Thermal-Energy Performance of Bulk Insulation Coupled with High-Albedo Roof Tiles in Urban Pitched Residential Roof Assemblies in the Hot, Humid Climate

Abstract

The high rate of heat transfer through the residential roof assembly aggravates the condition of indoor thermal discomfort. Bulk insulation can be installed in the assembly to improve thermal performance. However, although it can efficiently reduce diurnal heat transfer from the outdoor environment into the indoor space through the roof assembly, it can also suppress nocturnal heat transfer in the opposite direction. Alternatively, high-albedo roof tiles employ cool colors to reflect heat at the roof surface, whereas bulk insulation hinders the conduction of heat through the roof assembly. In light of the potential of high-albedo roof tiles and bulk insulation in reducing heat transfer, thermal-energy performance of an urban pitched residential roof assembly, which adopted varying configurations of high-albedo roof tiles and bulk insulation under a hot, humid climate, was evaluated. Energy savings were generated, which were 15.13% when the change from a conventional to a high-albedo roof surface was performed, and 17.00% when the installation of bulk insulation was performed on the high-albedo roof assembly.

1. Introduction

Buildings in countries that have hot, humid climates are exposed to intense solar radiation during the day, owing to the high altitude of the sun path [1,2]. In particular, the roof receives the highest amount of solar radiation in comparison to other components of the building envelope by virtue of the horizontal orientation and higher elevation of the roof [3,4].

Malaysia, which is located in Southeast Asia from 1° to 7° north of the equator [1], has a hot, humid climate throughout the year [1,5]. Its climate can be classified as a tropical rainforest climate, as per the Köppen–Geiger climate classification [6]. According to the annual moving averages reported in Tang [5] for selected urban areas, namely Kota Kinabalu, Kuantan, Kuching, Malacca and Subang Jaya, mean daily temperatures of Malaysia, from 1956 to 2016, ranged between 25.0 °C and 28.7 °C. Recently, the Malaysian Meteorological Department revealed that 38.6 °C was the highest peak daily temperature in Malaysia in 2020, which was recorded in Alor Setar [7]. Previously in 1998, a higher peak daily temperature of 40.1 °C was recorded in Chuping [7]. The average duration of exposure to sunshine throughout Malaysia ranged from six to eight hours per day [8,9].

Typical urban residential buildings in Malaysia are predominantly low-rise with pitched roof assemblies, where the heat transfer through the roof accounts for between 50% and 70% of the total heat gain in the indoor space beneath the roof [10]. The high rate of heat transfer through the residential roof assembly aggravates the condition of indoor thermal discomfort experienced by the occupants. Accordingly, dependence toward air conditioners increases, which is a huge concern, as air conditioners heavily consume energy [8,9].

Bulk insulation restricts the transfer of heat via conduction and convection by trapping air in millions of pockets within bulky materials that possess low density. Common bulk insulation materials employed for various applications, include, among others, cellulose, glass wool, mineral wool, polyester, polyisocyanurate, polystyrene and polyurethane. Bulk insulation products can be manufactured in various forms, which are, but not limited to, batts, loose-fills, rigid boards and rolls. Bulk insulation can be installed in the residential roof assembly to improve thermal performance owing to the presence of miniature air spaces that hinder heat conduction [11], which can potentially reduce the intensity and duration of the operation of air conditioners by the occupants [12]. Innovations pertaining to bulk insulation materials have been proposed by, among others, Husna et al. [13] and Ismail et al. [14], who adopted nano-materials that possess ultra-low thermal conductivity, as well as Nuruddin et al. [15], Farhan et al. [16] and Omar et al. [17], who adopted natural fibers, which are greener than synthetic fibers. Although bulk insulation has great potential in improving the thermal-energy performance of the roof assembly, its rate of adoption in Malaysia is still low [18]. Increases in cost related to the purchase, installation and maintenance of the insulation material, as well as lack of awareness and understanding of the long-term benefits of employing insulation, influence the decisions opted by homeowners [18]. Consequently, the omission of bulk insulation from the roof assembly may result in an increase in the rate of heat transfer through the roof assembly and into the indoor space. Hence, thermal-energy performance of the building during hours of high exposure to intense solar radiation will be negatively impacted.

Residential roof assemblies in Malaysia are typically lightweight and pitched. They comprise roof tiles, attic spaces and ceiling boards. For lightweight roofs, Malaysian Standard: Energy Efficiency and Use of Renewable Energy for Residential Buildings (MS 2680:2017) [19] recommends the installation of insulation within the roof assembly. Furthermore, a minimum thermal resistance (R-value) of 2.50 m²K/W has been set as a mandatory compliance criterion for lightweight roofs as stated in MS 2680:2017 [19], Green Building Index Assessment Criteria for Residential New Construction [20], and Selangor Uniform Building By-Laws [21].

Previous studies on building insulation paid more attention to wall insulation in cold climates [22], and less emphasis was given to roof insulation in hot climates. In Malaysia, previous research pertaining to thermal performance evaluation of insulation materials installed in residential roof assemblies is limited to the studies of Farhan et al. [16], Halim et al. [23], Irwan et al. [24,25], Ismail et al. [14], Morris et al. [26], Nuruddin et al. [15,27], Puad et al. [28] and Zakaria et al. [29]. Findings indicated that installing roof insulation efficiently reduced diurnal heat transfer from the outdoor environment into the indoor space through the roof assembly. Conversely, findings also revealed that the presence of insulation suppressed nocturnal heat transfer through the roof assembly, which is in the opposite direction to that of the diurnal heat transfer. Consequently, the nocturnal energy consumption owing to the use of air conditioners will increase in view of the fact that indoor thermal comfort has to be sustained throughout the night in order to facilitate adequate rest and sleep among the occupants.

Alternatively, Al-Obaidi et al. [4], Al Yacouby et al. [30] and Farhan et al. [31] studied the effect of high-albedo roofs without insulation under the climate of Malaysia. High-albedo roof tiles reflect heat at the roof surface, whereas bulk insulation hinders the conduction of heat through the roof assembly. Adoption of high-albedo roofs has been reported in Synnefa et al. [32] to be effective at increasing thermal-energy performance for widely differing climate classes. Prevalently, previous studies have attempted to increase the albedo of roof tiles by applying high-albedo coatings. The coatings can be classified according to their binders, such as cementitious or elastomeric coatings. Alternatively, the coatings can also be categorized according to their carriers, such as solvent- or water-based coatings [33]. Essentially, for application on high-albedo roofs, the coatings are required to possess superficial thermal-optical properties that are appropriate for maintaining, under exposure to solar radiation, surface temperatures that are appreciably lower than those of conventional roofs. In general, high-albedo coatings that possess pre-eminent thermal-optical properties are those that are white in color. However, as aesthetics of buildings cannot be disregarded, studies have been conducted to develop innovative coatings, such as those that possess solar-reflective surfaces with non-white colors, those that are thermochromic, or those that are doped with phase-change materials [33].

Despite the potential for improving the thermal-energy performance, adoption of high-albedo roofs in countries that are exposed to the tropical rainforest climate is still low [9,30]. In particular, within the region of Southeast Asia, research on the effect of high-albedo roofs is currently deficient. Exclusive of the studies that were conducted in Malaysia, which are Al-Obaidi et al. [4], Al Yacouby et al. [30] and Farhan et al. [31], the research is limited to the studies of Syuhada and Maulana [34] in Indonesia, Zingre et al. [35] in Singapore and Thongkanluang et al. [36] in Thailand.

Although Al-Obaidi et al. [4] and Al Yacouby et al. [30] studied the effect of high-albedo roofs by varying the color of the roof surface, their methodologies employed test cells that did not comply with the clauses in Uniform Building By-Laws 1984 (UBBL 1984) [37] for habitable rooms of residential buildings in Malaysia. The methodology adopted in Farhan et al. [31] later addressed the shortcomings of the test cells employed in Al-Obaidi et al. [4] and Al Yacouby et al. [30] but focused solely on the effect of high-albedo roofs without considering its coupling with insulation. The scope of Syuhada and Maulana [34] zoomed in on zinc roofs and excluded the adoption of roof tiles. Zingre et al. [35] adopted a methodology that concentrated on flat roofs and did not consider pitched roof assemblies that have attic spaces and ceiling boards. Thongkanluang et al. [36] focused on synthesizing a coating material for potential application on the surface of high-albedo roofs, without performing any study on heat transfer through the roof assembly that endures exposure to solar radiation.

Hence, new studies are required to address the shortcomings of previous research on the effect of high-albedo roofs, in particular, those that zoom in on pitched roof assemblies that have roof tiles, attic spaces and ceiling boards, together with the adoption of insulation, under a hot, humid climate. In light of the potential of high-albedo roof tiles and bulk insulation in reducing heat transfer, thermal-energy performance of an urban pitched residential roof assembly, which adopted varying configurations of high-albedo roof tiles and bulk insulation under a hot, humid climate, was evaluated.

2. Materials and Methods

The thermal-energy performance of an urban pitched residential roof assembly was evaluated by developing a building information model using Integrated Environmental Solutions <Virtual Environment> (IESVE), which is a building information modeling (BIM) tool. Thermal-energy and computational fluid dynamics (CFD) analyses were performed on the model. Varying configurations of high-albedo roof tiles and bulk insulation within the roof assembly were adopted. The roof was exposed to the hot, humid climate of Shah Alam in Malaysia, which is an urban area.

The second law of thermodynamics states that the total entropy, which is a measure of the disorder of a system and its environment, will never decrease. Therefore, heat transfer through the residential roof assembly will occur from the hotter to the colder bodies, as the building and its environment attempt to gain entropy over time and reach its maximum, which is when thermal equilibrium is achieved [38]. Accordingly, as outdoor and sky conditions change over time throughout the day, magnitude and direction of heat transfer through the roof assembly will continually change in conformity with the second law of thermodynamics.

In the present study, evaluation of thermal-energy performance considered the conduction, convection and radiation modes of heat transfer through the roof assembly. Thermal properties of materials that constitute the assembly were also taken into account. The rate of heat transfer by conduction (Q_conduction), convection (Q_convection) and radiation (Q_radiation) can be expressed by Fourier’s Law as per Equation (1) [39], Newton’s Law of Cooling as per Equation (2) [40] and Stefan–Boltzmann’s Law as per Equations (3) and (4) [41].

$$Q_{conduction}=kA\frac{dT}{dx}$$

(1)

where k is the thermal conductivity of the material expressed in W/mK, A is the cross-sectional area perpendicular to heat flow expressed in m², and $\frac{dT}{dx}$ is the temperature gradient expressed in K/m.

$$Q_{convection}=h_{c}A\left(T_s-T_f\right)$$

(2)

where h_c is the surface heat transfer coefficient, A is the surface area, T_s is the surface temperature, and T_f is the fluid temperature.

$$Q_{radiation}=\sigma A_1{\epsilon }_1\left(T_1{}^4-T_2{}^4\right)$$

(3)

$$Q_{radiation}=h_r{A}_1\left(T_1-T_2\right)$$

(4)

where σ is the Stefan–Boltzmann constant, h_r is the coefficient of heat transfer, A₁ is the area of the first surface, ɛ₁ is the emissivity of the first surface, T₁ is the absolute temperature of the first surface, and T₂ is the absolute temperature of the second surface.

The ability of the roof surface to reject solar heat, as indicated by the solar reflectance index (SRI), was also taken into consideration. SRI refers to the relative steady-state temperature of a surface with respect to a standard white, which is given the SRI value of 100, and a standard black, which is given the SRI value of 0, under standard solar and ambient conditions. It is calculated as per Equation (5) [42]. As its definition and method of calculation are based on the steady-state temperatures of a standard black, which has a reflectance of 0.05 and an emittance of 0.90, and a standard white, which has a reflectance of 0.80 and an emittance of 0.90, it is possible for SRI values to be slightly negative or exceed 100.

$$SRI=\frac{\left(T_{black}-T_{surface}\right)}{\left(T_{black}-T_{white}\right)}\times 100$$

(5)

where T_black, T_white and T_surface are steady-state temperatures of the standard black, standard white and material surface, respectively, which are derived from measured values of solar reflectance and infrared emittance of the material surface according to the calculations in the Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces (ASTM E1980-11) [43].

Adoption of high-albedo roof tiles and bulk insulation within the roof assembly were aimed toward developing an energy-efficient roof assembly. Monitoring of the energy-efficiency considered the cooling load and energy savings, which signify the level of indoor thermal comfort. A conceptual framework of the present study is outlined in Figure 1.

BIM was selected as the methodology, as it is capable of assisting the decision-making process when it comes to sustainable design of buildings. BIM has been employed in previous studies to perform sustainable design of residential buildings that are exposed to the hot, humid climate of Malaysia. Amir et al. [44], Gardezi et al. [45] and Jamaludin et al. [46] developed building information models of pre-determined types of residential buildings. Alternatively, building information models developed in Farhan et al. [31], Halim et al. [23], Irwan et al. [24,25] and Morris et al. [26] were those of test cells that represent the conditions of a habitable space in typical urban residential buildings in Malaysia. BIM simulation data that were collected from the building information models can be validated by conducting field measurements and comparing the measured data with their counterparts from the BIM simulation based on Equation (6) as in Vangimalla et al. [47].

$$P{D}_{SD-FM}=\frac{SD-FM}{FM}\times 100\%$$

(6)

where SD and FM are the simulation and field measurement data, respectively, and PD_SD−FM is the percentage difference between the simulation and field measurement data.

Acceptable PD_SD−FM values adopted in Vangimalla et al. [47] and Leng et al. [48] are 15% and 20%, respectively, which were determined based on the 10% to 20% acceptable range recommended in Maamari et al. [49].

In the present study, two test cells, as shown in Figure 2, were constructed at the site location of 3.07° N, 101.50° E in Shah Alam, as shown in Figure 3. The test cells are identical, barring the roof tile color, where red and white roof tile colors were adopted, as shown in Figure 4 and Figure 5, to represent conventional and high-albedo roofs, respectively. Thermocouples and data loggers were installed in the test cells to collect air and surface temperature data throughout the whole year of 2021.

Inspections were conducted prior to commencement of data collection, as well as once per week after the commencement, to ascertain the accuracy of data throughout the data collection. The inspections were conducted by comparing air and surface temperature data that were recorded using primary data loggers, which were mounted to the test cells, with those that were recorded using secondary data loggers of the same model that were minimally used. For the surface temperature data, their accuracy was further ascertained by performing supplementary comparisons between the temperatures that were measured using thermocouples, which were then recorded by the data loggers, with those that were manually measured using a thermal imaging camera.

The test cells were 4 m long, 4 m wide and 3 m high. The dimensions were selected as such to fulfil minimum size requirements as specified in UBBL 1984 [37], while minimizing the size of the test cells for feasibility of the experiment. The minimum base area, height and width of a habitable room in residential buildings were 11 m², 2.5 m and 2 m, respectively.

Conventional materials were employed to construct the test cells, as itemized in

Table 1. Materials employed to construct the test cells.

Component	Material	Density (kg/m3)	Specific Heat (J/kgK)	k-Value (W/mK)	Thickness (mm)
Roof	Cement Tile	1890	1000	0.836	10.0
Ceiling	Cement Board	720	1000	0.250	4.5
Window	Clear Float Glass	2800	800	0.810	6.0
Door	Solid Timber	702	2720	0.138	38.0
Wall	Cement Plaster	1690	840	0.533	18.0
	Clay Brick	1800	800	1.154	114.0
	Cement Plaster	1690	840	0.533	18.0
Floor	Reinforced Concrete	2400	1000	1.442	50.0

, inclusive of the density, thermal conductivity (k-value), specific heat and thickness of each material, with the aim of creating the conditions of a habitable space in typical urban residential buildings in Malaysia.

The test cell was modeled in IESVE as a building information model. An axonometric projection of the model is shown in Figure 6. Two-dimensional plan, front and rear, and left and right views of the model are shown in Figure 7.

Thermal-energy performance of the roof assembly of the building information model was evaluated by performing thermal-energy and CFD analyses using Apache and MicroFlo, respectively, in IESVE. SRI values of the roof surfaces were calculated from solar reflectance and infrared emittance values of roof surfaces of roof tile samples, which were obtained from laboratory measurements. Thermal-energy analysis was performed for a whole typical meteorological year (TMY) for the site location of the test cells at 3.07° N, 101.50° E in Shah Alam, Malaysia. Meteorological data, inclusive of solar irradiance, were generated by Meteonorm, based on data obtained from weather stations, geostationary satellites and globally calibrated aerosol climatology, as well as sophisticated interpolation models [50]. Roof-surface and attic-air temperature data generated from the thermal-energy analysis throughout the TMY were averaged to obtain annual-averaged 24-h profiles. The CFD analysis generated indoor temperature contours at peak diurnal outdoor temperature and the trough of nocturnal outdoor temperature. Configuration of the roof assembly was varied according to the roof tile color and presence of insulation to create three building information models as presented in

Table 2. Roof assembly configurations of building information models.

Building Information Model	Roof Tile Color	Bulk Insulation
Conventional	Red	Nil
High-Albedo	White	Nil
High-Albedo + Bulk Insulation	White	Mineral Wool

. Then, 100-mm thick mineral wool was employed within the roof assembly as bulk insulation, as it is commonly used for building insulation in Malaysia.

The evaluation of thermal-energy performance considered the operation of a unit of a 950-W air conditioner for cooling of the indoor space with a set-point temperature of 24 °C as recommended in Malaysian Standard: Energy Efficiency and Use of Renewable Energy for Non-Residential Buildings (MS 1525:2019) [51] and also adopted in Halim et al. [23] and Irwan et al. [24,25]. Daily, weekly and monthly indoor cooling profiles were configured based on the profiles adopted in Halim et al. [23], Irwan et al. [24,25], Tang and Chin [52] and Zakaria et al. [29], which also focused on residential buildings in Malaysia. Simulation settings were configured with the assumption that no occupants and furniture are present in the indoor space, and the door and windows are closed throughout the year.

The methodology of the study is elucidated in Figure 8 and Figure 9.

3. Results and Discussion

The site location of the test cells at 3.07° N, 101.50° E in Shah Alam, Malaysia is within the tropical rainforest region as per the Köppen–Geiger climate classification [6]. Consequently, the test cells are exposed to a hot, humid climate throughout the year [1,5]. Minimum, mean and maximum annual-averaged profiles of solar irradiance and outdoor air temperature throughout the TMY of the location are presented in Figure 10. For the most part, solar irradiance throughout diurnal periods is relatively high, particularly in the afternoon, owing to the high altitude of the sun path as mentioned in Alam et al. [1] and Al-Obaidi et al. [2]. Minimum, mean and maximum solar irradiance profiles peaked at 73.36, 587.52 and 1070.25 W/m², respectively. The solar irradiance culminated at 13:30, which is about halfway through the diurnal period. Inversely, there is zero solar irradiance throughout the nocturnal period from 20:30 to 5:30. Accordingly, as outdoor air temperature is directly impacted by solar irradiance, the trend of the outdoor air temperature profiles trailed those of solar irradiance. Minimum, mean and maximum profiles of the outdoor air temperatures peaked at 26.9, 31.23 and 36.00 °C, respectively, with the temperatures culminating from 15:30 to 16:30.

Solar reflectance and infrared emittance of the roof surfaces are, as measured in the laboratory, presented in Figure 11. SRI values, as shown in Figure 12, were calculated from the solar reflectance and infrared emittance values, as per Santamouris et al. [42] and ASTM E1980-11 [43]. The change from a conventional to a high-albedo roof surface has led to an increase in the solar reflectance from 0.20 to 0.73, no change in the infrared emittance at 0.90, and an increase in the SRI from 19 to 90. Application of white paint that brought about the high-albedo roof surface can significantly reduce heat transfer through the roof assembly, as, according to Raeissi and Taheri [53], the wavelength of light is reflected by the white pigment at the roof surface and, as a consequence, less solar radiation is absorbed.

Roof surface temperature (T_RS) profiles of the non-insulated conventional and high-albedo roof assemblies are compared in Figure 13. As T_RS is heavily influenced by solar irradiance, waveform of the T_RS profiles in Figure 13 bears resemblances to that of the solar irradiance profiles in Figure 10. The change from a conventional to a high-albedo roof surface has led to the reduction in T_RS throughout the diurnal segment, where T_RS culminated at 50.50 and 35.84 °C for the conventional and high-albedo roof surfaces, respectively. The strong peak reduction of −14.79 °C in T_RS, as illustrated in Figure 14, which presents the change in T_RS (∆T_RS), transpired owing to the relatively higher SRI of the high-albedo roof surface of 90, in comparison to that of the conventional roof surface of 19. The higher SRI resulted in a higher rate of reflection and accordingly a lower rate of absorption of solar radiation that is incident on the roof surface. As opposed to that of the diurnal segment, the increase in SRI did not influence T_RS throughout the nocturnal segment due to the absence of solar radiation throughout the nocturnal period.

Plot of T_RS versus solar irradiance for the non-insulated conventional and high-albedo roof assemblies are presented in Figure 15. Correlations between T_RS and solar irradiance are positive, with coefficient of determination (R²) values of 0.9662 and 0.8233, and gradients of 0.0446 and 0.0197, for the conventional (SRI = 19) and high-albedo (SRI = 90) roof assemblies, respectively. The lower R² and gradient for the high-albedo roof assembly signify that the change from the conventional to the high-albedo roof surface has led to the reduction in the influence of solar irradiance on T_RS by virtue of the higher rate of reflection and lower rate of absorption of solar radiation on the high-albedo roof surface in comparison to that on the conventional roof surface.

T_RS profiles of the non-insulated and insulated high-albedo roof assemblies are compared in Figure 16. The installation of bulk insulation within the high-albedo roof assembly has led to the reduction in T_RS throughout the diurnal segment, where the peak T_RS further declined from 35.84 to 32.63 °C. The presence of insulation has led to a further peak reduction in T_RS of −4.06 °C as illustrated in Figure 17, as the insulation material hinders heat conduction through the roof assembly [11].

Throughout the nocturnal segment, the absence of solar radiation has caused the heat conduction transfer to invert. The transposition occurred due to the reduction in the average effective sky temperature, which resulted in the radiation of heat from the roof surface to the sky during the nocturnal period. Accordingly, heat within the indoor space and roof assembly flows toward the roof tiles and attempts to escape the building to achieve thermal equilibrium as mentioned in Farhan et al. [31] and Tang and Chin [52]. Under the circumstances, the presence of insulation within the roof assembly contributed toward hampering the heat transfer out of the building. As a consequence, T_RS increased throughout the nocturnal period by up to 2.64 °C, as shown in Figure 17.

Indoor temperature (T_i) contours were generated at peak diurnal outdoor temperature (T_o) as shown in Figure 18, which was on 17 March at 16:00, and the trough of nocturnal T_o as shown in Figure 19, which was on 22 September at 6:00. The contours were generated for all of the configurations of the roof assembly, which were conventional, high-albedo, and high-albedo with bulk insulation. T_i contour for the conventional roof assembly discloses that, during peak diurnal T_o, the high T_RS, which culminated at 45.00 °C, caused the attic air temperature (T_AA) to elevate to within the range from 39.55 to 43.18 °C. Then, heat transfer into the indoor space resulted in the increase in room air temperature (T_RA) to within the range from 34.09 to 37.73 °C. T_i contour for the high-albedo roof assembly exhibits that, resulting from the change from a conventional to a high-albedo roof surface, at peak diurnal T_o, the range of T_RS greatly reduced to within the range from 25.00 to 39.55 °C. The decline in T_RS transpired due to the adoption of the high-albedo roof surface, which reduced heat transfer into the attic space. Accordingly, T_AA and T_RA reduced to within the range from 32.27 to 34.09 °C. T_i contour for the high-albedo roof assembly with bulk insulation reveals that the presence of bulk insulation caused T_AA and T_RA to further reduce to within the range from 30.45 to 32.27 °C by hampering the heat transfer from the roof surface to the attic space.

At the trough of nocturnal T_o, increase in the SRI of the roof surface did not influence T_RS, T_AA and T_RA by virtue of the absence of solar radiation, which induced heat radiation from the roof surface to the sky during the nocturnal period and is in agreement with Farhan et al. [31] and Tang and Chin [52]. T_RS ranged from 20.00 to 21.36 °C while T_AA and T_RA ranged from 21.82 to 22.27 °C. T_i contour for the high-albedo roof assembly with bulk insulation shows that, due to the presence of bulk insulation, T_RS, T_AA and T_RA increased owing to the obstruction of heat from escaping the building from the roof surface toward the sky during the nocturnal period. Consequently, heat transfer from the indoor space to the roof surface was hindered.

The change from a conventional to a high-albedo roof surface, followed by the installation of bulk insulation, has resulted in reduction of indoor annual cooling load of the building information model as presented in Figure 20. The indoor annual cooling load reduced from 2.67 MWh for the conventional roof assembly to 2.32 MWh for the high-albedo roof assembly. Installation of bulk insulation within the high-albedo roof assembly has led to further reduction of the indoor annual cooling load from 2.32 to 2.28 MWh. Energy savings of 15.13% have been generated when the change from a conventional to a high-albedo roof surface was performed, while 17.00% have been generated when the installation of bulk insulation was performed on the high-albedo roof assembly.

4. Conclusions

Thermal-energy performance of an urban pitched residential roof assembly, which adopted varying configurations of high-albedo roof tiles and bulk insulation under the hot, humid climate, was evaluated. Thermal-energy and CFD analyses were performed on a building information model.

Change from the conventional to the high-albedo roof surface has led to the reduction in the influence of solar irradiance on roof surface temperature due to the higher rate of reflection of solar radiation on the roof surface. However, the change did not influence the roof surface temperature throughout the nocturnal segment due to the absence of solar radiation. Installation of bulk insulation within the high-albedo roof assembly has led to further reduction in roof surface temperature throughout the diurnal segment. However, the reduction coincided with the increase in the roof surface temperature throughout the nocturnal period as heat transfers out of the building, owing to the absence of solar radiation that has caused the direction of heat conduction transfer to invert, which is hampered by the insulation material. Despite the negative impact of installing bulk insulation throughout the nocturnal period, on the whole, energy savings have been achieved, which are 15.13%, which is from 2.67 to 2.32 MWh when the change from a conventional to a high-albedo roof surface was performed, and 17.00%, which is from 2.32 to 2.28 MWh when the installation of bulk insulation was performed on the high-albedo roof assembly.

For future research, studies that consider the variation in height of the building and surrounding buildings and degree of the placement of high-albedo materials can be considered. Development of a solar-reflective coating that can further increase the solar reflectance of the roof surface and potentially eliminate dependence toward insulation is also recommended. Alternatively, engineering of novel materials that possess extremely low thermal conductivity, which can potentially be applied within the roof assembly with minuscule thicknesses, is proposed.