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Integration of thermal insulation coating and moving-air-cavity in a cool roof system for attic temperature reduction
Energy Conversion and Management 75 (2013) 241–248
Contents lists available at SciVerse ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Integration of thermal insulation coating and moving-air-cavity in a cool roof system for attic temperature reduction M.C. Yew a,⇑, N.H. Ramli Sulong a, W.T. Chong b, S.C. Poh b, B.C. Ang b, K.H. Tan b a b
Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history: Received 4 March 2013 Accepted 21 June 2013
Keywords: Air cavity Cool roof Environmental friendly Energy savings Thermal insulation coating Ventilated roof
a b s t r a c t Cool roof systems play a significant role in enhancing the comfort level of occupants by reducing the attic temperature of the building. Heat transmission through the roof can be reduced by applying thermal insulation coating (TIC) on the roof and/or installing insulation under the roof of the attic. This paper focuses on a TIC integrated with a series of aluminium tubes that are installed on the underside of the metal roof. In this study, the recycled aluminium cans were arranged into tubes that act as a movingair-cavity (MAC). The TIC was formulated using titanium dioxide pigment with chicken eggshell (CES) waste as bio-filler bound together by a polyurethane resin binder. The thermal conductivity of the thermal insulation paint was measured using KD2 Pro Thermal Properties Analyzer. Four types of cool roof systems were designed and the performances were evaluated. The experimental works were carried out indoors by using halogen light bulbs followed by comparison of the roof and attic temperatures. The temperature of the surrounding air during testing was approximately 27.5 °C. The cool roof that incorporated both TIC and MAC with opened attic inlet showed a significant improvement with a reduction of up to 13 °C (from 42.4 °C to 29.6 °C) in the attic temperature compared to the conventional roof system. The significant difference in the results is due to the low thermal conductivity of the thermal insulation paint (0.107 W/mK) as well as the usage of aluminium tubes in the roof cavity that was able to transfer heat efficiently. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Temperature rise increases the demand for air conditioning and energy consumption, leading to the increase of heat emission and further rise in temperature. Cool roof systems are an inexpensive method to save energy and to improve the comfort level in buildings located in hot climates. With today’s high energy prices, various studies have been carried out to reduce energy consumption by increasing energy efficiency. Undeniably, most buildings incur a number of considerable impacts to the environment. The emission of greenhouse gases and the formation of microclimates within urban areas are among the most prevalent [1]. The consumption of operational energy emits the most greenhouse gases during the entire lifetime of a building [2]. As a tropical country, Malaysia’s ambient air temperature and relative humidity lie in the ranges of 26–40 °C and 60–90%, respectively. This type of climate is non-conducive for human comfort and productivity since it leads to uncomfortable conditions [3].
⇑ Corresponding author. Tel.: +60 3 79676884; fax: +60 3 79675318. E-mail addresses: [email protected], [email protected] (M.C. Yew). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.06.024
In general, a common building is designed in such a way that roof tiles are at the uppermost region, followed by a gypsum board ceiling. The region between these two components is the attic space. Meanwhile, low cost houses and factories would have bare metal deck roofing. Concrete roof tiles are the most commonly used roofing material in Malaysia (85%) followed by clay tiles (10%) and metal deck (5%) [4]. Heat transmission through the roof can be reduced simply by providing insulation as a radiant and conductive heat barrier. In new roof constructions, aluminium foil glued to fibreglass or rockwool blankets are suspended directly under the roof with wire mesh. The benefit of ventilation on the thermal load in roofs has been the subject of numerous studies. Airflow in the cavity effectively carries heat and moisture to the outside and keeps the internal part of the roof cool and dry [5–9]. Roofs contribute tremendously to building heat gain compared to vertical surfaces such as walls, mainly because the roofs are exposed to the sun throughout the daytime [10]. Residential buildings in Malaysia, especially the low rise buildings are found to undergo high intensity heat transmission from the building envelope, whereby the roof represents around 70% of heat gain [11]. Heat from solar radiation is absorbed by the roofs and is transmitted through and trapped in the attic, resulting in a hot ceiling.
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The hot ceiling then radiates heat to the occupants inside a building. In buildings without a ceiling, heat from the roof is exposed directly to the occupants below [3]. Overall, the increase in both the building surface temperatures and the air temperature inside the buildings cause thermal discomfort for users especially for those in buildings without air conditioning. This results in higher energy demand for cooling and ventilation systems [12]. Both systems create two major problems whereby air conditioning systems require enormous electrical energy and thus increase the utility bills. Several research programmes have been carried out regarding insulation and reflective materials in roof designs. Ong concluded that insulation under the tile is preferred to above the ceiling from his field testing of several passive roof designs [13]. His study also proved that the roof solar collector (RSC) resulted in the coolest attic among his prototype, about 13 °C less than the standard non-insulated roof during the period from 2 to 3 pm (the highest temperature in the attic throughout the day). Investigation of the most suitable location of insulation on the roof was performed by Ozel and Pihtili. They found that the best load levelling was to place three pieces of the same thickness insulation at the outdoor surface, middle and indoor surface of the 20 cm thick roof construction (the gap between the insulation pieces was 10 cm) [14]. While minimizing heat transfer into the attic, the design of multi-layer green roof panel is proposed to induce the ventilation process of the building. Khedari et al. and Hirunlabh et al. have carried out an experimental study of a RSC towards natural ventilation [15,16]. The results of the study concluded that the optimum length of RSC of about 100 cm with a tilt angle of 30° was able to induce natural ventilation of 0.08–0.15 m3 s1 m2. Akasaka and Takeda have intensively studied a practical heat transfer calculation method for development of roofs with ventilated air layers to evaluate quantitatively the shading and insulating effects of various combinations of techniques [17]. Al-Sallal [18], Taylor et al. [19] and Mathews et al. [20] showed that there are substantial energy savings in both winter heating and summer cooling with insulated ceilings. Hatamiour et al. have studied some useful ancient energy technologies that have been used for natural cooling of buildings during summer in a hot and humid province in the south of Iran [21]. Chong et al. have studied the moving air path in the air gap [22]. Based on their investigation into the mechanism of heat transfer of the multilayered green roof system, the movement of the surrounding air into the air gap space ensures no aggregation of hot air as well as draws out the interior hot air rapidly to prevent heating of the attic. Yokoyama et al. have investigated the prediction of energy demands using neural network with model identification by global optimization [23]. The use of reflective materials on the building envelope is one of the most efficient ways to reduce the roof temperature. According to Synnefa et al. [24], for peak solar conditions (about 1000 W/m2) for an insulated surface and under a low wind condition, the temperature of a black surface with solar reflectance of 0.05 is about 50 °C higher than ambient air temperature. For a white surface with solar reflectance of 0.8, the temperature rise is about 10 °C. Surface temperature measurements demonstrated that a cool coating can reduce the concrete tile surface temperature by 7.5 °C and it can be 15 °C cooler than a silver grey coating. This novel integrated cool roof design is more eco-friendly compared to other cool roof designs because it uses recycled aluminium cans and chicken eggshell (CES) waste. CES waste, a byproduct of the aviculture industry has been highlighted recently because of its reclamation potential. Most of it is discarded in landfills without further processing. It is known that CES waste contains valuable organic and inorganic components which can be utilized in commercial products to create new value in these waste materials. This study highlights a useful bio-filler derived from CES waste and its potential role in the coating industry. Although there
have been several attempts to use CES components for various applications [25–27], its chemical composition and availability makes CES a potential source of filler for polymer composites [28–30]. The other advantages of using CES are that it is available in bulk quantity, inexpensive, lightweight and environmentally friendly. The main objective of this study is to evaluate a system that combines TIC with aluminium tubes for cavity ventilation as well as to optimize the performance of the roofing systems in terms of heat reflection and heat rejection. The performance will be gauged by measuring the various temperatures of the roof and attic. The aim will be to obtain lower attic temperatures that will result in a more comfortable living environment. Four small roof models representing the different roof system designs were constructed to evaluate the resistance to heat gain. Components that were tested included the TIC, cavity ventilation (MAC) and attic inlet. The performance of the four designs (i) roof coated with normal paint without MAC (Design A), (ii) roof coated with TIC without MAC (Design B) and (iii) combination of TIC and MAC with closed attic inlet (Design C) and (iv) combination of TIC and MAC with opened attic inlet (Design D) were studied and compared. 2. The green roof design 2.1. Mechanism of heat transfer of the cool roof system The mechanism of heat transfer of the cool roof system is shown in Fig. 1. The temperature difference between the environment which is cooler than the aluminium tube on the underside of the roof causes the natural ventilation in the cool roof cavities. The thermal performance of the integrated cool roof system was analysed by estimating two control volumes by the following Eqs. (1) and (2). The first control volume (CV1): The heat transfer between the environment and thermal insulation coating and deck:
Q s ¼ Q Rad;Out þ Q Conv;Out þ Q Cond
ð1Þ
where Qs is the heat from the halogen light bulbs (W), QRad,Out is the radiation heat reflected from top of roof (W), QConv,Out is the convection heat transfer from top of roof (W), and QCond is the conduction heat transfer into the roof deck (W). The second control volume (CV2): The heat transfer between the roof deck and aluminium channel and indoors (attic):
Q Cond ¼ Q Rad;In þ Q Conv;In þ Q ve
Fig. 1. Mechanism of heat transfer of the integrated cool roof system.
ð2Þ
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where Qcond is the conduction heat transfer from deck into the roof (W), QRad,In is the radiation heat transfer to inside of roof (W), QConv,In is the convection heat transfer into inside of roof (W), and Qve is the ventilation heat transfer out of the roof (W). The exhausted heat from the air removed through the aluminium channel can be obtained from the following equation:
_ p ðT out T in Þ Q ve ¼ mC
ð3Þ
_ is the rate of mass flow (kg/s), Cp is the specific heat at atmom spheric pressure (J/kg K), Tin is the indoor air temperature (K), and Tout is the environmental temperature in the shade (K). By considering Eqs. (1) and (2), it is revealed that Qcond is the heat transfer from CV1 to CV2 via the roof deck as shown in Fig. 2. At CV1, heat transfer is decreased due to the reflective coating while at the roof deck, heat transfer is reduced because of its low thermal conductivity. At CV2, the reduction in heat transfer is attributed to the hot air being vented out through the aluminium channel. Besides the highly reflective layer of coating, choosing a material with low thermal conductivity for the roof deck or adding a thermal insulation layer to the roof deck is an effective method to reduce the heat transfer between CV1 and CV2. Consequently, the rate of heat transfer from the environment to the attic decreases [31].
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the TIC is designed to reduce the surface temperature of the roof by reflecting heat. The thermal insulation paint was formulated using titanium dioxide pigment and chicken eggshell waste as bio-filler bound together by a polyurethane resin binder with low thermal conductivity (0.65 W/mK). The addition of high purity reflective titanium dioxide pigment increases solar reflection which results in enhanced thermal insulating function. 2.2.2. Moving-air-cavity (MAC) This roof system is designed to integrate aluminium tubes that provide the cavity ventilation of the roof. The moving air gap in the aluminium tube forces the hot air to flow out through the cavity which is located on the underside of the metal roof. Chong et al. have studied the mechanism of heat transfer in the air gap as shown in Fig. 3 [22]. Hot air rises due to the buoyancy effect. Hence, the plumes of hot air will promptly enter the air gap. The moving air from the exterior guides the air in the space all the way to the ridge before being released to the exterior of the building. In this study, the dispersion of hot air from the aluminium tubes and opened attic inlet keeps the interior temperature low for human comfort. A comfortable condition reduces the usage of air-conditioning systems in a building, thus saving electricity costs. 3. Experimental design
2.2. Main features of the green roof system 3.1. Materials and method 2.2.1. Thermal insulation coating (TIC) TIC plays an important role in promoting measures against heat island phenomenon and limiting carbon dioxide emissions. The heat island phenomenon is air pollution caused by changes in the surface heat balance in which the temperature in an urban region rises above that of the surrounding area and it is isolated like an island. It is caused by reductions in greenery and water surfaces and by an increase in artificial heat resulting from energy consumption. There are a number of different thermal insulation materials used in the building industry today. Conventional materials, such as glass wool, rock wool, expanded polystyrene (EPS) and extruded polystyrene (XPS), require a thick building envelope to reach a sufficiently low thermal transmittance. The effect of using TIC on the roof surface with white solar-reflective paint is a very efficient way to reduce heat discomfort conditions for a single-story building located in cities with hot and humid climates. TIC has the potential to contribute to the reduction of heat island phenomenon as well as carbon dioxide emissions. TIC creates new opportunities for the design of energy efficient buildings. The demand for thermal insulation products have significantly increased because of the high energy prices. With the focus on cost efficiency, there is growing interest in the use of TIC. In this study,
The compositions of the different thermal insulation paint formulations are listed in Table 1. The polyurethane resin, CES bio-filler, titanium dioxide and dispersing agent were mixed until homogenous using a high-speed disperse mixer. The thermal conductivity of the thermal insulation paint was measured using a single-needle (KS-1) sensor in conjunction with the KD2 Pro Thermal Properties Analyzer. The single-needle sensor has a diameter of 1.3 mm with a length of 60 mm and its thermal conductivity is in the range of 0.02–2.00 W/mK. In this study, the lowest value of thermal conductivity of the thermal insulation paint formulations will be selected as TIC. The outlet airflow was measured using heavy duty hot wire thermo-anemometer airflow meter with telescoping probe which is designed to fit into small openings. 3.2. Experimental set-up The specimens were in the form of cubical boxes with monopitch roofs. Each box was custom-made using Perspex with a thickness of 3.0 mm and had dimensions of 350 mm (length) 350 mm (width) 350 mm (height). The angle of inclination of the roof was
Fig. 2. Heat flow diagram of the integrated cool roof system.
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Fig. 3. The moving air path in the MAC.
Table 1 Different formulations of thermal insulation paints. Component
Formula A composition (wt.%)
Formula B composition (wt.%)
Formula C composition (wt.%)
Polyurethane resin Titanium dioxide pigment ES bio-filler Dispersant Thermal conductivity (W/mK)
63.5 15
63.5 18
63.5 20
20 1.5 0.129
17 1.5 0.117
15 1.5 0.107
about 30 ° from the horizontal with a perpendicular height of 202 mm and the attic inlet was provided via an open hinged door with dimensions of 10 cm 10 cm on one side of the Perspex box as shown in Fig. 4. The metal roofing sheets with a thickness of 0.5 mm (dimensions: 500 mm 450 mm) were coated with a 0.2 ± 0.05 mm layer of normal or thermal insulation paint using a gun sprayer and the sprayed sheets are shown in Fig. 5. The thermal insulation paint that has been sprayed on the metal roof is referred to as the TIC. For the ventilation system, aluminium cans (diameter: 66 mm and length: 110 mm) were joined to form tubes using epoxy. Four
cans were used for each tube (Fig. 6). The tubes were arranged closely and installed to cover the entire area under the metal roof. The gap between the metal roof and Perspex box were sealed around the edges using cloth tape. This paper is a study on the TIC, MAC and attic inlet of this cool roof system and their roles in enhancing the effectiveness of the system. Four small scale roof models, each representing the various roof designs (A, B, C and D as shown in Fig. 7) were fabricated and tested side-by-side. The integrated cool roof experimental set-up is shown in Fig. 8. The RTD air probe and surface temperature sensor (accuracy ±0.5 °C) were employed for air and surface temperature measurement, respectively, as shown in Fig. 9. They were mounted to the upper and lower surface of the metal roofs by using aluminium tape. All data were logged at 1 min intervals using E-Log data logger. Data was recorded continuously until the temperatures became constant.
4. Results and discussion 4.1. Metal deck roof with normal coating (closed attic inlet) Results in Fig. 10 show that the maximum temperatures of the attic, roof upper and lower surfaces were 42.4 °C, 64.0 °C and 64.5 °C, respectively. The roof lower surface was 0.5 °C higher than the roof upper surface due to the heat trapped within the Perspex box, while the heat from the roof upper surface dissipated easily
Fig. 4. Dimensions of the small scale roof design and the measuring points.
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Fig. 5. The coated roof with (a) normal paint and (b) thermal insulation paint.
Fig. 6. Dimensions of the aluminium tube moving air channel.
into the surrounding environment. The temperatures became constant after 18 min of testing. The maximum difference in temperature between the attic and roof upper surface was 21.5 °C. The rate of roof temperature increase was 9.3 °C/min in the first 3 min. The rapid increase was due to the low reflectivity of the normal coating on the metal surface.
Fig. 7. Four small scale roof designs.
4.2. Metal deck roof with TIC (closed attic inlet) Results for this roof design are shown in Fig. 11. The roof upper and lower surfaces reached maximum temperatures of 60 °C and 57 °C, respectively. The roof lower surface was 3 °C cooler than the TIC coated upper surface as a result of the reflective barrier inhibiting heat transfer by thermal radiation from the upper to the lower surface. The rate of increase in roof temperature was about 5.8 °C/min in the first 3 min. The highest temperature reached in the attic was 33.9 °C. The temperature of the attic, roof upper and lower surface became constant after 19 min of testing. The maximum difference in temperature between the attic and roof upper surface was 26 °C for this cool roof system. 4.3. Metal deck roof with TIC and MAC (closed attic inlet) Fig. 12 shows that the maximum temperature of the roof upper surface was 60 °C whereas for the aluminium tube lower surface
Fig. 8. Experimental set-up of the integrated cool roof.
on the underside of the roof was only 35.5 °C. The highest temperature reached in the attic of this integrated system was 30.5 °C. The temperature became constant after 12 min of heating. The maximum difference in temperature between the attic and roof upper
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reached in the attic was 29.6 °C for this integrated system with its attic inlet opened. The temperature became constant after 12 min of testing. The temperature achieved in the attic of this system was close to the ambient temperature of 27.5 °C. Results for the different roof designs were recorded and tabulated in Table 2. The time taken for designs C and D to reach a constant temperature was merely 12 min compared to designs A and B which took 18 and 19 min, respectively. 4.5. Comparison of roof and aluminium tube lower surface temperatures
Fig. 9. (a) RTD air probe and (b) surface temperature sensor.
Fig. 14 shows that the temperature of the metal roof coated with normal coating increased more rapidly than the metal roof coated with TIC because of its lower reflectivity and larger thermal mass. The maximum difference of 7.5 °C was due to the reflective barrier of the insulation coating inhibiting heat transfer by thermal radiation from roof upper to lower surface. The aluminium tube lower surface of both roof designs C and D had almost the same maximum temperature of 35.5 °C. The difference in temperature between designs A and D was 29 °C because of the greater convective and radiant heat losses in Design D. 4.6. Comparison of attic temperatures Fig. 15 compares the attic temperatures of the four roof designs. Design A had the highest attic temperature which reached a max-
Fig. 10. Performance of the metal roof with normal coating (closed attic inlet).
Fig. 12. Performance of the metal roof combined with both thermal insulation coating and MAC (closed attic inlet).
Fig. 11. Performance of the metal roof with thermal insulation coating (closed attic inlet).
surface was 29.5 °C. The addition of the aluminium tube in the cavity of the roof was shown to significantly improve the attic temperature reduction due to its efficient heat transfer mechanism and the outlet airflow is about 0.22 m/s. The detailed mechanism of this system is explained in Fig. 1. 4.4. Metal deck roof with TIC and MAC (opened attic inlet) For this roof design, the maximum temperature of the roof upper surface and aluminium tube lower surface was 60 °C and 35.5 °C, respectively as shown in Fig. 13. The highest temperature
Fig. 13. Performance of the metal roof combined with both thermal insulation coating and MAC (opened attic inlet).
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M.C. Yew et al. / Energy Conversion and Management 75 (2013) 241–248 Table 2 Comparison of temperatures of the four roof designs. Roof model
Attic maximum temperature (°C)
Upper surface roof maximum temperature (°C)
Lower surface roof maximum temperature (°C)
Aluminium tube lower surface maximum temperature (°C)
Stabilized temperature (min)
Design Design Design Design
42.4 33.9 30.5 29.6
64 60 60 60
64.5 57.0 – –
– – 35.5 35.5
18 19 11 11
A B C D
(i) The metal roof with normal coating resulted in the highest roof temperature. (ii) The metal roof with thermal insulation coating resulted in the lowest roof temperature. (iii) The combined system of TIC and MAC with opened attic inlet resulted in the coolest attic.
Fig. 14. Comparison of roof and aluminium tube lower surface temperatures.
The combination of TIC and MAC with opened attic inlet showed a significant improvement with a reduction of up to 13 °C (from 42.4 °C to 29.6 °C) in the attic temperature compared to the normal roof system. This combination is the most effective due to the synergistic effect of heat reflection and hot air rejection. The positive results indicate that the aluminium tube in the cavity of the roof is an important component as it is able to efficiently transfer heat. The installation of this eco-friendly integrated cool roof system will enhance the comfort of building occupants without the environmental-damaging effects of conventional cooling methods.
Acknowledgements
Fig. 15. Comparison of attic temperatures.
imum of 42.4 °C. The difference in attic temperature of designs A and D was 13 °C. Design D obtained the coolest attic temperature of 29.6 °C. This result showed that the application of thermal insulation coating on the upper roof surface combined with the MAC while having the attic inlet opened was the most effective. The significant difference in the results is due to the low thermal conductivity of the thermal insulation paint (0.107 W/mK) as well as the installation of aluminium tubes in the roof cavity that efficiently transfers heat (airflow about 0.22 m/s). The difference in attic temperature of designs C and D was merely 0.6 °C. This result indicated that the small attic opening did not have significant influence on the reduction of attic temperature. For design D, the attic temperature did not exceed 30 °C throughout the test.
5. Conclusions Four different small scale cool roof models were tested indoors, side-by-side under halogen light bulbs. The results showed the following:
The authors would like to thank the University of Malaya for the assistance provided in the patent application of this design (Publication No. PI2012700831), and the research grant allocated to further develop this design under the project UM.C/625/1/HIR/090 (High Impact Research Grant) and RG113-11AET (University of Malaya Research Grant). Special appreciation is also credited to the Malaysian Ministry of Higher Education (MOHE) for the Exploratory Research Grant Scheme (ER023-2012A). The authors would also like to thank Yew Ming Kun, Mohammad Reza Hassan and Abdullah Mamoon for their valuable contribution throughout this project.
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Contents lists available at SciVerse ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Integration of thermal insulation coating and moving-air-cavity in a cool roof system for attic temperature reduction M.C. Yew a,⇑, N.H. Ramli Sulong a, W.T. Chong b, S.C. Poh b, B.C. Ang b, K.H. Tan b a b
Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history: Received 4 March 2013 Accepted 21 June 2013
Keywords: Air cavity Cool roof Environmental friendly Energy savings Thermal insulation coating Ventilated roof
a b s t r a c t Cool roof systems play a significant role in enhancing the comfort level of occupants by reducing the attic temperature of the building. Heat transmission through the roof can be reduced by applying thermal insulation coating (TIC) on the roof and/or installing insulation under the roof of the attic. This paper focuses on a TIC integrated with a series of aluminium tubes that are installed on the underside of the metal roof. In this study, the recycled aluminium cans were arranged into tubes that act as a movingair-cavity (MAC). The TIC was formulated using titanium dioxide pigment with chicken eggshell (CES) waste as bio-filler bound together by a polyurethane resin binder. The thermal conductivity of the thermal insulation paint was measured using KD2 Pro Thermal Properties Analyzer. Four types of cool roof systems were designed and the performances were evaluated. The experimental works were carried out indoors by using halogen light bulbs followed by comparison of the roof and attic temperatures. The temperature of the surrounding air during testing was approximately 27.5 °C. The cool roof that incorporated both TIC and MAC with opened attic inlet showed a significant improvement with a reduction of up to 13 °C (from 42.4 °C to 29.6 °C) in the attic temperature compared to the conventional roof system. The significant difference in the results is due to the low thermal conductivity of the thermal insulation paint (0.107 W/mK) as well as the usage of aluminium tubes in the roof cavity that was able to transfer heat efficiently. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Temperature rise increases the demand for air conditioning and energy consumption, leading to the increase of heat emission and further rise in temperature. Cool roof systems are an inexpensive method to save energy and to improve the comfort level in buildings located in hot climates. With today’s high energy prices, various studies have been carried out to reduce energy consumption by increasing energy efficiency. Undeniably, most buildings incur a number of considerable impacts to the environment. The emission of greenhouse gases and the formation of microclimates within urban areas are among the most prevalent [1]. The consumption of operational energy emits the most greenhouse gases during the entire lifetime of a building [2]. As a tropical country, Malaysia’s ambient air temperature and relative humidity lie in the ranges of 26–40 °C and 60–90%, respectively. This type of climate is non-conducive for human comfort and productivity since it leads to uncomfortable conditions [3].
⇑ Corresponding author. Tel.: +60 3 79676884; fax: +60 3 79675318. E-mail addresses: [email protected], [email protected] (M.C. Yew). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.06.024
In general, a common building is designed in such a way that roof tiles are at the uppermost region, followed by a gypsum board ceiling. The region between these two components is the attic space. Meanwhile, low cost houses and factories would have bare metal deck roofing. Concrete roof tiles are the most commonly used roofing material in Malaysia (85%) followed by clay tiles (10%) and metal deck (5%) [4]. Heat transmission through the roof can be reduced simply by providing insulation as a radiant and conductive heat barrier. In new roof constructions, aluminium foil glued to fibreglass or rockwool blankets are suspended directly under the roof with wire mesh. The benefit of ventilation on the thermal load in roofs has been the subject of numerous studies. Airflow in the cavity effectively carries heat and moisture to the outside and keeps the internal part of the roof cool and dry [5–9]. Roofs contribute tremendously to building heat gain compared to vertical surfaces such as walls, mainly because the roofs are exposed to the sun throughout the daytime [10]. Residential buildings in Malaysia, especially the low rise buildings are found to undergo high intensity heat transmission from the building envelope, whereby the roof represents around 70% of heat gain [11]. Heat from solar radiation is absorbed by the roofs and is transmitted through and trapped in the attic, resulting in a hot ceiling.
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The hot ceiling then radiates heat to the occupants inside a building. In buildings without a ceiling, heat from the roof is exposed directly to the occupants below [3]. Overall, the increase in both the building surface temperatures and the air temperature inside the buildings cause thermal discomfort for users especially for those in buildings without air conditioning. This results in higher energy demand for cooling and ventilation systems [12]. Both systems create two major problems whereby air conditioning systems require enormous electrical energy and thus increase the utility bills. Several research programmes have been carried out regarding insulation and reflective materials in roof designs. Ong concluded that insulation under the tile is preferred to above the ceiling from his field testing of several passive roof designs [13]. His study also proved that the roof solar collector (RSC) resulted in the coolest attic among his prototype, about 13 °C less than the standard non-insulated roof during the period from 2 to 3 pm (the highest temperature in the attic throughout the day). Investigation of the most suitable location of insulation on the roof was performed by Ozel and Pihtili. They found that the best load levelling was to place three pieces of the same thickness insulation at the outdoor surface, middle and indoor surface of the 20 cm thick roof construction (the gap between the insulation pieces was 10 cm) [14]. While minimizing heat transfer into the attic, the design of multi-layer green roof panel is proposed to induce the ventilation process of the building. Khedari et al. and Hirunlabh et al. have carried out an experimental study of a RSC towards natural ventilation [15,16]. The results of the study concluded that the optimum length of RSC of about 100 cm with a tilt angle of 30° was able to induce natural ventilation of 0.08–0.15 m3 s1 m2. Akasaka and Takeda have intensively studied a practical heat transfer calculation method for development of roofs with ventilated air layers to evaluate quantitatively the shading and insulating effects of various combinations of techniques [17]. Al-Sallal [18], Taylor et al. [19] and Mathews et al. [20] showed that there are substantial energy savings in both winter heating and summer cooling with insulated ceilings. Hatamiour et al. have studied some useful ancient energy technologies that have been used for natural cooling of buildings during summer in a hot and humid province in the south of Iran [21]. Chong et al. have studied the moving air path in the air gap [22]. Based on their investigation into the mechanism of heat transfer of the multilayered green roof system, the movement of the surrounding air into the air gap space ensures no aggregation of hot air as well as draws out the interior hot air rapidly to prevent heating of the attic. Yokoyama et al. have investigated the prediction of energy demands using neural network with model identification by global optimization [23]. The use of reflective materials on the building envelope is one of the most efficient ways to reduce the roof temperature. According to Synnefa et al. [24], for peak solar conditions (about 1000 W/m2) for an insulated surface and under a low wind condition, the temperature of a black surface with solar reflectance of 0.05 is about 50 °C higher than ambient air temperature. For a white surface with solar reflectance of 0.8, the temperature rise is about 10 °C. Surface temperature measurements demonstrated that a cool coating can reduce the concrete tile surface temperature by 7.5 °C and it can be 15 °C cooler than a silver grey coating. This novel integrated cool roof design is more eco-friendly compared to other cool roof designs because it uses recycled aluminium cans and chicken eggshell (CES) waste. CES waste, a byproduct of the aviculture industry has been highlighted recently because of its reclamation potential. Most of it is discarded in landfills without further processing. It is known that CES waste contains valuable organic and inorganic components which can be utilized in commercial products to create new value in these waste materials. This study highlights a useful bio-filler derived from CES waste and its potential role in the coating industry. Although there
have been several attempts to use CES components for various applications [25–27], its chemical composition and availability makes CES a potential source of filler for polymer composites [28–30]. The other advantages of using CES are that it is available in bulk quantity, inexpensive, lightweight and environmentally friendly. The main objective of this study is to evaluate a system that combines TIC with aluminium tubes for cavity ventilation as well as to optimize the performance of the roofing systems in terms of heat reflection and heat rejection. The performance will be gauged by measuring the various temperatures of the roof and attic. The aim will be to obtain lower attic temperatures that will result in a more comfortable living environment. Four small roof models representing the different roof system designs were constructed to evaluate the resistance to heat gain. Components that were tested included the TIC, cavity ventilation (MAC) and attic inlet. The performance of the four designs (i) roof coated with normal paint without MAC (Design A), (ii) roof coated with TIC without MAC (Design B) and (iii) combination of TIC and MAC with closed attic inlet (Design C) and (iv) combination of TIC and MAC with opened attic inlet (Design D) were studied and compared. 2. The green roof design 2.1. Mechanism of heat transfer of the cool roof system The mechanism of heat transfer of the cool roof system is shown in Fig. 1. The temperature difference between the environment which is cooler than the aluminium tube on the underside of the roof causes the natural ventilation in the cool roof cavities. The thermal performance of the integrated cool roof system was analysed by estimating two control volumes by the following Eqs. (1) and (2). The first control volume (CV1): The heat transfer between the environment and thermal insulation coating and deck:
Q s ¼ Q Rad;Out þ Q Conv;Out þ Q Cond
ð1Þ
where Qs is the heat from the halogen light bulbs (W), QRad,Out is the radiation heat reflected from top of roof (W), QConv,Out is the convection heat transfer from top of roof (W), and QCond is the conduction heat transfer into the roof deck (W). The second control volume (CV2): The heat transfer between the roof deck and aluminium channel and indoors (attic):
Q Cond ¼ Q Rad;In þ Q Conv;In þ Q ve
Fig. 1. Mechanism of heat transfer of the integrated cool roof system.
ð2Þ
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where Qcond is the conduction heat transfer from deck into the roof (W), QRad,In is the radiation heat transfer to inside of roof (W), QConv,In is the convection heat transfer into inside of roof (W), and Qve is the ventilation heat transfer out of the roof (W). The exhausted heat from the air removed through the aluminium channel can be obtained from the following equation:
_ p ðT out T in Þ Q ve ¼ mC
ð3Þ
_ is the rate of mass flow (kg/s), Cp is the specific heat at atmom spheric pressure (J/kg K), Tin is the indoor air temperature (K), and Tout is the environmental temperature in the shade (K). By considering Eqs. (1) and (2), it is revealed that Qcond is the heat transfer from CV1 to CV2 via the roof deck as shown in Fig. 2. At CV1, heat transfer is decreased due to the reflective coating while at the roof deck, heat transfer is reduced because of its low thermal conductivity. At CV2, the reduction in heat transfer is attributed to the hot air being vented out through the aluminium channel. Besides the highly reflective layer of coating, choosing a material with low thermal conductivity for the roof deck or adding a thermal insulation layer to the roof deck is an effective method to reduce the heat transfer between CV1 and CV2. Consequently, the rate of heat transfer from the environment to the attic decreases [31].
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the TIC is designed to reduce the surface temperature of the roof by reflecting heat. The thermal insulation paint was formulated using titanium dioxide pigment and chicken eggshell waste as bio-filler bound together by a polyurethane resin binder with low thermal conductivity (0.65 W/mK). The addition of high purity reflective titanium dioxide pigment increases solar reflection which results in enhanced thermal insulating function. 2.2.2. Moving-air-cavity (MAC) This roof system is designed to integrate aluminium tubes that provide the cavity ventilation of the roof. The moving air gap in the aluminium tube forces the hot air to flow out through the cavity which is located on the underside of the metal roof. Chong et al. have studied the mechanism of heat transfer in the air gap as shown in Fig. 3 [22]. Hot air rises due to the buoyancy effect. Hence, the plumes of hot air will promptly enter the air gap. The moving air from the exterior guides the air in the space all the way to the ridge before being released to the exterior of the building. In this study, the dispersion of hot air from the aluminium tubes and opened attic inlet keeps the interior temperature low for human comfort. A comfortable condition reduces the usage of air-conditioning systems in a building, thus saving electricity costs. 3. Experimental design
2.2. Main features of the green roof system 3.1. Materials and method 2.2.1. Thermal insulation coating (TIC) TIC plays an important role in promoting measures against heat island phenomenon and limiting carbon dioxide emissions. The heat island phenomenon is air pollution caused by changes in the surface heat balance in which the temperature in an urban region rises above that of the surrounding area and it is isolated like an island. It is caused by reductions in greenery and water surfaces and by an increase in artificial heat resulting from energy consumption. There are a number of different thermal insulation materials used in the building industry today. Conventional materials, such as glass wool, rock wool, expanded polystyrene (EPS) and extruded polystyrene (XPS), require a thick building envelope to reach a sufficiently low thermal transmittance. The effect of using TIC on the roof surface with white solar-reflective paint is a very efficient way to reduce heat discomfort conditions for a single-story building located in cities with hot and humid climates. TIC has the potential to contribute to the reduction of heat island phenomenon as well as carbon dioxide emissions. TIC creates new opportunities for the design of energy efficient buildings. The demand for thermal insulation products have significantly increased because of the high energy prices. With the focus on cost efficiency, there is growing interest in the use of TIC. In this study,
The compositions of the different thermal insulation paint formulations are listed in Table 1. The polyurethane resin, CES bio-filler, titanium dioxide and dispersing agent were mixed until homogenous using a high-speed disperse mixer. The thermal conductivity of the thermal insulation paint was measured using a single-needle (KS-1) sensor in conjunction with the KD2 Pro Thermal Properties Analyzer. The single-needle sensor has a diameter of 1.3 mm with a length of 60 mm and its thermal conductivity is in the range of 0.02–2.00 W/mK. In this study, the lowest value of thermal conductivity of the thermal insulation paint formulations will be selected as TIC. The outlet airflow was measured using heavy duty hot wire thermo-anemometer airflow meter with telescoping probe which is designed to fit into small openings. 3.2. Experimental set-up The specimens were in the form of cubical boxes with monopitch roofs. Each box was custom-made using Perspex with a thickness of 3.0 mm and had dimensions of 350 mm (length) 350 mm (width) 350 mm (height). The angle of inclination of the roof was
Fig. 2. Heat flow diagram of the integrated cool roof system.
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Fig. 3. The moving air path in the MAC.
Table 1 Different formulations of thermal insulation paints. Component
Formula A composition (wt.%)
Formula B composition (wt.%)
Formula C composition (wt.%)
Polyurethane resin Titanium dioxide pigment ES bio-filler Dispersant Thermal conductivity (W/mK)
63.5 15
63.5 18
63.5 20
20 1.5 0.129
17 1.5 0.117
15 1.5 0.107
about 30 ° from the horizontal with a perpendicular height of 202 mm and the attic inlet was provided via an open hinged door with dimensions of 10 cm 10 cm on one side of the Perspex box as shown in Fig. 4. The metal roofing sheets with a thickness of 0.5 mm (dimensions: 500 mm 450 mm) were coated with a 0.2 ± 0.05 mm layer of normal or thermal insulation paint using a gun sprayer and the sprayed sheets are shown in Fig. 5. The thermal insulation paint that has been sprayed on the metal roof is referred to as the TIC. For the ventilation system, aluminium cans (diameter: 66 mm and length: 110 mm) were joined to form tubes using epoxy. Four
cans were used for each tube (Fig. 6). The tubes were arranged closely and installed to cover the entire area under the metal roof. The gap between the metal roof and Perspex box were sealed around the edges using cloth tape. This paper is a study on the TIC, MAC and attic inlet of this cool roof system and their roles in enhancing the effectiveness of the system. Four small scale roof models, each representing the various roof designs (A, B, C and D as shown in Fig. 7) were fabricated and tested side-by-side. The integrated cool roof experimental set-up is shown in Fig. 8. The RTD air probe and surface temperature sensor (accuracy ±0.5 °C) were employed for air and surface temperature measurement, respectively, as shown in Fig. 9. They were mounted to the upper and lower surface of the metal roofs by using aluminium tape. All data were logged at 1 min intervals using E-Log data logger. Data was recorded continuously until the temperatures became constant.
4. Results and discussion 4.1. Metal deck roof with normal coating (closed attic inlet) Results in Fig. 10 show that the maximum temperatures of the attic, roof upper and lower surfaces were 42.4 °C, 64.0 °C and 64.5 °C, respectively. The roof lower surface was 0.5 °C higher than the roof upper surface due to the heat trapped within the Perspex box, while the heat from the roof upper surface dissipated easily
Fig. 4. Dimensions of the small scale roof design and the measuring points.
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Fig. 5. The coated roof with (a) normal paint and (b) thermal insulation paint.
Fig. 6. Dimensions of the aluminium tube moving air channel.
into the surrounding environment. The temperatures became constant after 18 min of testing. The maximum difference in temperature between the attic and roof upper surface was 21.5 °C. The rate of roof temperature increase was 9.3 °C/min in the first 3 min. The rapid increase was due to the low reflectivity of the normal coating on the metal surface.
Fig. 7. Four small scale roof designs.
4.2. Metal deck roof with TIC (closed attic inlet) Results for this roof design are shown in Fig. 11. The roof upper and lower surfaces reached maximum temperatures of 60 °C and 57 °C, respectively. The roof lower surface was 3 °C cooler than the TIC coated upper surface as a result of the reflective barrier inhibiting heat transfer by thermal radiation from the upper to the lower surface. The rate of increase in roof temperature was about 5.8 °C/min in the first 3 min. The highest temperature reached in the attic was 33.9 °C. The temperature of the attic, roof upper and lower surface became constant after 19 min of testing. The maximum difference in temperature between the attic and roof upper surface was 26 °C for this cool roof system. 4.3. Metal deck roof with TIC and MAC (closed attic inlet) Fig. 12 shows that the maximum temperature of the roof upper surface was 60 °C whereas for the aluminium tube lower surface
Fig. 8. Experimental set-up of the integrated cool roof.
on the underside of the roof was only 35.5 °C. The highest temperature reached in the attic of this integrated system was 30.5 °C. The temperature became constant after 12 min of heating. The maximum difference in temperature between the attic and roof upper
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reached in the attic was 29.6 °C for this integrated system with its attic inlet opened. The temperature became constant after 12 min of testing. The temperature achieved in the attic of this system was close to the ambient temperature of 27.5 °C. Results for the different roof designs were recorded and tabulated in Table 2. The time taken for designs C and D to reach a constant temperature was merely 12 min compared to designs A and B which took 18 and 19 min, respectively. 4.5. Comparison of roof and aluminium tube lower surface temperatures
Fig. 9. (a) RTD air probe and (b) surface temperature sensor.
Fig. 14 shows that the temperature of the metal roof coated with normal coating increased more rapidly than the metal roof coated with TIC because of its lower reflectivity and larger thermal mass. The maximum difference of 7.5 °C was due to the reflective barrier of the insulation coating inhibiting heat transfer by thermal radiation from roof upper to lower surface. The aluminium tube lower surface of both roof designs C and D had almost the same maximum temperature of 35.5 °C. The difference in temperature between designs A and D was 29 °C because of the greater convective and radiant heat losses in Design D. 4.6. Comparison of attic temperatures Fig. 15 compares the attic temperatures of the four roof designs. Design A had the highest attic temperature which reached a max-
Fig. 10. Performance of the metal roof with normal coating (closed attic inlet).
Fig. 12. Performance of the metal roof combined with both thermal insulation coating and MAC (closed attic inlet).
Fig. 11. Performance of the metal roof with thermal insulation coating (closed attic inlet).
surface was 29.5 °C. The addition of the aluminium tube in the cavity of the roof was shown to significantly improve the attic temperature reduction due to its efficient heat transfer mechanism and the outlet airflow is about 0.22 m/s. The detailed mechanism of this system is explained in Fig. 1. 4.4. Metal deck roof with TIC and MAC (opened attic inlet) For this roof design, the maximum temperature of the roof upper surface and aluminium tube lower surface was 60 °C and 35.5 °C, respectively as shown in Fig. 13. The highest temperature
Fig. 13. Performance of the metal roof combined with both thermal insulation coating and MAC (opened attic inlet).
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M.C. Yew et al. / Energy Conversion and Management 75 (2013) 241–248 Table 2 Comparison of temperatures of the four roof designs. Roof model
Attic maximum temperature (°C)
Upper surface roof maximum temperature (°C)
Lower surface roof maximum temperature (°C)
Aluminium tube lower surface maximum temperature (°C)
Stabilized temperature (min)
Design Design Design Design
42.4 33.9 30.5 29.6
64 60 60 60
64.5 57.0 – –
– – 35.5 35.5
18 19 11 11
A B C D
(i) The metal roof with normal coating resulted in the highest roof temperature. (ii) The metal roof with thermal insulation coating resulted in the lowest roof temperature. (iii) The combined system of TIC and MAC with opened attic inlet resulted in the coolest attic.
Fig. 14. Comparison of roof and aluminium tube lower surface temperatures.
The combination of TIC and MAC with opened attic inlet showed a significant improvement with a reduction of up to 13 °C (from 42.4 °C to 29.6 °C) in the attic temperature compared to the normal roof system. This combination is the most effective due to the synergistic effect of heat reflection and hot air rejection. The positive results indicate that the aluminium tube in the cavity of the roof is an important component as it is able to efficiently transfer heat. The installation of this eco-friendly integrated cool roof system will enhance the comfort of building occupants without the environmental-damaging effects of conventional cooling methods.
Acknowledgements
Fig. 15. Comparison of attic temperatures.
imum of 42.4 °C. The difference in attic temperature of designs A and D was 13 °C. Design D obtained the coolest attic temperature of 29.6 °C. This result showed that the application of thermal insulation coating on the upper roof surface combined with the MAC while having the attic inlet opened was the most effective. The significant difference in the results is due to the low thermal conductivity of the thermal insulation paint (0.107 W/mK) as well as the installation of aluminium tubes in the roof cavity that efficiently transfers heat (airflow about 0.22 m/s). The difference in attic temperature of designs C and D was merely 0.6 °C. This result indicated that the small attic opening did not have significant influence on the reduction of attic temperature. For design D, the attic temperature did not exceed 30 °C throughout the test.
5. Conclusions Four different small scale cool roof models were tested indoors, side-by-side under halogen light bulbs. The results showed the following:
The authors would like to thank the University of Malaya for the assistance provided in the patent application of this design (Publication No. PI2012700831), and the research grant allocated to further develop this design under the project UM.C/625/1/HIR/090 (High Impact Research Grant) and RG113-11AET (University of Malaya Research Grant). Special appreciation is also credited to the Malaysian Ministry of Higher Education (MOHE) for the Exploratory Research Grant Scheme (ER023-2012A). The authors would also like to thank Yew Ming Kun, Mohammad Reza Hassan and Abdullah Mamoon for their valuable contribution throughout this project.
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