Green roofs, external and internal walls - Hulfarin Keren & Nbewany Sally

‫עבודה סמינריונית‬
‫תפקיד הצמחייה במבנה בראיית קיימות:‬
‫גגות ירוקים, קירות חוץ ופנים ירוקים‬
‫קארן הולפרין - 584839823‬
‫סאלי נבואני – 892039520‬

‫סמסטר א' תש"ע 0102-9002‬ ‫|‬ ‫תגית כלימור‬ ‫|‬ ‫ארץ, עיר, חי, צומח – קיימות בראיה כוללת‬

Abstract

In the era of global warming and increasing population despite limited territories on the urban
cities, the dilemma of how to create and sustain a healthy urban ecosystem becomes more
prominent, and the demand of finding alternatives in construction that contribute to the
improvement of the climate and life quality on the urban city is rising.
Among the alternatives offered today on the green construction are green roofs and green
walls, which technologies are very well established in many countries throughout Europe.

The objective of this study is to examine the benefits and how do green walls and roofs effect
the microclimate of the city and other aspects of the urban life.
This study provide a review of these benefits of green roofs and green walls technologies.

Although costly, the installation of green roofs and walls could be justified in the long run if
the environmental benefits and other savings are considered.

With this study we can contribute to clarify the main advantages and disadvantages of the
green roofs and walls technologies, processes and study results that were already made. This
may lead to increase the installation of green roofs and walls.

Green Roofs and Walls' initiatives around the world provide developers, communities
and government with modeling of green roof performance, materials and installation
services.

2 ‫תפקיד הצמחייה במבנה בראיית קיימות: גגות ירוקים, קירות חוץ ופנים ירוקים‬

Table of Contents

1.0 - Introduction………………………………………………………………………….……4
1.1 - Presentation of the subject………………………………….........................….4
1.2 - Purpose of the study..………………………………...………………….…..…..4
1.3 – Research question / objectives………………….................................……...5

2.0 – Brief summary of the theoretical background of the subject…..…...………….…...6

3.0 – Definitions and applications…………………………………………………….…….8
3.1 – Green Roof definition …………...……………………...………………….….8
3.2 – Green wall definition……………………………………………………..……..9
3.3 – Benefits of the green roofs and walls..…………………………………...….10
3.4 – New Buildings and retrofits……………………………...……………………..21
3.5 – Installation and maintenance…………………………………………………..22
3.6 – Barriers to Retrofitting…….......……………………………………………..23

4.0 – Literature review……………………………………………………………………….25
4.1 - Green roof retrofitting in Europe……………………..………………………..……25
4.2 - Green roof retrofitting in Canada and North America..……………………..……25
4.3 - Green roof retrofitting in other parts of the world…..………………………..……26
4.4 - Green roof retrofitting in U.K…..……………………..………………………..……26
4.5 - Performance of green roofs and walls in different climates……………………..27

5.0 – Case Studies……………………………………………….………………………..…29
5.1 - Comparison between the examples under criteria……..…….…...…29
5.2 - Results of the examples………………....……………………………...29

6.0 – Discussion ……………………………..………………………………………………30

7.0 - Conclusions…………….……………………………………………………………....32

References……………..……..………………………………………………………………33


1.0 – Introduction

The work is a description study of “Green Roofs and Walls” with their advantages and
disadvantages, and their effects in the structures, in the microclimate of the city and in urban
life.
The high concentration of buildings in cities, their structure, materials and lack of vegetation
have altered the climatic characteristics of urban spaces and has resulted in many
environmental issues. The buildings and hardscape surfaces make cities into giant, heat
absorbing sinks, creating an "Urban Heat Island Effect" which contributes to air pollution,
increased energy usage and heat-related illnesses. Lack of green space in cities means
rainwater runoff carries contaminants from rooftops into our waterways, often releasing
untreated sewage with it.
Cities increasingly struggle to cope with the urban heat island effect, stormwater
runoff, altered weather patterns, air pollution, noise, and the loss of tree canopy, greenspace, and
wildlife habitat. There is growing demand to develop methods that integrate environmental
and economic assessment of more sustainable technologies incorporated into
commercial and residential buildings.
The rising ecological concerns for building design attempt to create harmony between
buildings and their surroundings through mitigating their negative impact on the environment.
The greening of buildings is essentially one of these ecological measures
Green roofs and walls, which are specially designed landscapes on the top or on the front of
the buildings, can provide solutions to many of these problems, as well as enhance the quality
of life for urban residents; for quality of life means mitigation of the common problems of urban
environments cited above – noise, air pollution, heat (thermal comfort) .
The moderation of the heat in the local environment of the urban cities is an issue that have
extremely importance to all of us, and that's the reason of the choice of the work's subject, and could
mean not only the sustainability of the cities, but also the potential of occupying them without the
mortality risk caused by excessive heat.
The objective of this study is clarify the pros and cons of this green technology and
make it clear not only to the people who work or study the subject, but also for those who don't
know anything about the subject and to the governments and decision-makers of the countries, to
encourage them to increase the initiatives of installation of Green Roofs and Walls and know their
potential.
By placing vegetation in the built space of the urban fabric (in this case, urban surfaces
such as building envelope), raised urban temperatures can decrease in the human habitats
themselves. The coverage with vegetation could alter the microclimate of the built environment,
as well as the local climate of the city.
The development of contemporary approaches to green roof and wall technology began in
urban areas of Germany over thirty years ago. In the United States, many green roofs are now
in place. Chicago and Atlanta have municipal buildings retrofitted with green roofs.
City governments, large corporations, and other entities with significant financial resources are
sponsoring green roof programs and leading the way in promoting the adoption of green roof
technologies.

This study contributes to architects, designers, urban planners and landscapers, whose role
in this subject is to improve the life quality in urban cities, taking vegetation as a strong
resource to mitigate the heat, air pollution, reduce storm water runoff and its increased pollutant-
carrying potential, and as a consequence, achieve energy savings for cooling buildings.


2.0 - Theoretical background
Definition of the fields of the knowledge

Sustainability 1
Due to some adverse impacts caused by urban cities, such as energy consumption in
building operations and its consequent carbon emissions, and degraded water quality with urban
runoff and is consequent exacerbate flooding, a motivation to implement new designs and
construction management strategies that contribute to a more sustainable built environment is
increasing. The introduction of a technology such as Green roofs and walls may enable a building
to be more environmentally sustainable.

Green Roofs and Walls 2
Green roofs and walls are being incorporated in the urban landscapes at some places
in the world, for example Toronto, Chicago, Tokyo, Berlin, Madrid, London and New York City.
Some of the advantages of this ecological measure:
- create better thermal conditions and mitigate the Urban Heat island effect
- achieve energy saving for cooling buildings (reducing the heat flux through the roof)
- improve air quality
- make potential savings in urban storm water runoff reduction (reduction in storm
water treatment fees)
- reduce greenhouse gas emissions by the photosynthesis process
There are two major types of rooftop greenery: extensive green roofs and intensive
green roofs.
Extensive green roofs are low cost and low height. Minimal maintenance is required
and inspection is performed 1-2 times a year. Plants selected tend to be of low maintenance and
self-generative type.
Intensive green roofs are heavier and higher cost, with intensive planting and higher
maintenance. Regular garden maintenance, such as fertilizing, watering and weeding is required.
Plants selection range from bushes to trees.

Cooling Effect of Green Roofs and Walls and reduced energy use
Particularly in hot climate cities, the evaporative cooling effect of the Green Roofs and
Walls is effective in reducing the heat that penetrates the building, principally on the room
underneath the roof. It was confirmed by field measurements that the surface temperature of the
roof slab can be reduced to its half during the day, by which a 50% reduction in heat flux into the
room could be expected 3 .
The evaporative heat transfer of the green roofs act as a heat sink and the radiative
energy absorbed by the green roof is smaller then that absorbed by the concrete roof, so the
energy fluxes on a green surface offer lower surface and air temperatures, compared to those
produced by concrete surfaces. Because of that, air masses enter the canyon much cooler from
the vegetated roofs, and so the canyon air temperatures decrease. This occur because of the
evapotranspirational rate from plants and the lower surface temperatures of vegetated surfaces,


which are responsible not only for lowering the air temperature, but also for lowering
surface temperatures of surfaces not covered with vegetation.
Apart from creating outdoor conditions more “human-friendly”, green roofs can
decrease cooling load demands inside the building very significantly due to the microclimatic
modifications; in some cases, the cooling load decreases reach 100% (in places where little
cooling load is needed, cooling demand is reduced to zero).
Green Roofs can cool the microclimate around them, that can lead to important energy
savings for cooling 4 .

Air Pollution Removal 5
Vegetation can reduce air pollutants through a dry deposition process (its foliage is an
effective sink for air pollutants) and has an indirect effect on pollution reduction by modifying
microclimates (it reduces the use of electricity for air conditioning, so the emission of pollutants
from power plants decreases due to reduced energy use). But, in a densely populated city, the
percentage of impervious areas is big, sometimes it reaches more than 90%; green roof is a
solution to this dilemma, since rooftops are usually 40-50% of the impermeable area in a city.
However, green roofs cannot be used as the only measure in air pollution controls because of its
high cost.
Storm Water Runoff reduction 6
Green roofs have been implemented as a low impact technology for controlling storm
water. It reduces storm water runoff and its increased pollutant-carrying potential.
A study from the U.S. Department of Energy (2007) found that green roofs absorb,
filter, and retain about 75% of the precipitation that falls on the roof. This could be described as a
reduction of non-permeable surfaces, that leads to a decreased rain water runoff, which would
lead to a combined sewer overflow event. This reduction of non-permeable surface area, while
increasing the permeable surface area, would ensure delayed release of stored rainwater. This
would effectively “hold” the runoff and release it over time, which would allow the sewers time to
transport the excess water away from the building. As a result, the municipality and the taxpayers
would save money by not having to enlarge the storm and sanitary sewer systems to handle large
water events. This savings would also include the reduction of construction and operations of river
filtration systems to clean the runoff as it washes toxins and other harmful solutions into the local
rivers and lakes.
Green roofs in an urban setting can effectively be used to reduce peak runoff rates
during small storm events by up to 26%. However, green roofs themselves cannot solely provide
stormwater management in an entire watershed. Other rain-gathering technologies (such as rain-
barrels, porous-pavements, and retention tanks) should be used to minimize the runoff caused by
rain events. Green roofs reduce the CSO (the combination of storm sewers and sanitary sewers)
of the entire surface area of the building footprint by 40%. With this reduction, one can assume
that if all structures in the representative neighborhood have this technology, a 40% reduction in
total flow could be implied. CSO reduction also translates into a reduction in water pollution costs.


3.0 – Definitions and applications

3.1 – Green Roof definition
Green Roofs are roofs with a vegetated surface and substrate, an engineered roofing
system that enables the growth of vegetation on conventional rooftops. This green roof
technology includes:
-Insulation above the roof structure
-a waterproofing membrane, often with root repellent inserted
-a drainage layer, sometimes with built-in water reservoirs
-a landscape or filter cloth to contain the roots and the soil
-a specialized growing medium
-the plants

Most green roof systems fall into two categories, extensive and intensive. This categorization
is based on soil depth, but determines a variety of factors including weight, cost, maintenance,
plant selection, and function.

Table 1: Comparison between Extensive and Intensive Green Roof Systems
Extensive Green Roof Intensive Green Roof
Brief -thin soil, little or no irrigation, stressful conditions - deep soil, irrigation system, more
Description for plants favorable conditions for plants
Advantages - lightweight - greater diversity of plants and
- suitable for large areas habitats
- suitable for roofs with 0-30° slope - good insulation properties
- low maintenance - can simulate a wildlife garden on
- often no need for irrigation and drainage systems the ground
- relatively little technical expertise needed - can be made very attractive
- often suitable for retrofit projects - often visually accessible
- can leave vegetation to develop spontaneously - diverse utilization of roof
- relatively inexpensive (recreation, growing food, as open
- looks more natural space)
- easier for planning authority to demand green
roofs be a condition of planning approvals
Disadvantages - more limited choice of plants - greater weight loading on roof
- usually no access for recreation or other uses - need for irrigation and drainage
- unattractive to the some people, especially in systems; hence, greater need for
winter energy, water, materials,…
- higher cost
- more complex systems and
expertise required
Source: adapted from Johnston, 1996, p.54


Table 1 provides generic information only. Each individual green roof system will be a
combination of intensive and extensive, depending on factors such as: location, structural
capacity of the building, budget, material availability and client needs.
There are other sub-classifications of green roofs, which include: semi-intensive green roofs,
earth sheltered buildings (where the earth covers all or part of the building) and hydroponic
systems.
Green Roofs can be classified as being "accessible" or "inaccessible". An accessible green
roof is an open space for people use as a garden or terrace. These types of green roof often
involve surface planting, pathways, seating, play areas and shade structures, and can provide
an important social benefit to their users and increase the market value of the building.
An inaccessible green roof is only accessible for periodic maintenance. The green space can
be viewed but not used, and the roofs can be flat, curved or sloped up to 30°.

3.2 – Green wall definition

Green walls, also called "vertical gardens", define the growing of plants on, up, or against the
façade of a building or on an interior wall of the building. Strategies for vertical garden
development include: planting in planter boxes (at grade, attached to walls, on window ledges
and as part of horizontal and vertical sun screens over windows, doors or glazed areas) and
planting in a vertical hydroponic system. Suitable plants include a wide variety of perennial
and annual vines as espaliered trees.
A vertical garden is a system with many of the benefits of a green roof, and often without the
added weight or cost implications. It has more potential to impact the area per building,
because the greening of a building's façade encompasses four times the area of the roof, and
this can be increased to 20 times the area of the roof if the building is high.
Wind can be a limiting factor in the effectiveness of vertical gardens, especially for plants and
planter boxes located above eight floors.

Green Façades
- are made up of climbing plants growing directly on a wall or of designed supporting structures
- The plant shoot system grows up on the side of the building while being rooted to the ground

ACTIVE LIVING WALL
- the modular panels are often made of polypropylene plastic containers, geotextiles,
irrigation systems, a growing medium and vegetation
- 'Active living walls‘ – a new concept: wall is integrated into a building's air circulation system
- draw air through the root system of the wall: beneficial microbes degrade the pollutants in the air
before returning the new, fresh air back to the buildings interior
- Have biofilters that increase the capacity of air filtration
- Do water reuse: the plants may purify a little the polluted water (such as greywater) by absorbing
the dissolved nutrients - bacteria mineralize the organic components to make them available to the
plants


- suitable in arid areas: the circulating water on a vertical wall is less likely to evaporate than in
horizontal gardens
- could also function for urban agriculture or urban gardening
-may be built as a work of art for its beauty
- Sometimes is built indoors to help cure sick building syndrome

Passive living walls (INACTIVE WALL)
- do not have any means of moving the air into the root system where most of the degradation of
pollutants occur
- have no mechanized air circulation
-their impact on air quality are scientifically questionable
- promote as much free air circulation as possible

3.3 – Benefits of the green roofs and walls
Urban landscapes create imbalances in the natural ecosystem. These imbalances result, in
large measure, from factors such as:
concentrated human populations
- the introduction of areas of hard and impermeable surfaces, which are devoid of flora
and fauna
- the importation of energy and other resources from outside of the city
- the creation of waste products which cannot be reintegrated into the ecosystem as a
resource, resulting in water, soil and air pollution
Through the reintroduction of plants on the walls and roofs of buildings, some of the lost
equilibrium in the urban ecosystem can begin to be rebuild. The "greening" of the urban
regions can also help to better manage many of the wastes that are generated in the air
and water and so contribute to improved human and ecosystem health.
There are some environment, social and economic benefits caused by green roofs and
walls technologies in an urban ecosystem.

3.3.a – Improvement of Air Quality
Urban areas tend to perpetuate their own air pollution. During the summer months, when
the concrete, glass, stone and asphalt surfaces of roads and buildings are heated, vertical
thermal air movements are created and the dust and dirt particles found on the ground and
in the air are carried and spread.
A green wall will block the movement of dust and dirt particles along the sides of a building
and filter them. A green roof will reduce the amount of energy available for heating, which
decreases the tendency towards thermal air movement and will also filter the air moving
across it. Air particulates tend to get trapped in the leaves and branches surface areas of
plants and when it rains, they get washed into the soil/substrate below. Plants are also
known to absorb gaseous pollutants through photosynthesis and sequester them in the
leaves (which fall to the ground in autumn to create humus) 7 .

Minke, 1982, p.11 7


Studies have shown that treed urban streets have only 10-15% of the total dust particles
found on similar streets without trees 8 . This air cleansing quality of green roofs and walls
directly decreases summer smog and other forms of air pollution. The widespread use of
these technologies would also extend the life of all urban infrastructures that are
susceptible to damage from air pollution.

3.3.b – Climate change – mitigation and adaptation
Urban areas are a significant source of greenhouse gas (GHG) emissions, with space/air
conditioning playing a significant role in urban energy demand 9 . If widely implemented,
green roof and walls can provide an effective and proven methods for governments,
companies and building owners to reduce these GHG emissions through direct shading of
individual buildings, improving insulation values and reducing the Urban Heat Island Effect
(section 3.3.f).
Energy cost savings are very difficult to standardize accurately since every building and
installation is different. GHG emission reductions and associated cost savings depends on
a number of factors, such as:
-specific siting on the building
-climate
-choices of roofing materials and design
-insulation
-mechanical system
-thickness of growing medium
-primary sources of energy used to meet heating and cooling needs
-types of plants used
-extent of alternative functions of green roofs and walls, such as displacing the need for
cooling towers and local food production

Even though estimates of climate change mitigation specifically due to green roofs and
walls installations are not readily available, the following examples provide some
illustrative data:
-Approximately one-third of a home's basic thermal unit demand for heating in winter is
created by the wind, which makes the outside walls colder and reduces the effectiveness
of insulation. Protecting a house from wind can reduce the wind chill factor by 75% and cut
the heating demand by 25%.
In the summer, every degree (F) of summer heat requires an additional 5-7% of cooling
energy. Hence, a 10° F reduction in the outside air temperature achieved through the
judicious arrangement of shade trees (green roofs and walls), can reduce energy
consumption for air-conditioning by 50-70% 10 .

Johnston, 1996, p.10 8
Mercier, 1998 9
Gaudet, 1995, p.24 10


-Canadian studies have shown that vines lower the inside temperatures as effectively as
shade trees if allowed to grow on south or west walls 11 . Studies from Britain have also
shown that over a one year period, energy costs for a conventional house can be reduced
by as much as 25%. This is achieved through the reduction of wind penetration by, for
example, careful perimeter planting of deciduous species of trees. These trees also
provide summer shading and winter solar gain 12 . Similar results could be expected with a
green wall.

3.3.c – Temperature regulation
Climate can be understood at four basic levels:
1. Climatic Zones – broad, geographic bands, affected primarily by large water bodies land
mass and distance from the equator. Significant changes to climatic zones, such as global
warming, can occur only on a global scale.

2. Regional Climate – refers to the regional variations within climatic zones, with
accompanying differences in annual temperatures, sunniness, snowfall, rainfall, wind, etc.

3. Local Climate – occupies a smaller footprint. The Urban Heat Island Effect (section
3.3.f) is an example of an anthropogenically produced local climate.

4. Microclimate – are site-specific, for example: rooftop will often have a different
microclimate from the grade surrounding the building. Microclimate is directly influenced
by a variety of elements on and around the site (land contour, vegetation, water, soil
conditions and buildings) which affect the site's sunniness, warmth or coldness, humidity,
wind, snowdrift and runoff patterns and degree of wind chill. By manipulating these site
elements, the microclimate of a site can be substantially changed.

Much of the sun's energy falling upon a typical concrete, asphalt or hard surface is
reradiated as heat. Using a layer of vegetation to intercept the sunlight can reduce this
heat. Of the sun's light energy that falls on a tree leaf, 2% is used in photosynthesis, 48%
is passed through the leaf and stored in the plant's water system, 30% is transformed into
heat (used in transpiration) and only 20% is reflected. Since a large amount of incident
radiation on a plant canopy is used for evapo-transpiration, plants on vertical and
horizontal surfaces are able to regulate wild temperature swings. On a warm summer day
their absorption of energy lowers the temperature of the shaded surface and regulates
humidity while at night and in the winter, they give off energy/heat. This can reduce the
amount of sun-energy falling on a hot summer day by up to 90% 13

Gaudet, 1985, p.29 11
Johnston, 1996, p.14 12
Minke, 1982, p.11-12 13


In a city, the impact of evapo-transpiration and shading can significantly reduce the
amount of heat that would otherwise be re-radiated by building and street surfaces.

3.3.d – Creation of Microclimates
Changing a site's microclimate through rooftop and vertical greening can have a complex
and layered effect on urban climate. Green roofs and walls create their own specific
microclimate, different from surrounding conditions, both around the building and at grade.
Depending on height, orientation and the location of surrounding buildings, the roof is
subject to extreme temperature swings (hot during the day and cool at night), with
constants exposure to sunlight and wind – a desert-like climate suitable only to specific
types of plants. Although this can be tempered by additional irrigation and greater soil
depth, a green roof is closer to an arid or alpine environment than it is to the surrounding
environment grade. This means that designers and installers must have a specialized
knowledge of the flora and fauna best suited to these conditions. Once established, the
green roof will have a noticeable impact on the heat gain and loss of the building beneath
it, as well as the humidity, air quality and reflected heat in the surrounding neighborhood.
In conjunct with other green installations, the green roof will also play an important role in
altering the climate of the city as a whole. The same can be said for green walls. One
German source remarked that a "healthy" urban climate could be achieved by greening
only 5% of all roofs and walls within the city 18 .

3.3.e – Plants and Building Insulation
The role of insulation is to slow down the rate of heat transfer between the inside and outside
of a building, which is a function of the difference between the inside and outside
temperatures. Insulation mitigates the impact of this temperature differential. In the winter,
insulation slows down the rate of heat transfer to the inside. The greening of vertical and
horizontal surfaces has long been used as a technique for insulating buildings through exterior
temperature regulation. The insulation value of a building's "skin" (roof and wall) can be
increased in several ways:
-by trapping an air layer or "pillow" within the plant mass, the building surface is cooled in
summer and warmed in winter
-by covering the building with vegetation, the summer heat is prevented from reaching the
building's skin, and in the winter, the internal heat is either prevented from escaping, reflected
or absorbed
-since wind decreases the energy efficiency of a building in winter by 50% 19 , a plant layer will
act as a buffer that keeps wind from moving along a building surface
With a green façade, it is created an exterior insulation, and the added cost of interior or
traditional exterior insulation is avoided. The need to re-apply finish surface materials or
cladding and the loss of space resulting from thicker walls can be avoided too through the use
of green facades. Besides that, the insulation applied to the exterior of buildings is much more
effective than interior insulation, especially during the summer months.

Minke, 1982, p. 718
Minke, 1982, p.14 19

Ideally, if winter heat-loss is a concern, one should choose evergreen plants that do not lose
their leaves during the winter months. However, a protected southern or western exposure,
this seasonal loss of foliage could serve as a passive solar technique. This would be
advantageous since the lack of foliage would allow the sun to heat up the wall during the day.
During the summer it is important to place green facades on the southern and particularly
western exposures, since they receive the largest amount of incoming solar energy.
Installation of south and west-facing "green" window shades can add significant insulation to
buildings with large window exposure, since much of the summer heat gain is from the
absorption of incoming solar energy by walls and other objects in a room. With plants shading
the windows, the heat will not reach the interior.
With a green roof, the insulation value is in both the plants and the growing medium. It is
unclear which of the two has the most benefit since much depends on the depth of the
growing medium and type of plants chosen. An extensive application is much more effective
as an insulator than an intensive one. A layer of mixed grass performs better than a layer of
limited-species grass, which in turn is better than a layer of low-growing sedum.
Under a green roof, indoor temperatures (without cooling) were found to be at least 3-4°C
lower than hot outdoor temperatures between 25-30°C. Green roofs can also play a role in
pre-cooling the make-up air that is required by most mechanical systems. As the outdoor air
temperature in the summer is often warmer than the exhausted, internal air it is replacing, the
air needs to be pre-cooled before it is allowed into the building. A green roof and the strategic
planting of specific vegetation to shade the intake valves will lower the air temperature at roof
level, thereby reducing the air temperature, the demands on air-conditioning equipment and
result in net energy savings.

3.3.f – Moderation of the Urban heat island Effect
The "Urban Heat Island Effect" is a macroclimate caused by the difference in temperatures
between a city and the surrounding countryside. This difference is mainly due to the expanse
of hard and reflective surfaces in urban areas, which absorb incoming solar radiation and re-
radiate it as a sensible heat 20 . In the surrounding areas, there is a higher proportion of
"greened" surface area, which is able to absorb and transform this radiation into biomass and
latent heat. Re-radiated heat, waste heat generated by industry, vehicles and mechanical
equipment and increased levels of air pollution, have combined to raise urban temperature
levels up to 8°C warmer than their surroundings on warm summer evenings. And if estimates
are correct, global warming will exacerbate the Urban Heat Island Effect by raising summer
temperatures an additional 5°C.
Higher urban temperatures increase the instability of the atmosphere, which in turn can
increase the chance of rainfall and severe thunderstorms. In cities already plagued by
overextended stormwater systems and combined sewage overflows, the problems caused by
severe rainfall are likely to worsen with global climate change.
Higher temperatures also have a direct effect on air quality, since heated air stirs up dust and
airbourne particulates as it rises. On a hot summer day, a typical gravel-covered roof in middle
Europe tends to heat up by 25°C, to between 60°-80°C. This temperature increase means that

Johnston, 1996, p.1120


a vertical column of moving air is created over each roof. Studies have shown that there is no
vertical thermal air movement over grass surfaces. These surfaces will not heat up more than
25°C. Air movement along vertical heated surfaces is even greater than over horizontal
surfaces. With strategic placement of green walls, plants can create enough turbulence to
break vertical air flow which cools the air at the same time it slows it down 21 .
3.3.g – Carbon Dioxide and Oxygen Exchange
Plants play a crucial role in the survival of life on our planet. Through the
photosynthesis process, which takes place within green leaves and stems, plants
convert carbon dioxide, water and sunlight/energy (solar energy) into oxygen and
glucose. Plants supply humans and other animals with oxygen and food, and animals,
in turn, produce the carbon dioxide and manure required by the plants.
Although the production of oxygen is an important contribution of urban greening, in
evaluating this benefit we should take into consideration some facts:
-plants only produce oxygen during the daytime; at night, the process reverses so that
they take in oxygen and give off carbon dioxide (but there is still a net increase in
oxygen)
-the decomposition of organic matter on top of, and within, the growing medium also
requires oxygen
-in extensive green roof systems, where the plants and grass layer is allowed to dry up
during the summer, with no additional irrigation, the plants are unable to participate in
photosynthesis.
By increasing the amount of biomass in an urban area, green roofs and walls can
contribute to reducing the carbon dioxide levels produced by vehicles, industry and
mechanical systems, leading to improved air quality and reduced respiratory
problems.

3.3.h – Stormwater Management
The stormwater is diverted from the cities through constructed stormwater systems, since
much of the surface area in a city is paved or covered with buildings, and most of precipitation
can't be infiltrated into the ground or be intercepted by vegetation.
This has created a number of problems, such as:
-contamination of stormwater, which, as it runs off impermeable surfaces, it picks up
particulates, pesticides, oil, grease, heavy metals, and garbage from roads, roofs and
pavements before it reaches the drain. In some cities, stormwater is the number one cause of
water pollution in local rivers.
-Combined Sewage Overflows (CSO). Many stormwater systems run parallel to a city's
sewage system, overflowing into the sewage system if they cannot handle the volume of
water during heavy rainfall. During a storm event, diluted raw sewage is discharged into the
local streams and rivers, resulting in beach closures and other negative impacts.
-increase in water temperatures, that negatively impacts aquatic plants and animals and
encourage algae blooms
-severe flooding, often resulting in loss of human life due to the high volume of run off

Minke, 1982, p.12 21


One solution to this problem involves the enlargement or expansion of stormwater
infrastructure, which can be a costly process. Some of other alternatives include the
disconnecting of downspouts, increased use of swales adjacent to parking lots, constructed
wetlands, rain barrels, cisterns, retention ponds and requiring the use of porous pavement.
These solutions require associated inspection and maintenance, and the cost of additional
surface area at grade. Green roofs and walls can provide viable alternatives where, in older
urban areas, there is a lack of suitable land at grade to address alternative stormwater
management approaches.
If sufficiently implemented in an urban area, green roof systems can help to improve
stormwater management. Green roofs absorb 75% of precipitation that falls on them, meaning
a discharge reduction to 25% of normal levels. Runoff that occurs is also stretched out over
several hours, helping to reduce the risk of flash flooding 22 and the frequency of combined
sewage overflow events. Most of the stormwater is stored by substrate and then taken up by
plants, through which it is returned to the atmosphere through evapotranspiration.
Green walls interrupt rainfall and delay runoff, especially during heavy rainstorms with strong
winds, where the rain is falling more horizontally than vertically.
If widely implemented, green roofs and walls provide new opportunities to address urban
stormwater management in a manner that also results in other environmental and social
benefits.

3.3.i – Water Filtration and Quality Improvement
Greened surfaces not only retain much of the precipitation that falls on them, they also
moderate the temperature of the water and act as natural filters for any of the water that
happens to runoff. Heavy metals and nutrients carried by the rain end up being bound in the
substrate instead of being discharged. Studies show that as much as 95% of cadmium,
copper and lead, and 16% of zinc have been taken out of rainwater by green roof systems 23 .

3.3.j – Other water-based Benefits
In addition to the generic benefits from green roof systems, they can also be design to
perform particular functions, which help to increase the return on investment for the
building owner. For example, the use and reuse of water can be integrated into the
design of a green roof system in a number of different ways:
-A factory of biodegradable laundry products in Belgium has 2 acres of native grasses
and wildflowers on its roof. Effluent produced by the factory is treated in an on-site
sewage pond at grade and then filtered through the green roof while at the same time
acting as irrigation and nutrient source for the plants.
-In the new inner-city development of Block 103 in Berlin, nearly 40% of the roofs have
been greened and many of the facades have been planted with climbers and vertical
gardens, including a unique "vertical swamp", which cleans the building's grey water

Johnston, 1996, p.17 22
Johnston, 1996, p.12 23


through a system of planters filled with swamp grasses and aquatic plants. Water is
released in measured amounts form the roof into the top planter and then through a
system of pipes and drains, is filtered through successive layers of grasses. Upon
reaching ground level the cleansed water can be reused 24 .
-The Possman Cider Cooling and Storage Facility in Frankfurt, Germany, uses a
water-based heat exchange system to cool the building. The green roof was designed
as a marsh with a rain and roof water collected in an underground cistern pumped
through the building to collect heat, run through the vegetation layer on the roof for
cooling and filtration and then recycled back into cistern.
-Roofs can also be used as water collectors and storage "cisterns" by using a floating
layer of plants to decrease evaporation and act as a filter. By recycling the stored
rainwater, whether in the building or on the site, companies like Possman eliminate
the need to purchase water from other sources and consequently, are able to realize
cost savings 25 .

3.3.l – Sound Insulation
Soil, plants and the trapped layer of air between the plants and the building surface can be
used to insulate for sound. Sound waves produced by machinery, traffic and airplanes can be
absorbed, reflected and deflected. The substrate tends to block lower frequencies while the
plants block higher frequencies. Tests have shown that a 12 cm. layer of substrate can reduce
sound by 40dB; 20 cm. can reduce sound by 46dB 26 (with some reductions as high as
50dB) 27 .

3.3.m – Building Envelope Protection and Life Extension
Green roofs have been proven to protect the roofing membrane against ultra-violet (UV)
radiation, extreme temperature fluctuations and physical damage from recreation or
maintenance.
On a roof, temperatures can swing over the course of a day. A 10cm. thick green roof layer
can reduce this range of temperatures, thus ensuring less expansion and contraction stress
on the roof membrane, which in turn reduces cracking and aging. The longer life-span
decreases the need for re-roofing and the amount of waste material bound for landfill, both of
which are direct cost savings for the building owner. Reducing building waste also helps to
conserve municipal landfill capacity.
It is not true that green walls will damage the wall they are covering; as long as the original
cladding is in good repair, even vines that cling with their roots will not threaten the integrity of
the wall. If the wall is not in good conditions or if periodic maintenance of the cladding is
required, vines can be grown up on another structure that is kept separate from the wall itself
or on a bottom-hinged system, which can be pivoted towards the ground while keeping the
plants intact. Green facades protect the exterior finishes and masonry from UV radiation,
avoiding the wear and tear caused by moisture and temperature differentials.

Johnston, 1996, p.75-76 24
Johnston, 1996, p.12 25
Minke, 1982, p.1526
Hooker, 1994, p.3 27


In cold climates, a facade can heat up to 60°C and then cool to minus 10°C; with a layer of
plants, temperatures will only fluctuate between 30° to 5°C 28 . Plants will also increase the air
tightness of windows and cladding by decreasing the effect of wind pressure 29 .

3.3.n – Aesthetic Improvements
Urban greening has long been promoted as an easy and effective strategy for beautifying the
built environment. Studies have shown that from earliest recorded times, Western cultures
have conditioned their citizens to appreciate nature and to have negative associations with
cities and their aesthetic.
A layer of plants can enhance good design or disguise bad design 30 . Plants can add visual
interest to plain walls and roofs, soften industrial and commercial properties and allow a new
building to blend in better with rural or suburban surroundings.

3.3.o – General Health Benefits and Horticultural Therapy
Visual contact with vegetation has been proven to result in direct health benefits,
cause psychological well-being and reduces the stress of urban living. Psychological
studies have confirmed these beliefs by clearly demonstrating that the restorative
effect of natural scenery holds the viewers' attention, diverts their awareness away
from worrisome thoughts and elicits a meditation-like state.
Windowless rooms or rooms with little or no visual access to the outdoors are disliked
and cause stress, especially in the workplace and in healthcare facilities. People living
in high-density developments are known to be less susceptible to illness if they have a
balcony or terrace garden 31 . This is partly due to the additional oxygen, air filtration
and humidity control supplied by plants. Trees in a park setting can filter out up to 85%
of airbourne particulates, with the leaves of climbing plants providing an equally large
surface area capable of filtering out dust and pollutants.
There are therapeutic benefits from the act of caring for plants. The variety of sounds,
smells, colors and movement provided by plants, although not quantifiable, can add
significantly to human health and well being.

3.3.p – Improved Safety
A garden or amenity space on a roof is often considered safer than one located at grade for
the following reasons:
-there is less vandalism because access to the roof is usually restricted to building tenants or
employees.
-the public services and utilities that may hamper garden installation or digging on the ground
do not exist on the roof
-pollution levels on the roof are lower than at street level
-soil quality, including contamination through heavy metals, hydrocarbons and the use of
pesticides and herbicides, can be controlled since everything has to be sourced and then

Minke, 1982, p.18 28
29
Minke, 1982, p .25
Johnston, 1996, p.32 30
31
Johnston, 1996, p.27


brought up to the roof (this is of particular relevance if the green roof is being used for food
production).

3.3.q – Recreation/amenity Space
Studies have shown that leisure activities in natural settings such as gardens and
parks are important for helping people cope with stress and in meeting non-stress-
related needs 32 . Green roofs and walls can help to address the lack of green space in
many urban areas.
Many urban residents consider the roofs and walls of buildings as a city's greatest
unexplored resource. Finding new ways to utilize roof and wall space can generate
added economic impetus and make cities more livable by providing significant
amounts of accessible outdoor recreation or amenity space close to work and home.
Greening is often the only legal and also one of the least expensive ways that an
individual tenant can personalize or change the exterior of their building, apartment
unit or exterior living space.

3.3.r – Community Building
Apartment buildings, although densely populated, rarely become "communities"
unless the residents are able to rally around a common goal or against a common
problem. The creation of shared gardens allow residents to feel ownership of their
building and meet neighbours in a relaxed setting. The propensity of apartment
dwellers to grow plants on their balconies attests to the potential for vertical gardens to
help build communitites.

3.3.s – Job Creation and production of food
Green roofs and walls installations can create and enhance a lot of job markets, like suppliers
and manufacturers of roof membranes, root repellent layers, drainage layers, landscaping
cloth, irrigation systems, light-weight soils, professionals as designers, landscapers, engineers
and contractors, besides maintenance companies.
There is great potential for producing high quality food in urban areas. Many places have
realized this potential and grow significant amounts of food for local consumption.
There are a lot of benefits associated with increasing local food production, like:
- increased access to food by everyone, including lower income, inner-city communities
-fresher produce
-local economic opportunity in growing food, processing them and their distribution
-decreased travel and environmental costs, such as greenhouse gases generated by long
range transportation and cooling requirements
-improved control of soil, fertilizers and pesticides
The use of green roofs and walls to produce high quality food and other alternative uses of
wasted building space hold a big promise through the application of rooftop and vertical
garden technologies.

Ulrich, 1992, p.98 32


3.3.t – Preservation of Habitat and Biodiversity
With the growing of the cities around the world, buildings are replacing natural habitats. This
cause plants, animals and insects to adapt, find other locations to live or become extinct.
Green roofs and walls can be designed as acceptable alternative habitats although they
should never be considered as substitutes for natural habitat or as a justification to destroy
natural habitat at grade 33 .
Green roofs are also specifically designed to mimic endangered ecosystems/habitats. In
Germany, 20% of all endangered plants are arid/semi-arid grassland plants, conditions
specific to an extensive green roof installation. Dryness, heat, frost and lack of oxygen are
rooftop conditions that are very similar to the dry grassland ecosystem which have been
seriously degraded by fertilizing, irrigation and other forms of human interference.
Extensive green roofs, because of their lack of human intervention, are more protected and
can become home to sensitive plants that easily damage by walking and to bird species that
only nest on the ground. The soil on an inaccessible green roof is also less likely to be
disturbed, so it becomes a safer habitat for insects as well. The deeper the soil the more
insect diversity the green roof will support.
The animals and invertebrates found on a green roof tend to be highly mobile, not only
because they have to be able to reach the roof in the first place, but because the varying and
intense temperature and moisture levels force them to move from one location to the next 34 .
Green walls can also create important habitat for birds and insects. Birds eat insects as well
as the berries and fruits produced by certain vertical garden plants.

3.3.u – ECONOMIC BENEFITS
The typical economic benefits and opportunities for building owners that implement green
roofs and walls include:
-increase of insulation of the walls and roof of the building, resulting in energy cost savings for
heating and cooling and leading to reductions in greenhouse gas emissions
-protection of the roof membrane and the wall cladding, resulting in a longer material life span,
decreased maintenance and associated savings in replacement costs
-increase in stormwater management may offset these costs elsewhere in a development by,
for example, reducing the need for stormwater management ponds or reducing fees. In
Germany, residents with green roofs receive discounted rates.
-increase in property values. Green roofs and walls offer vary similar visual and environmental
benefits; aesthetic appeal can directly increase the value and marketability of a property.
Urban beautification will also have as impact on tourism and the way visitors view the city
-The cleansing of wastewater and the growing of herbs and other urban agricultural products
can add economic value to underutilized roofs and walls. Other example, the implementation
of gardens by hospitals can improve patient recovery rates, which translates into cost savings
in health care. Noise reduction benefits can also help offset additional costs for buildings
where noise control is an issue.

Johnston, 1996, p.49 33
Johnston, 1996, p.69 34


Standard cost savings through the greening of roofs and walls are often related to reduced
heating and cooling costs, but pay back periods are medium to long-term. The installation of a
green roof or a green wall requires an up-front capital investment, especially in a retrofit
situation, that will be returned through cost savings 35 . If the concept is included at the
beginning of the design phase for a new building, a green roof or wall can be installed at little
or no extra capital cost 36 .
The life cycle costs would be increased by the maintenance of the garden, but would be
decreased by the extended durability and minimized maintenance of the building
envelope 37 and by the savings in energy costs.
A green roof or wall becomes even more viable where the price of land, or the lack of
available adjacent land, prevents the creation of garden or green space at grade. By providing
green space, developers, building owners and companies are often more effective at
attracting and retaining buyers and tenants 38 .
There are some economic benefits for the community, like:
-job creation in design, manufacturing, installation and maintenance
-increased livability of cities, including worker productivity and creativity
-air quality improvements that have an impact on human health and well being
-ability to retain and treat stormwater runoff, that can help decrease operational expenditures
on related urban infrastructure
-the benefits of experiences with nature and vegetation decrease the need for health care
services 39

Green roofs and walls can be located in courtyards, terraces, balconies and rooftops and will
increase property values.

3.4 – New Buildings and retrofits
There are several application issues in green roofs that must be pointed. The requirement for
additional loading capacity is one of the main factors in determining both the viability and cost
of a green roof. If a green roof is part of the initial design of the building, the additional loads
can be accommodated easily and at relatively minor costs. If a green roof is installed on an
existing building, then the design is limited to the current carrying capacity of the roof, unless
the owner is prepared to upgrade the structure which can be a significant investment.
If the green roof is to be accessible to the public, certain safety requirements must be added,
including guardrails, adequate access and exiting, depending on roof size and number of
occupants, lighting, fire safety equipment and structural integrity.
These elements are much easier and less expensive to include in the initial design of the
building than after the building is constructed.

Johnston, 1996, p.12 35
Johnston, 1996, p.71 36
Hooker, 1994, p.3 37
Johnston, 1996, p.12 38
Ulrich, 1992, p.101 39


The real limit to retrofitting is the structural capacity of the existing roof and the building
structure. If the roof and building can take extra load due to spare capacity and the budget for
the project can afford the construction, a retrofit is a feasible option. There are difficulties
associated with safely determining the spare capacity of the roof can arise, depending on the
materials and type of construction.
Retrofitting green roofs, as opposed to using on new buildings, has the greatest potential for
the maximum impact on environmental issues, considering the relatively small roof areas of
new build projects compared with existing buildings. In addition, retrofitting can be combined
with a renovation already taken place, allowing the additional cost to be relatively low.

3.5 – Installation and maintenance

In green roof installations, the most crucial element is the roof membrane. Organic materials
are not roof-proof, but this problem can be solved by placing a metal foil between the layers.
Parapets, edges, skylight, mechanical systems, vents and chimneys should be protected with
a gravel drainage layer and sometimes a weeping drain pipe.
Without greening, flat roofs are 50% more susceptible to damage after 5 years than slightly
sloped roofs (5% slope), because water tends to pool instead of running off. If the drainage
layer isn't sufficient or if drainage routes become blocked, green roofs can cause some flat
roofs to leak due to continuous contact with water or wet soil. With insufficient drainage, the
plants will also be susceptible to the impact of wide degrees of variability in the moisture
content of the soil (for example, with too much water, the soil can go sour and the plants can
drown or rot) 40 .
If the slope on the roof is greater than 20°, the roofer needs to ensure that the plant layer
does not slip through its own weight, especially when is wet. This can be prevented through
the installation of horizontal strapping, placed either under the membrane or laid on top of the
membrane.
In vertical garden applications, vines require very little maintenance once established, and
crop plants, such as beans and lettuce, require the same level of maintenance as they would
in a garden at the ground. Since vertical gardens can be designed to keep plants from direct
contact with a building's wall, no additional maintenance on the wall is required. Vines, which
are often grown directly on the wall, will not damage a surface that is already in good
condition. Rather, vertical gardens will actually reduce the damage caused by rapid
temperature variations such as freeze-thaw cycle, acid rain, ice accretion and pollution.
In retrofitting situations, there are many variables when designing a green roof, such as type
and depth of substrate, type of planting, pitch, selection of drainage layer, etc. All these
influence the many impacts green roofs can have, sometimes in conflicting ways. For
example, the greater the pitch, the less water is retained, which is positive if considering
retrofitting on a roof with little spare structural capacity but not if the primary need is to prevent
runoff. Another example is that of a green roof designed to be a Sustainable Urban Drainage

Minke, 1982 40


System and retain the maximum water, which if not carefully considered could result in the
substrate rotting plants or reducing the effectiveness of insulation.

3.6 – Barriers to retrofitting

Structural capacity
Although this is the primary issue if considering retrofitting, people's perception are far more
negative in this respect than they need to be. A survey of various professionals carried out in
London showed that while 92% of developers agreed that "the physical structure of many
buildings prevents the establishments of green roofs", only 27% of the engineers questioned
held the same opinion 41 .

Cost
While many building and renovation projects are carried out with an approach that recognizes
long-term gains and the advantages of investing in initially more expensive options, there is
still a reluctance to invest in fitting green roofs. One reason for this is that the benefits green
roofs and walls have to offer, financial and environmental, are not widely known as they could
be. Another reason is that some building owners may simply not care about increased life of
the roof and long-term gains if it reaches beyond their probable ownership.
In terms of cost, the argument for retrofitting is strong if the existing roof needs repairing or
updating anyway. This is a trend in Germany, where architects are more likely to recommend
a green roof when the old one needs replacing, rather than include one in a new project with a
tight budget 42

Maintenance
Most people believe green roofs require more attention and maintenance than other roofs.
Although intensive roofs require as much maintenance as any garden, extensive green roofs
need little extra maintaining than other flat roofs covered with paving slabs. It is recommended
maintenance twice a year, as with all roofs.
The main issues of retrofitting initially seem to be technical, but in fact the major factors
preventing more retrofitting are the perceptual barriers that effect planning, policies and
general attitudes of potential clients and the public. There are many successful examples of
retrofitted roofs around the world, so there is a very strong argument that green roofs can be
feasibly retrofitted 43 .

41
Living Roofs, 2005
Herman, 2003 42
43
Living Roofs, 2005


4.Literature review – retrofitting around the world

4.1 – Green roof retrofitting in Europe
Germany is at the center of green roof technology around the world and home to the major
manufacturers of green roof systems. In the 1980s, official government support was offered to
building projects incorporating green roofs and now 43% of cities have incentives for green
roof building. Part of the reason of this development was the Federal Nature Protection Act,
which affected the building codes, obliging all new developments to replace and enhance any
landscape which might be changed as a result 44 .
Another reason was the publication in 1982 of the first green roof standards by the Landscape
Development and Landscape Construction Research Society. These standards have been
updated over time, reflecting lessons learned from many projects.
Emphasis on the importance of green roofs has changed over the years as more has been
learned, and the initial ecological drive evolved into the prominence of energy saving and
effectiveness. The concentration is on stormwater retention; most of the 2.5 million square
maters of green roofs fitted between 2000 and 2001 (60-65% of which were retrofitted) were
done for this reason 45 .
There are government grants, state grants and city grants, as well as a large number of
organizations offering support and guidance for those wishong to incorporate green roofs into
new and existing buildings.
Retrofitting in Germany is common. The ideas and technologies involved are familiar in the
building world, and architects and engineers have no inhibitions to retrofit.
Similar legislations exist in other countries such as Switzerland, Sweden, Norway and Austria.
In Switzerland, 25% of all new community developments must incorporate green roofs.
Retrofitting is also common and there are many examples of hospitals which have had green
roofs retrofitted. In Sweden, a non-governmental organization (IGRI) coordinates research on
his issue and has close links with the International Green Roof Journal.

4.2 – Green roof retrofitting in Canada and North America
The green roof industry in Canada and in North America has been growing interest and
awareness in recent years. The first financial incentive in Canada has been initiated by
Quebec's Energy Board: the green Roof Financial Incentive program 46 .
Some retrofit projects have objectives such as one planned for a head injury recovery centre
in Toronto, where horticultural therapy will be used to speed patients' recovery and also
reduce drug use. In Portland, stormwater runoff is an important issue as polluted water
entering the rivers damage local salmon stock 47 .
Recent changes to design codes in Ontario, Canada, have also increased the capacity of
many existing buildings 48 .

Dunnet, 2004 44
Herman, 2003 45
Liu & Baskaran, 2003. 46
Dunnet, 200447
Peck & Kuhn, 2000 48


4.3 – Green roof retrofitting in other parts of the world
In Singapore and Japan, interest of green roofs on the urban heat island effect has
encouraged research and changes. All new buildings taking up an area greater than 1000m²
must replenish at least 20% of the developed site with vegetated areas, which encourage the
use of green roofs 49 . However, examples of retrofitted buildings are not so prevalent.
Areas in southeast Asia and South America with tropical climates could potentially benefit
from the application of green roofs to problems such as flooding, water erosion and drainage.
However, there are problems with green roofs that arise in climates of prolonged and heavy
rainfall, such as saturated soil, heavy erosion causing damage and the possibility of an
increased risk of malaria if the roofs attract mosquitoes 50 and no retrofitted projects in these
areas are known of.
4.4 – Green roof retrofitting in the U.K.
Projects incorporating green roofs in the U.K. are not as frequent as in the Northern Europe,
which has a similar temperate climate. Green roofs appear on high profile buildings such as
environmental centers or on purpose built sustainable housing (such as BedZed project in
Beddington), and increasingly green roofs appear on buildings such as schools – either on
new buildings or extensions made.
In terms of retrofitting green roofs, a number of building owners who have opted to retrofit as
part of a necessary roof renovation.
There are no financial incentives (local or national) that encourage the use of green roofs in
new-build or retrofit projects. However, in 2005 a report¹ was made by the London mayor to
promote "living roofs", in which it is stressed the importance of outdoor access and the
potential for converting roofs into gardens. Retrofitting green roofs is encouraged in this
report, as well as new-build projects.
Although this does not solve all of the problems associated with taking retrofitting forward, the
report is part of a larger move to guide planners and architects. This green roof movement in
London is partly due to the "Urban Renaissance" programme initiated in 1999 by the
government, aiming to "revitalize town and cities" 51 . It builds on a recognition of the current
and future pressure in Britain on urban spaces (with an estimated 3.8 million new households
to be incorporated by 2021).
Although these movements and task forces do not specifically target green roofs as the only
solution, they provide a promising climate in which green roofs could become more widely
recognized.

There are many examples of retrofitting around the world, which show us that it is feasible.
Countries such as Germany have integrated green roof technology into wider aims of social
and environmental improvement. For many years these benefits have been accepted and
encouraged financially by the government as well as state and local authorities. Any practical
issues have become part of common knowledge among architects, engineers and roof
installers and there is greater public awareness, understanding and acceptance.

Liu & Baskaran, 2003 49
Dunnet, 2004 50
Frith, 2003 51


4.5 - Performance of green roofs and walls on different climates 52
A study of thermal benefits of green roofs and walls was made in 9 cities with different
climates, and a before and after measurement and an experiment (a model) were carried out,
to explore the thermal effects of the greening of building. The measurements were carried out
on a hot day on the hottest month of the year for each place.
The thermal effects of green roofs and walls have different results for different climates. To
study the thermal effects, we have to take into consideration not only the climates themselves, but
also other parameters like the urban canyon geometries, wind directions, canyon orientations.

Some conclusions in relation to the climates:
-The measurements have proved that the hottest and most arid of all the climates
examined, like a desert climate, benefit the most from green walls and roofs. The hotter
and drier a climate is, the more important the effect of green walls and green roofs on
mitigating urban temperatures is.
-The greatest temperature decreases with green roofs and walls: for the hotter climate and
with high solar radiation
-Places more humid, like a rain forest climate, reach smaller decreases than places more
arid when roofs and walls are covered with vegetation (Fig. 1 and 2).
-The lowest surface asphalt temperature decreases: in much colder and with lower
insolation Moscow (maximum decrease of 0.9ºC)
-The colder climates, like temperate, subartic and continental climates, benefit the least
with the green walls and roofs.

Some conclusions in relation to the decreases in temperature and other parameters:
-Air temperatures lower significantly (in all climates examined) when walls and roofs are
covered with vegetation
-Canyon air temperatures lowers the most when both walls and roofs are covered with
vegetation, because air masses enter the canyon much cooler from the vegetated roofs
-When only walls are covered with vegetation, air masses enter the canyon heated by the
plain roofs, which absorb great amounts of summer insulation
-Surface temperature decrease is significantly, both inside the canyon and at roof level
-Roof surface temperature lower even more due to the greatest amount of solar radiation
horizontal surfaces receive in summer
-The orientation doesn’t affect temperature decreases so significantly when vegetation
covers its vertical surfaces and roofs (despite the fact that it plays an important role in
temperature distributions in the canyon)
-The magnitude of the effect strongly depends on the geographic latitude (examples of
Hong Kong-22.16N and Athens-37.59N)
-The amount and geometry of vegetation is more important than the canyon’s orientation
-Only when the amounts of solar radiation received by the vertical planes differs
significantly, the orientation have an important role in temperature decreases due to
vegetation

52
Alexandri, 2008


- The largest cooling load decreases occur for the combination of green roofs and wall
-For the green roofs and walls cases, cooling load decreases are very significantly, while
for the case of only green walls it is less dramatic
-This difference between the cooling loads of these two cases are smaller for humid
climates and greater for arid climates, due to the different humidity concentrations in the 2
climatic groups
-The cooling of the microclimate by the vegetated surfaces can lead to energy savings for
cooling, depending on the climatic type and the amount of vegetation on the building
- In places when little cooling load is needed, its demand can be reduced to zero when
building surfaces are covered with vegetation; in other cases, energy savings vary from
35% to 90%
-For Savanna climate and the humid subtropical climate, cooling load decreases for the
case of green roofs and walls reach 100% - after the surfaces covered, no cooling load is
needed
-The temperate and continental cool summer climates are not affected, because no
cooling load is needed for the examined day, even before vegetation was placed
For green roofs and walls cases:
-Riyadh (desert climate) has a high cooling load decrease (90% - lowering from 12 to 5
hours its cooling demand) and Montreal (subartic climate) too (85% - lowering from 8 to 4
hours its cooling demand)
-Mumbai (rain forest climate) reaches a 72% decrease (lowering from 11 to 6 hours its
cooling demand)
-Athens (mediterranean climate) and Beijing (steppe climate) reaches a 66% and 64%
decrease (lowering by 4 and 3 hours its cooling demand)

FIGURE 1
FIGURE 2


5.Case studies
Qualitative/Quantitative Changes due to Study
Study Location Monitoring green roof recommendation Criteria
Kohler et Berlin, As early as 1984 surface Green roof reduced surface temp. But also The complex
al. (2002) Germany temperatures of a green more importantly reduced the max- temp composition of green
roof were monitored. amplitude by half. roofs represents a
The surface temp; decisive additional
shadowed surface temp buffer zone; the
of gravel; shadowed lowering roof temp.
surface temp of green And added insulation
roof; temp of substrate; effect are undeniably
ambient air temp. were all positive for indoor
measured climate; the durability Reducing temp.-
of flat roof is increased Saving energy
significantly usage
Wong NH Singapore A "before and after" The green roof tends to experience lower The substrates could
et al. measurement was carried surface temperature than the original exposed be light in colour (with
(2007) out. Six weather roof surface, especially in areas well covered the tendency to
parameters, ambient air by vegetation. A maximum temperature absorb less heat
temperature, relative difference of 18° C was observed. during the day time).
humidity, solar radiation, Coverage of greenery
wind speed, wind in most systems is not
direction and rainfall, extensive. It is
were monitored with the believed that better
use of the Hobo weather thermal performance
station, which was placed of rooftop greenery
in the centre of the could be achieved
rooftop. Two sets of when extensive
Yokogawa data loggers vegetation is planted.
were employed to record
the surface temperatures.

Alexandri Around the A two-dimensional, Lowering urban temperatures when, the hotter The substrates could
& Jones world prognostic, micro scale and drier a climate is, the greater the effect of be light in colour (with
(2008) model has been used, vegetation on urban temperatures. Also humid the tendency to
developed for the climates can benefit from green surfaces, absorb less heat
purposes of this study. especially when both walls and roofs are during the day time). It
The climatic covered with vegetation. Temperature is believed that better
characteristics of nine decrease due to vegetation is primarily affected thermal performance
cities, three urban canyon by the vegetation itself (amount and geometry), of rooftop greenery
geometries, two canyon more than the canyon orientation in hot could be achieved
orientations and two wind periods. when extensive
directions are examined. In general, the larger amounts of solar radiation vegetation is planted.
The thermal effect of a surface receives, the larger its temperature
green roofs and green decreases are when it is covered with
walls on the built vegetation. For the low air velocities inside the
environment is examined canyon, the wind direction does not have any
in both inside the canyon significant effect on temperature decreases due
and at roof level. to vegetation

Graham Vancouver Evaluating the Green roofs protect stream health and reducing Retrofit to counteract
and Kim stormwater management flood risk to urban areas. green roof climate change and
(2003) benefits; water balance redevelopment on existing buildings could help land use densification,
Modmel to restore watershed health over time. Green to restore watershed
roofs able to filter contaminants out of rainwater
that has flowed across the roof Surface, they
can also degrade contaminants, either by direct
plant uptake, or by binding them within the Stormwater
growing medium itself saving and
Rowe et Michigan Slope and substrate On average green roofs can retain 61% of total filtering
al. (2003) depth influence on runoff rainfall. During light rain events (<2mm
quantity; Model daily), their green roof retained up to
TE525WS tipping bucket approximately 98% of rainfall, whereas the
rain gauges; 2 months same green roof was capable of retaining only
50% of the heavy rain events (when rainfall
>6mm).


Qualitative/Quantitative Changes due Study recommendation
Study Location Monitoring to green roof Criteria
Yok and substrate, air; HOBO data Reduction of surface temperatures by 15- Application of green roofs in
Sia, Singapor loggers, infrared radiometer 20 degrees C; visible light (glare) from urban areas for reasons such
e (Thermo tracer TH7102WX, green roofs lowered by 12-56%; air quality as: reduced ambient air
(2005) NEC Japan); HOBO Weather improvements noted in sulphur dioxide by temperature, improved air
Station for humidity, solar quality and reduced glare
radiation, wind speed and 37%; nitrous acid by 21%; but nitric acid from buildings
rainfall; air quality measured increased by 48%; PM 2.5 and PM 10
with annular denuder
increased (possibly from re-suspended
system (URG, Chapel Hill,
chips related to gravel ballast and bare
NC, USA), particle counter
spots on green roof) and particle number
(TSI, St. Paul, MN, USA) and
concentration decreased by 6% on green
air quality with an aerosol
roofs.
sampler (Airmetrics, Eugene, Air quality
OR) and to measure black

cargon mass Aethalometer

(Magee Scientific
Currie, UFORE – Urban Forest Air contaminant reductions between Recommends the application
(2005) Toronto Effects Model from varying levels of vegetation in one of urban vegetation at grade
Northeaster Forest Service, neighborhood in Toronto over a one year and/or elevated surfaces to
Research Station, Syracuse, period mitigate air pollution with
New York–quantified resulting population health
vegetation effects on air benefits.
contaminants based on one
year of data from
Environment Canada’s 3
local weather stations in
Toronto
Peck et Quebec The rooftop is landscaped The residents enjoy the use of the green
al City, with roof. Provide new access to a rooftop that
(1999) Quebec a garden of succulents, was not used before. It was designed to
ground covers, bulbs and create an accessible
herbs. The green terrace for people living in the co-op
roof utilizes Soprema (which is located on a main commercial
SOPRADRAIN PSE for street with no green spaces in the
drainage, a Soprema surrounding area).
SOPRAFILTRE filter, and the
growing medium is Soprema
SOPRAFLOR-X.
Environmen
tal, Social
Peck et Toronto, The green roof was built Each year more tenants use the green Lower costs by not using
al Ontario using cedar planters lined roof (tenants have formed gardening western red cedar and do
(1999) with insulation and filter cloth group) and more perennials get planted. some surface planting (i.e.
and contains triple mix soil, Birds and insects are now found on the put a lawn on the roof).
vegetables, fruit trees, berry roof. The success has not been quantified,
bushes and vines, except that the roof terrace is used and
ornamental annuals and vandalism has decreased after the first
perennials. year.

Jones Seven roof sites were initially Green roofs in urban London are providing Any opportunities for erecting
(2002) London selected as suitable for study. useful and interesting habitats for interpretation panels and
Each was visited once in invertebrates and other wildlife. Many of display boards should be
spring and once in late these invertebrates are species adapted vigorously taken up; they will
summer. An additional site to harsh dry or well-drained conditions and further educate the local
was visited only once, in late which are obviously benefiting from the residents, commuting office
summer. peculiar habitat afforded by the roofs. workers and the passing Environmen
Several very unusual and uncommon public tal,
species have been found that have not ecological
otherwise been recorded in the London in general, of the ecological
area and which, it would seem, have taken value of the green roofs and
advantage of a new type of niche not
available elsewhere in the capital. brown fields in our cities.


6. Discussion – Disadvantages

Green roofs not only add aesthetic appeal to the unused roof space that is available in most
urban areas; they also provide many benefits. Green roofs can protect the roofing membrane
from exposure to ultra violet radiation and hail damage. They can reduce energy demand on
space conditioning, and hence greenhouse gas emissions, through direct shading of the roof,
evapotranspiration and improved insulation values. If widely adopted, green roofs could
reduce the urban heat island which would further lower energy consumption in the urban area.
They can also be used as part of the stormwater management strategy in the urban area. Part
of the rain is stored in the growing medium temporarily, and to be taken up by the plants and
returned to the atmosphere through evapotranspiration. Green roofs delay runoff into the
sewage system, thus help to reduce the frequency of combined sewage overflow (CSO)
events, which is a significant environmental problem for many major cities in North America.
The plants and the growing medium can also remove airborne pollutants picked up by the
rain, thus improving the quality of the runoff. In addition, green roofs can improve air quality,
provide additional green space in urban areas, and increase property values 53 . As was in this
work, and many researches, there are many advantages for green roofs. On the other hand,
there are few of such roofs. Some of them are:
Costs: Green roofs are more expensive to install than white roofs 54 . Moreover, the
performance of the green roof will increase, with respect to the environmental benefits, if each
of the of green roof technology characteristics increases in quantity. However the cost of
green roof increases with the increase in the depth and area coverage. Also, for existing
buildings structural load limitations impose a restriction on the nature of the green roof that
can be implemented.

Costs: Green roofs are more expensive to install than white roofs 55 . Moreover, the
performance of the green roof will increase, with respect to the environmental benefits, if each
of the of green roof technology characteristics increases in quantity. However the cost of
green roof increases with the increase in the depth and area coverage. Also, for existing
buildings structural load limitations impose a restriction on the nature of the green roof that
can be implemented.

Structural load: existing buildings structural load limitations are an important criterion and
therefore it prevents green roofs with deep growing medium. In order to allow deep growing,
the roof structure should pass strengthening.

Benefit quantified: Not all benefits of green roofs can be quantified at this time, and in many
cases, it is hard to isolate variable in order to enumerate and investigate the social and
environmental benefits of green roofs 56 . Hence, Many questions remain to be answered

Liu & Baskaran, 200353
54
Gaffin et al, 2003, p9
Gaffin et al, 2003, p9 55
Banting et al, 2005, p62. 56


regarding the uncertainty of the benefits, impact of less than 100% green roof coverage,
impact of building specific constraints, the quantification of costs leading to a complete cost
benefit analysis, quantification of other social benefits and consideration of the effect of
alternative technologies that may be able to perform one or more of the functions of a green
roof. These questions are important and will need to be considered in further studies. Policy
decisions regarding green roofs will need to consider the impact of these questions.

Exceeding temperature: When the substrate is very dry, the substrate temperature can
exceed the surface temperature of the original exposed roof 57

New architecture: The green roofs create new advantages in building architecture that still
need to be investigated. Adding a green roof to a building needs new architectural
consideration that still not defined well.

Substrates 58 :
The colour of substrates is mostly dark and this will absorb more solar heat and easily incur
higher surface temperatures.
A prolonged drought period can make the substrate very dry and evaporative cooling effect is
marginal.
Thermal capacities of substrates of the extensive roof garden systems are rather small
compared with the intensive system. With thin layer and lightweight substrate, the heat can
easily build up during the day time and dissipate at night. This is the reason why the
fluctuation of surface temperatures of the substrate surface is so large.

57
Wong et al, 2007, p53
Wong et al, 2007, p3058


7.Conclusion

The necessity to recover green space is becoming increasingly critical to maintain
environmental quality. Vegetated or green roofs and walls are one potential remedy for this
problem. Establishing plant material on rooftops and on interior and exterior walls of the
buildings provides numerous ecological, economic, social and environmental benefits, as
described in this work.
The construction and maintenance of green roofs provide business opportunities for
landscape contractors, irrigation specialists, and other green industry members while
addressing the issues of environmental stewardship 59
For architects, this provides a way for new structures to have stormwater management built in
to their designs 60 .
These typically unused spaces can become a way to reclaim habitat that was lost as a result
of construction while also aiding in the protection of our environment through more sustainable
practices.
Although these numerous benefits, some conclusions from studies should be taken in to
account:
1. The temperature distribution through the thickness of the lawn is influenced not only by the
heat flow but also by moisture content, and the water (vapor and liquid) diffuses through the
lawn. The solar radiation is absorbed through the lawn layer and affects the results of the roof
significantly 61 .
2. The impacts of different types of vegetation may vary. Those with relatively extensive
greenery coverage led to better thermal performance 62 .

Despite the many social, economic and environmental benefits of these technologies and their
widespread use throughout many countries in the world, Israel have been slow to implement
them.
This work provides a review of the benefits of green roofs and walls technologies. With this in
mind, here is a question for further studies:
With the proposal of a National Action Plan, will Israel be capable, in a medium or long-term
period, to generate a new industry of these technologies that will improve too the health and
liveability of our urban environments?

Green roofs and walls provide a lot of advantages – and some disadvantages that have to be
taken into consideration – in areas such as air quality, reduction of greenhouse gases, water
quality and quantity improvements and economic benefits for building owners and for the
community.
Will these benefits present a strong case for federal and municipal government policy and
program support to create a strong and viable market for these technologies throughout
Israel?

Getter, 2006; Peck et al, 1999 59
Carter, 2005 60
Onmura, 2001 61
Wong, 2007 62


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3. Onmura, S., Matsumoto, M. and Hokoi, S. (2001). " Study on evaporative cooling effect of roof lawn gardens".
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