A Study on Transparent Type Envelope Material in Terms of Overall Thermal Transfer, Energy, and Economy for an Office Building Based on the Thai Building Energy Code

Abstract

Energy consumption in buildings has increased significantly as population and economic activities are concentrated in urban areas. Air conditioning accounts for a significant percentage of energy consumption in buildings, especially in tropical climates. The main area where heat can be transferred into the building is through glass windows. Thus, this study aims to evaluate feasibility in terms of overall thermal transfer value (OTTV), energy, and economics for retrofitting different glass materials in an office building in Thailand by using building energy code (BEC) software. The software uses Thailand’s building energy code as the standard to evaluate the energy performance of the case study building in comparison with different glass types used in retrofitted cases. From an economic perspective, the internal rate of return (IRR) and discounted payback periods (DPP) were used as determining indexes. The results demonstrated that retrofitted windows with the best energy-efficient glass might achieve energy performance, but installation cost can reduce economic feasibility, while the glass with the second lowest heat transfer coefficient can reduce the OTTV by 68.89% and building energy consumption by 16.87%. However, it can achieve the highest economic performance with 10.70% IRR and DPP at 11.83 years. Therefore, the balance between energy and economic factors must be considered to provide energy-efficient and investment-friendly glass materials for retrofit projects. In addition, the study focuses specifically on tropical climates. Thus, the finding may not be reflected similarly for buildings located in different regions.

1. Introduction

The greatest global problems at present are the energy and environmental crises resulting from a decade of rapid worldwide population and economic growth. Energy is the main driving force that keeps the industry running to further expand each country’s economic sector. In addition, the shift of jobs from rural agriculture to urban industry and commerce has led to an increasing number of residential and commercial buildings. Buildings require a significant amount of energy to provide functions and comfort to occupants. A survey by the International Energy Agency (IEA) [1] found that the majority of energy use in buildings mainly consists of air-conditioning systems, followed by lighting systems and other equipment, which is in line with the survey from the US Energy Information Administration [2]. The amount of energy consumed in an HVAC system depends on many factors, but the main factor is the thermal transfer from the environment through the building envelope. For buildings in tropical climates, heat transfer through the building materials increases the cooling load, the energy demand of the air-conditioning system, and the energy consumption of the entire building [3]. Therefore, changing building materials can achieve high energy efficiency and can be a potential measure to improve energy efficiency.

In the case of Thailand, a similar trend in energy usage in buildings has also been observed [4]. Thus, the Department of Alternative Energy Development and Efficiency (DEDE), the Ministry of Energy, has set the goal to improve the country’s energy efficiency by presenting an Energy Efficiency Plan 2018–2037 (EEP 2018). The goal of the policy is to reduce the energy intensity (EI) by 30% in 2030 from 181,238 ktoe in the case of business as usual (BAU) to 126,867 ktoe in 2030 [5]. To achieve this goal, the Energy Conservation Promotion Act was passed in 1992 [6]. One of the components of this act is the Building Energy Code (BEC), which establishes minimum building energy performance requirements for both new and renovated buildings that have floor areas greater than 2000 m². The energy code evaluates the following six parameters of the building: the building skin, lighting system, air-conditioning system, water heating appliance, alternative energy performance, and the entire building’s energy consumption.

From the literature review, energy consumption in a building in a tropical climate mainly consists of a cooling load for an air conditioning system [7]. The energy demand for HVAC systems depends on many building factors, such as the building envelope, climate, and occupant behavior [8,9]. The aggregate result from profiling office building energy usage patterns reveals that building energy consumption throughout the day varied depending on the operation of lighting and air conditioning systems [10]. The heat generated from a lighting system in an office building is large enough to have a significant impact on the air conditioning system by increasing the building’s cooling load [11]. The COVID-19 pandemic and remote work have reduced the usage of office space [12]. Even post-COVID-19, the level of energy consumption in office-type buildings has not been raised to pre-COVID-19 levels [13]. However, a new factor that directly impacts energy usage in office buildings located in urban areas is the Urban Heat Island (UHI) effect [14]. The urban microclimate from the effects of UHI can result in higher temperatures, around 4 degrees, compared to suburbs [15] and also increase energy consumption in buildings by up to 25% [16].

For the envelope, OTTV is the most important parameter for determining the amount of heat from the environment that can penetrate a building envelope [17,18]. Many studies have shown that improved building envelopes with highly energy-efficient materials have the potential to decrease building energy consumption by reducing heat transmittance through the building skin [19]. As a result, the cooling loads and air conditioning energy consumption were reduced [20]. The main factor for material thermal properties is thermal conductivity, which has a direct correlation with efficiency in building envelopes [21]. In hot climates, changing the building envelope to energy-efficient material can significantly reduce energy usage in the building and can additionally decrease emissions [22]. Therefore, architects and engineers must select suitable materials for each building. A previous study also demonstrated that the thermal properties of an opaque material can significantly affect energy consumption and indoor comfort, especially in tropical climates [23].

Thermal transfer mainly occurs through building windows and can affect the energy efficiency of the building [24]. The reason is that transparent materials have a higher thermal transfer value and result in higher energy demand for the cooling load compared to opaque materials [25]. Therefore, new technologies for transparent materials, such as reflective coatings [26], vacuum glazing [27,28], evacuated glazing [29], transparent insulation [30], and movable window insulation [31], have been developed and applied in the building envelope to reduce direct heat transfer from the environment. The studies on changing glass material into energy-efficient material with nano aerogel glazing [32] and nano vacuum insulation panel [33] have demonstrated that it can achieve both energy performance and economic viability. Shading can also reduce OTTV on the transparent material by limiting the amount of solar heat in the building and reducing the cooling load, which in turn reduces the building energy consumption [34]. The ratio of windows to walls is another factor that significantly affects the energy performance of a building [35]. However, the shading and window-to-wall ratio is the factor that cannot easily address in the building retrofitted case due to the constraining of the old building skin. In addition, the higher cost of retrofitted buildings with better energy-efficient materials can potentially decrease investment feasibility [36]. Therefore, the target thermal characteristics for building materials used in renovation must be optimized in the aspect of economic feasibility in order to make the project attractive for investment [37].

From the literature review, the thermal characteristics of glass materials have a significant impact on thermal transmittance into buildings, especially in tropical climates. Sunlight heat through the windows can affect the building’s cooling load and then increase the energy usage in the air conditioning system, which results in raising the whole building’s energy consumption. To improve buildings’ energy efficiency, retrofitted windows with glass that have better thermal properties can significantly reduce thermal transfer through the building envelope. The objective of this study was, therefore, to evaluate the performance of high-efficiency glass types in terms of energy savings and economic feasibility. The software based on Thailand’s building energy code, or BEC, developed by the Ministry of Energy, Thailand, has been used as an evaluation tool. The case study involved an office building in Bangkok, Thailand. Thailand’s building energy code has been used as a basis for comparative studies between different glass types. In addition, the two economic indicators, such as internal rate of return (IRR) and discounted payback period (DPP), were chosen as determinants of investment attractiveness. The contributions of the study can be summarized as follows:

• Five different glass types with distinguishing materials, components, and thermal characteristics were used to retrofit the current windows of the office building.
• The entire building energy performance using the different types of glass materials was evaluated and compared based on the Thai building energy codes.
• Feasibility analysis of the economic aspects of each type of glass material retrofitted in a building.

The content of this paper can be separated into six parts. The research problems and a literature review are presented in Section 1. Section 2 are discussed the details of Thai building energy codes. The case study building and the building components of the envelope subsystem are presented in Section 3. The energy performances of different glass materials and their economic feasibility are discussed in Section 4 and Section 5, respectively. Finally, Section 6 summarises the research results and concludes the study.

2. Building Energy Code

The Thai building energy code (BEC) is part of the Energy Conservation Promotion Act of 1992, which mandates the standard for both new or renovated buildings with a total area under a single roof of more than 2000 m² [2]. The BEC has created two pathways for a building to meet the requirement. The first way is to evaluate four sub-systems consisting of the building envelope, lighting system, air-conditioning system, and hot water system (if installed). If all subsystems meet the standard, the Energy Ministry declares that the building is in compliance with BEC. However, if some subsystems (other than the hot-water system) do not meet the requirements, the code allows the building to be evaluated via a second pathway. The whole building’s energy consumption is compared to that of the reference building, which is calculated using a similar characteristic building with the standard allowance as an input. To comply with this standard, the energy consumption must be lower than that of the reference building. In addition, the energy produced from renewable energy can be used to directly reduce building energy usage. These pathways provide leeway for building design.

2.1. Envelope System

The BEC categorises different buildings types into three distinct groups depending on the hours of operation, which are summarized in

Table 1. The building type and allowable thermal transfer value according to the standard.

Classification of Buildings	Operating Hour	OTTV (W/m2)	RTTV (W/m2)
Office, School	8 A.M.–4 P.M. (8 h)	≤50	≤15
Theater Department store, Convention building, Entertainment complex	10 A.M.–10 P.M. (12 h)	≤40	≤12
Hotel, Hospital, Condominium	24 h (more than 12 h)	≤30	≤10

. The standard evaluated the envelope performance by using the Overall thermal transfer value (OTTV) and Roof thermal transfer value (RTTV) parameters. These two indexes determined the amount of thermal value from the environment in the building through material usage in walls, windows, and roofs. Different sets of OTTV and RTTV apply to these different categories due to different coefficients in the calculation equations, such as air leakage, occupant behavior, and sun direction during operating hours.

For building envelope, the OTTV can be calculated by determining the heat transfer coefficient (U-value) of the component and material used in skin building [6]. The U-value can be calculated by inverting the total thermal resistance of the component, as shown in Equation (1). The material thermal resistance was calculated by dividing material thickness by the thermal conductivity coefficient, as shown in Equation (2). For different opaque components in the section of the envelope that has more than one layer of material, the combined thermal resistance of the component can be calculated by using Equation (3). The air around the interior and exterior of the material can provide a small thermal resistance. The value provided by the BEC for the calculation is 0.12 m²·k/W in the case of the interior air film and 0.044 m²·k/W in the case of exterior air film

$$U=\frac{1}{R_T}$$

(1)

$$R=\frac{\Delta x}{k}$$

(2)

$$R_T=R_o+\frac{\Delta x_1}{k_1}+\frac{\Delta x_2}{k_2}+\dots +\frac{\Delta x_n}{k_n}{R}_i$$

(3)

where

• $R$ = material thermal resistance (m²·k/W);
• $R_T$ = component thermal resistance (m²·k/W);
• $R_i$ = thermal resistance of the interior air film (0.12 m²·k/W);
• $R_o$ = thermal resistance of the exterior air film (0.044 m²·k/W);
• $\Delta x_n$ = material thickness (m);
• $k_n$ = material thermal conductivity coefficient (W/m²·K).

The calculation of the OTTV for the BEC focuses only on a section with air-conditioning systems installed [6]. The calculation assumes that the heat from the environment through building skin generates a cooling load for the air-conditioning systems. Thus, in a section of a building without air conditioning, this effect does not occur, so these sections do not need to be evaluated according to BEC. The OTTV of the building envelope section can be calculated by using Equation (4). For the entire building envelope, the calculation was performed using the weighted arithmetic mean of each envelope section, as expressed in Equation (5).

$$OTT{V}_i=\left(U_W\right)\left(1-WWR\right)\left(T{D}_{eq}\right)+\left(U_f\right)\left(WWR\right)\Delta T+\left(WWR\right)\left(SHGC\right)\left(SC\right)\left(ESR\right)$$

(4)

$$OTTV=\frac{\left(A_{w1}\right)\left(OTT{V}_1\right)+\left(A_{w2}\right)\left(OTT{V}_2\right)+\cdots +\left(A_{wi}\right)\left(OTT{V}_i\right)}{A_{w1}+A_{w2}+\cdots +A_{wi}}$$

(5)

where

• $OTTV$ = overall thermal transfer value of the whole building (W/m²);
• $OTT{V}_i$ = overall thermal transfer value in each envelope section (W/m²);
• $U_W$ = heat transfer coefficient in case of opaque component (W/m²·°C);
• $WWR$ = Ratio between window area and wall area;
• $T{D}_{eq}$ = temperature difference equivalent (°C);
• $U_f$ = heat transfer coefficient of the transparent component (W/m²·°C);
• $\Delta T$ = differential temperature between building interior and exterior (°C);
• $SHGC$ = solar heat gain coefficient;
• $SC$ = shading coefficient;
• $ESR$ = effective solar radiation (W/m²);
• $A_{wi}$ = area of the wall section (m²).

2.2. The Lighting Power Density for Lighting System

The energy consumption of lighting systems in buildings is evaluated against the BEC using the lighting power density (LPD) as an index. The building code does not evaluate lighting quality in the working space. However, a separate labor law prescribes the minimum illuminance requirements for each type of working space [38]. In the standard, LPD is the summation of the power consumption of the building’s lighting system divided by the building’s floor area. However, the calculation did not include the parking area because the required illuminance for the parking area is much lower than that for the working area. Therefore, the LPD results for a building with a large parking area may not reflect the actual performance of the lighting system. In addition, display lighting or movable lighting was not included in the calculation. The building categories and maximum LPD allowances are summarized in

Table 2. The building type and light power density allowance according to the standard.

Classification of Buildings	LPD (W/m2)
Office, School	≤10
Theater, Department store, Convention building, Entertainment complex	≤11
Hotel, Hospital, Condominium	≤12

.

2.3. Air-Conditioning System

The energy performance of each air-conditioning unit installed in the building is evaluated against the BEC. The standard divides air conditioners into two categories: split-type and central units. The split-type air-conditioners have a separate cooling unit inside the building and a compressor on the outside of the building; these air conditioners are commonly used in residential and small commercial buildings in Thailand [39]. For split-type air-conditioners, the seasonal energy efficiency ratio (SEER) is used as an indicator to evaluate energy performance. The evaluation process for central air conditioning was divided into two sections. First, the cooling unit must have a coefficient of performance (COP) that is higher than the energy code allowance. Second, the other components, such as the pump, cooling tower, and fan coil unit, must have a combined power consumption of less than 0.5 kW per refrigerator ton. All air conditioning units were split in the case study building. Therefore, only the energy code requirements for split-type air-conditioners were discussed.

The minimum requirements for the SEER value for the split-type air-conditioners in the BEC are summarized in

Table 3. Allowable efficiency of air-conditioners, according to BEC.

	Size of Air Conditioning Unit	Conventional Label 5 (Without Star)	New Label 5 (1 Star)	New Label 5 (2 Stars)	New Label 5 (3 Stars)
SEER of Fixed speed air conditioning unit	8000 W (27,296 BTU/h.)	12.85–13.84	13.85–14.84	14.85–15.84	≥15.85
	8000–12,000 W (27,296–40,944 BTU/h.)	12.40–13.39	13.40–14.39	14.40–15.39	≥15.40
SEER of Variable speed air conditioning unit	8000 W (27,296 BTU/hr.)	15.00–17.49	17.50–19.99	20.00–22.49	≥22.50
	8000–12,000 W (27,296–40,944 BTU/h.)	14.00–16.49	16.50–18.99	19.00–21.49	≥21.50

. The SEER value is based on the Thailand electric appliance standard [40], that applied to electrical appliances sold in the Thai market, which BEC incorporates into its own code. The EGAT 5-star label has different steps depending on the level of energy efficiency achieved by the electrical appliances, ranging from level 5 with no stars to level 5 with 3 stars. For the split-type air-conditioning units, the SEER allowance is divided into fixed-speed and variable-speed (inverter) air-conditioning units. This is due to the different technologies and the possibility of achieving higher efficiency with inverter air-conditioners. Thus, the increased bar with higher SEER compared to fixed-speed air-conditioners.

2.4. Whole Building Energy Consumption

For the total energy consumption of the building, the calculation can be performed with the BEC by combining the annual energy usage as shown in Equation (6). From the equation, the energy consumption for an air conditioning system is calculated based on the cooling load, which includes factors such as the building envelope heat source, lighting, number of occupants, air leakage, and electrical equipment. Coefficient values for the heat source affecting the cooling load and hours of operation for each building type are summarized in Table 4. In addition, the code allows for the use of renewable energy by subtracting the total building energy use of the building from the annual energy production from solar, heat, or other types of renewable energy. The evaluation process of the BEC was performed by comparing the annual building energy consumption to reference buildings with similar floor areas and standard values for subsystems.

$$\begin{array}{c}{E}_{pa}={\displaystyle {\displaystyle \sum }_{i=1}^n}\left[\begin{array}{c}\frac{A_{wi}\left(OTT{V}_i\right)}{CO{P}_i}+\frac{A_{ri}\left(RTT{V}_i\right)}{CO{P}_i}\hfill \\ +A_i\left\{\frac{C_l\left(LP{D}_i\right)+C_e\left(EQ{D}_i\right)+130C_o\left(OCC{U}_i\right)+24C_v\left(VEN{T}_i\right)}{CO{P}_i}\right\}\hfill \end{array}\right]{\eta }_h\hfill \\ +{\displaystyle {\displaystyle \sum }_{i=1}^n}{A}_i\left(LP{D}_i+EQ{D}_i\right){\eta }_h-\left(PVE+HEE+ORE\right)\hfill \end{array}$$

(6)

where

• $EQD$ = other equipment energy usage per area (W/m²);
• $OCCU$ = number of occupants in building (1/m²);
• $VENT$ = air leakage (l/s/m²);
• $COP$ = air conditioning unit coefficient of performance;
• $PVE$ = annual electricity production from photovoltaic;
• $HEE$ = annual heat energy usage in building;
• $ORE$ = annual electricity production from renewable energy sources;
• $C_l$ = coefficient lighting system cooling load;
• $C_e$ = coefficient electrical equipment cooling load;
• $C_o$ = coefficient occupants cooling load;
• $C_v$ = coefficient air ventilation cooling load;
• ${\eta }_h$ = total number of operating hours per year.

Table 4. Cooling load coefficient using whole building energy consumption calculation.

Classification of Buildings	${\mathit{C}}_{\mathit{l}}$	${\mathit{C}}_{\mathit{e}}$	${\mathit{C}}_{\mathit{o}}$	${\mathit{C}}_{\mathit{v}}$	${\mathit{\eta }}_{\mathit{h}}$
Office, School	0.84	0.85	0.90	0.90	2340
Theater Department store, Convention building, Entertainment complex	0.84	0.85	0.90	0.90	4380
Hotel, Hospital, Condominium	1.0	1.0	1.0	1.0	8760

Table 4. Cooling load coefficient using whole building energy consumption calculation.

Classification of Buildings	${\mathit{C}}_{\mathit{l}}$	${\mathit{C}}_{\mathit{e}}$	${\mathit{C}}_{\mathit{o}}$	${\mathit{C}}_{\mathit{v}}$	${\mathit{\eta }}_{\mathit{h}}$
Office, School	0.84	0.85	0.90	0.90	2340
Theater Department store, Convention building, Entertainment complex	0.84	0.85	0.90	0.90	4380
Hotel, Hospital, Condominium	1.0	1.0	1.0	1.0	8760

3. Research Methodology

The study process consists of evaluating the case study building, which is modeled after an actual office building located in Bangkok, Thailand, with the current envelope in compliance with Thailand’s building energy code using the BEC software. The BEC software is a web-based building energy analysis program capable of simulating whole building energy, including envelope, lighting, air-conditioning, hot water, and renewable energy system based on the location of the evaluated building. In addition, it also assesses compliance with the Thailand building code automatically. The result from the building code evaluated will be used as a basis for comparison in case of changing window material with different glass components. Five different glass components with distinctive characteristics have been selected for each case study. The assessment has been performed on an aspect of OTTV and whole building energy consumption to select suitable glass material that passes the standard passageway one. Finally, the economic performance of the selected glass material will be assessed using IRR and DPP as an indicator for feasibility determination

3.1. Case Study Building

The building used in the case study is an office building located in Bangkok, Thailand. It consists of six floors with a total usable area of 5100 m² divided into two zones. The area with air-conditioning is 3475 m², and the area without air-conditioning is 1675 m². Thus, the case study building underwent a BEC assessment based on an area under a single roof of more than 2000 m² according to standard criteria. The envelope area that has air conditioning is used in the calculation because only areas with air conditioning for environmental heat can affect the cooling load. The case study building has a window-to-wall ratio (WWR) of 0.49 with an opaque area of 711.89 m² and a transparent area of 703.9 m², as shown in the simple SketchUp diagram in Figure 1.

3.2. Material in Building Envelope

The thermal characteristic of opaque and transparent materials used in the case study building can be summarized as shown in

Table 5. The thermal properties of the materials used in the case study building envelope.

Opaque Material
Material	Thermal Conductivity (W/m·K)	Density (kg/m3)	Specific Heat (kJ/kg·K)
Concrete plaster	0.72	1860	0.84
Autoclaved Aerated Concrete	0.476	1280	0.84
Gypsum plates	0.282	800	1.09
Fiberglass insulation	0.033	32	0.96
Reinforced concrete	1.442	2400	0.92
Alu carbon sheet	5.54	1375	0.897
Steel	47.6	7840	0.50
Transparent material
Material	Thickness (m)	U-Value (W/m2·K)	SHGC
Clear float glass	0.06	5.74	0.82
Air gap
	Types	Thickness (m.)	Air gap resistance (m2·C/W)
Air gap (roof)	High radiation level	0.6

. The main opaque material is concrete, which is mainly used for the building walls, whereas steel and Alu carbon sheets are scattered throughout the building envelope. For the air gap, it can act as a natural insulator, blocking heat from passing through building skin.

The case-study building had five different opaque components for the walls and one type of roof, as summarized in

Table 6. Material composition of the case study building.

	Diagram	Detail	Thickness (m.)	U-Value (W/m2·K)
Wall 1		Concrete	0.42	0.956
Wall 2		Autoclaved Aerated Concrete	0.01	1.461
		Concrete block	0.18
		Autoclaved Aerated Concrete	0.01
Wall 3		Alu carbon Sheet	0.004	3.073
		Air gap	0.2
		Alu carbon Sheet	0.004
Wall 4		Autoclaved Aerated Concrete	0.01	2.991
		Concrete block	0.08
		Autoclaved Aerated Concrete	0.01
Wall 5		Steel Sheet	0.003	6.093
Glass 1		Clear float glass	0.06	5.74
Roof 1		Reinforced concrete	0.25	0.231
		Air Gap	0.6
		Fiberglass insulation	0.075
		Gypsum plates	0.009

. The first type is pure concrete with a thickness of 42 cm, which is a column and beam. The second opaque component is a 20 cm thick concrete wall, and the fourth type also has a similar component with a slightly smaller thickness of 10 cm. As for the window and door frames, the third type was an aluminum sheet with an air gap between the sheets, and the fifth type is a steel sheet, which has second highest and highest U-values, respectively. This is owing to the thermal properties of the material, which has high U-values. However, the areas with these two types of opaque components accounted for only a small portion of the building envelope and did not significantly affect the OTTV. In contrast, the transparent material used in the case study is float glass, which has two times the thermal transmittance compared to opaque components. Thus, retrofitting the glass material can significantly reduce the heat transferred into the building from the environment and reduce the energy consumption of the air-conditioning systems and the energy consumption of the entire building.

4. Energy Performance

The BEC software version 2.6.0 provided by the Ministry of Energy was used to evaluate whether the office building met Thai building energy codes. The case study building does not have a hot water system or renewable energy sources. Therefore, the three subsystems to be evaluated are the building envelope, lighting system, and air-conditioning system for the first pathway and the total energy consumption of the building in the second pathway.

4.1. Assessment of the Base Case Building

The BEC evaluation result of the base-case office building without modifications is summarized in

Table 7. BEC evaluation of the case study office building using base case glass.

Description		Standard	Case Study Building	Result
First Passageway	OTTV (W/m2)	≤50.00	112.63	Failed	Failed
	RTTV (W/m2)	≤10.00	3.73	Passed
	LPD (W/m2)	≤10.00	7.62	Passed
	Seasonal Energy Efficiency Ratio (SEER)	≥12.40	12.89	Passed
	Hot Water System	No Installation on Building
Second Passageway	Hot Water System	No Installation on Building			Failed
	Whole Energy Building Consumption (kWh/year)	≤226,021.83	264,474.22	Failed

. The results generated by the BEC software show that the case-study building passes only two subsystem evaluations when the first pathway is used. The lighting equipment installed in the working area consumed less power per square meter than the standard. For example, the lighting power density (LPD) value of 7.62 W/m² was within the standard allowance (less than 10 W/m²). Each unit of the air-conditioning system of the case-study building was assessed using the standard by comparing the SEER to the standard (greater than 12.40). The air-conditioning system used in the building had an average SEER of approximately 12.89, which was consistent with the standard.

However, the envelope system of the case study building did not meet the OTTV and was 70.53 W/m², which was significantly higher than the standard allowance for office buildings (less than 50 W/m²). However, the RTTV was within the standard allowance (less than 10 W/m²) at 3.73 W/m². Therefore, the case study building needs to be evaluated using the second pathway. Comparing the total energy consumption, the total annual energy consumption of the case study building was 264,474.22 kWh/yr., slightly higher than the standard reference building (less than 226,021,83 kWh/yr.). Thus, the building did not comply with the Thai energy codes.

This result indicates that the envelope material used on the wall and windows is not sufficiently energy-efficient to meet the standard value. The main factor causing a significant amount of thermal transfer is the transparent material of the window. This allows heat from the environment to enter the building and causes higher energy consumption for cooling loads in air-conditioning systems. In the current case study of building envelope components, the windows were only equipped with clear glass. Different glass materials with unique properties were selected as replacements to reduce thermal transfer and energy usage in cooling loads, and each type of glass was evaluated to find efficient materials for building windows.

4.2. Assessment of the Glass Material Retroffited Case Building

Based on the BEC evaluation, the OTTV in the case study did not meet the BEC. One of the methods to improve building OTTV is to replace the material of the envelope with a material with a lower heat transfer coefficient (U-value). As shown in Table 4, glass had the second highest U-value after the steel frame. However, the envelope area of the glass was much higher than that of the steel frame, with a WWR of 0.49. Thus, changing the glass material can significantly improve the OTTV of the building. Two significant thermal characteristics of transparent material are U-value and SHGC. So, the retrofitted case considers these two factors when selecting replacement material. Five different glass that has been used in Thailand markets are selected for retrofitted, and their properties can be summarized in

Table 8. Thermal properties of the selected glass material for retrofitted case.

	Material	Composition	Thickness (m)	U-Value (W/m2·K)	SHGC
Glass Type 1	Clear float glass		0.06	5.25	0.82
Glass Type 2	Ocean green float glass		0.06	5.25	0.60
Glass Type 3	Clear reflective glass		0.06	4.65	0.46
Glass Type 4	Ocean green reflective glass		0.06	3.99	0.28
Glass Type 5	Double-glazed glass with air gap		0.06–0.12–0.06 *	1.66	0.25

^* Thickness value consists of outer glass panel-air gap-inner glass panel.

. The diagram shown depicts the composition of glass material with more than one layer, which is also included in the table. The glass material currently used for the windows of the case study building was a 6 mm thick clear glass. The retrofitted glass material consisted of five unique glass compositions, labeled glass types 1–5. Glass type 1 is a clear float glass but has slightly different thermal characteristics. Glass type 2 has a similar U-value to type 1 with lower SHGC due to ocean green color coating. Glass type 3 is a heat-reflective glass that can reflect much of the solar heat from the interior of a building, reducing the U-value and SHGC value. Glass type 4 is similar to glass type 3, with a reflective coating on the ocean green glass. Therefore, the U value and SHGC have been slightly reduced. Glass type 5 is double-glazed with an air gap between panels, giving the best U-value and SHGC compared to other glass materials.

To evaluate the effect of glass material on the energy consumption of the building, the lighting, air conditioners, and other equipment in the building were set similarly to the base case. Only the glass material in the window section of the building envelope was replaced with different materials. The results of the evaluation using BEC software for the four main aspects compared to the standard are listed in

Table 9. Evaluation of the energy demand of the subsystems of the building envelope and the whole building using different types of glass.

Description	OTTV (W/m2)			Whole Energy Building Consumption (kWh/Year)
Standard	≤50.00	Reduction (%)	Compliance	≤226,021.84	Reduction (%)	Compliance
Case study building	112.63	-	Failed	264,474.22	-	Failed
Glass Type 1	70.53	37.38%	Failed	224,156.76	0.83%	Passed
Glass Type 2	57.30	49.13%	Failed	210,638.65	6.81%	Passed
Glass Type 3	47.45	57.87%	Passed	200,572.35	11.26%	Passed
Glass Type 4	35.04	68.89%	Passed	187,901.82	16.87%	Passed
Glass Type 5	27.68	75.42%	Passed	180,373.75	20.20%	Passed

. It can be seen that the newer glass can provide a better value in terms of OTTV compared to the base case. Glass type 1 is a clear glass material that has a similar U-Value and lower SHGC value to the glass currently used in the base-case buildings. Thus, changing this type of glass may lower the OTTV, but not enough to meet the compliance level. Glass type 2 is a green glass similar to type 1. To meet the energy code, the glass material must be coated with a reflective substance, as shown in glass type 3, where the OTTV is within the standard allowance. Looking at the second pathway, clear float glass with better thermal properties can reduce the energy consumption of the whole building to the standard value. However, the reduction rate for the OTTV and energy consumption of the whole building is not linearly correlated because other factors must be taken into account when calculating the energy whole building’s energy consumption, but the OTTV is directly influenced by material thermal characteristic.

5. Economic Feasibility

In this section, a study is conducted on the economic feasibility of retrofitting a building window with glass material. This study uses economic reference data from Thailand, including inflation rate, electricity price, and material and labor costs, to evaluate the economic feasibility.

5.1. Economic Indexes

Two economic indexes were used to evaluate the feasibility of retrofitting glass materials in building windows for a 25-year project lifetime. Factors such as installation costs, labor costs, and revenue in the form of energy cost savings were used as the basis for the analysis. Typically, the payback period (PP) can be used as a simple indicator to evaluate the economic potential. However, actual investment returns lose value over time due to factors such as inflation, degradation of income, and the value of the interest rate. To account for these parameters, the DPP is used as an index to determine the period over which the investment returns occur by accounting for the time value of money. The DPP can be calculated using the discounted cash flow shown in Equation (7) to determine the period over which the investment will earn a return, as displayed in Equation (8) [41].

$$\mathrm{Discounted} \mathrm{Cash} \mathrm{Flow}=\frac{\mathrm{Actual} \mathrm{Cash} \mathrm{inflow}}{{\left(1+i\right)}^n}$$

(7)

$$DPP=A+\frac{B}{C}$$

(8)

where

• $i$ = rate of interest used to determine the present value of future cash flows;
• $n$ = period in which cash inflows are related;
• $A$ = last period with negative cumulative cash flow;
• $B$ = absolute value of the discounted cumulative cash flow at the end of period A;
• $C$ = discounted cash flow during the period after A.

For the second index, the IRR is used to estimate shows the annual percentage return of the project investments. To calculate the IRR, the NPV of future cash flows must first be determined. By definition, the NPV is the gap between the cash flow in terms of present value and the original cost of investment over a projected lifespan. A positive NPV indicates that the present value of the project’s cash flow exceeds its cost and has potential for investment. The NPV can be calculated using Equation (9) [42]. IRR, by definition, means the discount rate that causes the NPV to be zero. Thus, the IRR can be obtained by solving Equation (10) [42].

$$NPV={\displaystyle \sum }_{t=1}^T\frac{C_t}{{\left(1+k\right)}^t}-C_o=0$$

(9)

$${\displaystyle \sum }_{t=1}^T\frac{C_t}{{\left(1+IRR\right)}^t}-C_o=0$$

(10)

where

• $C_t$ = net cash inflows during period t;
• $C_0$ = total initial investment cost;
• $t$ = the number of time periods;
• $k$ = discount rate.

5.2. Economic Data

The economic parameter requirements for the evaluation are summarized in

Table 10. The material and installation costs for retrofitted office buildings with different glass types.

	Glass Type 3	Glass Type 4	Glass Type 5
Glass Area (m2)	703.90	703.90	703.90
Glass Cost (USD/m2) *	33.32	43.57	85.43
Frame and Labor Cost (USD/m2) *	0.31	0.31	3.43
Glass Material Cost (USD) *	25,330.41	35,397.16	66,925.14
Inflation Rate (%)	6	6	6
Energy Cost (USD/kW·h) *	0.13418	0.13418	0.13418

* The exchange rate is approximately 1 THB = 0.029 USD (April 2023).

. The electricity cost derived from MEA category 3 medium-type businesses was 0.13418 USD per kWh, including the service charge, float time, and taxes [43]. The glass material cost was obtained from the glass manufacturer, and the labor cost for installing the different types of windows was obtained from the Comptroller General’s Department (CGD), Ministry of Finance [44]. The total cost of installation also increased the absorbed cost by 7%, including packaging, transportation of manpower, and materials. In addition, the case study assumes that there is no cost for installing the frame in the retrofitted case for the retrofitted glass material only.

Glass material types 1 and 2 were not selected for the economic evaluation because the material failed the BEC subsystem evaluation (pathway 1) in relation to the building envelope. Therefore, only three glass types are compared in terms of the economic feasibility of retrofitted glass materials for buildings. The Glass material types 3 and 4 were single monolithic heat-reflective glass with differences in color. Thus, the material cost is on a similar level with green color glass for type 4, a bit higher than clear glass in type 3. Glass type 5, a double-layer glass with low-e coating, has significantly higher material and labor costs up to 10 times that of single-layer clear glass. Labor costs for the single-layer glass were similar. However, for glass types 3 and 4, the material costs for reflective glass are three to four times higher than for glass without a coating.

5.3. Economic Result

An economic analysis of an office building where different glass materials were used for the windows is summarized in

Table 11. Summary of economic indexes for retrofitted office buildings with different glass types.

		Glass Type 3	Glass Type 4	Glass Type 5
Energy Reduction per Year (kW·h)		63,901.87	76,572.40	84,100.47
Energy Cost Reduction per year. (USD)		8574.35	10,274.48	11,284.60
In case only glass material	Total Installation Cost (USD)	25,330.41	35,397.16	66,925.14
	IRR (%)	33.83%	28.98%	16.49%
	DPP (years)	2.88	3.45	6.37
In case of material included frame	Total Installation Cost (USD)	80,446.12	88,490.73	150,836.48
	IRR (%)	9.57%	10.70%	5.54%
	DPP (years)	10.41	9.51	15.08
	cumulative cash flow(USD)	86,954.88	112,102.72	69,477.94

. The case study is divided into the case where only the glass material is retrofitted and the case where both the glass material and frame are retrofitted, as costs differ significantly.

The result from the evaluation shows that the different glass materials make a significant difference in the degree of energy savings and, thus, a reduction in the energy cost of the building. Glass type 3 is a clear glass material with a reflective coating that prevents the heat from entering the building interior. Thus, retrofitting this type of glass material can reduce energy consumption and cost per year. When retrofitting with only the glass material, the IRR was the highest at 33.83%, and the DPP was 2.88 years. However, when the cost of the retrofitted frame is included, the initial investment increases significantly, resulting in a lower IRR of 9.57% with a DPP of 10.41 years. Glass type 4 is a similar reflective glass with a green color. The cost of the material is higher than that of glass type 3, but the frame cost is similar. Thus, the retrofitted glass material, including the frame, provided the best index value with an IRR of 10.70% and a DPP of 9.51 years. For glass materials with a double layer in glass type 5, the installation cost is much higher than single-layer glass due to the complex window frame. Glass type 5 has the best thermal characteristics with double-layer glass, which in turn results in the highest reduction in energy consumption for the entire building. However, the cost is significantly higher, with a reduced rate of approximately 50% more than glass type 4, and the installation cost is two times higher. This reflects the lower economic indexes compared to reflective glass. This result is also displayed when considering the cumulative cash flow throughout the 25-year project lifetime. Glass type 4 can provide the project owner with the highest cumulative cash flow, while the best energy performance in glass type 5 can obtain the lowest amount of cumulative cash flow due to the highest installation cost.

6. Conclusions

Air conditioning is the main system in the building that requires the largest amount of electricity in buildings due to cooling load from heat entering the room through the windows. Therefore, the thermal characteristics of glass directly affect the energy consumption of an entire building. Therefore, the objective of this study is to conduct a feasibility study on different types of retrofitted glass materials for building windows. The Thai building energy code was used as an evaluation tool and basis for comparison between the different case studies. The results show that windows retrofitted with glass material with a lower U-value can provide better OTTV for the building envelope subsystem. In addition, a reduction in OTTV means a reduction in energy consumption for the entire building. The reduction in energy consumption translates into energy cost savings for buildings with retrofitted windows that can offset the installation costs of new glass materials. However, the economic parameter for evaluating feasibility depends not only on energy savings but also on the cost of retrofitted glass. This discrepancy is evident in the case of glass type 5, where the energy savings from replacing the glass increase by up to 20.20%, while the economic parameter is worse than that of the other glass types. For the case study building, glass type 4 provides the best economic result with an IRR of 10.70%, a payback period of approximately 9.51 years, and annual energy savings of approximately 16.87%. In addition, the renovation project with glass type 4 can achieve a cumulative cash flow of up to 112,102.72 USD through a 25-year project lifetime. This results from the discrepancy between cost and savings, as the installation cost for glass type 5 is up to six times higher than for glass type 3, whereas the percentage of energy savings is only three times higher. A balance must be struck between energy savings and installation costs to conduct a feasibility study for project owners who wish to invest in retrofitted glass. The result from the evaluation of changing glass material in building windows demonstrated that energy saving through retrofitted buildings is feasible. Thus, the renovation of an aging building with energy efficient envelope with additional energy measurement can be evaluated in future studies in order to provide the project owner with a feasibility study to achieve a net zero energy building. In addition, this study applied specifically to buildings located in the tropical climate. Thus, the assessment of performance in different climates can be further addressed in future work.