Thermal Performance of Residential Roofs in Malaysia: Experimental Study Using an Indoor Solar Simulator

Abstract

Previous researchers have detailed the problems in measuring the thermal resistance value of a whole roof assembly under hot conditions due to the uncertainty of the outdoor environment. Currently, no established method exists to experimentally investigate an entire thermal roof performance under a steady-state condition. This article details the properties of the indoor solar simulator and the research methods undertaken to measure the thermal resistance value of roof assembly. The indoor solar simulator utilizes 40 halogen bulbs to accurately replicate sun radiation. Thermocouples and heat flux sensors are installed at several locations on the roof assembly to quantify the heat transmission occurring through it. The thermal resistance value is determined by adding up the average difference in temperature across the external and internal roof surfaces and dividing the total amount by the total of all averaged heat fluxes. Subsequently, this study investigates the thermal efficiency of residential roof assemblies that comprise various insulation materials frequently employed in Malaysia, including stone wool, mineral glass wool, reflective bubble foil insulation, and radiant barriers. The analysis showed that the roof configurations with bubble foil reflective insulation produce superior thermal resistance values when coupled with enclosed air space or mass insulation, with thermal resistance values ranging between 2.55 m²K/W and 3.22 m²K/W. It can be concluded that roof configurations with bubble foil reflective insulation resulted in high total thermal resistance and passed the minimum thermal resistance value of 2.5 m²K/W under the Malaysian Uniform Building By-Law 38 (A) requirements. Furthermore, the radiant barrier produced a high thermal resistance value of 2.50 m²K/W when installed parallel to a 50 mm enclosed air space, emphasising the crucial function of an enclosed air space next to a reflective foil to resist the incoming heat radiation. The findings from this research can help building professionals determine the optimum insulation for residential building roofs in Malaysia.

1. Introduction

Malaysia experiences hot and humid conditions all year long. Buildings in Malaysia depend highly on air-conditioning to keep indoor environments cool and comfortable. As a result, commercial and residential buildings are responsible for 12% of Malaysia’s total energy consumption [1]. Due to this problem, passive strategies have been introduced to lower building energy consumption and elevate indoor thermal comfort. Local building authorities have advised using roof insulation to reduce the heat intake into a building through the roof component. Malaysian local authority has introduced a new by-law 38A (2) of the Uniform Building By-Laws (UBBLs), which states that the roof of all non-residential and residential buildings must achieve a thermal transmittance (U-value) lower than 0.4 W/m²K or must achieve a thermal resistance (RSI value) higher than 2.5 m²K/W [2]. However, a precise method to experimentally evaluate a roof assembly’s RSI value or U-value is yet to be established.

1.1. Research Background

The roof is one of the main components of a building envelope responsible for high heat gain. It is constantly exposed to solar radiation, especially for buildings with large and poorly insulated roof surfaces [3]. Poorly insulated roofs can contribute to a large amount of air conditioning usage. For example, a previous study showed that the solar radiation absorbed by a roof surface contributes to more than 40% of the top floor spaces’ electrical energy during hot summer [4]. In tropical countries, building sectors consume roughly 57% of the nation’s total electrical energy, and 60% of the electrical energy in the building sector is from air conditioning systems [5]. Past research proves that roof insulation is an effective passive cooling method to reduce heat intake through the roof and reduce building energy consumption [6].

Thermal insulation is a proven passive technique for lowering heat intake through the roof component. Typically, there are two main thermal insulation types: mass insulation (or bulk insulation) and reflective foil [7]. A reflective foil is generally an aluminium sheet with low emissivity (ε ≤ 0.05) and high reflectivity characteristics. It is proven to reflect 95–99% of radiative heat transfer. Reflective foil is divided into two categories: radiant barriers and reflective insulation. A radiant barrier is a single sheet of aluminium foil with a reflective surface.

In contrast, reflective insulation is aluminium foil attached to foam or air bubbles. Incorporating air bubbles and foam in reflective insulation traps air and thus minimises heat transfer via conduction and convection [8]. In a roof assembly, the low emissivity side of the radiant barrier or reflective insulation must be installed parallel to an enclosed air space to efficiently reflect the incoming heat transfer, as seen in Figure 1.

Mass insulation, such as mineral glass wool and stone wool, is another common insulation in Malaysian buildings. Both mineral glass wool and stone wool have similar manufacturing processes. The only difference is that mineral glass wool is manufactured from natural and recycled glass, whereas stone wool is made from basalt. Although both insulations claim to have similar thermal properties, stone wool products have better fire-resistant performance than mineral glass wool [10]. Mineral glass wool and stone wool have small, isolated, and trapped air pockets. The trapped air pockets prevent heat gain via convective heat transfer by preventing heat from transferring from one air pocket to the next. If the insulation is compressed, the air pockets will be extinguished, disrupting the thermal performance [11]. Hence, in the building envelope, the mineral glass wool or stone wool must be installed with spacers to ensure that they are not compressed and that the thermal performance remains optimum.

1.2. Previous Work

In Malaysia, residential buildings such as landed houses commonly use roof tiles with attic space as the roof assembly. Concrete roof tiles are widely used as they are durable, cost-effective, and easy to maintain. Many studies in the literature have investigated the effectiveness of roof insulations.

Past literature has discussed the effectiveness of reflective foil with low emissivity as a roof insulation method in both summer and winter weather conditions. Shrestha et al. [12] studied an entire roof attic to study its performance in various climatic conditions in a large-scale climate simulator. Low emissivity reflective foil with various emissivity (ε = 0.02, 0.03, and 0.23) was used for the roof insulation. The study shows that the low emissivity reflective foil managed to lower the heat intake into the attic by 19.9% to 49.8% under summer conditions. In winter, the reflective foil decreased the thermal waste from the attic by 5.7% to 10.0%. Fantucci and Serra [13] compared the thermal performance of reflective paint and reflective insulation in a pitched roof of a residential building under construction in Italy. It was found that a reduction in indoor summer heat of between 10% and 53% can be achieved depending on the surface emissivity.

Similarly, Ferreira and Corvacho [14] used a detached house to compare the thermal performance of TRB, a radiant barrier attached below the truss/rafter, and HRB, a radiant barrier attached horizontally to the ceiling. Although HRB could perform well and reduce heat flux transmission by 70–73% during summer days, the TRB was considered a better option as it abolished the problem of dust accumulation on the radiant barrier. This is because dust accumulation on the HRB could reduce the emissivity, thus reducing the thermal performance of the radiant barrier. Michels et al. [15] used a pitched roof of a residential house in Brazil to examine the thermal efficiency of a radiant barrier under clay roof tiles assembly. The study showed that the radiant barrier managed to lower the heat flux transmission by up to 70% and reduced the ceiling temperature by 8 °C during the summer days. Teh et al. [16] compared the thermal performance of bubble and woven aluminium foil in two identical test cells with gable roofs. The heat flux transmission reduction for both reflective foils was roughly 80–90% compared to uninsulated roofs. Furthermore, the attic air temperature reduction increases when the air space thickness between the roof tiles and the reflective foil is thicker.

Previous research detailed the thermal roof performance of roofs insulated with mass insulation and reflective foil. For example, Medina [17] studied the effectiveness of combining several ceiling mass insulation levels with radiant barriers using computer simulation. The simulation results revealed that the radiant barrier’s contribution in reducing the heat transfer through the ceiling dropped from 42% to 25% when the ceiling insulation increased from 1.94 m²K/W to 5.28 m²K/W. Therefore, the mass insulation level must be optimized to maximize the reflective insulation’s effectiveness. D’Orazio et al. [18] compared the thermal performance of a pitched roof of a single-story building with and without reflective insulation. The study found that reflective insulation decreases heat flux transmission by 17% compared to the roof without reflective insulation.

Furthermore, the reflective insulation reduced 10% of heat loss during cold nights compared to uninsulated roofs. The researchers also detailed that combining high levels of mass insulation and reflective insulation will reduce the effectiveness of the reflective insulation. Hauser et al. [19] used two gable roofs in Germany to compare the thermal roof performance of reflective insulation and mineral wool. During the winter, the reflective insulation loses double the heat energy of mineral wool. This shows mineral wool saves more heating energy than reflective insulation during winter. Furthermore, the calculated R-value of the roof system with mineral wool was three times higher than that of reflective insulation. Amer [20] investigated the combination of 5 cm thick glass wool and aluminium foil as the insulation method below the roof. The combination of glass wool and aluminium reduced the indoor air temperature by 2.6 °C compared to the ambient temperature. Ong [21] experimented with a 50 mm thick stone wool and a radiant barrier to insulate cement tiles on a small gable roof. It was found that the combination of the stone wool and radiant barrier was able to reduce the attic temperature to as low as 30 °C when the insulations are installed directly below the cement roof tiles. Muhieldeen et al. [22] examined the optimum thickness of glass wool as the roof insulation to reduce indoor air temperature and heat flux transmission. The results showed that the glass wool thickness of 25 mm, 50 mm, and 75 mm managed to lower the indoor air temperature by 1.0 °C, 1.3 °C, and 1.5 °C, and heat flux transmission by 3%, 12%, and 20%, respectively. Although the 75 mm thick glass wool reduced indoor air temperature and heat flux transmission, the 50 mm glass wool is the most optimum as it is the most cost-effective, with a return on investment of 27.40% per year.

Few previous research has attempted to calculate insulated roof systems’ thermal resistance (RSI value). However, it is still very challenging for the researchers to determine the RSI value of a steady-state condition in summer via empirical measurement. This is due to the significant temperature fluctuations and unstable climate boundary conditions during summer days [19,23]. Therefore, this paper aims to evaluate the RSI value of thermally insulated roof assemblies using a novel indoor solar simulator.

1.3. Research Objectives

This study uses a novel indoor solar simulator to assess the thermal efficiency of thermally insulated roof assemblies for residential buildings. Furthermore, this research compares the effectiveness of several types of insulation commonly used for residential roofs in Malaysia, such as bubble foil reflective insulation, radiant barriers, mineral glass wool, and stone wool, using a novel indoor solar simulator in a steady-state laboratory environment.

2. Laboratory Measurement—Indoor Solar Simulator

The objective of the indoor solar simulator is to examine the thermal performance of roofs in a controlled indoor laboratory setting under steady-state conditions. The experimental measurement needs to occur in a steady-state condition so that the RSI value of the whole roof assembly can be calculated with high accuracy. There is minimal air movement inside the laboratory; thus, this study considers the wind effect negligible. The advantage of the indoor solar simulator is that any desired roof configuration can be assembled and assessed in a shorter period with high accuracy. As seen in Figure 2, 40 units of halogen lamps are fixed above the roof assembly to simulate solar radiation. Previously, the invention of the novel solar simulator was patented with the grant number MY-194877-A.

2.1. Sensors and Data Acquisition System

To measure the thermal response of the roof assembly, thermocouples, and heat flux sensors were placed at several locations of the roof assembly. The properties of the sensors are as detailed in

Table 1. Sensors used for the measurement using the indoor solar simulator.

Equipment	Manufacturer	Function	Accuracy	Range	Reference
K-type thermocouple	RS Components (London, UK)	To obtain the temperature of a surface	±1 °C	−200–1000 °C	[24]
Heat Flux Sensor (GHT-2C-ENV#0695)	International Thermal Instrument Company, Inc. (Del Mar, CA, USA)	To evaluate heat flux transmission across a surface	±1%	0–3155 W/m2	[25]
Pyranometer	Kipp & Zonen (Delft, The Netherlands)	Measure solar irradiance	±3%	0–2000 W/m2	[26]

. This research used K-type thermocouples to quantify the surface temperature of different roof surfaces. In addition, heat flux sensors (GHT-2C-ENV#0695) were used to quantify the heat flow transmission across different roof surfaces. Three units of thermocouples were placed on the upper surface of the roof tiles (T_1A, T_1B, and T_1C), three units of thermocouples were placed at reflective foil’s lower surface (T_2A, T_2B, and T_2C), and three units of thermocouples were placed on the gypsum board (T_3A, T_3B, and T_3C). The purpose of having three thermocouples on the surfaces is to obtain the average surface temperature. Besides that, one heat flux sensor unit is positioned on the bottom surface of the reflective foil (q₁) and one heat flux sensor unit is located beneath the gypsum board (q₂). The sensors transmit signals every minute, and the Arduino controller and data acquisition system acquire the readings. The roof inclination angle is adjusted to 15°. The total roof size that can be assembled on the indoor solar simulator is 3 m in length and 1.5 m in width. Figure 3 shows the roof assembly setup and the sensor positions on the roof assembly.

2.2. Properties of the Concrete Roof Tile and Insulation Materials

Concrete roof tiles are commonly used for residential buildings in Malaysia. Depending on the roof configuration, concrete roof tiles are usually insulated with thermal insulations such as mass insulation (mineral glass wool or stone wool) and reflective foil (radiant barrier and reflective insulation). The properties of concrete roof tiles and the insulation materials used in this research are detailed in

Table 2. Specification of the concrete roof tile and insulation materials.

Material	Specification
	Extruded concrete roofing tile with fused pigmented colour coating [27]; Thickness: 12 mm; Thermal conductivity = 0.836 W/mK; RSI value = 0.014 m2K/W.
Concrete roof tile
	Stone wool is made of basalt—a type of volcanic stone. It is non-combustible, which also can be used for thermal insulation, fire, and sound protection. At a thickness of 50 mm and a density of 40 kg/m3, it has an RSI value of 1.39 m2K/W and thermal conductivity of 0.036 W/mK [28].
Stone wool
	Mineral glass wool is commonly used as insulation for metal deck roofs and roof tiles. At a thickness of 50 mm and density of 16 kg/m3, it has a thermal conductivity of 0.0366 W/mK and RSI value of 1.35 m2K/W [29].
Mineral glass wool
	The radiant barrier is an aluminium foil laminated to a high-density polyurethane woven fabric. One side of the surface is pure aluminium with a low emissivity of 0.02. Another side of the surface is made out of metalized aluminium. The thickness of the radiant barrier is 0.17 mm and has a density of 160 g/m2 [30]. It has an RSI value of 0.000 m2K/W.
Radiant barrier
	Bubble foil reflective insulation is aluminium laminated to a polyethylene bubble sheet. Both sides of the surfaces are made out of pure aluminium with a low emissivity value of 0.01. The bubble pockets have a diameter of 10 mm and a density of 380 g/m2. The thermal conductivity is 0.0537 W/mK, and the RSI value of the material is 0.1694 m2K/W. The thickness of the foil is 9 mm [31].
Bubble foil reflective insulation
	Gypsum plasterboard as the ceiling. It has a thickness of 12 mm. Density: 9.7 kg/m2; Thermal conductivity: 0.25 W/mK; RSI value: 0.05 m2K/W.
Gypsum ceiling board

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3. Methodology

3.1. Test Method Validation Using Outdoor Roof Tile Surface Temperature

The concrete roof tiles’ outdoor surface temperature value was used to justify the test method undertaken using the indoor solar simulator. Figure 4 displays the measurement of the surface temperature of roof tiles on a typical hot and clear bright day in Malaysia. The average temperature recorded on that particular day was 30 °C, while the highest temperature was 33 °C. Obtaining the steady-state figure of heat flux transmission was challenging due to the dynamic nature of the outside environment.

Figure 5 shows the surface temperature of a concrete roof tile measured on a clear and sunny afternoon in Malaysia. It can be seen that the concrete roof tile surface achieves a constant peak surface temperature of 57–61 °C during the hottest time of the day. The halogen lamps’ wattage of the indoor solar simulator is adjusted using a computerized dimmer controller until the concrete roof tile surface achieves a stable temperature of 57–61 °C, similar to the surface temperature achieved by the outdoor measurement.

3.2. Method of Data Collection

Once the halogen lamps are powered up, the Arduino data acquisition system will constantly record and store the readings from the thermocouples and heat flux sensors. As shown in Figure 6, it takes about 4 h for the readings to reach a steady state condition. Once steady state measurements are achieved, the test proceeds for another 3 h to ensure that constant and reliable data are collected. The total time of measurement is 7 h. The steady state values of the surface temperatures and heat flux transmissions are retrieved to compute the RSI value. Each reading was taken three times to ensure the measurements were accurate and reliable.

Since the test was carried out in a controlled laboratory environment, the relative humidity does not vary significantly between tests. As shown in Figure 6, the relative humidity inside the laboratory reaches a steady state at 82%, which is common in hot and humid climates.

3.3. Method of RSI Value Calculation

The RSI value was calculated using the average method formula detailed in ISO 9869-1:2014 [32]. The total RSI value of the entire roof assembly is the summation of the thermal resistance of the reflective air space, RSI_A, and the thermal resistance of the attic space, RSI_B.

$${\mathrm{R}\mathrm{S}\mathrm{I}}_{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}={\mathrm{R}\mathrm{S}\mathrm{I}}_{\mathrm{A}}+{\mathrm{R}\mathrm{S}\mathrm{I}}_{\mathrm{B}}$$

(1)

$${\mathrm{R}\mathrm{S}\mathrm{I}}_{\mathrm{A}}=\frac{{\mathrm{T}}_{{1\_}_{\mathrm{a}\mathrm{v}\mathrm{g}}}-{\mathrm{T}}_{{2\_}_{\mathrm{a}\mathrm{v}\mathrm{g}}}}{{\mathrm{q}}_1}$$

(2)

$${\mathrm{R}\mathrm{S}\mathrm{I}}_{\mathrm{B}}=\frac{{\mathrm{T}}_{{2\_}_{\mathrm{a}\mathrm{v}\mathrm{g}}}-{\mathrm{T}}_{{3\_}_{\mathrm{a}\mathrm{v}\mathrm{g}}}}{{\mathrm{q}}_2}$$

(3)

where

• RSI_A is the RSI value of the reflective air space, m²K/W;
• RSI_B is the RSI value of the attic air space, m²K/W;
• RSI_Total is the RSI of the entire roof assembly, m²K/W;
• T_{1_avg} is the average surface temperature of roof tiles, K;
• T_{2_avg} is the average surface temperature of reflective foil, K;
• T_{3_avg} is the average surface temperature of the ceiling, K;
• q₁ is the heat flux transmission at the reflective air space, W/m²;
• q₂ is the heat flux transmission at the ceiling, W/m².

3.4. The Residential Roof Configurations Investigated in This Study

Table 3. The visual representation and composition of the residential roof configurations.

Roof Configuration	Visual Representation	Composition
A0		Concrete roof tiles; Attic space; Gypsum ceiling board (thickness 12 mm).
A1		Concrete roof tiles; Enclosed air space of 25 mm; Radiant barrier (thickness 0.17 mm); Attic space; Gypsum ceiling board (thickness 12 mm).
A2		Concrete roof tiles; Enclosed air space of 50 mm; Radiant barrier (thickness 0.17 mm); Attic space; Gypsum ceiling board (thickness 12 mm).
B1		Concrete roof tiles; Enclosed air space of 25 mm; Bubble foil reflective insulation (thickness 9 mm); Attic space; Gypsum ceiling board (thickness 12 mm).
B2		Concrete roof tiles; Enclosed air space of 50 mm; Bubble foil reflective insulation (thickness 9 mm); Attic space; Gypsum ceiling board (thickness 12 mm).
C1		Concrete roof tiles; Mineral glass wool (50 mm); Radiant barrier; Attic space; Gypsum ceiling board (thickness 12 mm).
C2		Concrete roof tiles; Mineral glass wool (50 mm); Bubble foil reflective insulation (thickness 9 mm); Attic space; Gypsum ceiling board (thickness 12 mm).
D1		Concrete roof tiles; Stone wool (50 mm); Radiant barrier (thickness 0.17 mm); Attic space; Gypsum ceiling board (thickness 12 mm).
D2		Concrete roof tiles; Stone wool (50 mm); Bubble foil reflective insulation (thickness 9 mm); Attic space; Gypsum ceiling board (thickness 12 mm).

details the illustrations and compositions of the roof residential configurations studied in this research. The roof configurations chosen for study in this research are commonly found types of roof assemblies for residential buildings in Malaysia. Figure 7 and Figure 8 show the actual assembly of the investigated roof configurations on the indoor solar simulator.

4. Validating the Test Results Obtained from the Indoor Solar Simulator in Relation to the ASHRAE Standard

This section explains the validation of the indoor solar simulator in relation to the RSI values published in the ASHRAE Handbook of Fundamentals Chapter 26, where the RSI values of the enclosed air space adjacent to a surface with low emissivity are listed [33]. The RSI values are based on reducing radiative, convective, and conductive heat transfer in the enclosed air space.

The error bars of the data obtained through the indoor solar simulator are depicted in Figure 9. These bars are found to be within the ASHRAE Standard error margin, which signifies that the error is minimal and thus deemed acceptable. Errors from the measured values are expected because the values in the ASHRAE Standard abided by several simplified assumptions for ideal conditions, such as assuming that the enclosed air space is entirely airtight and unventilated and ignoring any heat transfer from the metal fasteners inside the system [34]. These ideal conditions are challenging to replicate in an empirical condition, which causes errors between the experimental measurement via indoor solar simulator and the ASHRAE Standard. The validation of the novel indoor solar simulator in relation to the ASHRAE standard has been published and can be seen in previous research [35].

5. Result and Discussion

This section presents the roof tiles-to-ceiling temperature reduction, heat flux transmission at the reflective foil, heat flux transmission at the ceiling, and the total RSI values of every roof configuration tested in this study. The section also evaluates the thermal roof performance achieved by the radiant barrier, mineral glass wool, stone wool, and bubble foil reflective insulation. The section additionally provides an analysis of the impact that changing the thickness of the enclosed air space has on the thermal performance of the roof, as well as the consequences of substituting mass insulation with an enclosed air space.

5.1. The Roof Tiles-to-Ceiling Temperature Reduction, T₁–T₃

Figure 10 shows the roof tiles-to-ceiling temperature reduction for all roof configurations. It can be seen that the pairing of bubble foil reflective insulation with mineral glass wool and stone wool (D1 and D2) has the highest temperature reduction, followed by the pairing of radiant barrier with mineral glass wool and stone wool (C1 and C2). This shows that the low thermal conductivity of mineral glass wool and stone wool can retard high amounts of heat transmission and produce high roof tiles-to-ceiling temperature reduction. The temperature reduction achieved through stone wool is always slightly higher than that of mineral glass wool when combined with a radiant barrier or bubble foil reflective insulation by 1.62% to 2.50%, as seen in

Table 4. The percentage difference in roof tiles-to-ceiling temperature reduction between mineral glass wool and stone wool.

Roof Tiles-To-Ceiling Temperature Reduction, T1–T3 (K)
Type of Reflective Foil	Mineral Glass Wool	Stone Wool	Percentage Difference (%)
Radiant barrier	32.84	33.68	2.50
Bubble foil reflective insulation	34.18	34.75	1.62

. This is because stone wool has lower thermal conductivity than mineral glass wool. Furthermore, the roof tiles-to-ceiling temperature reduction achieved by the radiant barrier is always lower than the bubble foil reflective insulation by 3.06–6.48%, as seen in

Table 5. The percentage difference in the reduction in roof tiles-to-ceiling temperature when comparing radiant barrier and bubble foil reflective insulation.

Roof Tiles-To-Ceiling Temperature Reduction, T1–T3 (K)
Enclosed Air Space Thickness or Insulation Type	Radiant Barrier	Bubble Foil Reflective Insulation	Percentage Difference (%)
25 mm air space	29.04	30.18	3.76%
50 mm air space	29.11	31.13	6.48%
Mineral glass wool	32.84	34.18	3.93%
Stone wool	33.68	34.75	3.06%

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5.2. The Heat Flux Transmission at the Reflective Foil and the Ceiling, q₁ and q₂

Figure 11 shows the heat flux transmission at the reflective foil of all roof configurations. It clearly shows that the combination of reflective foil and mass insulation (C1, C2, D1, and D2) has higher heat flux transmission at the reflective foil than the combination of reflective foil with an enclosed air space (A1, A2, B1, and B2). This is because replacing the enclosed air space with mineral glass wool or stone wool induces higher conductive heat transfer and thus increases the heat flux transmission at the reflective foil. The lowest heat flux transmission at the reflective foil was achieved by bubble foil reflective insulation with a 50 mm enclosed air space (B2), followed by a radiant barrier with a 50 mm enclosed air space (A2), bubble foil reflective insulation with 25 mm enclosed air space (B1), and radiant barrier with 25 mm enclosed air space (A1). This highlights the significance of installing the reflective foil parallel to an enclosed air space to reflect the heat at the reflective foil effectively.

By increasing the thickness of the enclosed air space from 25 mm to 50 mm, the heat flux transmission at the reflective foil is reduced by 27.02% for the radiant barrier and 23.49% for bubble foil reflective insulation, as detailed in

Table 6. The percentage difference of 25 mm enclosed air space and 50 mm enclosed air space for radiant barrier and bubble foil reflective insulation.

Heat Flux Transmission at the Reflective Foil, q1 (W/m2)
Type of Reflective Foil	25 mm Enclosed Air Space	50 mm Enclosed Air Space	Percentage Difference (%)
Radiant barrier	19.52	14.25	27.02
Bubble foil reflective insulation	18.19	13.92	23.49

. The bubble foil reflective insulation has lower heat flux transmission at the reflective foil by 2.34–6.84% than the radiant barrier, as seen in

Table 7. The percentage difference of heat flux transmission at between the reflective foil of radiant barrier and bubble foil reflective insulation.

Heat Flux Transmission at the Reflective Foil, q1 (W/m2)
Enclosed Air Space Thickness or Insulation Type	Radiant Barrier	Bubble Foil Reflective Insulation	Percentage Difference (%)
25 mm air space	19.52	18.19	6.84
50 mm air space	14.25	13.92	2.34
Mineral glass wool	27.04	26.13	3.35
Stone wool	24.78	23.94	3.38

. A higher percentage difference was observed between the radiant barrier and bubble foil reflective insulation at an enclosed air space of 25 mm than at 50 mm. This is because the bubble pockets on the bubble foil reflective insulation become the dominant factor in disrupting the conductive heat transfer in smaller enclosed air spaces. At the 50 mm enclosed air space, the influence of the bubble pockets in reducing the heat gain minimizes as the influence of a thicker enclosed air space becomes more influential than the bubble pockets. Furthermore, replacing the 50 mm enclosed air space with mass insulation induces the heat flux transmission at the reflective foil by 41.9% and 47.3%, depending on the roof configuration, as seen in

Table 8. The comparison of the heat flux transmission at the reflective foil between 50 mm enclosed air space, mineral glass wool, and stone wool.

Heat Flux Transmission at the Reflective Foil, q1 (W/m2)
Type of Reflective Foil	50 mm Air Space	Mineral Glass Wool (50 mm)	Stone Wool (50 mm)
Radiant barrier	14.25	27.04 (↑ 47.3%)	24.78 (↑ 42.5%)
Bubble foil reflective insulation	13.92	26.13 (↑ 46.7%)	23.94 (↑ 41.9%)

Note: ↑ indicates percentage increment of heat flux transmission by mineral glass wool and stone wool compared to 50 mm air space.

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Figure 12 shows the roof configuration’s heat flux transmission at the ceiling. The least heat flux transmission at the ceiling was achieved by bubble foil reflective insulation with a 50 mm enclosed air space (B2), followed by bubble foil reflective insulation with a 25 mm enclosed air space (B1), bubble foil reflective insulation with stone wool (D2), and bubble foil reflective insulation with mineral glass wool (D1). Figure 12 shows that the bubble foil reflective insulation plays a vital role in minimizing the heat flux transmission at the ceiling. It performs superior radiant barrier by 27.83–43.15% depending on the roof configuration, as detailed in

Table 9. The percentage difference of heat flux transmission at the ceiling between the radiant barrier and reflective insulation.

Heat Flux Transmission at the Ceiling, q2 (W/m2)
Enclosed Air Space Thickness or Insulation Type	Radiant Barrier	Bubble Foil Reflective Insulation	Percentage Difference (%)
25 mm air space	9.25	5.26	43.15
50 mm air space	7.27	5.25	27.83
Mineral glass wool	8.87	5.61	36.76
Stone wool	8.72	5.51	36.85

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5.3. The Total Thermal Resistance of All Roof Configurations, RSI_Total

Figure 13 shows the total RSI values of all roof assemblies investigated in this study. The highest total RSI value was achieved by bubble foil reflective insulation with a 50 mm enclosed air space (B2), followed by bubble foil reflective insulation with a 25 mm enclosed air space (B1), bubble foil reflective insulation with stone wool (D2), bubble foil reflective insulation with mineral glass wool (D1), and radiant barrier with 50 mm air space (A2). These five roof configurations passed the minimum RSI value of 2.5 m²K/W for a lightweight roof, as required in UBBL 38 (A). Figure 13 shows the strong influence of bubble foil reflective insulation in enhancing the thermal performance of a residential type of roof assembly, mainly due to the reduction in heat flux transmission in the attic space, as observed in Figure 12. All four roof configurations which use bubble foil reflective insulation (B1, B2, D1, and D2) passed the minimum RSI of 2.5 m²K/W. Furthermore, the results highlighted the significance of installing the reflective foil parallel to an enclosed air space, as B1 and B2 achieved the two highest RSI values. This observation is also seen in roof configuration A2, which shows that the radiant barrier with a 50 mm enclosed air space can pass the minimum RSI value of 2.5 m²K/W without mineral glass wool or stone wool.

6. Conclusions

The experimental investigation of the thermal roof performance of residential roofs in Malaysia using a novel indoor solar simulator has been detailed in this research. An earlier section of this article provided a comprehensive explanation of the process used to verify the accuracy of the indoor solar simulator by comparing it to the external surface temperature of a concrete roof tile. On a typical afternoon in Malaysia, an outdoor roof tile surface achieves a peak constant surface temperature between 57 °C and 61 °C. In order to achieve a high degree of accuracy in the measurement conducted with the indoor solar simulator, halogen lamp wattage is altered by using a computerised dimmer controller until the concrete roof tiles achieve a peak surface temperature of about 57 °C to 61 °C.

Then, this research was compared several types of thermal insulation, such as a radiant barrier, bubble foil reflective insulation, mineral glass wool, and stone wool. Some of the significant results are:

• Bubble foil reflective insulation is much more effective than a radiant barrier in enhancing the thermal efficiency of residential roof assemblies. This is because the heat flux transmission at the ceiling is reduced by 27.83–43.15% when bubble foil reflective insulation is used.
• When evaluating the thickness of an enclosed air space of 25 mm and 50 mm, it can be seen that the enclosed air space of 50 mm outperforms the 25 mm gap in lowering the heat flux transmission at the reflective foil by a range of 23.49% to 27.02%.
• By adding more insulating material into the enclosed air space, the heat flux transmission at the reflective foil is enhanced. The addition of mineral glass wool and stone wool to the reflective foil results in a 41.9–47.3% increase in heat flux transmission relative to a 50 mm thick enclosed air space. Consequently, the presence of an air space next to the radiant barrier or bubble reflective insulation decreases the amount of heat transmitted through the reflective foil.
• Regarding total thermal resistance, it can be concluded that roof configurations with bubble foil reflective insulation resulted in high total thermal resistance and passed the minimum RSI value of 2.5 m2K/W under the UBBL 38 (A) requirements. Most of the roof configurations with radiant barriers did not pass UBBL 38 (A), except for that with an enclosed air space of 50 mm (A2). This shows that it is essential to have a thick, enclosed air space parallel to the radiant barrier to reduce heat gain effectively.

Finally, the novel indoor solar simulator enables accurate and efficient testing of steady-state roof thermal performance. The indoor solar simulator, on the other hand, has several constraints. Restricted by the rig’s size, the roof assembly that can be tested is limited to 1.5 m wide and 3 m long. In addition, the maximum height of the attic space is no more than 500 mm, and the shape is fixed with the ceiling parallel to the roof tiles. Therefore, gable and hip roof types cannot be assembled and tested using the indoor solar simulator. Finally, the unique indoor solar simulator could provide a more comprehensive means to evaluate the thermal performance of roofs in a shorter period of time. The findings from this research can help building professionals determine the optimum insulation for residential building roofs in Malaysia.