, reduce the amount of water evaporated and the benefits of
evapotranspiration are lost. During warm sunny summer days in rural areas, the moist or
shaded surfaces will remain close to air temperature while the exposed dry surfaces in
urban areas can be heated to temperatures as high as 90°F hotter than the air. The
difference in daytime surface temperatures between rural and urban areas ranges between
18 and 20°F (EPA 2008). Surface urban heat islands are strongest when the sun is shining
during the day; but can also be observed at night.
Different studies using thermal remote sensing have measured the daytime surface
temperatures in urban areas. The following maps were created by Robert Simmon using
data from the NASA Landsat Program, in order to assess New York City’s heat in the
summer during daytime hours. The data was captured on August 14, 2002 at 10:30 AM
(Scott 2006). The map labeled Figure1 illustrates temperatures, with cooler temperatures
in blue and hotter temperatures in yellow. The map labeled Figure 2 illustrates the
location of vegetation throughout the city, with light green indicating a sparse amount of
vegetation and dark green representing dense vegetation.
Figure 1: Temperature Map (Scott 2006)
Figure 2: Vegetation Map (Scott 2006)
The maps illustrate a correlation between areas with dense vegetation and cooler
temperatures, suggesting the existence of the urban heat island effect.
2.2 Energy consumption, GHG emissions and Heat Island Effect
Why should individuals be concerned about the urban heat island effect? The heat
island effect negatively affects urban communities in various ways. Higher temperatures
in the summertime lead to an increase in energy demand for cities, when compared with
surrounding rural communities. Urban areas experience the burden of higher air
conditioning costs in the summer and higher heating costs in the winter. One study found
that the heat island effect was responsible for 5-10% of the peak electricity demand for
the cooling of buildings in cities (Akbari 2005). Localized urban areas produce an
augmented amount of greenhouse gas emissions and air pollution. The increase in energy
demand produces greater greenhouse gas emissions from the power plants supplying the
electricity (EPA 2008).
Impervious surfaces in cities result in a higher percentage of runoff and lower
percentage of evapotranspiration than areas with natural ground cover. The hot
impervious surfaces can transfer their stored heat to storm water, which will eventually
make its way through storm drains and eventually to rivers, streams and lakes where the
water temperatures of these bodies are also raised. The fluctuation in water temperature
can adversely affect the ecosystem (EPA 2009). Higher heat can also lead to the quicker
degradation and rutting of pavements. (Mallickm 2009). There are also increased risks of
heat-related illnesses (cramps, exhaustion, heat-stroke and respiratory difficulties) and
even heat-related mortality. (Changnon 1996).
Heat islands vary from area to area depending on various factors. A city’s size,
structure and function, the geographic location and weather, the time of day and season,
population size, anthropogenic heat generation, levels of air pollution and land use all
take a role in determining the type of heat island the urban area will experience (Voogt
2004). Urban communities can partake in various mitigation strategies in order to reduce
the heat island effect. Both domestically and internationally, cities have utilized cool
pavements, increased tree cover and vegetation, cool and green roofs to counteract the
heat island effect. This study focuses on green roofs and their potential to combat the heat
island effect and reduce the energy consumption in older academic buildings on Duke
University’s campus in Durham, North Carolina.
2.3 Green Roofs
Green roofs, also known as vegetated roof covers, are comprised of natural
materials, soil and vegetation, installed on top of conventional flat or sloping roofs to
provide a sustainable and cheap way to reduce the heat absorbed by infrastructures. A
green roof will contain a waterproofing layer, drainage and growing vegetation, covering
the entirety of a roof surface, thus removing the need for shingles, tiles, or other manmade roofing materials. There are two types of green roofs: intensive and extensive (EPA
2008).
Intensive green roofs are the heavier of the two and require a structure that can
withstand the extra loads on the roof. These green roofs are typically designed as an
outdoor amenity. The elaborate gardens may include shrubs, trees, small ponds,
fountains, paths and benches, and are open for public use. Soil depths range from as low
as 6 to 8 inches and as deep as 15 feet; the intensive green roofs require elaborate
irrigation and drainage systems.
Extensive green roofs are vegetative roof covers that are lighter and composed of
shallower growth medium. The plant selections for this system are limited because of
structural constraints. This type of green roof minimizes the inputs installed on a rooftop
in order to provide environmental benefits while minimizing costs. Figure 3 illustrates the
various types of green roof systems and how intensive and extensive green roofs vary by
the installation components, types of plants, irrigation systems, weight, saturated weight
and water retention capabilities (Shauman 2010).
Figure 3: Green Roof Systems (Shauman 2010)
2.3.1 Green Roofs: Costs
Because of the greater amount of layers and intricacy, intensive roofs require a
higher capital investment ($25-40+/sq ft) and have higher long-term maintenance costs.
Extensive green roofs are the lighter weight and simpler designed roofs that typically cost
$5-$25/sq ft (EPA 2008). There are various factors driving costs which lead to the wide
range of cost estimates. The design and specifications of a project, the type of existing
roof, roof accessibility and the type of new roof system required in re-roofing with a roofroot-repelling membrane, green roof system (the type and depth of growing medium,
square footage of the green roof), the types of plants and season of installation,
installation and labor and maintenance costs (typically only for the first two years) are all
factored in to the extensive green roof cost range. These factors are cost drivers for
intensive roofs, with the addition of an irrigation cost component and higher and longterm maintenance costs (Peck 2010).
A green roof recently installed atop a Duke University Hospital building with a
roof area equal to ~6000 square feet, cost $17 to $20/sq ft in roofing assembly
installation, which includes the price for the greening component ($8 to $10/sq ft). Xero
Flor America’s pre-vegetated mats were used in the green roof because they have a
synthetic fiber in them, which makes it easy and inexpensive if a portion of the green roof
needs to be replaced. The design of intensive roofs do not allow for such simplified and
cheap installations and replacement options. The goal of an extensive green roof is to
design a roof that requires little to no maintenance and added structural support.
Extensive green roofs are less expensive and the preferred type when retrofitting an older
building (Pennigar 2010).
2.3.2 Green Roofs: Benefits
There are many documented benefits of green roofs. They supply an otherwise
dry region with thermal mass and evaporative cooling. Soil and plants also provide a
natural insulation for the building structure, which aids in reducing energy use and
minimizing utilities costs for businesses. Soil and plants used on the edifice absorb water,
thus reducing storm water runoff. Green roofs also help reduce costs and the production
of landfill-bound garbage by eliminating the use of petroleum-based shingles. Green
roofs have numerous incentives for business owners as well. For example, many hotels
have utilized green roofs to help with storm-water management, growing fresh produce
for in-house use in restaurants, and for cutting costs because self-sustaining roofs require
little to no maintenance (Cannarsa 2008). Aside from the environmental and economic
benefits, green roofs are aesthetically pleasing, can help in promoting sustainable
community gardens and in some instances can serve as additional city green spaces, as
demonstrated by New York’s High Line, a park built on an out of use railway in
Manhattan (Friends of the High Line 2009).
2.3.3 Green Roofs and Energy
Buildings in the United States represent 38.9% of primary energy use and 72% of
national electricity consumption, according to the Environmental Information
Administration (2008). Buildings also account for 38% of all CO2 emissions in the US
(USGBC 2008). According to the American School and University’s Annual
Maintenance and Operation Study, the average total energy/utilities cost per student in
the US rose from $232.55 in 2007 to $400 in 2009. This represents a 72% increase in
energy costs in only two years. As energy consumption and subsequent costs increase, so
do the CO2 emissions from power plants. If Universities are to reduce their carbon
footprint, changes need to be made to older buildings in order to reduce their impact on
the environment.
Researchers have determined that vegetated roofs are able to reduce the heatisland effect (Akbari 2009) as well as significantly reduce energy consumption and costs
(Hui 2002). A Canadian study determined that installing a 32,000 square foot green roof
on a commercial building in Toronto would reduce the building’s total cooling by 6% and
the heating energy usage by 10% for a complete energy usage reduction of 21,000 kWh
(Bass 2001). A similar study published in Building and Environment found that green
roofs reduced building temperatures and increased the thermal comfort outdoors
(Alexandri 2008).
4. OBJECTIVES
The goals of this study are:
1) To design an implement a method to quantify the reduction in energy
consumption that can be achieved by the installation of extensive green roofs
in existing academic buildings.
2) To apply this method to the analysis of two building in Duke University.
3) To provide information that will contribute to the discussion on how to reduce
Duke University’s carbon footprint.
5. METHODS
In this study we explore the benefits of retrofitting old academic buildings with
green roofs by analyzing in detail two buildings at Duke University: Physics and Teer. In
order to assess the energy reduction benefits that can be obtained from green roof
installation, it was necessary to gather data on current energy consumption and then
forecast future energy use after a green roof installation. Building energy models were
created using eQuest (Quick Energy Simulation Tool), a freeware building energy
modeling and analysis tool that runs hourly simulations of a described building based on
the user’s specifications. The model produces the estimate energy consumption for a
variety of end uses, such as: general equipment (ie: computers, phones, printers), lighting,
space cooling, space heating, refrigeration, hot water, pumping, and ventilation.
The model takes as inputs data on various building specifications that were
collected for both the Physics building and the Teer building: cooling system, heating
system, hot water, thermostat settings, minimum design airflow, electricity, hours of
HVAC operation, energy consumption profile (light and computers), and structure of
building (dimension and material of windows, doors, rooms, and dimension and thickness
of building envelope.)
Inputting appropriate regional weather data is also an important factor in creating
the eQuest building model. It is common practice to utilize the Heating Degree Days
(HDD) and Cooling Degree Days (CDD) in determining the heating and cooling
requirements for a selected building in a specific location because energy consumption is
considered to be directly proportional to the number of HDD and CDD calculated for that
location. HDD are utilized to determine the demand for energy that will arise in order to
heat a building. These indices are calculated by subtracting the daily outside temperature
from a base temperature of 65°F. Similarly, CDD are utilized to determine the demand
for energy that will arise in order to cool a building. These indices are calculated by
subtracting the base temperature of 65°F from the daily outside temperature.
5.1 Estimating energy use
In order to determine that the eQuest model was accurate in predicting energy
consumption in a building, the model was calibrated using historical data. The following
figure represents a comparison of actual gas consumption (historical data from 2007)
with the eQuest Simulation for that year. This demonstrates that eQuest successfully
predicted the energy consumption required for heating based on the specifications input
for the Physics Building.
2
Observed
(Historical Data)
1.5
1
Predicted (eQuest
Simulation)
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Physics Building: Gas Consumption
Figure 4: Physics Building Model Calibration
There are various challenges to precisely estimating energy usage for the
buildings. Thermostat settings on the fourth floor of the Teer building are set at 76o F for
cooling and 69o F for heating; but the top floor of the Physics building is currently set by
individual room controls (Palumbo 2010). Average temperature settings were determined
by conducting surveys of the thermostat settings in every room and hallway area (99
spaces total). Observed temperature settings and inquiry responses may not be accurate,
and can lead to variability in estimation of energy consumption.
Another challenge that arises is determining the efficiency of HVAC systems. For
example, chilled water is used for all building HVAC and process cooling in the Physics
building. Yet some of the HVAC systems in this building are older and inefficient,
requiring cooling year round; all cooling that occurs during the winter months is not
necessarily process cooling (Palumbo 2010). If these inefficient systems (fan coil 3-pipe)
were replaced with newer more efficient systems, greater energy reductions may be
obtained.
5.2 Estimating energy use after a green roof installation
Once the building model produced a forecast of the energy consumption, we were
able to add the input for the green roof specifications and re-run the simulation. The
following characteristics of the growing medium were used as inputs in the eQuest
model: thickness, conductivity, density, and specific heat. The results of the second
simulation, when compared to the first set of results, demonstrate the energy reduction
(and subsequent monetary savings) that can be achieved through green roof installation
on the specific building.
6. DATA
6.1 Building Characteristics
Physics Building (120 Science Drive)
The first subject of study is the Physics Building located on the Duke University
campus in Durham, North Carolina. Built in 1948, with extensions added on in 1963, the
Physics building was selected as a case study because it possesses the attributes of a
typical college or university building and it was built before sustainable building design
and construction practices were the norm. The building houses the Physics and
Mathematics Departments, has various computer labs, classrooms, administrative and
student offices, and laboratories.
The Physics Building’s size is approximately 122,679 GSF, with four floors
(subbasement, basement, first floor and second floor). The roof is comprised of a
combination of Built-Up Roof and Membrane Roof and measures approximately 38,000
GSF (Gusa 2010).
Figure 5: Aerial View Physics Building (Google maps 2010)
Nello L. Teer Building (110 Science Drive)
The second subject of study is the Nello L. Teer Building located on the Duke
University campus in Durham, North Carolina. Built in 1984, the Teer Building housed
the Vesic Library for Engineering, Mathematics and Physics until 2008. Teer houses
administrative offices, a student services center, two auditoriums, laboratories,
conference rooms, computer labs and the Pratt Information Technology Help Desk.
The Teer Building’s size is approximately 43,392 GSF, with five floors (the
basement, first, second, third and fourth floors). The roof is comprised of a built-up roof
and measured approximately 11,500 GSF (Gusa 2010)
Figure 6: Aerial View Teer Building (Google maps 2010)
6.2 Data on current energy use
Historical data on the buildings’ utility consumption and cost data for the past
three years was obtained from Duke University’s energy manager from the Facilities
Management Department. The cooling, heating and electricity costs were provided,
divided by months- from July 2007 to June 2010. The following graphs illustrate the
energy consumption (chilled water, electricity and steam), broken down by monthly
averages over three years, for the Physics building and Teer building.
Physics: Energy Consumption (CHW)
140,000
100,000
80,000
60,000
40,000
20,000
Monthly Averages
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CHW (Ton-hrs)
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CHW (Ton-hrs)
Figure 7: Physics Building Energy Consumption (CHW)
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Electricity (kWh)
Physics: Energy Consumption (Electricty)
Monthly Averages
Elec (kWh)
Figure 8: Physics Building Energy Consumption (Electricity)
Physics: Energy Consumption (Steam)
2,000,000
1,000,000
500,000
Monthly Averages
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Steam (lbs)
1,500,000
Steam (Lbs)
Figure 9: Physics Building Energy Consumption (Steam)
50000
45000
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35000
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15000
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CHW (Ton-hrs)
Teer: Energy Consumption (CHW)
Monthly Averages
CHW (Ton-hrs)
Figure 10: Teer Building Energy Consumption (CHW)
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Electricity (kWh)
Teer: Energy Consumption (Electricty)
Elec (kWh)
Figure 11: Teer Building Energy Consumption (Electricity)
Teer: Energy Consumption (Steam)
600000
400000
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500000
Monthly Averages
Steam (Lbs)
Figure 12: Teer Building Energy Consumption (Steam)
For the purposes of this study, we focused on the potential energy reductions that could
be achieved for space heating and cooling in the Physics and Teer buildings.
6.3 Energy use and weather
Historical weather data was obtained from the National Oceanic and Atmospheric
Administration’s National Weather Service (NOAA) for the weather station located at
latitude 35’52’N, longitude 78’47’W. Heating Degree Days (HDD) and Cooling Degree
Days (CDD) were calculated and provided within the data.
6.4 Characterizing the Physics and Teer buildings using eQuest
The following is a list of the various building specifications that were collected
for both the Physics building and the Teer building, and input to eQuest for the two
building energy simulations:
Figure 13: Building Characteristics
7. RESULTS
The eQuest building simulations for Physics and Teer both indicate that green
roofs are the better choice for roof installation. Our findings are illustrated in the tables
below. For the Physics building the annual cost of natural gas consumption (for heating)
decreased by $6,000, or about 4 percent, and the annual cost of chilled water (for cooling)
decreased by $12,000, or about 6 percent. For the Teer building the annual cost of natural
gas consumption (for heating) decreased by $3,000, or about 6 percent, and the annual
cost of chilled water (for cooling) decreased by $2,000, or about 4 percent. These results
could change if various inputs were altered. With the Physics building, for example, if the
thermostat levels were controlled and set by facilities and the inefficient portions of the
HVAC system were replaced then the building’s energy consumption would decrease
before any green roof installation occurred. Therefore, the initial costs for the model
would be significantly lower and the energy reduction for simulation results may not be
as great they were forecasted in this study.
$350,000
$300,000
$250,000
~$144k
~$138k
~$190k
~$178k
$200,000
$150,000
$100,000
$50,000
conventional
Annual cooling costs
green roof
Annual heating costs
Figure 14: Physics Building: Conventional and Green Roof Energy Comparison
$100,000
$80,000
$60,000
$40,000
$20,000
$0
conventional
Annual cooling costs
green roof
Annual heating costs
Figure 15: Teer Building: Conventional and Green Roof Energy Comparison
7.1 Net Present Value of investments
In order to determine the real value of a green roof system in the long term, a Net
Present Value (NPV) comparison was calculated for the green roof and the conventional
roof. As previously mentioned, installation costs as well as maintenance costs may vary
greatly for both green and conventional roofs. Due to this variability, we conduct an NPV
for the high and the low range of costs.
Green roofs are expected to have a lifespan of 40 years whereas a conventional
concrete or build-up membrane roof is expected to have a lifespan of 20 years. In order to
get an accurate and equal comparison of the two, we assume that in the 21st year the
conventional roof requires new installation (and subsequently incurs installation costs).
We assume a 4 percent discount rate, based on published literature of similar green roof
NPV analyses (Carter 2008). Maintenance costs for the green roof were only applied for
the first two years, with a low cost of $1.25 and a high cost of $2.00 a square foot (Peck
2003) because subsequent maintenance requirements (irrigation, fertilizing and weeding)
incur minimal cost and labor. Maintenance costs for the conventional roof were applied
for every year (with the exception of the 1st and 21st years) and set to an average cost of
~2% of initial installation costs per year (Warseck 2010). The full break down is as
follows: The Physics building- high cost of $0.25 per square foot and low cost of $0.15
per square foot. Teer building- high cost of $0.25 per square foot and low cost of $0.14
per square foot (Madsen 2010). The energy costs were kept constant.
The following table illustrates the lifetime costs comparison of a conventional
roof and a green roof, for the Physics building and the Teer building at Duke University.
Figure 16: NPV Comparison.
After analyzing the NPV results for the high and low end analyses, it is clear that
installing a green roof would be the cheaper option both on the Physics building and the
Teer building.
8. DISCUSSION
There are various improvements that can be made within the two buildings
analyzed in order to obtain energy reductions before any green roof installation is
considered. The thermostat settings for the Physics building should be locked at a certain
setting, as they are in the Teer building, and not left up to the discretion of individuals
with varying behavioral habits. Furthermore, inefficient systems that require cooling
during the winter months should be replaced for newer efficient systems.
After analyzing the energy reductions that may be obtained from installing a
green roof, we suggest selecting other buildings on Duke University’s campus for case
study analysis. It would be prudent to select a smaller building and initiate a test green
roof plot to monitor the energy reductions achieved after installation. While eQuest
estimates on larger buildings such as Physics and Teer appear to indicate great savings
potential from the installation of green roofs, a pilot green roof would provide the
opportunity to collect real-time measurements of the temperature decreases of the
ambient air and the roof. We suggest adopting methods similar to those of the green roof
thermal study conducted at Michigan State University in 2006 (Rugh 2010). We also
recommend analyzing other drivers for installing green roofs, such as air quality
improvement, storm water management and the aesthetic appeal (specifically the benefits
in the healthcare sector and work productivity improvement).
Literary Citations
Akbari, H. (2005). “Energy Saving Potentials and Air Quality Benefits of Urban Heat
Island Mitigation.” LBNL-58285. Berkeley, CA: Lawrence Berkeley National
Laboratory
Alexandri, E. and Jones, P. (2008). “Temperature decreases in an urban canyon due to
green walls and green roofs in diverse climates.” Building and Environment 43, pp 480493.
Bass, Brad and Bas Baskaran. (2001). “Evaluating rooftop and vertical gardens as an
adaptation strategy for urban areas.” CCAF Impacts and Adaptation Progress Report
April 1, 1999-March 31, 2001. Institute for Research in Construction.
http://www.roofmeadow.com/technical/publications/BBass_GreenRoofs_2001.pdf
Burba, George. (2010). “The Encyclopedia of Earth: Evapotranspiration.” http://www.eo
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