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PLEA2013 - 29th Conference, Sustainable Architecture for a Renewable Future, Munich, Germany10-12 September 2013

Effects of Roof Treatment on Thermal Performance of Residential Buildings in Dhaka

TASNEEM TARIQ1, PROF. DR. ZEBUN NASREEN AHMED1

1Department of Architecture, Bangladesh University of Engineering and Technology (BUET),

Dhaka, Bangladesh ABSTRACT: Energy consumption in buildings has tremendously increased in Bangladesh due to high population growth along with rapid Urbanization. Sustainable design of building elements can have great impact on reducing energy consumption of Bangladesh. Extensive field work has been done on several buildings with roofs of different compositions (flat concrete, water proofing-lime terracing, roof with water body, double roof of corrugated sheet, roof with plants on bamboo structure and using pot plants) of residential areas in Dhaka. The combined effect on Air Temperature in overheated period (April) has been investigated, under the assumption that the temperature of interior space of the floor below the roof being studied, is an indicator of Space Performance. These data have been correlated, to find out the effects of different treatments of roof on indoor temperature of buildings. This result was observed with the highest values of solar radiation for the period. An average gradient of around 2.7ºC between double roof and flat concrete slab, most commonly used, has been observed on clear sky days. This study provides recommendations, on the basis of the investigation, about sustainable roof design, aiming to reduce the pressure on energy needs, thereby helping to create more sustainable residential environments in Dhaka. Keywords: thermal performance, roof treatment, energy, Air temperature, double roof

PROLOGUE To ensure environmental comfort in indoor spaces, passive techniques have become an increasingly important issue. Thermal Performance of interior spaces, which is an extremely important issue in the context of sustainability in cities [1], is seen to have a significant role for efficient living. In residential areas humans need thermally comfortable environments to rest and revive after the day’s work. Where energy or power supply is scarce, like in Dhaka [2], passive climatic control should be applied, for achieving thermal comfort. Over 15-25% of all savings, in both developed and developing countries, is invested in dwelling construction for shelter, security and comfortable living conditions [3]. Overheating is a growing environmental concern for tropical cities like Dhaka [4], though rules and controls, to achieve thermal comfort in residential environments, are non-existent [2]. This paper is based on a scientific investigation on thermal performance of various roofs, a key building design element, as this horizontal surface receives the highest solar radiation throughout the year in Dhaka [5]. The positive thermal effects of roofs are usually described by the reduction of the thermal transmittance into the interior space of the building [6], which forms the basis of ‘Thermal Performance Ranking’ of residential roofs, adopted in this paper. PROBLEM STATEMENT The Intergovernmental Panel on Climate Change (IPCC) predicts a rise of the mean annual temperature of about 3.3°C per century in the world. The effect of climate

change is being felt in the tropics, and Bangladesh is one of the most vulnerable countries [8, 9]. Temperature trends during the last 60 years (1950-2010) shows a tendency of increase in the average day temperature [10, 11] (Fig. 1). Fig. 2 shows the temperature rise in Bangladesh using historical data of some selected meteorological stations [10]. The mean annual temperature here has increased during the period of 1895-1980 by 0.31°C over the past two decades [12].

Figure 1: Temporal variation of annual maximum temperature of Bangladesh during1950-2010. (Source: Climate division, Bangladesh Meteorological Department, Agargaon, 2011.)

Figure 2: Observed trend in surface air temperatures for Bangladesh. (Source: Bangladesh Met Department, 2010)

Dhaka lies just north of the Tropic of Cancer, displaying characteristics of “Composite” climate, with approximately one-third of the year being hot-dry (mean max 33.6 ºC), two thirds warm-humid (mean max 31.3 ºC), while there is a cool-dry season [9]. Urban Dhaka is also subject to an urban heat island effect [13]. Dhaka has a high rate of residential development, to cater to the huge influx of rural to urban migration [1]. Residential areas form almost 27% (highest) of built areas [14]. There is a gap between power generation (4.5MW) and demand (6.5MW), resulting in frequent power-cuts [15], and situation deteriorate further during summer. Fig. 3 shows the variation of electricity consumption throughout a year, which peaks in the summer [14].

Figure 3: Relationship between Met data and energy consumption data, Year 2005. (Source: Met Data and PDB)

A Building Roof is the horizontal skin through which a man-made indoor space and outdoor space interacts, and it receives the highest amount of solar radiation in the tropics (5329Wh/m2) [4, 5]. It is the building component most exposed to the climatic elements [4, 16]. Solar radiation particularly on clear days affects the thermal behaviour of roof more than any other part of the structure [4]. Under warm ambient conditions in the tropics, the roof significantly affects the indoor air temperature in adjacent interiors [4, 16]. Air temperature is the most important environmental factor and main criterion of human comfort [4]. Passive means of achieving thermal comfort inside the building is the best solution to provide a healthy and energy efficient indoor environment [17, 18, 19]. Dhaka’s Energy consumption Data (Fig. 4) shows, domestic consumption is the highest, making it imperative to initiate passive measures for sustainability [14].

Figure 4: Energy consumption pattern of year 2005 of Dhaka city. (Source: Data from Power Distribution board)

Most of the buildings in the urban areas of Bangladesh (specially in Dhaka) usually have flat concrete roofs [4]. Some roofs have 75mm layer of lime terracing which provides some degree of insulation. There are a few examples of double roofs, rooftop swimming pools and roof gardening. As there are no standards of sustainable roof design, this paper aims to evaluate the performance of various roof treatments and identify the most efficient residential roofs of the city. OBJECTIVE OF THE RESEARCH This research investigates the effectiveness of various roof treatments, and their potential to achieve passive climatic control, by reducing the indoor air temperature, for residential buildings in warm-humid season in Bangladesh. METHODOLOGY OF THE RESEARCH To frame the knowledge base, previous thermal condition researches, relevant to climatic condition of Dhaka have been studied. Six residences with six types of roofing have been selected for the study. At each spot, measurements of air temperature (average value of five readings) were recorded at three different positions (at outdoor , roof surface and indoor space) at around 2 P.M. on the three days of survey (April, 2011) by Pocket Weather Meter (kestrel 3000). These days were characterised by clear skies, dry weather, high solar altitude angle, high solar intensity and high duration of sun-shine. The study was primarily based on, field data measurement, observations, discussions with inhabitants, comparison and analysis of data. The air temperature difference between the exterior (roof surface) and interior spaces was considered a significant indicator of the performance of the roof. Within the limited period of time for the study, residential areas of similar physical features were chosen, to minimize the impact of surroundings on the temperature variations. FIELD INVESTIGATION The six survey spots with six different types of treatments are based on the flat concrete roof (except Spot 4). The survey spots are as below: Spot 1: the basic flat concrete roof, Spot 2: the flat concrete roof with water proofing-

lime terracing Spot 3: the flat concrete roof with water body, Spot 4: double roof with corrugated sheet and thin

bamboo layer (locally known as ‘Chatai’), Spot 5: the flat concrete roof with plants on bamboo

structure (locally known as “Macha”) and Spot 6: the flat concrete roof with pot plants.

85%

0%3%

12%

0%

Domestic (85%)

irrigation (0%)

Light industrial (3%)commercial (12%)

Street Light &Water Pump (0%)

spaces (spaces (recorded. from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and their general surroundings. recorded air temperature at roof surface was recorded from Position 2recorded of the interior of topmost floor.relative humiditythree daysvariable humiditycollected from meteorological station. Table 1: Collected Air Temperature in

Spot 1: Location

Spot 2: Location

At each spot, tspaces (spaces (recorded. from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and their general surroundings. recorded air temperature at roof surface was recorded from Position 2recorded of the interior of topmost floor.relative humiditythree daysvariable humiditycollected from meteorological station.

Table 1: Collected Air Temperature in

Spot 1: Location

Positions

Spot 2: Location

Positions

At each spot, tspaces (immediately bspaces (1m recorded. from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and their general surroundings. recorded air temperature at roof surface was recorded from Position 2recorded of the interior of topmost floor.relative humiditythree daysvariable humiditycollected from meteorological station.


Spot 1: Location

PositionsP1P2P3

Spot 2: Location

PositionsP1P2P3

At each spot, timmediately b1m

recorded. Figures inset in Table 1 show the positions from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and their general surroundings. recorded from Position 1air temperature at roof surface was recorded from Position 2 (P2)recorded from of the interior of topmost floor.relative humiditythree days variable during humidity, Wind speed collected from meteorological station.


Flat concrete roofLocation Dhanmondi 28

Positions P1 P2 P3

Roof with water proofing limeLocation East Rajabazar

Positions P1 P2 P3

At each spot, timmediately b1m above the

Figures inset in Table 1 show the positions from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and their general surroundings.

from Position 1air temperature at roof surface was recorded from

(P2), while the ifrom Pos

of the interior of topmost floor.relative humidity

of recordingduring

Wind speed collected from meteorological station.


Flat concrete roofDhanmondi 28

Roof with water proofing limeEast Rajabazar

At each spot, the air temperatures for both inimmediately b

above the Figures inset in Table 1 show the positions

from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and their general surroundings.


, while the iPosition

of the interior of topmost floor.relative humidity (62%)

of recordingduring this period.





he air temperatures for both inimmediately below the roof surface) and outdoor

above the Figures inset in Table 1 show the positions



, while the iition

of the interior of topmost floor.(62%)

of recordingthis period.




Day 130.831.431.2


Day 133.834.233

he air temperatures for both inelow the roof surface) and outdoor

above the roof and at roof surface) were Figures inset in Table 1 show the positions


from Position 1 (P1), air temperature at roof surface was recorded from

, while the iition 3 (P3)

of the interior of topmost floor.(62%) of a

of recording. Only the wind speed was this period.

Wind speed and air temperature collected from meteorological station.



Day 1 30.8 31.4 31.2


Day 1 33.8 34.2 33


oof and at roof surface) were Figures inset in Table 1 show the positions

from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and their general surroundings. Outdoor air temperature was

(P1), air temperature at roof surface was recorded from

, while the indoor air temperature was (P3)

of the interior of topmost floor.of air w. Only the wind speed was

this period. and air temperature

collected from meteorological station.


Flat concrete roof

Roof with water proofing lime

1

12m



from which the temperatures were recorded for each of the six cases, along with their locations in Dhaka and

Outdoor air temperature was (P1), 1m above the

air temperature at roof surface was recorded from ndoor air temperature was

(P3), 1m above the floor level of the interior of topmost floor. It was observed that the

ir was. Only the wind speed was

Solar radiation, and air temperature



Roof

Day 2

Roof with water proofing limeRoof Height

Day 2

18m

12m




Outdoor air temperature was 1m above the


1m above the floor level It was observed that the as nearly constant in the

. Only the wind speed was Solar radiation,

and air temperature collected from meteorological station.

Table 1: Collected Air Temperature in ºC

Roof Height

Day 230.431.430.8

Roof with water proofing limeRoof Height

Day 233.333.832.1






1m above the floor level It was observed that the

nearly constant in the . Only the wind speed was



ºC at different spots

Height

Day 2 30.4 31.4 30.8

Roof with water proofing lime-Roof Height

Day 2 33.3 33.8 32.1









different spots

Height

-terracingRoof Height




Outdoor air temperature was 1m above the roo




Solar radiation, and air temperature were also

different spots

18m

Day 3

terracing12m

Day 3

P3PP




Outdoor air temperature was roof level




Solar radiation, relative were also

different spots

18m

Day 330.930.930.8

terracing 12m

Day 332.733.231.8

P3 P2 P1

P3 P2 P1

he air temperatures for both indooelow the roof surface) and outdoor



Outdoor air temperature was level




relative were also

different spots

Day 3 30.9 30.9 30.8

Day 3 32.7 33.2 31.8

door elow the roof surface) and outdoor



Outdoor air temperature was level;




relative were also

Spot Location

Spot 4: Location

Spot 5: Location

Spot Location

Positions

Spot 4: Location

Positions

Spot 5: Location

Positions

Spot 3: Location

PositionsP1 P2 P3

Spot 4: Location

PositionsP1 P2 P3

Spot 5: Location

PositionsP1 P2 P3

Roof with Location Uttara

Positions

DLocation East Rajabazar

Positions

Roof with plant on bamboo structureLocation Uttara

Positions

Roof with Uttara

Double roofEast Rajabazar

Roof with plant on bamboo structureUttara

Roof with Uttara

Day 1

ouble roofEast Rajabazar

Day 1

Roof with plant on bamboo structureUttara

Day 1

Roof with W

Day 136.237

34.8

ouble roof East Rajabazar

Day 134.537.632.6

Roof with plant on bamboo structure

Day 135.235.234.3

Water body

Day 1 36.2 37

34.8

ouble roof with East Rajabazar

Day 1 34.5 37.6 32.6


Day 1 35.2 35.2 34.3

ater bodyRoof Height

with Corrugated sheet

Roof Height


Roof Height

21m

4.6

ater body Roof Height

Day 2

Corrugated sheetRoof Height

Day 2

Roof with plant on bamboo structureRoof Height

Day 2

21m

4.6m

21m

Roof Height

Day 236

36.934.7


Day 234.336

32.8


Day 235.835.134.2

21m

“Chatai” inside the Double Roof

m

21m

Roof Height

Day 2

36.9 34.7


Day 2 34.3

32.8


Day 2 35.8 35.1 34.2


Roof Height




21m

Day 3

Corrugated sheet 4.6m

Day 3

Roof with plant on bamboo structure 21m

Day 3

“Chatai” inside the

21m

Day 336.537 35

4.6m

Day 333.836.432.7

21m

Day 336.336.134.2

P3

P1P2


Day 3 36.5

Day 3 33.8 36.4 32.7

Day 3 36.3 36.1 34.2

P3

P1 P2


P1P2P3

P3

P1 P2

P1 P2 P3

Spot Location

TEMPERATURE DATA ANALYSISTable roof surface and indoor for Table 2

below the outdoor temperature and roof surface temperature, except in Spot 1 (roof). Roof surface temperature for all the cases were higher than, either the outdoor or indoor

33.5

34.5

35.5

36.5

37.5

33

33.5

34

34.5

35

35.5

36

36.5

Spot Location

Positions

TEMPERATURE DATA ANALYSISTable roof surface and indoor for

Table 2

The results show that indoor temperature is always

below the outdoor temperature and roof surface temperature, except in Spot 1 (roof). Roof surface temperature for all the cases were higher than, either the outdoor or indoor

30

30.2

30.4

30.6

30.8

31

31.2

31.4

31.6

33.5

34

34.5

35

35.5

36

36.5

37

37.5

33

33.5

34

34.5

35

35.5

36

36.5

Spot 6: Location

PositionsP1 P2 P3

TEMPERATURE DATA ANALYSISTable 2 shows roof surface and indoor for

Table 2: Changing Pattern of Temperature

The results show that indoor temperature is always below the outdoor temperature and roof surface temperature, except in Spot 1 (roof). Roof surface temperature for all the cases were higher than, either the outdoor or indoor

Outdoor Temp (P1)

30.8

31.4

31.2

Day 01

36.2

37

34.8

Day 01

35.235.2

34.3

Day 01

Roof with pot plantsLocation West Rajabazar

Positions

TEMPERATURE DATA ANALYSISshows

roof surface and indoor for

: Changing Pattern of Temperature

Spot 1:

Spot 3:

Spot 5:


Outdoor Temp (P1)

Day 01

Day 02

Roof with pot plantsWest Rajabazar

TEMPERATURE DATA ANALYSISshows



Spot 1:

Spot 3:

Spot 5:


Outdoor Temp (P1)

30.4

31.4

30.8

Day 02

36

36.9

34.7

Day 02

35.8

35.1

34.2

Day 02


Day 1

TEMPERATURE DATA ANALYSISshows the changes of temperature



Spot 1:

Spot 3:

Spot 5:


Outdoor Temp (P1)

Day 03

36.3

36.1

34.2

Day 03


Day 134

35.133.2

TEMPERATURE DATA ANALYSISthe changes of temperature




30.9

30.8

Day 03

36.5

37

35

Day 03

36.1

34.2

Day 03


Day 1

35.1 33.2





Roof surface Temp (P2)

Roof with pot plants


roof surface and indoor for the




12m

Roof with pot plants Roof Height

Day 2


the period


The results show that indoor temperature is always below the outdoor temperature and roof surface temperature, except in Spot 1 (the basic roof). Roof surface temperature for all the cases were higher than, either the outdoor or indoor


30

31

32

33

34

35

36

37

38

31.5

32

32.5

33

33.5

34

34.5

35

35.5

36

12m

Roof Height

Day 234.935.133


period


The results show that indoor temperature is always below the outdoor temperature and roof surface

the basic roof). Roof surface temperature for all the cases were higher than, either the outdoor or indoor


34.5

37.6

32.6

Day 01

34

35.1

33.2

Day 01

Roof Height

Day 2 34.9 35.1 33

TEMPERATURE DATA ANALYSIS the changes of temperature

period for the six cases.


Spot 2:

Spot 4:

Spot 6:


the basic roof). Roof surface temperature for all the cases were higher than, either the outdoor or indoor


34.5

37.6

32.6

Day 01

35.1

33.2

Day 01

Roof Height

the changes of temperature

for the six cases.

Spot 2:

Spot 4:

Spot 6:


the basic flat concrete roof). Roof surface temperature for all the cases were higher than, either the outdoor or indoor

Indoor Temp (P3)

34.3

36

32.8

Day 02

34.9

35.1

33

Day 02

12m

Day 3

of outdoor, for the six cases.

Spot 2:

Spot 4:

Spot 6:


flat concrete roof). Roof surface temperature for all the cases were

temperature

Indoor Temp (P3)

Day 02

12m

Day 334.535.533.1




temperature

Indoor Temp (P3)

33.8

36.4

32.7

Day 03

34.5

35.5

33.1

Day 03

PP3P

Day 3 34.5 35.5 33.1




temperature

Indoor Temp (P3)

Day 03

P1 P3 P2

of outdoor,



temperature

Indoor Temp (P3)

values, except in Spot 5 (plant on bamboo structure), where outdoor temperature was seen to be higher than the other values. COMPARATIVE ANALYSIS As thedifferent survey dayused for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface temperatures wdifferent roof by thesurface and indoorvalue

Figure

corrugated sheet and thin bamboo layer)highest temperature highestwith water body)displayed significant temperature differences. The results on the other two days were consistent with the above. perform

at 5 (roof with plant on bamboo structure)concrete roof)are contextmost comost according to this study

reducedmeans to allow cooler interiorstemperaturetemperature have been

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

values, except in Spot 5 (plant on bamboo structure), where outdoor temperature was seen to be higher than the other values.

COMPARATIVE ANALYSIS As thedifferent survey dayused for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface temperatures wdifferent roof by thesurface and indoorvalue.

Figure

This corrugated sheet and thin bamboo layer)highest temperature highestwith water body)displayed significant temperature differences. The results on the other two days were consistent with the above. perform

This

at Spot (roof with plant on bamboo structure)

concrete roof)are considered less contextmost comost according to this study

Undoubtedly

reducedmeans to allow cooler interiorstemperaturetemperature have been

0.20

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Flat Concrete


COMPARATIVE ANALYSIS As there were variations in thedifferent survey dayused for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface temperatures wdifferent roof by the differencesurface and indoor

.

Figure 5: Temp. Differences between roof surface and

This comparisoncorrugated sheet and thin bamboo layer)highest temperature highest difference of 4with water body)displayed significant temperature differences. The results on the other two days were consistent with the above. So these three treatmentperform more efficiently in Dhaka

This comparisonSpot 2 (roof with plant on bamboo structure)

concrete roof)considered less

context. most common most incompetent according to this study

Undoubtedlyreduced, means to allow cooler interiorstemperaturetemperature have been

0.20.6

0.1

Flat Concrete Roof

Spot 1


COMPARATIVE ANALYSIS re were variations in the

different survey dayused for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface temperatures wdifferent roof

differencesurface and indoor

: Temp. Differences between roof surface and

comparisoncorrugated sheet and thin bamboo layer)highest temperature

difference of 4with water body)displayed significant temperature differences. The results on the other two days were consistent with the

So these three treatmentmore efficiently in Dhaka

comparison (roof with water

(roof with plant on bamboo structure)concrete roof)

considered less As

mmon incompetent

according to this study

Undoubtedly if attention is paid to

means to allow cooler interiorstemperature temperature have been

0.1

Flat Concrete Roof with Water

Spot 1



different survey dayused for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface temperatures wedifferent roof treatments (Fig. 5)

differencesurface and indoor



difference of 4with water body)displayed significant temperature differences. The results on the other two days were consistent with the


comparison(roof with water

(roof with plant on bamboo structure)concrete roof) respectively.

considered less s the

mmon roof type incompetent

according to this study

Undoubtedlyattention is paid to

means to allow cooler interiors differences between surface and interior

temperature have been

1.2

1.7

Roof with Water proofing

terracing

Spot 2



different survey dayused for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface

ere examined. treatments (Fig. 5)

differences surface and indoor



difference of 4with water body) and displayed significant temperature differences. The results on the other two days were consistent with the


comparison(roof with water

(roof with plant on bamboo structure)respectively.

considered less the flat concrete roof type

incompetent according to this study

Undoubtedly, significantattention is paid to

means to allow cooler interiorsdifferences between surface and interior

temperature have been

1.71.4

Roof with Water proofing-Lime

terracing

Spot 2

values, except in Spot 5 (plant on bamboo structure), where outdoor temperature was seen to be higher than


different survey days, exact temperature values used for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface

re examined. treatments (Fig. 5)

between the temperature of roof surface and indoor, as an indicator of its insulation


comparison showscorrugated sheet and thin bamboo layer)highest temperature difference

difference of 4and S

displayed significant temperature differences. The results on the other two days were consistent with the


comparison also (roof with water


considered less efficient flat concrete

roof type incompetent in

according to this study,

significantattention is paid to


temperature have been presented in

2.2

Roof with Water Roof with Water

Day 01



, exact temperature values used for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface


between the temperature of roof , as an indicator of its insulation


showscorrugated sheet and thin bamboo layer)

differencedifference of 4 deg

Spot displayed significant temperature differences. The results on the other two days were consistent with the


also shows, (roof with water-proofing lime terracing)


efficient flat concrete

roof type in Dhaka lower the

significantattention is paid to


presented in

2.2 2.22

Roof with Water Body

Spot 3

Day 01







shows thatcorrugated sheet and thin bamboo layer)

differencedegC

pot 6 (roof with pot plants) also displayed significant temperature differences. The results on the other two days were consistent with the


shows, proofing lime terracing)

(roof with plant on bamboo structure)respectively. Thus,

efficient as insulationflat concrete

in Dhaka lower

he result is

significant energy consumption can be attention is paid to


presented in

2

Roof with Water Corrugated Sheet with Double Roof

Day 02




re examined. The effecttreatments (Fig. 5)



that Spot 4corrugated sheet and thin bamboo layer)

difference on (roof with pot plants) also

displayed significant temperature differences. The results on the other two days were consistent with the

So these three treatments at Spotmore efficiently in Dhaka

shows, proofing lime terracing)

(roof with plant on bamboo structure)Thus,

as insulationflat concrete roof

in Dhaka lowering

result is

energy consumption can be attention is paid to proper roofing


presented in

4

2.2

Corrugated Sheet with Double Roof

Spot 4

Day 03


re were variations in the temperatures of the


The effecttreatments (Fig. 5) was



pot 4corrugated sheet and thin bamboo layer)

in general with the day 01

(roof with pot plants) also displayed significant temperature differences. The results on the other two days were consistent with the

s at Spotmore efficiently in Dhaka’s climate

shows, lowest difference was proofing lime terracing)

(roof with plant on bamboo structure) these three treatments

as insulationoof,

in Dhaka [4], and is ing indoor temperature

result is alarming

energy consumption can be proper roofing


presented in Table

2.2

2.7


Spot 4

Day 03


temperatures of the , exact temperature values

used for the analysis. In such a situation, the differences between the indoor, outdoor and the roof surface

The effectwas, therefore, judged



pot 4 (double roof with corrugated sheet and thin bamboo layer)

in general with the day 01


s at Spot’s climate

lowest difference was proofing lime terracing)

(roof with plant on bamboo structure), and Spot these three treatments

as insulation, happens to be, and is

indoor temperaturealarming


means to allow cooler interiors. differences between surface and interior

Table

0.9



Spot 5


temperatures of the , exact temperature values


The effectivenesstherefore, judged



(double roof with corrugated sheet and thin bamboo layer) displayed the

in general with the day 01. S


s at Spots ’s climate.


, and Spot these three treatments

as insulation in happens to be, and is

indoor temperaturealarming


The averagedifferences between surface and interior

Table 3.

0.9 0.9

1.9


Spot 5


temperatures of the , exact temperature values were


ivenesstherefore, judged



(double roof with displayed the

in general with the Spot


3,


, and Spot these three treatments

in the happens to be, and is deemed

indoor temperaturealarming.

energy consumption can be proper roofing, as passive


1.91.9


Roof with Pot


temperatures of the were


iveness of therefore, judged


: Temp. Differences between roof surface and indoor


in general with the pot 3 (roof


, 4 and

lowest difference was proofing lime terracing), Spot

, and Spot 1these three treatments

the Dhakahappens to be

deemedindoor temperature

energy consumption can be , as passive


1.92.1

Roof with Pot Plants

Spot 6


temperatures of the were not


of the therefore, judged


indoor


in general with the (roof


and

lowest difference was , Spot 1 (flat

these three treatments Dhaka

happens to be the deemed the

indoor temperature



2.4

Roof with Pot Plants

Spot 6


temperatures of the not


the therefore, judged



in general with the (roof


and 6

lowest difference was , Spot

(flat these three treatments

Dhaka the the

indoor temperature


The average differences between surface and interior

Table 3: Average Temperature difference between surface and interior spaces

Spot 1: Spot 2:

Spot 3: Spot 4:

Spot 5: Spot 6:

REMARKS The ranking of thermal performance of roofs (Fig. 6) has been suggested on the basis of average temperature difference identified in this investigation.

Figure 6: Ranking of Roof Treatments on Thermal performance

Double Roof (Ranking 1) displayed highest efficiency at reducing internal heat gain. It lowered the interior temperature from the exterior value by about 3 degC, generating temperatures 2.7 0C lower than the most commonly used flat concrete roof. The roof with water body (Ranking 2), reduced the internal temperature by about 2.1 degC, producing interiors with temperatures 1.8 ºC lower than with the flat-concrete roof. The Water content has a positive impact on heat transfer reduction. Roof having pot plants, sharing the same ranking (Ranking 2), also decreased temperatures

by about 2.1ºC. The soil content of the pots, increases the thermal capacity insulation value, and thus helps to a great extent to reduce the interior temperature. Roof with water proof lime terracing (Ranking 3), is much less effective as a passive control tool. The roof with plant on bamboo structure (Ranking 4), while creating a cooler microclimate on the roof, was not found effective as a passive cooling tool to reduce internal heat gain. Ranked at 5, the flat concrete roof was found to be the most inefficient at reducing internal heat gain, decreasing the temperature by only about 0.3ºC on average.

Of the types examined, two of them, the one with pot

plants (Ranked 2) and the other with bamboo structure (Ranked 5), are non-structural solutions and can be instituted on demand. It is obvious that the additional capacity insulation of the pot plants is more effective in controlling temperature, than the resistance insulation of the bamboo structure with plants.

Double roof with corrugated sheeting and bamboo

layer (Ranked 1), is not a common practice in urban Dhaka, and has potential only if redesigned to suit the context. The Roof with water body (Ranked 2) was found very effective in contributing to cooling of spaces, again by virtue of increased thermal capacity insulation. But a water body container requires a substantial initial budget, while also increasing the structural load on buildings. Therefore, this cannot be suggested as a viable strategy to adopt in general. Placing pot plants (Ranked 2) on the roof top, works effectively to reduce the internal air temperature and also creates cooler microclimate on the roof. But it occupies the flat roof top, which is used as a social space, and therefore not encouraged by the inhabitants. DOUBLE ROOF FOR URBAN RESIDENCES Given the above discussion, for the Dhaka context, it is recommended that double roof with a local material named “Chatai” (made of woven bamboo) may be installed under the ceilings of the top floors of residential buildings. The “Chatai” is a very low-cost material and adds negligible load to the structure. A similar scheme was conceptualized by Givoni in a previous study, for houses with metallic sloped roofs in developing countries [4, 20].

The suggestion here is for retrofitting the large amount of multistoried residences with flat roofs, with a layer of “Chatai”, forming a sort of false ceiling at the top floors of residences. If operable, the efficiency can be further increased. During the daytime, the space above it contains the heat, while during the night, when the concrete roof has cooled down, the ceiling can be adjusted.

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For new constructions (Fig. 7), the upper floor can be made higher than the other floors and the layer of “Chatai” can be installed below the roof (Type 01). Its efficiency can also be improved by installing an operable roof [4] at the roof level, which can further decrease heat transfer (Type 02). Cooling the roofs has potential benefits on energy demand and power peaks [21]. The additional double roof while providing some passive cooling, would also create a social space for the inmates to gather.

Figure 7: Installation of “Chatai” (Type 01) and both “Chatai” and “operable roof” (Type 02) for new construction. (Source: By Authors) CONCLUSION The selection of an adequate roof configuration influences the thermal condition of the space. But we do not have proper research-based building by-laws, to ensure desirable thermal conditions in urban residential environment [5]. The authors have monitored the thermal performance of roofs by observing the passive cooling capacity of various roof types. The outcome of this study may be implemented in roofs of residential buildings for achieving better thermal performance, thus, helping to reduce energy consumption of Dhaka city. The study can form baseline data, that can be used as a guideline for the formulation of Building Laws to ensure sustainable comfort conditions in residential buildings of Bangladesh. Appropriate computer simulation studies would help cross-checking with actual performance, for further authenticity and validity. ACKNOWLEDGEMENTS The Authors acknowledge support of the Department of Architecture, BUET, where the M.Arch course formed the impetus and inspiration of the research cited in this paper. REFERENCES 1. Ahmed, K.S., (1995). Approaches to Bioclimatic Urban Design for the Tropics with special Reference to Dhaka, Bangladesh. Ph. D. thesis (unpublished), Environment Studies Programme, Architectural Association. London,UK. 2. Roy, G.S., (2010). Thermal Environment in Residential Areas of Metropolitan Dhaka: Effects of Densification. M.Arch Thesis, Department of Architecture, BUET. Dhaka.

3. Renaud, B., (1984). Housing and Financial Institutions in Developing Countries. Prepared for International Union of Building Societies and Savings Associations. Chicago, Illinois. 4. Mridha, A. M.M.H., (2002). A study of Thermal Performance of Operable Roof Insulation, with special reference to Dhaka. M.Arch Thesis (Unpublished), Department of Architecture, BUET. Dhaka, Bangladesh. 5. Ahmed, Z.N., (1994). Assessment of Residential Sites in Dhaka with respect to Gains. Ph. D. Thesis (unpublished), De Montfort University in Collaboration with the University of Sheffield, UK. 6. Roche1,P.,L., Carbonner, E., Halstead, C., (2012). Smart Green Roofs: Cooling with variable insulation. PLEA2012 - 28th Conference. Lima, Perú, November 7-9. 7. IPCC (2007). Climate Change 2007. The physical science basis: IPCC Working Group I Fourth Assessment Report: Summary for Policymakers, IPCC. Geneva. 8. Huq, S., (2001). Climate Change and Bangladesh. Science, 2001(294): p. 1617. 9. UNDP (United National Development Programme), (2007). Country-in-focus: Bangladesh. UNDP RCC web bulletin (2). 10. Shajahan, A., (2012). An Investigation of indoor Thermal Comfort Range for Rural Houses of Dhaka Region. M. Arch Thesis(Unpublished), Dept. of Architecture, BUET, Dhaka. 11. Bangladesh Meteorological Department, Climate Division, Agargoan, Dhaka, 2012. 12. Divya, Mehritra, R. (1995). Climate Change and hydrology with emphasis on the Indian subcontinent. Hydrologic Sciences Journal, 40: p. 231-241. 13. Mojumder, A.U., (2000). Thermal Performance of Brick Residential Buildings of Dhaka City. M. Arch. Thesis (unpublished), Department of Architecture, BUET. Dhaka. 14. Hossain, M.M., Towards Sustainable Urban Environment: An Investigation on the Relationship between Electrical Energy Consumption and Urban Morphology in Context of Dhaka City. Unpublished Report, Department of Architecture, BUET. Dhaka, Bangladesh. 15. The World Bank (2011), Bangladesh: Lighting Up Rural Communities, [Online], Available:http://www.worldbank.org.bd/WBSITE/EXTERNAL/COUNTRIES/SOUTHASIAEXT/BANGLADESHEXTN/0,,contentMDK:22888125~menuPK:295765~pagePK:2865066~piPK:2865079~theSitePK:295760,00.html. [ March, 2012]. 16. Ahmed, Z.N., (1987). The Effects of Climate on the Design and Location Buildings in Bangladesh. M. Phil. Thesis from Sheffield City Polytechnic with the University of Sheffield, UK. 17. Supic, P, (1982) .Vernacular architecture: a lesson of the past for the future, Energy and Buildings, 5: p. 43–54. 18. Sharples, S., Malama, A. (1996). Thermal performance of traditional housing in the cool season in Zambia. Renewable Energy, 8: p. 190–193. 19. Li, J. (1996). The bioclimatic features of vernacular architecture in China. Renewable Energy, 8: p. 629–636. 20. Givoni, B., (1994). Passive and Low Energy Cooling of Buildings. Van Nostrand Reinhold, New York. 21. Akbari, H.; Pomerantz, M. and Taha, H. (2001). Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy, 70: p. 295-310. 22. Mallick , F. H., (1994). Thermal Comfort for Urban Housing in Bangladesh. Ph.D.Thesis (Unpublished) Environment and Energy Studies programme, Architectural Association, London.

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Chatai Operable Roof Chatai

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