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© JournalsPub 2023. All Rights Reserved
9
ISSN: 2456-2335
Volume 11, Issue 1, 2023
DOI (Journal): 10.37628/IJCEP
International Journal of
Construction Engineering and
Planning
http://civil.journalspub.info/index.php?journal=IJCE&page=index
Research
IJCEP
Terrace Garden - Alternative for Passive Cooling
Shivaansh M. Cairae1, Avitesh2*
Abstract
Rising development of concrete jungles has questioned the sustainability and ecological aspect of many
cities and even countries. The paper bridges the gap on how a single green element – a terrace garden
helps in dealing with this crisis and harness the concept of sustainability. Urban greenery and urban
landscaping, is a growing trend and using these terraces as an aesthetic aid is one of the wide range
benefits it has to offer, we have seen numerous buildings accepting this idea and, in some cases, using
them as an external factor to add structural connection i.e. an eco-bridge. The research gap addresses
the idea of making buildings around the urban greenery, to maintain balance of concrete to nature and
addressing the essential need of this element as a passive cooling technique, which will help in
increasing the thermal resistivity of the building via creating a naturally insulated structure, i.e.
Terrace garden. Buildings which will be harnessing this architectural practice will not only create a
sustainable micro-climate and a passive cooling effect but will also help in overall carbon reduction,
less thermal loss and come in compliance with the efficiency regulatory regulations.
Keywords: Terrace Gardening, Sustainability, Green Element, Urban Heat Island, Thermal
Transmittance, Thermal Resistivity, Passive Cooling.
INTRODUCTION
“Rooftop gardens are cultivated areas of greenery found above offices, apartment buildings and
homes. They can serve as a living, green area, a place to play, an escape from the sun, or any
combination of these[1]. Sky garden is not a very new concept, some of ancient civilisation’s quest to
integrate greenery into cities at height, The Hanging Gardens of Babylon of the 6th century BC is an
example as being, a series of planted terraces that were supported on stone arches which was around 23
metres above ground. Rooftop gardens are also referred to as sky gardens or sky terraces, and act as an
X-factor element in a building.
In addition to improving the building's aesthetic value, a roof garden can regulate the building's
temperature, provide outdoor enjoyment and wildlife habitat, and even produce edible plants. Rooftop
farming refers to the practise of growing produce on top of a building. Container gardening,
aeroponics, hydroponics, and green roofs are
common tools for this. Between tall buildings,
"aero-bridges" can be constructed on top of the
roofs or on the ground Figure 1. [2].
An urban heat island comprises of all high-
density urban areas comprising a city. A massive
pocket of warm or in some areas, hot air is created
due to rapid heating up of man-made materials.
Apart from this, cities face noise pollution and air
pollution, which significantly lowers the air-quality,
causing multiple health and environmental issues.
As a result, HVAC and other forms of cooling
*Author for Correspondence
Avitesh
E-mail: aravitesh13@gmail.com
1Student, Department of architecture, Sushant University,
Gurugram, Haryana, India.
2Assistant Professor, Department of architecture, Sushant
University, Gurugram., Haryana, India.
Received Date: November 21, 2022
Accepted Date: December 02, 2022
Published Date: December 31, 2022
Citation: Shivaansh M. Cairae, Avitesh. Terrace Garden - Alternative for
Passive Cooling. International Journal of Construction Engineering and
Planning. 2023; 11(1): 9–29p.
Terrace Garden - Alternative for Passive Cooling Shivaansh and Avitesh
© JournalsPub 2023. All Rights Reserved
10
equipment have to exert more effort for longer periods of time. Rooftop gardens have been advocated
by architects and landscape designers as a solution to lessen the need for artificial energy. Greener
and more cost-effective than conventional air conditioning systems, rooftop gardens lower
temperatures and improve air quality by blocking the sun's rays [3-9].
Figure 1. Ferri, Benedetta, Roof terrace landscape design.
(Source: https://i.pinimg.com/originals/cf/56/22/cf56228ad4b0c02f9540038c9d4a678d.jpg )
In the modern times as well, countries like Singapore and Hong Kong, which have less space as
compared to India and other European countries, have taken up the idea of rooftop gardens and
implemented them with mixed use and hybrid typologies, making the whole architecture work in
harmony.
Using natural landscaping as the basis of what will define the buildings, has started to spring up in
many countries and is visible in different architectural techniques as well, such as, Bio-mimicry, etc.
METHODOLOGY
Due to infrequent use of this element in the current architectural practices in India, developing the
idea will require studying the present case studies and characteristics affecting the area i.e. contextual
study, based on readings from reports, magazines and research paper. On collation of data from these
studies, an evaluation will be made with respect to the materials and their respective thermal resistance
(R-Value) and conductance (U-value) values, further giving a comparison to analyse which type of
vegetation will give the best result, in a particular context of area.
Formulation of Urban Heat Islands
The increasing temperatures around the world can only be attributed to manmade causes. The
warming is localised, with certain parts of our continent heating up more quickly than others. The term
"urban heat island" (UHI) is commonly used to describe these [4].
Building Typologies - Most dominant building typology in India is Residential, due to increase in
population. This has led to cramped up spaces and uncontrolled and unplanned development.
Unplanned human activities affect the overall environment and due to rise in density of people and this
uncontrolled development, it has given rise to multiple Urban Heat Islands, for example in Delhi, the
National capital Region itself, The UHI have become significant and thus, has initiated an inspection of
the overall health of the residents of urban areas. The temperature of these UHI of Delhi has been around
3.8°C to 7.8 °C. higher at 3:00 pm and around 2.8°C to 8.3°C higher at 9:00 pm and between 4.1°C and
5.6°C in early mornings Figure 2 [1].
International Journal of Construction Engineering and Planning
Volume 11, Issue 1
ISSN: 2456-2335
© JournalsPub 2023. All Rights Reserved
11
Figure 2. Cairae, Shivaansh M.
Methodology Flow Chart
Due to the increasing unplanned density, the scope of sustainable architectural practices in these areas
become less successful, because of prevalent negative contextual attributes of these dense areas, hence
a need to bring a sustainable approach which is easily and readily available almost all the time and gives
a sense of well-being to the residents and users as well is in dire need[10-23]
Terrace Gardens or Green Roofs, are an efficient way to counter many of the prevalent problems,
these gardens are easy to set up and greatly influence and help in bringing down the thermal levels of
building, harnessing rainwater and give a mental relief to the residents as well. Inferring from the studies
regarding typology, this green element is suitable to be placed and planned before and after the
construction of the building; Residential and MNC office typologies are most suitable for implementing
this architectural element. As an additional benefit, a concrete-built mass employing a green roof, also
ticks many of the requirements for a GRIHA rating.
Area Context
Context of a building plays a major role in determining whether a terrace garden will bring more
benefits or limitations. Context is further observed as densely packed (Figure 3) and Well-planned areas
(Figure 04), limitations in consideration to the terrace gardening, usually arise in densely packed
settlements. Some commonly observed are –
1. Less Sunlight i.e. due to height variations in a close proximity often leads to an unstable plantation
and in some cases might backfire by damaging the plants overtime.
2. Cramped up areas often lead to poor water management such as storage and disposal, which end up
damaging the overall structure of the building due to water damage.
3. Accessibility i.e. if building is multi floored and is inhabited by different families, the access to the
Terrace Garden - Alternative for Passive Cooling Shivaansh and Avitesh
© JournalsPub 2023. All Rights Reserved
12
terrace is usually limited to or restricted to the top floor residents, Furthermore, creating a conflict
of maintenance of the terrace plantation with other residents of the building.
4. Keeping in mind the Urban Heat Island effect, it is advisable to keep the building typology in an
area which is less dense in planning, which helps narrowing it down to, Office buildings,
commercial centres (such as malls), hotels and residential areas which are well planned and not
densely packed.
Figure 3. Cairae, Shivaansh M., densely packed settlement (representational).
Figure 4. Cairae, Shivaansh M., well planned settlement (representational).
An Urban Heat Island effect refers to urban areas showing a significant rise in temperature as
compared to the surrounding landmass. It has been scientifically demonstrated that as cities grow
without proper planning, local temperatures rise because new sources of heat generate more heat than
are removed by existing sinks (passive heat exchangers). Over the previous few decades, a warming
trend has been noted in the Delhi National Capital Region. Such a trend is noticeable during the night-
time temperatures, which is again a reflection of change in land-use pattern and additional heat source
that may enhance the overall urban heat Figure 5.
Increased levels of CO2 and aerosols in the air trap the heat and do not allow any cooling to happen,
thus making the area a perpetual greenhouse.
1. Air pollutants present in urban concrete masses have major ingredients like aerosols and CO2 that
absorb infrared radiations and re-radiate the long wave radiations, thus trapping the heat waves near
the earth's surface, leading to a rise in temperature
2. Large concrete masses form a major heat production zone with very little or no heat sink areas like
waterbodies, green covers. Etc.
3. Traffic conditions affect the urban heat islands severely
4. Industrial activities and factories emit pollutants that contribute to a rise in local temperatures Air-
conditioning systems used in buildings in urban areas reject a substantial amount of heat. Other
International Journal of Construction Engineering and Planning
Volume 11, Issue 1
ISSN: 2456-2335
© JournalsPub 2023. All Rights Reserved
13
pollutants, with the help of sunlight, warm the earth's surface, whereas air conditioners operating
during night time further add heat to the outdoor surroundings, thus significantly increasing the
night time local temperature [4].
Thermal Conductivity and Resistivity
To create a conclusive evaluation, three models were made based on different type of vegetations,
precisely, Annual/ Grass, Perennials/ Ornamental Grasses/ Shrubs and Shrubs/ Small trees. Further,
evaluated in comparison to their Thermal Resistivity (R-Value), Thermal Transmittance (U-Value) and
their overall Insulation properties.
Because the area for a Terrace garden differs from building to building, a comparison was set on the
outlines of Thermal Conductivity and Resistivity of the materials that are used to set up a rooftop garden
on a flat roof in accordance to the three cases as mentioned earlier.
R-Value (Thermal Resistance)
The rate at which heat travels across a surface area per unit of time when there is a uniform
temperature difference between two surfaces of a certain material or construction. R-value is measured
in terms of m2.K/W. [6]
U-Factor (Thermal Transmitance)
The rate at which a given temperature gradient across an area of a material or structure causes thermal
energy to be transferred via that area and the boundary air layers. The U value is expressed in terms of
watts per square metre per kelvin. [6]
These values help in determining whether how much a material is good at retaining the overall
thermal energy and helps in distinguishing whether a material or set of material, layered onto each other
are acting as a good insulation or not.
U-Value is inversely proportional to the R-Value, i.e. if a material has a greater value of Thermal
resistivity, it’ll have lower value to that of Thermal conductivity or transmittance. Hence, making it a
good insulator and a preferred material to be used in a building construction.
Preferred materials for building construction in order to make it more compliant to the efficiency
norms put up by different standards and organisations such as ECBC, GRIHA and EIA, require them
to be good insulators having as lower the U-Value as possible. Some materials taken into consideration
for this evaluation include Table 1.
Table 1. Application in building roofing and materials.
Application in Building Roofing
Materials
Roof Construction
• R.C.C
• Precast hollow slabs
Waterproofing Layer
• Bitumen
• Polyurethane (Board)
Protection Layer
• Cement Screed
Drainage Layer
• Drainage Board (Geotextile/ Polypropylene)
Insulation Layer
• Polyisocyanurate (PIR panel)
• Polyurethane (Foam)
Substrate Layer
• Gravel
• Gravel based soil
• Soil
Vegetation
• Grass or Annual plants
• Perennial/ Ornamental Grass/ Shrubs
• Shrubs/ Small trees
Terrace Garden - Alternative for Passive Cooling Shivaansh and Avitesh
© JournalsPub 2023. All Rights Reserved
14
Roof
Although area of context, weather and climate play a vital role in determining whether a building is
able to adapt to the surroundings and act as an efficient structure; materials, techniques, location of the
built mass, the elements of design used and the way construction has been done play a major role in
determining the energy efficiency of the interior and the overall exterior of a built mass or a building.
An architect is responsible to foresee that the building design, elements of the building, materials
employed and the techniques used, all contribute to the building being a sustainable, energy efficient
and also comply to the regulatory norms for saving maximum energy, while having lower carbon
emittance and lower carbon footprint. Often times, one of the key elements for ensuring this outcome
goes unnoticed, i.e. the roof of the building.
Roof is an important element of the building, as it is the only element which receives constant sunlight
over the whole surface area and often ends up absorbing the overall heat of the building as it rises up
through the day, in a hot and arid environment and on the other hand unable to retain the overall heat
inside the building, in a cold and humid context. Thus, an architect has to make sure that the technique,
typology and material of the roofing being used will give the best thermal transmittance (U-Value) and
thermal resistivity (R-Value) values, which in turn contributes and helps in making the building abide
by more sustainability and efficiency regulations, and getting certified by different efficiency evaluation
bodies such as ECBC, GRIHA and EIA.
Shape
Although roofing can be done in many shapes, forms and via different techniques, the most
commonly used shapes are flat roofing and sloped roofing. Shape of roof plays a major part in
determining the overall heat gain or thermal insulation of the building and area of the roof further adds
onto the overall conclusive value of the thermal insulation achieved by the building.
Flat roofs are plain horizontal roofs which could be constructed from different easily acquired
materials, this type of roofing has been dominant in the construction of different types of buildings. The
aforementioned types of roof, are all models and experiments based on this type of roofing system.
Although easily designed and constructed, Flat roofs tend to be more susceptible to get direct sunlight
and for more duration of time, than any other element of the building, hence resulting in maximum solar
gain via exterior. Similarly, the interior workflow of the building also contributes significantly, as the
heat emitted rises up, in turn adding to the already hot roof, thus increasing the overall temperature of
the building; which in turn leads to excessive use of Air-conditioning or HVAC system, thus critically
affecting the overall efficiency of the built mass[24]
These roofs if inaccessible to residents or users, are usually covered with PVC or vinyl material. But
for an accessible roof, the materials used include Cement, Bitumen, Brick tiles, etc. Although, in the
latter case, due to material properties and colour of the roof, these roofs can sometimes touch
temperatures upward of 80 degree Celsius or more, depending on the typology and internal ventilation.
Due to the less efficient thermal output from this conventional roofing system, Architects and Designers
came up with different ways to counter this and create more energy efficient and sustainable roofing
systems based on this type.
Terrace Gardening or Roof Gardening, is one of the most eco-friendly, thermally susceptible and
sustainable method from the aforementioned different types. Apart from helping in overall efficiency
of the building, the element also helps in creating a natural connection and a micro-climate within the
building, giving an effortlessly modifiable landscaping option to the user and significantly adds to the
overall aesthetic of the building.
One of the other most commonly used roofing type is, Sloped roof. Combinations of which include,
Single sloping i.e. lean-to or shed; and Double sloping i.e. Triangle, Gable or pitched roof. The angle
International Journal of Construction Engineering and Planning
Volume 11, Issue 1
ISSN: 2456-2335
© JournalsPub 2023. All Rights Reserved
15
at which the roof is pitched, the direction in which the slope is facing and the properties of construction,
majorly affects the overall heat gain in the interior side, subsequently affecting the living spaces. This
type of roof is generally seen in residential buildings, but not as dominantly as flat roof Figure 6 and 7.
Figure 6. Cairae, Shivaansh M., Flat roof gardening (Representational).
Figure 7. Cairae, Shivaansh M., Sloped roof gardening (Representational).
If we create a comparison to that of Flat roof type building, sloping roofs tend to have equal to or
more than, the value of incident solar radiation, due to their excessive and inaccessible surface area.
Although, this value can vary significantly based on the materials used, and is even more variable in
cases of different sloping angles, which have a range of 0o, 15o, 30o, 45o and 60o; and the direction in
which the slope is facing, i.e. if the slope is oriented due South side, the incident solar radiation gain
will be maximum and directly proportional to its area, Similarly, a slope oriented towards the North
will receive the solar radiation but for lesser duration of the day. Therefore, strategic placement of this
type of roof and using materials which can further help in increasing the thermal resistivity of the
element are critical to the building, in order to get the maximum thermal efficiency out of it.
Similar to flat roofing, terrace or roof gardens can be one alternative to get more efficient readings,
but it’s not an easy model that a user can set up after the construction of a sloped roof. Some
comparisons drawn for sloped roof against the flat roof system show that, the landscaping done is not
easily modifiable, not easily accessible due to sloping and thus create issues of maintenance of the
garden as well as the whole structure on which it’s being supported. Negligence of maintenance can
backfire the effects of introducing a rooftop garden to the structure, by creating water induced damage,
as discussed earlier in the paper and algae formation on the affected area, thus damaging the aesthetic
feel and structural property of the roof.
Terrace Garden - Alternative for Passive Cooling Shivaansh and Avitesh
© JournalsPub 2023. All Rights Reserved
16
Shading Devices
Based on the discussions done previously, we know certain techniques and methods will help in
prolonging the building efficiency. Apart from these methods, properly designed and curated, sun
control and shading devices help to minimize the peak heat gain of the building, furthermore, improving
the natural lighting quality of the interior living spaces and at the same time helping in bringing down
the overall usage of different cooling methods employed. “Depending on the amount and location of
fenestration, reductions in annual cooling energy consumption of 5% to 15% have been reported. Sun
control and shading devices can also improve user visual comfort by controlling glare and reducing
contrast ratios.” [16,25]
These shading devices increase the wellbeing of the building inhabitants as well as saving the
building structure from different type of loads. These shading devices mostly include, window
overhangs or Chajja(s), awnings and trellises, which are constructed along the building; external
shading devices which are added or further modified after the building has been completed, include
building basic structural shed over the rooftop or terrace of the building. These external devices usually
are fit in the confined area and are bound by regulations of the present structure, i.e. they can’t span
more than the surface area of the roof on the top. Due to these limitations, at times these structures are
made overlooking the quality aspect, furthermore these structures are not maintained properly which in
turn severely affects their life span, rendering them as a temporary solution to the heat gain[22].
Building orientation with respect of sun path is important aspect to design effective shading devices.
For example, simple fixed overhangs are very effective at shading south-facing windows in the summer
when sun angles are high. However, the same horizontal device is ineffective at blocking low afternoon
sun from entering west-facing windows during peak heat gain periods in the summer.
Therefore, a wide range of design elements and components cater to the solar control and shading,
these are:
• Landscape features such as mature trees or hedge rows;
• Exterior elements such as overhangs or vertical fins;
• Horizontal reflecting surfaces called light shelves;
• Low shading coefficient (SC) glass; and,
• Interior glare control devices such as Venetian blinds or adjustable louvers[16].
These devices and methods are further quantified and compared by the, Coefficient of shading factor.
This factor varies according to different shading options used, their projection factor and latitude of site
and application on-site.
Vegetation
Vegetation or plantation of trees and various other plants, help immensely in cooling down the
surrounding area, introduction of this has been able to counter as well as reduce the urban heat island
effect, as talked in the previous case study.
Trees and vegetation lower surface and air temperatures by providing shade and through
evapotranspiration. Shaded surfaces, for example, may be 20–45°F (11–25°C) cooler than the peak
temperatures of unshaded materials[2]. Evapotranspiration, alone or in combination with shading, can
help reduce peak summer temperatures by 2–9°F (1–5°C).
This mitigation strategy comes into play when it is implemented in consideration to locations and
strategic points all around the building, and also works as a natural shading device, be it surrounding
areas i.e. pavements or parking lots, and areas on the building i.e. the rooftop, window curtaining or on
Chajja’s.
International Journal of Construction Engineering and Planning
Volume 11, Issue 1
ISSN: 2456-2335
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17
The use of trees and vegetation in the urban environment brings benefits beyond mitigating urban
heat islands including:
• Reduced energy use: Trees and vegetation that directly shade buildings decrease demand for air
conditioning.
• Improved air quality and lower greenhouse gas emissions: By reducing energy demand, trees and
vegetation decrease the production of associated air pollution and greenhouse gas emissions. They
also remove air pollutants and store and sequester carbon dioxide.
• Enhanced storm water management and water quality: Vegetation reduces runoff and improves
water quality by absorbing and filtering rainwater.
• Reduced pavement maintenance: Tree shade can slow deterioration of street pavement, decreasing
the amount of maintenance needed.
• Improved quality of life: Trees and vegetation provide aesthetic value, habitat for many species,
and can reduce noise.
The cost inferred by introduction of this process is mainly associated with setting up the plantation
and maintenance of the vegetation. These further include the initial cost of buying the plants, which can
vary from different user to different typology of structure in context, planting of saplings and
maintaining the plantation via irrigation, pruning and pest or disease control.
U-Value
U-value measures the rate of heat transfer through the envelope or through a particular section of
construction due to a temperature difference between the indoors and outdoors; also referred as an
overall coefficient of heat transmission, this value helps in identifying the conductance to heat flow or
the measure of how efficiently or how inefficiently a component or material transmits heat flow through
itself by taking in comparison the internal temperature and external temperature of the material
employed in a building or structure.
If a material has the rate of transmittance of heat relatively slower or more difficult, it causes the U-
value to decrease, hence a lower U-Value is preferred while calculating or comparing. U- value is
calculated by doing the reciprocal of the Thermal Resistance Value (R-value).
R-value is the resistance to the conductive heat flow, and inversely to the U-Value, if the R-Value is
high, the performance of insulation is considered to be better.
Equation for calculating the R-Value and U-Value:
R = d/k
Where,
R = Thermal Resistance (m2K/W)
d = Thickness of material (m, in Metres)
k = Thermal conductivity of the material (W/m K)
For multi-layered components or multiple materials, the equation used is:
RT = (d1/k1) + (d2/k2) + (d3/k3) +…. + (dn/kn) = 1∑n dn/kn
Where,
RT, denotes the total Thermal Resistance value (m2K/W).
For taking out the U-Value,
U = 1/R
Where,
U = Thermal Transmittance (W/m2K)
Terrace Garden - Alternative for Passive Cooling Shivaansh and Avitesh
© JournalsPub 2023. All Rights Reserved
18
R = Thermal Resistance (m2K/W)
For taking out the U-Value for multiple or multi-layered materials, the equation used is:
UT = 1/RT
Where,
UT, denotes the total Thermal Transmittance value.
Units system used kept in mind while calculating U-Value are:
1. Metric (SI) unit, which is W/m2K, i.e. the rate of heat flow (in Watts) through 1m2 of a structure
when there is a temperature difference across the structure of 1 Degree (K or Co).
2. Imperial Unit for quoting the value, is BTU/hr.ft2.F
For conversions,
• Multiplying the Imperial Unit (BTU/hr.ft2.F) by 5.675 will convert the value to Metric Unit
(W/m2K).
• Similarly, dividing the Metric Unit (W/m2K) by 5.675 will convert the value to Imperial Unit
(BTU/hr.ft2.F).
These conversions apply when referring to U-values of different countries, with different codes
adhering to their respective unit system.
The U-value vary for each particular material and can be found in the Energy conservation Building
Code (ECBC), Environmental Impact Assessment (EIA), IGBC and ASHRAE Fundamentals, for
Indian Subcontinent.
U-values play a major role in determining the overall efficiency of a building. In order to comply
with the regulations of the above-mentioned documents and bodies keeping a track of energy efficiency,
the building need to consider certain points:
The building design must be in compliance to show that the overall carbon dioxide emission rate does
not exceed the minimum set up level or the target CO2 rate (TER).
The area that takes into consideration the importance of the U-values includes the DFEE and the
TFEE, basically stating that the loss of energy through the building fabric for the whole building, i.e.
Dwelling fabric energy efficiency (DFEE) must not be worse or exceeding the maximum allowance of
dwelling Target fabric energy efficiency (TFEE).
ANALYSIS
A further quantitative analysis was done, with respect to the U-values and R-values of the material
layers taken in consideration in making a terrace garden (refer to Table 1). Thermal conductivity (k)
values were taken in reference to Architect’s Pocket Book, ECBC 2017 and ASHRAE [3].
Roof Construction
Materials/Techniques taken into consideration and compared are, Reinforced Cement Concrete
(R.C.C.), for which thermal conductivity is taken for Concrete 25/50 and Precast Hollow Slab, for which
thermal conductivity value was taken for Concrete 30/60 Figure 8.
The calculations for which are as follows:
1. Reinforced Cement Concrete [R.C.C.]
Where the,
Layer Thickness (d) = 150mm/0.15m
Thermal Conductivity (k) = 1.3957 W/mK ≈1.4 W/mK
For calculating R-value,
R = d/k
International Journal of Construction Engineering and Planning
Volume 11, Issue 1
ISSN: 2456-2335
© JournalsPub 2023. All Rights Reserved
19
R = 0.15/1.4 = 0.107 Km2/W
R ≈ 0.11 Km2/W
For calculating U-value,
U = 1/R = 1/0.11
U = 9.09 W/m2K
Figure 8. Cairae, Shivaansh M., Roof Construction layer.
2. Precast Hollow Slab
Where,
Layer Thickness (d) = 100mm/0.1m
Thermal Conductivity (k) = 1.4107 W/mK ≈1.4 W/mK
For calculating R-value,
R = d/k
R = 0.1/1.4 = 0.07 Km2/W
R = 0.07 Km2/W
For calculating U-value,
U = 1/R = 1/0.07
U = 14.28 W/m2K
Waterproofing Layer
Materials taken into consideration and compared are, Bitumen and Polyurethane Board Figure 9. The
calculations for which are as follows:
Figure 9. Cairae, Shivaansh M., Waterproofing layer.
Terrace Garden - Alternative for Passive Cooling Shivaansh and Avitesh
© JournalsPub 2023. All Rights Reserved
20
1. Bitumen
Where,
Layer Thickness (d) = 12-15mm/0.012-0.015m
Thermal Conductivity (k) = 0.500 W/mK
For calculating R-value,
R = d/k
R = 0.015/0.500 = 0.03 Km2/W
R = 0.03 Km2/W
For calculating U-value,
U = 1/R = 1/0.03
U = 33.3 W/m2K
2. Polyurethane [Board]
Where,
Layer Thickness (d) = 100mm/0.1m
Thermal Conductivity (k) = 0.025 W/mK
For calculating R-value,
R = d/k
R = 0.1/0.025 = 4 Km2/W
R = 4 Km2/W
For calculating U-value,
U = 1/R = 1/ 4
U = 0.25 W/m2K
Protection Layer
Material taken into consideration is Cement Screed (laid to slope) Figure 10. The calculations for
which are as follows:
Figure 10. Cairae, Shivaansh M., Protection layer.
1. Cement Screed
Where,
Layer Thickness (d) = 30mm/0.03m
Thermal Conductivity (k) = 1.3957 W/mK ≈1.4 W/mK
International Journal of Construction Engineering and Planning
Volume 11, Issue 1
ISSN: 2456-2335
© JournalsPub 2023. All Rights Reserved
21
For calculating R-value,
R = d/k
R = 0.03/1.4 = 0.02 Km2/W
R = 0.02 Km2/W
For calculating U-value,
U = 1/R = 1/0.02
U = 50 W/m2K
Drainage Layer
For this layer, Drainage board which is made from Geotextile or Polypropylene material is used.
Thermal Conductivity value is taken for Polypropylene solid Figure 11. The calculations for which are
as follows:
Figure 11. Cairae, Shivaansh M., Drainage layer.
1. Drainage Board [Geotextile/ Propylene]
Where,
Layer Thickness (d) = 10mm/0.01m
Thermal Conductivity (k) = 0.17-0.22 W/mK ≈ 0.22 W/mK
For calculating R-value,
R = d/k
R = 0.01/0.22 = 0.045 Km2/W
R = 0.045 Km2/W
For calculating U-value,
U = 1/R = 1/0.045
U = 22.2 W/m2K
Insulation Layer
Materials taken into consideration and compared are, Polyisocyanurate Panel (PIR Panel) and
Polyurethane Spray Foam (PUF) Figure 12. Calculations for which are as follows:
1. Polyisocyanurate Panel [PIR Panel]
Where,
Layer Thickness (d) = 20-50mm/0.02-0.05m
Thermal Conductivity (k) = 0.0364 W/mK
For calculating R-value,
Terrace Garden - Alternative for Passive Cooling Shivaansh and Avitesh
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R = d/k
R = 0.05/0.0364 = 1.37 Km2/W
R = 1.37 Km2/W
For calculating U-value,
U = 1/R = 1/1.37
U = 0.73 W/m2K
Figure 12. Cairae, Shivaansh M., Insulation layer.
2. Polyurethane Spray Foam [PUF]
Where,
Layer Thickness (d) = 30-120mm/0.03-0.12m
Thermal Conductivity (k) = 0.0372 W/mK
For calculating R-value,
R = d/k
R = 0.12/0.0372 = 3.22 Km2/W
R = 3.22 Km2/W
For calculating U-value,
U = 1/R = 1/3.22
U = 0.310 W/m2K
Substrate Layer
Now layer depth and material of the substrate used, completely depends upon the vegetation it is
catering to. Similarly, the thickness is taken with respect to the vegetation, which is explained later for
each plantation. Hence, the calculations done further are co-related to the plantation being considered
for this research i.e. Grass/Moss Grass/Annual small plants, Perennial/Ornamental Grass/ Shrubs
(plants, less than 3 ft. in height) and Small Trees/ Shrubs (hardwood, less than 6 ft. in height).
1. Grass/ Moss grass/ Annual small plants
Grass requires 6 inches or 15 centimetres of top soil in order to grow freely, as their roots usually
tend to grow about 12 centimetres. Whereas, Gravel and Gravel based soil are not recommended as the
roots are unable to hold or firm their grasp and as a result not grow properly Figure 13.
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ISSN: 2456-2335
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Figure 13. Cairae, Shivaansh M., Grass on Soil Substrate.
Taking substrate as soil, calculations are as follows:
Where,
Layer Thickness (d) = 150mm/0.15m
Thermal Conductivity (k) = 1.729 W/mK
For calculating R-value,
R = d/k
R = 0.15/1.729 = 0.086 Km2/W
R = 0.086 Km2/W
For calculating U-value,
U = 1/R = 1/0.086
U = 11.6 W/m2K
2. Perennial/Ornamental Grass/ Shrubs
Thickness or minimum soil depth depends on the overall height attainable by the plant typology. For
this type, plant height ranges from 16”-24” i.e. 406 to 609 mm. Hence, the required soil depth should
be 12”-18” thick i.e. 304 to 457 mm Figure 14-15.
Figure 14. Cairae, Shivaansh M., Perennial/Ornamental grass/Shrubs on Gravel based soil Substrate.
Figure 15. Cairae, Shivaansh M., perennial/ornamental grass/shrubs on soil substrate.
Taking substrate as Gravel based soil, calculations are as follows:
Where,
Layer Thickness (d) = 457mm/ 0.457m
Thermal Conductivity (k) = 0.520 W/mK
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For calculating R-value,
R = d/k
R = 0.457/0.520 = 0.87 Km2/W
R = 0.87 Km2/W
For calculating U-value,
U = 1/R = 1/0.87
U = 1.15 W/m2K
Taking substrate as soil, calculations are as follows:
Where,
Layer Thickness (d) = 457mm/ 0.457m
Thermal Conductivity (k) = 1.729 W/mK
For calculating R-value,
R = d/k
R = 0.457/1.729 = 0.26 Km2/W
R = 0.26 Km2/W
For calculating U-value,
U = 1/R = 1/0.26
U = 3.84 W/m2K
3. Small Trees/ Shrubs
For this type, plant height is taken less than 6’ i.e. 1828mm. Hence, the required soil depth is taken
to be 3’ thick i.e. 914mm Figure 16-18.
Figure 16. Cairae, Shivaansh M.,
Small trees/Shrubs on Gravel
based soil Substrate.
Figure 17. Cairae, Shivaansh M.,
Small trees/Shrubs on Soil
Substrate.
Figure 18. Cairae, Shivaansh M.,
Small trees/Shrubs on Gravel
Substrate.
Taking substrate as Gravel based soil, calculations are as follows:
Where,
Layer Thickness (d) = 914mm/ 0.914m
Thermal Conductivity (k) = 0.520 W/mK
For calculating R-value,
R = d/k
R = 0.914/0.520 = 1.75 Km2/W
R = 1.75 Km2/W
For calculating U-value,
U = 1/R = 1/1.75
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U = 0.57 W/m2K
Taking substrate as soil, calculations are as follows:
Where,
Layer Thickness (d) = 914mm/ 0.914m
Thermal Conductivity (k) = 1.729 W/mK
For calculating R-value,
R = d/k
R = 0.914/1.729 = 0.528 Km2/W
R ≈ 0.53 Km2/W
For calculating U-value,
U = 1/R = 1/0.53
U = 1.88 W/m2K
Taking substrate as Gravel, calculations are as follows:
Where,
Layer Thickness (d) = 914mm/ 0.914m
Thermal Conductivity (k) = 0.360 W/mK
For calculating R-value,
R = d/k
R = 0.914/0.360 = 2.53 Km2/W
R ≈ 2.53 Km2/W
For calculating U-value,
U = 1/R = 1/ 2.53
U = 0.39 ≈ 0.4 W/m2K
Analysis Overview
In order to understand the proper convective heat transfer which is affected by the plants or their
canopy’s, we further need to refer to Leaf area index of a plant (LAI). Because of its correlation with
light and energy capture, leaf area index (LAI) is an important measure for characterising different plant
canopies. LAI is calculated by dividing the total leaf area by the soil surface area per unit. These
measurements are mainly used to study the canopy structure of a plant, by taking in account either one
of the sides of the leaf and its subsequent area coverage over the ground. This value helps in identifying
the light reflected and the area shaded by the canopy of a leaf with respect to the plant.
Taking into account the considered materials, which showed better thermal efficiency and examining
with respect to the 3 different types of plantation, we find out the total U-factor Table 2 and Figure 19-21:
Figure 19. Cairae, Shivaansh M.,
Grass/Moss grass/Annual
Figure 20. Cairae, Shivaansh M.,
Perennial/Ornamental
grass/Shrubs
Figure 21. Cairae, Shivaansh M.,
Small trees/Shrubs.
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Table 2. Preferred Materials for Terrace-garden construction.
Application in Building Roofing
Preferred Materials and their Thermal values
1. Roof Construction
R.C.C
R1 = 0.11 Km2/W
U1= 9.09 W/m2K
2. Waterproofing Layer
Polyurethane (Board)
R2 = 4 Km2/W
U2= 0.25 W/m2K
3. Protection Layer
Cement Screed
R3 = 0.02 Km2/W
U3= 50 W/m2K
4. Drainage Layer
Drainage Board (Geotextile/ Polypropylene)
R4 = 0.045 Km2/W
U4= 22.2 W/m2K
5. Insulation Layer
Polyurethane Spray Foam (PUF)
R5 = 3.22 Km2/W
U5= 0.310 W/m2K
6. Substrate Layer
Soil for Grass/ Moss Grass/ Annual plants
R6 = 0.086 Km2/W
U6= 11.6 W/m2K
Gravel based Soil for Perennial/Shrubs
R6 = 0.87 Km2/W
U6= 1.15 W/m2K
Gravel for Small Trees/Shrubs
R6 = 2.53 Km2/W
U6= 0.4 W/m2K
Where,
Rn – ‘n’ denotes the number of layers; from roof construction till substrate layer.
For U-factor or Total U-value,
UT = 1/R1+R2+…. +Rn OR UT = 1/RT
For Grass/Moss Grass/Annual small plants,
RT = R1 + R2 + R3 + R4 + R5 + R6
RT = 0.11 + 4 + 0.02 + 0.045 + 3.22 + 0.086
RT = 7.481 Km2/W
hence,
U-Factor = 1/ RT = 0.13 W/m2K
For Perennial/Ornamental Grass/ Shrubs,
RT = R1 + R2 + R3 + R4 + R5 + R6
RT = 0.11 + 4 + 0.02 + 0.045 + 3.22 + 0.87
RT = 8.265 Km2/W
hence,
U-Factor = 1/ RT = 0.12 W/m2K
For Small Trees/ Shrubs,
RT = R1 + R2 + R3 + R4 + R5 + R6
RT = 0.11 + 4 + 0.02 + 0.045 + 3.22 + 2.53
RT = 9.925 Km2/W
hence,
U-Factor = 1/ RT = 0.10 W/m2K
The above evaluation gives us three different U-factors each for their respective Vegetated roof models.
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ISSN: 2456-2335
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Relative Analysis of U-factor
In order for a roof to be compliant with ECBC (2017), it has to adhere with the maximum assembly
U-factors of the roof, keeping in check that the roof insulation under evaluation should be a part of
structural slab and not as a part of false ceiling.
This evaluation will adhere to values for all types of buildings except educational institutions and
hospitality. The U-factor (W/m2K) for this category under following weather conditions are as follows:
1. Composite – 0.33
2. Hot and dry – 0.33
3. Warm and humid – 0.33
4. Temperate – 0.33
5. Cold – 0.28
Comparing these values with the 3 vegetated roof models, we can say;
• Vegetated roof with grass/Moss grass/annual, create an overall thermal difference of 39.4% more,
under all weather conditions except cold and 46.4% more, in case of cold weather, with respect to
maximum permissible values.
• Vegetated roof with Perennial/ornamental grass/shrub, create an overall thermal difference of
36.4% more under all weather conditions except cold and 42.8% more in case of cold weather, in
adherence to aforementioned.
• Similarly, vegetated roof with Small trees/Shrub, create an overall thermal difference of 30.3%
more under all weather conditions except cold and 35.7% more in case of cold weather, in adherence
to aforementioned.
Hence, from this comparison we can conclusively say that introduction of a vegetated roof/terrace
garden on roof, can significantly contribute in lowering the thermal transmittance well below the
permissible values, thus improving the thermal characteristics of a building remarkably, consequently
bringing down the Urban Heat island effect in its immediate vicinity. Thus, acting as a sustainable
alternative for the passive cooling of the building and the location.
CONCLUSION
Terrace gardening is one of the most effective ways of eliminating the effects caused by present urban
sprawl and increasing the thermal efficiency of a built mass, as discussed. There have been many
different experiments in consideration to the flat roof model of a building with respect to the area and
context of the site, a further research could be done on similar effects in respect to different shapes and
types of roof, for instance, Sloped roof.
To make a building even more efficient, common technique of under deck insulation can also be
taken up as a compulsion. Under Deck Insulation is a common solution to thermal gain in almost all
types of buildings, as it is done in interior part of the building. Introducing this in addition to a terrace
garden will increase the overall thermal resistivity of the building, owing to the multiple and dense layer
system of different materials.
Apart from insulating the interior even more from the heat gain, it’ll help in increasing the overall
efficiency of the heating, ventilation and cooling (HVAC) system, which will further lower the cooling
time consequently reducing the energy bill. Furthermore, HVAC systems should be kept in check and
be well maintained and serviced in order to make sure that the efficiency of the building is uniform.
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