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The most important factor affecting thermal comfort of people in hot humid climate like Malaysia is high level of solar radiation, air temperature and humidity. This country exposed towards solarradiation more than 10 hours/day throughout the year. The solar radiation propagated heat in the indoor spaces through the building envelope.Low slope roof in Malaysia typically trap heat due to the absence of roof ventilation, the heat takes long time to be released. The heat will be transmitted to the area underneath the roof and affect the living space area. For this study, 10° roof angle with 50% and 100% opening had been chosen as retrofit roofing system to investigate the effect of the roof openings towards the air temperature underneath. 0% roof opening (fully closed) roofing system has been chosen as the basecase studyfor this experimental work. The results obtained indicate that the existence of roof opening is crucial to reduce the air temperature in the living area. By having roof opening, the air temperature could be improved about 3 to 6°C for 50% opening and 2 to 10°C for 100% for 10° roof angles. The roof opening has a great potential to create the heat losses and allow the air movement through the roof and indirectly decrease the indoor air temperature.
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International Journal of Applied Engineering Research
ISSN 0973-4562 Volume 9, Number 23 (2014) pp. 20099-20110
© Research India Publications
http://www.ripublication.com
Paper Code: 28423 - IJAER
Study on the Effect of the Roof Opening on the Temperature
Underneath
Ibrahim, S.H.1, Azhari, N.A.2, Nawi, M.N.M.3, Baharun, A.4 and Affandi, R.5
1S.H., Ibrahim, Senior Lecturer, Department of Civil Engineering,
Faculty of Engineering, Kota Samarahan, Sarawak,
Malaysia, 94300 Kota Samarahan,
Sarawak, Malaysia (corresponding author, phone: 082-58 3283;
e-mail: ihalipah@feng.unimas.my).
2N.A. Azhari. undergraduate student, Department of Civil Engineering,
Faculty of Engineering, Kota Samarahan, Sarawak, Malaysia
3M.N M. Nawi,., Senior Lecturer, School of Technology Management and Logistic,
Universiti Utara Malaysia, 06010 Sintok, Kedah, Malaysia
4A Baharun,. Associate Professor, Department of Civil Engineering,
Faculty of Engineering, 94300 Kota Samarahan, Sarawak, Malaysia,
5R, Affendi, Lecturer, Department of Civil Engineering, Faculty of Engineering,
94300 Kota Samarahan, Sarawak, Malaysia
Abstract
The most important factor affecting thermal comfort of people in hot humid
climate like Malaysia is high level of solar radiation, air temperature and
humidity. This country exposed towards solarradiation more than 10 hours/day
throughout the year. The solar radiation propagated heat in the indoor spaces
through the building envelope.Low slope roof in Malaysia typically trap heat
due to the absence of roof ventilation, the heat takes long time to be released.
The heat will be transmitted to the area underneath the roof and affect the
living space area. For this study, 10° roof angle with 50% and 100% opening
had been chosen as retrofit roofing system to investigate the effect of the roof
openings towards the air temperature underneath. 0% roof opening (fully
closed) roofing system has been chosen as the basecase studyfor this
experimental work. The results obtained indicate that the existence of roof
opening is crucial to reduce the air temperature in the living area. By having
roof opening, the air temperature could be improved about 3 to 6°C for 50%
opening and 2 to 10°C for 100% for 10° roof angles. The roof opening has a
20100 Ibrahim, S.H. et al
great potential to create the heat losses and allow the air movement through
the roof and indirectly decrease the indoor air temperature.
Keywords: Roofing system, heat, ventilation, hot humid climate, indoor air
temperature
INTRODUCTION
Malaysia which located in hot humid climate exposed with the solarradiation more
than 10 hour/day throughout the year. The sun radiation propagated heat in the
interior spaces through the building envelope. The most critical part of the building is
the roof when this area heatedbysolar radiation and high emissivity. Heat accumulated
in the roof influence the indoor temperature and indirectly affects the indoor comfort
conditions of the occupants, especially during daytime. In order to maintain the indoor
temperature, most of the people use the mechanical means such as air-chiller unit or
fan that for sure will incur cost [i]. Study had been conducted in hot-humid region
shows that thermal comfort in the building could not be achieved most of the time
unless when the mechanical cooling is used [ii;iii;iv].
Building envelope has play important roles to determine the quality of indoor
conditions regardless of transient outdoor conditions. Because of being in direct
interaction with the external environment conditions, building envelope is defined as
the interface of energy loses or gains. In order to reduce energy usage in buildings, the
energy requirements of buildings system must be minimized. The efficiency of energy
use must be increased and in this context, energy efficient retrofit of building
envelope can be implemented with the improvement of thermo physical properties of
the envelope. Several researchers had carried out studies on the improvement of
building envelope and their impact on building energy usage [v;vi]. This kind of
studies suggested different type of retrofit design such as applyingdifferent types of
glasses for windows, using recycling cooling water catchment system, usinga well-
insulated and sealed building envelope with appropriately selected materials to
minimize heating and cooling loads[vii;viii;ix]. While the building envelope retrofit
design should reduce heating and cooling loads to aminimum level, it must also be
practical to build and make sense from a cost and functional standpoint.
The roof is the building envelope mostexposed to the climate elements.The
impactof solar radiation on clear days has effects on the roof more thanany other part
of the building components.In the hot humid regions, the roof hastremendous effects
on the indoor climate and it is generally believed that the roof is the main heating
element in the building. Infact, the roof serves as a potential route ofheat gain which
is mostly dependent on its angles and materials due toincorrectdesigned [x].
Theinference has greatereffecton indoorclimate.
Ramly & Hussain [xi]and Ibrahim [xii] stated that the roof angles affect the
performance of thermal comfort inside the house. Normally, the low-slope roof is
exposed to the full amount of horizontal solar radiation. Since the surfaces are opaque
to thermal radiation, the solar radiation that is not reflected is absorbed and
transmitted to the area underneath.Study conducted by Masiri et al.[xiii]show that
Study on the Effect of the Roof Opening on the Temperature Underneath 20101
recycling cooling water catchment system on the zinc roof can reduce the air
temperatures to 31°C. The concept of recycling cooling water catchment system is
similar to the roof pond system where the experiment was conducted by Wongsuwan
et. al.[xiv]. It was shown that the roof pond system reduced temperature inside the
experimental house by approximately 2°C to 4°C for a 12 hour period. However the
designs rely on the energy usage to operate the pumping system. The use of water
ponds on roofs for house cooling in arid regions is well known [9;xv;xvi]. The water is
cooled during the night by evaporation and radiation heat losses, and is protected
against solar irradiation. Although the study shows positive result, the roof pond only
practical under usual sky conditions and awareness should be taken on esthetical
value and wind damage.
A number of studies had been done by researchers in investigating the type of
roofing system such as a green roof [xvii;xviii]. The results indicate that green roof
performed better compared to asbestos, clay and flat roof.Wonget. al[xix]reported that
the green roof in Singapore helped to reduce the thermal radiating effect experienced
with bare roof. However, green roofing system demand high maintenance and it has
to consider a heavy load is added to the roof structure. The analysis in high-rise
buildings that have small roof area to total facade area ratio, shows that instead of
green roof applications, painting the roof with light colors in warm climates and dark
colors in cold climates is more effective in terms of energy and cost efficiency [xx;xxi].
Clay and concrete tile amongst the popular materials used for the roofing system for
most of the residential building in Malaysia. This material is definitely slow the heat
absorbance during the day and radiates underneath during night-time. However, this
material is muddle and requires maintenance, and leads to higher cost.Heat is
transmitted through air cavity by convection and radiation. Malaysian attics have
become less and less ventilated with intention to prevent the spread of fire, keep out
the rain, birds, and vermin. Thus, the moisture that escapes through the ceiling is not
removed and create condensation problem. The air space above the ceiling insulation
should be provided to allow air to circulate over the insulation and carry away
moisture escaping through the ceiling from the house.
Advanced stack ventilation strategies that has been developed and tested have
been discussed by Ismail and Abdul Rahman [xxii] includes solar induced ventilation,
wind-stack driven ventilation and fan induced stack ventilation. They also
investigated the performance of hybrid solar-wind turbine ventilator (HTV) in the real
building and under real weather conditions by conducting full-scale field
measurement. The study revealed that an innovation of ventilators devices which uses
both wind and solar energy are not only succeeded to increase the ventilation rate, but
also significant to ensure more consistent rotation of the device, thus make it more
reliable to be used even in the low wind velocity region. However, this mechanical
ventilation system does not work as expected. Normal operation may be interrupted
for numerous reasons including equipment failure, poor design and maintenance. In
addition, the installation and particularly maintenance cost for the operation may be
very high.
Hence, design of a retrofit roofing system is the effective method to improve
energy consumption and provides a proper ventilation to release the heat gain.
20102 Ibrahim, S.H. et al
Traditional Malay house which commonly using the nipa palm leaves(Nypa frutican.
sp) as roof material is one of the ideal design for hot humid climate andthe concept
has been used as good reference for most of the designer in hot humid region [18;xxiii].
The traditional concept of roof has prevailed without changes for a very long time.
The traditional way shows good performance of the roofs in terms of release the heat
from the internal area of the roof; however, through the passage of time, the
production of nipa for roofing has lessened. In this study, the similar concept has been
applied to the system of new design by using corrugated metal roofing.Therefore
corrugated metal roofing is the best option since the material is available in the
market.This technological evolution in roof systems opens an opportunity window for
new designs.
Description of Model Testing
A small scale model was constructed with one side of the roof with an adjustable tilt
angle of 10° with 0%, 50% and 100% openingsof batten width to investigate the
influence of air movement on the heat transfer thought the roof as shown in Figure 1,
2 and 3. The differences opening of batten were tested in order to investigate which of
the opening ventilation are more effective to maximize the air ventilation. This
opening was designed to releases the heated air from the roof system by the mean of
natural ventilation. The small scale model had dimensions of 0.5 m x 0.5 m and height
is 1.0 m, and the walls were made by plywood of 5 mm thickness. No windows or any
openings on all facades, except at the front wall.Figure4 shows the location for
measurement inside the model house. The thermocouples were placed to measure the
surface temperature and air temperature (Elevation 1) located 300mm from the
ground level.
Figure 1: 0% Roof Opening (Fully closed)
Study on the Effect of the Roof Opening on the Temperature Underneath 20103
Figure 2: 50% roof opening
Figure 3: 100% roof opening
Figure 4: the location for surface and air temperature measurements inside the
model house.
20104 Ibrahim, S.H. et al
Testing Methodology
The model was located at the open space area under the normal condition as shown
Figure 5. The air temperatures inside the roof were measured using thermocouples.
The model testing was instrumented using T-type thermocouples that connected to
data logger. These were installed to measure air and surface temperatures at the
selected locations as shown in Figure 4. Air temperature measurements were taken
using suspended thermocouples. The outdoor temperatures and air velocities were
measured using anemometer with 1m distance from the model house.A number of
experimental works had been conducted for this study but in this paper only three (3)
experiments will be discussed. These experiments using different size roof openings
for 10°roof angleswere chosen to investigate the effect of retrofit design towards the
air temperature and air velocity inside the roof. Surface temperatures, air temperatures
and air velocities were measured for everyexperiment at Elevation 1. The wind
velocity and direction outside the building were measured at a 1.00 m distance from
the model testing by the time measured wind velocity under the roof surface. Before
measurements for each experimental configuration were taken, a simple smoke test
was conducted inside the model to monitor the airflow.
Figure5:Instrument of Data Collection.
Result and Analysis
Figure 6, 7 and 8show the surface and air temperaturesof 10° roof angle for 0% (fully
closed), 50% and 100% opening. The results show that regardless of the percentage of
the roof opening, the highest value of roof surface temperature occurred at 12.00 noon
and higher than the air temperature at Elevation 1. This condition happened due to
higher solar radiation in the midday compared to early morning or late afternoon.
During midday, the sun is positioned high in the sky and the path of the sun's rays
through the earth's atmosphere is shortened. Consequently, less solar radiation is
scattered or absorbed, and more solar radiation reaches the earth's surface.The data
indicate that the air temperature inside the model house influenced by the roof surface
temperature.
Study on the Effect of the Roof Opening on the Temperature Underneath 20105
Referring to Figure 6, the roof surface temperature could reach until 62.4°C at
12.00 pm and air temperature around 45°C. Due to the absence of permanent
ventilation in the roof, the outside air velocity could not contribute to any internal air
movement. Therefore the heat was trapped inside the roof and affects the air
temperature below.
Figure 6:Comparison of 0% Roof Opening between Roof Surface and Elevation
1
Figure 7and 8show the result for roof surface and air temperatures at Elevation 1
for 50% and 100% roof openings. It can be seen that the size of the openings play an
important role as a source of air movement. By providing opening in the roof, the
airflow increased and reduces the roof surface and air temperature at Elevation 1.
Data presented in Figure 7 and 8 show that air velocities improved between 0.24-
0.36m/sdue the existence of the roof openings. When 50% and 100% roof opening
were tested, the wind velocity is significantly increases. Therefore, wind velocity in
100% opening is higher than 50% due to the increment of percentage of the opening.
Thus, the ventilation of the roof can be improved to overcome the inadequacy of
natural ventilation.
20106 Ibrahim, S.H. et al
Figure 7:Comparison of 50% Roof Opening between Roof Surface and Elevation
1
Figure 8:Comparison of 100% Roof Opening between Roof Surface and
Elevation 1
Figure 9 shows the comparison of surface temperature distributions of roof,
using different percentage of opening for 10 degree angle of the roof. The highest
surface temperature occurred when the roof is fully-closed (0%), followed by 50%
roof opening and the lowest is for 100% opening.
Study on the Effect of the Roof Opening on the Temperature Underneath 20107
Figure 9:Comparison of surface temperature for different percentage of roof
opening
It can be seen in Figure 10 that the air temperature at 0% during peak hour is
about 46°C. However, by having opening as the retrofit design for roofing system, the
air temperature could be improved by about 3 to 6°C for 50% opening and 2 to 10°C
for 100%. Heat accumulate in model testing can reduce by natural ventilation through
the roof opening. Generally, it is found that increasing roof opening has reduced the
indoor temperatures. The inside air temperature showed a distinct improvement.
Therefore, it shows that the effect of roof ventilation is based on percentage of
opening.
20108 Ibrahim, S.H. et al
Figure 10:Comparison of air temperature at Elevation 1 for different percentage
of roof opening
Conclusion
The results show that the indoor air temperature highly influenced by the roof surface
temperature and air velocity inside the model house. These parameters affected the air
temperature through the process of conduction and radiation. Based on the result of
the base case (fully closed), which the experiment was conducted in stagnant
condition, the indoor temperature at Elevation 1 can reached 46°C during the peak
hour. The results indicate the temperature at Elevation 1could be reduced about 4-8°C
by providing the roofopening. The roof performance has a great potential as it is able
to create the heat losses through the roof. The result shows that the 100% roof
opening gives the best performance to increase indoor temperature..
It demonstrates that building the retrofits design on roof ventilation can lead to
significant reductions in air temperature underneath. This design could lead to low-
cost roofs that also provide passive cooling system even cheaper than conventional
roofs. The design has followed a holistic approach to achieve a new roof paradigm. In
this sense, this design is well adapted to a new generation of environmentally friendly
buildings. However, the retrofit designs on roof ventilation not often undertaken due
to lack of research conducted on this area. However the design should be carefully
assessed so that the costs and benefits are understood and that appropriate measures
are taken to ensure that the long-term moisture performance of the retrofitted roof
assembly is not adversely affected.
Study on the Effect of the Roof Opening on the Temperature Underneath 20109
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... The recommended thermal comfort level ranges from 25 to 28°C . Based on a previous study on concrete terrace houses in Malaysia, indoor temperatures are only comfortable to the occupants for a few hours every day (Ibrahim et al., 2014a). The same study also discovered that the indoor temperature could reach more than 30°C during the daytime. ...
... Due to this, occupants rely on mechanical means to cool their houses, which incur energy consumption costs to power the electrical components. A previous study concluded that openings in the roof's surface could help to reduce the indoor air temperature (Ibrahim et al., 2014a). This study focuses on roofing because it plays an important role in controlling the amount of heat transmitted from the roof surface into the internal area. ...
... Natural ventilation offers a free cooling system and could help to provide comfort for the occupants (Heiselberg et al., 2001). Studies of roof ventilation have shown that openings in the roof can help to reduce the indoor temperature up to 8°C compared unventilated roofs (Ibrahim et al., 2014a). The openings in a roof can provide two ways of naturally transferring the hot air from the indoors to the outdoors, known as wind-driven ventilation and stack effect ventilation. ...
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... (continued on next page) • With 50% and 100% openings in the roof, room air temperature may be reduced up to 3-6°C and 2-10°C respectively (Ibrahim et al., 2014). (Ciampi et al., 2005;Ibrahim et al., 2014). ...
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... Researchers agreed on the capability of a ventilated roof approach to reduce the indoor temperature and minimize the amount of heat transfer (Endriukaityte et al., 2005;Trinuruk et al., 2007;Qasim et al., 2010;Al-Obaidi et al., 2014a;Ibrahim et al., 2014b). Proper design of a ventilated roof could also help in maintaining the indoor temperature within the thermal comfort zone, thereby reducing electricity consumption. ...
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... This proposed material has low emittance and thermal conductivity with high transmittance properties. Natural ventilation (Faggianelli et al., 2014;Ibrahim et al., 2014;Wan and Yik, 2004) and daylighting (Al-Obaidi and Mazran Ismail, 2014;Castanheira et al., 2015) is the most effective passive cooling technique. In a building design, average window to ground ratio should be around 0.44 for proper daylight and natural ventilation. ...
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The objective of this research was to provide the Room comfort by means of Solar Reflective Paint (SRP) on the roof and thereby reducing the consumption of electrical energy. Solar heat diffuses through roo f into the room resulting in human discomfort. Human discomfort leads to the usage of electrical energy. Valu able energy has to be conserved. Three different samples of solar reflective paint were collected and their properties were studied. The sample suitable for th e research was selected. A standard roof design was selected for analysis. Using the properties of sola r reflective paint and the roof design, a model was created and was analyzed by using ANSYS. The temperature of the living room was brought down considerably of about 7°C by implementing the solar reflecting pain t on the roof top. Power consumption can be drastic ally reduced by using SRP on industries as well as resid ential buildings. By using SRP, the usage of Air conditioning units can be minimized thereby keeping the global warming in control.
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Five identical test structures have been fabricated for studying passive techniques for better comfort conditioning in arid areas. As 50% of the heat load in the building is from the roof, therefore, it has been treated with different passive techniques, viz. painting of roof by white paint, thermal insulation underneath roof, shallow pond with movable insulation and soaked gunny bags with water. It has been found that the fall in roof and ambient temperatures inside the test structures were in increasing order for roofs treated with thermal insulation, painted with white paint, shallow pond with movable thermal insulation and evaporative cooling.
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Thermal models are presented for the typical methods of passive cooling of roof in the arid region of Rajasthan, India. The periodic analysis for hourly variation of roof cooling are presented for bare roof, insulation beneath the roof, evaporative cooling above the roof and roof pond with movable insulation system. Calculations were carried out for a typical summer day of May for Jodhpur (India). The models have been tested with the published experimental data of roof temperature. The predicted values of roof temperature obtained from the proposed models exhibited good agreement with the coefficient of correlation ranging from 0.94 to 0.96 and standard error 1.76-2.80. The 0.05 m of water depth of roof pond with movable insulation is appropriate for better comfort condition in arid region.
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