Research on the Current Situation and Calculation Method of Carbon Emissions Assessment for Building Curtain Walls

Abstract

Curtain wall systems stand out as a pivotal domain within the construction sector’s endeavors towards energy efficiency and carbon mitigation. To refine the evaluation framework for carbon emissions within this industry, this paper explores the calculation and assessment method for building curtain walls. The article first reviews the current research status regarding carbon emissions from materials and the impact of curtain walls on buildings in the operational stage. Based on lifecycle theory, the carbon emissions from building curtain walls are divided into six stages: material acquisition, processing and production, installation and construction, transportation, use and maintenance, and dismantling. On this basis, this paper proposes a method for calculating carbon emissions from building curtain walls. Following that, a case study is conducted using a specific glass curtain wall project for illustrative analysis. The results indicate that the carbon emissions from the material acquisition stage constitute approximately 90% of the total, serving as the primary source of carbon emissions for glass curtain walls. Furthermore, the scientific application of photovoltaics can significantly reduce the carbon emission levels of building curtain walls. Finally, an analysis was conducted on the current issues existing in the evaluation of carbon emissions.

1. Introduction

To restrain global climate change and its negative impacts, 178 contracting parties worldwide jointly signed the Paris Agreement in 2015, which sets long-term goals that will guide all nations to achieve carbon neutrality [1,2]. The United States and the European Union plan to achieve carbon neutrality in 2050 [3,4]. China plans to reach a carbon peak before 2030 and carbon neutrality before 2060 [5]. The construction industry is one of the primary contributors to carbon emissions. In China, the total carbon emissions from the entire process of construction nationwide amounted to 40.7 billion tCO₂, accounting for 38.2% of the country’s total carbon emissions in 2021 [6]. Worldwide, the building sector accounts for almost 38% of global energy-related carbon emissions, with at least 20% of these coming from the materials production industry [7].

Curtain walls are an important component of building carbon emissions, especially for public buildings. On the one hand, aluminum alloy and silicate glass, as the primary materials for building curtain walls, have high carbon emission factors. On the other hand, compared to the minimum service life of the main structure, which is usually around 50 years, the design lifespan of curtain walls is typically 25 years, implying the need for at least one retrofitting. According to research by the World Business Council for Sustainable Development on carbon emissions from different types of buildings, the embodied carbon emissions of external envelope structures account for approximately 10% to 31% of the total construction carbon emissions [8]. Therefore, the calculation and evaluation of carbon emissions from building curtain walls are of significant importance to the construction industry.

Currently, two main types of commonly used carbon accounting methods exist. The first one is the “bottom-up” model, which is based on process analysis and is also known as Life Cycle Assessment (LCA). It serves as the core theory for calculating and evaluating carbon emissions in the construction field. The second is the “up-bottom” model, which is based on input-output analysis and is mainly applicable to the economic sector. The LCA method has garnered recognition with several high-profile accounting standards. Examples include the first global accounting standard, the Greenhouse Gas Protocol, developed by the World Resources Institute (WRI), and the IPCC Guidelines for National Greenhouse Gas Inventories by the Intergovernmental Panel on Climate Change (IPCC). These standards have contributed to the formation of two widely influential life cycle assessment theoretical frameworks: SETAC and ISO. In recent years, many scholars have conducted research on the accounting and analysis of the complete lifecycle of carbon emissions of buildings using methods such as input-output analysis [9,10] or LCA [11,12,13]. In 2019, China released the national standard GB/T 51366-2019 Standard for Calculation of Building Carbon Emissions [14], which divides building carbon emissions into the operational stage, the construction and dismantling stage, and the building materials production and transportation stage. The standard specifies detailed calculation methods to guide carbon emission calculations in the construction sector. However, for non-structural components such as curtain walls, there are still many gaps in carbon emission calculations.

As non-structural components, curtain walls differ significantly from the main structure in terms of construction methods, material usage, and engineering calculations. Firstly, the weight of the non-structural curtain wall components is relatively small compared to the main structural system, while the area is large. Therefore, area-based units are more appropriate than mass-based units for carbon emission calculations. Secondly, carbon emissions from the main structural system mainly consider major materials such as steel and concrete, making it difficult to account for small components within curtain walls, which are crucial for the curtain wall industry. Additionally, the carbon emission calculation for the main structural system makes it difficult to compare carbon emissions for different types of curtain walls. Therefore, there is a lack of specific carbon emission calculation methods for building curtain walls.

To fill the gap in carbon emission calculation methods for building curtain walls and guide the development of carbon emission evaluation and reduction strategies, this study summarizes the current status of carbon emissions from building curtain walls. Based on the LCA theory and the GB/T 51366-2019 Standard for Calculation of Building Carbon Emissions [14], a method for calculating carbon emissions from building curtain walls was proposed, in which the carbon emission was divided into six stages: material acquisition, processing and production, transportation, installation and construction, use and maintenance, and dismantling. For the convenience of application in curtain walls, the carbon emission calculation method in this paper uses area (m²) as the benchmark unit. Then, a glass curtain wall project was analyzed as a case study, detailing the process of a carbon emission calculation. Finally, the main issues in the current research and evaluation work were analyzed in this paper, and suggestions were made for addressing these problems and proposing directions for future development.

2. Research Status of Carbon Emissions from Building Curtain Walls

2.1. Material Carbon Emissions

During the operational phase of buildings, curtain walls contribute negligibly to carbon emissions. Therefore, the contribution of building curtain walls to carbon emissions primarily occurs during the construction phase, especially due to the embodied carbon emissions of building materials. Categorized by panel materials, curtain walls encompass glass, metal, stone, and artificial board varieties. The construction of curtain walls varies in accordance with the architectural form, necessitating the determination of material quantities specific to each project.

Currently, the curtain wall market is still predominantly occupied by glass curtain walls. These mainly comprise aluminum alloy framework profiles, insulating tempered glass panels, and sealing materials. According to rough estimates provided by the U.S. Department of Energy [15], the embodied energy of a standard aluminum alloy-framed glass curtain wall stands at 734.5 kWh/m², with a corresponding embodied CO₂ emission of 322.7 kg/m². Furthermore, scholars have extensively investigated the production processes and carbon emission levels associated with the frame materials of glass curtain walls [16,17], which include wood, aluminum, PVC, wood–aluminum composites, fiberglass, and others. By comparing glass curtain wall systems with different frame materials but identical specifications, Azari and Kim [18] found the carbon emission levels of frame curtain wall systems, from low to high, as follows: wood frame, steel frame, and aluminum alloy. The production of aluminum and its alloys, recognized as a high-energy and high-emission industry, emits greenhouse gases per unit product at rates 12.4 and 2.3 times higher than steel and copper [19], respectively. Aluminum alloy profiles remain predominant in glass curtain wall frameworks. Over 90% of their carbon emissions throughout the lifecycle originate from the production phase, specifically from ore to aluminum ingot, with surface corrosion treatments accounting for approximately 5%. Statistics indicate that China’s carbon emission factor for electrolytic aluminum is around 11,200 kgCO₂e/t, significantly higher than Russia’s at about 3300 kgCO₂e/t and the United States’ at approximately 6900 kgCO₂e/t. As a comparison, the carbon emission factor of concrete C30 in building materials is 295 kgCO₂e/t, and the carbon emission factor of steel is 2050 kgCO₂e/t in China [14]. This substantial difference in carbon emission factors is attributed to the energy structure of aluminum production, with China relying mainly on thermal power and Russia on hydropower. Additionally, the process of extruding aluminum metal into aluminum profiles for glass curtain walls results in carbon emissions of approximately 11,131.04 kgCO₂e/t in China [20].

The carbon emissions throughout the lifecycle of glass panels mainly stem from the energy consumption in the production process, particularly from sand to float glass production, accounting for 80%. The deep processing process of tempered glass contributes 15%, while the remaining emissions arise from the assembly process of hollow glass. Yan et al. [21] investigated over 300 flat glass production lines in China and, based on 2015 production conditions, found that the carbon emissions per unit weight of boxed glass were approximately 52.46 kg, equivalent to 1049 kgCO₂e/t. Additionally, Yu et al. [22], based on extensive research data, determined that the carbon emissions for flat glass were 1130 kgCO₂e/t, tempered glass emissions were 1530 kgCO₂e/t, and laminated glass emissions were 1280 kgCO₂e/t. In addition to controlling carbon emissions during the production process, the recycling and utilization of materials are also effective ways to reduce glass carbon emissions. Statistics show that when the glass recycling rate reaches 50%, production process carbon emissions can be reduced by 42%, and when the recycling rate reaches 90%, production process carbon emissions can be reduced by 75% [23]. The efficiency of material recycling and reuse, in addition to the recycling rate, also depends on the quantity of raw materials produced, which is another major factor influencing the implicit carbon emission levels of building curtain walls. In 2019, China’s waste flat glass production was 98.67 million tons, with a recovery of 59.16 million tons, yielding an almost 60% recovery rate, higher than the international level of 35%. Unfortunately, due to the lack of data on carbon emissions from the process of sand to glass liquid in glass production, it is difficult to estimate the contribution of glass recycling to carbon emission reduction.

Sealant, including silicone structural sealant, weather-resistant sealant, and secondary sealant for insulating glass, is another material widely used in glass curtain walls. According to industry data [24], Ethylene Propylene Diene Monomer emits 2670 kgCO₂e/t, while silicone sealant emits 2910 kgCO₂e/t. Despite their high carbon emission factors, sealant materials significantly improve curtain wall performance in energy conservation. Compared to carbon dioxide released during production, silicone resin provides an average benefit nearly nine times higher before the end of its service life, with applications in insulating glass showing benefits exceeding 27 times.

In comparison to glass curtain walls, metal curtain walls exhibit a similarly significant level of carbon emissions. Industry data [24] suggests that 2 mm aluminum veneer emits around 159.6 kgCO₂e/m², equivalent to 29,600 kgCO₂e/t. In contrast, stone curtain walls have relatively lower carbon emissions. Zhao et al. [25] calculated that the carbon emissions during the production stage of 30 mm stone panels were only 18.75 kgCO₂e/t when converted, owing to the low energy consumption inherent in the mechanical processing of stone materials. In recent years, the rapid development of artificial panel curtain walls, such as porcelain panels, ceramic panels, PC panels, and UHPC panels, has been observed. The carbon emissions during the production stage of UHPC panels are approximately 1245.84 kgCO₂eq/m³, which is 1.58 times that of ordinary concrete. Despite the relatively high carbon emissions, artificial panel materials typically offer superior performance advantages. However, due to significant differences in the production processes of artificial panels, current carbon emission data remains incomplete.

The evaluation of embodied carbon emissions in building curtain walls should encompass not only the emissions generated during the material production phase but also integrate considerations of material longevity and recyclability. Currently, in China, the design lifespan of building curtain walls is 25 years, primarily limited by factors such as the surface corrosion resistance level of frame materials, the aging limit of sealants for insulating glass panels, and the fatigue performance of hardware systems in opening parts. Especially for organic sealant materials serving as sealing or structural connection functions, the industry generally recognizes a lifespan ranging from 10 to 25 years, directly determining the lifespan of building curtain wall systems. However, with technological advancements, the international community has been researching and achieving breakthroughs in the goal of achieving a 50-year lifespan for sealant materials [26]. Considering current engineering practices and technological levels, the requirement for building curtain wall materials to have the same lifespan as the system is realistic and feasible. This requirement can significantly reduce the carbon emission levels of building curtain walls caused by their lifespan.

According to the standards in China [14,24], the carbon emission data, which were mainly from comprehensive industry surveys, and crucial carbon emission factor data pertaining to primary materials have been scrutinized and compiled for the benefit of the industry, as delineated in

Table 1. Carbon Emission Factors for Main Materials of Building Curtain Walls.

Materials		Values	Units
Profile	Electrolytic aluminum	20,300	kgCO2e/t
	Ordinary carbon steel	2050	kgCO2e/t
	Hot-rolled carbon steel small-profiled sections	2310	kgCO2e/t
	Hot-rolled medium-sized carbon steel profiles	2365	kgCO2e/t
	Hot-rolled medium-thick carbon steel plates	2400	kgCO2e/t
	Hot-rolled carbon steel H-beams	2350	kgCO2e/t
	Hot-rolled carbon steel rebars	2340	kgCO2e/t
	Hot-rolled carbon steel seamless pipes	3150	kgCO2e/t
	Cold-drawn carbon steel seamless pipes	3680	kgCO2e/t
	Timber	310	kgCO2e/t
Panel	Flat glass	1130	kgCO2e/t
	Aluminum sheet and strip	28,500	kgCO2e/t
	Copper sheet	218	kgCO2e/m2
	Carbon steel hot-dip galvanized sheet/coil	3110	kgCO2e/t
	Carbon steel electro-galvanized sheet/coil	3020	kgCO2e/t
	Aluminum-plastic composite panel	8.06	kgCO2e/m2
	Copper-plastic composite panel	37.1	kgCO2e/m2
Fireproof insulation materials	Rockwool board	1980	kgCO2e/t
Sealing materials	Ethylene Propylene Diene Monomer sealing strip	2670	kgCO2e/t
	Silicone sealant	2910	kgCO2e/t
	Polyurethane foam	4330	kgCO2e/t
Fasteners and hardware materials	Mild steel	2050	kgCO2e/t
	Carbon steel	1960	kgCO2e/t
	Stainless steel	6800	kgCO2e/t
	Galvanized steel	2487	kgCO2e/t
Packaging materials	Plastic film	2570	kgCO2e/t
	Corrugated paper	1230	kgCO2e/t
Other materials	Tap water	0.168	kgCO2e/t
	High-density polyethylene	2620	kgCO2e/t
	Low-density polyethylene	2810	kgCO2e/t
	Polyvinyl chloride	7300	kgCO2e/t
	Linear low-density polyethylene	1990	kgCO2e/t

.

2.2. Impact of Carbon Emissions on Construction Operation Phase

In the operational phase of buildings, curtain walls contribute negligibly to direct carbon emissions. However, the energy-saving performance of curtain walls significantly affects the carbon emissions of buildings. Curtain walls facilitate thermal exchange between indoor and outdoor environments through mechanisms such as conduction, radiation, and convection, due to solar radiation and temperature differentials. To maintain indoor comfort, heating and air conditioning systems are necessary, the energy consumption of which is significantly influenced by the energy performance of curtain walls.

Based on experiments, theoretical analyses, and simulation software such as Energy Plus 8.2.0, PKPM V3.1, and DeST3.0, the insulation [27,28,29], air tightness [30,31,32], and shading performance [33,34,35] on building energy consumption of building curtain wall have been extensively investigated. The results underscore the pivotal role of curtain walls in building energy consumption. In 2021, China implemented the mandatory general specification GB55015-2021, titled General Specification for Building Energy Conservation and Utilization of Renewable Energy [36], which stipulates that the carbon emission intensity of newly constructed residential and public buildings should be reduced by an average of 40% compared to the energy-saving design standards implemented in 2016. Additionally, it requires that the carbon emission intensity should be reduced by an average of at least 7 kgCO₂/(m²·a). To ensure the achievement of the target, it is crucial to enhance the energy efficiency of building curtain wall systems, which account for over 50% of energy consumption. Globally, the potential for energy savings from the construction of high-performance buildings and energy retrofitting of existing building envelope structures exceeds the total energy consumption of all G20 countries in 2015, with an accumulated energy saving of approximately 330 EJ by 2060.

However, due to the complexity of energy consumption during the operational stage of buildings, it is challenging to contain the carbon emissions of this part within the curtain walls. Therefore, it is usually considered in carbon emissions of the whole building. Similarly, the calculation method proposed in this paper only considers the implicit carbon emissions. It should be noted that when evaluating the carbon emissions of building curtain walls, it should be based on the same performance standards. Curtain wall types with lower emissions but inferior performance may not necessarily have an advantage in actual carbon emission levels. This aspect requires further in-depth research for reasonable consideration.

3. Calculation Method of Carbon Emissions of Curtain Walls

3.1. Framework and System Boundaries

Referring to the calculation method for building carbon emissions and considering the engineering characteristics of building curtain walls, the life cycle of building curtain walls is divided into six stages, consisting of material acquisition, processing and production, installation and construction, transportation, use, and dismantling; this includes the entire life cycle from raw materials to waste disposal, as shown in Figure 1. In practical engineering, various response measures may be taken in retrofitting, including minor repairs, energy-saving renovations, safety improvements, and overall refurbishments. Considering the uncertainty of renovation, the carbon emission calculation method proposed in this paper adopts a conservative approach, treating the carbon emissions caused by renovation as if it were a complete refurbishment. Considering that the area is commonly used as the engineering measurement unit in the design, construction, and budgeting of building curtain wall projects and to meet the habits of the industry and facilitate comparisons under different engineering conditions, the carbon emission calculation method in this paper uses area (m²) as the benchmark unit.

Due to differences in energy consumption pathways, it is challenging to quantify which portion of this carbon emission reduction originates from the curtain wall. In contrast, with photovoltaic curtain walls, the contribution to reducing carbon emissions from the curtain wall can be clearly identified. Therefore, in the carbon emission calculation method proposed in this paper, the influence of curtain walls on building carbon emissions during the operational phase is not considered, and photovoltaic curtain walls are treated separately.

3.2. Calculation Method

Based on the different materials of curtain wall panels, they can be roughly categorized into glass curtain walls, metal curtain walls, stone curtain walls, and artificial board curtain walls. Due to the significant differences in materials and construction methods among different types of curtain walls and the possibility of multiple types of curtain walls being used in the same project, the carbon emissions are calculated separately for each type of curtain wall in this paper. Then, the total carbon emissions of the entire curtain wall project are obtained by adding up the emissions from each type as follows:

$$GH{G}_{\mathrm{cw},\mathrm{all}}={\displaystyle \sum _j^{n}GH{G}_{\mathrm{cw},j}{A}_j(N_j+1)}.$$

(1)

In the equation, $GH{G}_{\mathrm{cw},\mathrm{all}}$ represents the total carbon emissions of curtain walls over the lifespan of a single building or buildings in kgCO₂e; $GH{G}_{\mathrm{cw},j}$ represents the carbon emissions per unit area of the j-th type of curtain wall in kgCO₂e; $A_j$ represents the area of the j-th type of the curtain wall in m²; $N_j$ represents the number of replacements of the j-th type of curtain wall over the entire lifespan.

According to the delineation of different stages within the boundary of the building of the curtain wall, the carbon emissions per unit area of different types of curtain wall can be represented as follows:

$$GH{G}_{\mathrm{cw},j}=GH{G}_{\mathrm{gain},j}+GH{G}_{\mathrm{proc},j}+GH{G}_{\mathrm{tran},j}+GH{G}_{\mathrm{cons},j}+GH{G}_{\mathrm{use},j}+GH{G}_{\mathrm{aban},j},$$

(2)

In the equation, $GH{G}_{\mathrm{gain},j}$ represents the carbon emissions per unit area of the curtain wall during the material acquisition stage, $GH{G}_{\mathrm{proc},j}$ represents the carbon emissions per unit area of the curtain wall during the processing and production stage, $GH{G}_{\mathrm{tran},j}$ represents the carbon emissions per unit area of curtain wall during the transportation stage, $GH{G}_{\mathrm{cons},j}$ represents the carbon emissions per unit area of the curtain wall during the installation and construction stage, $GH{G}_{\mathrm{use},j}$ represents the carbon emissions per unit area of curtain wall during the use and maintenance stage, and $GH{G}_{\mathrm{aban},j}$ represents the carbon emissions per unit area of the curtain wall during the dismantling stage. For building curtain walls, the carbon emissions at different stages can be categorized into three parts: materials, energy consumption, and combustion of fossil fuels, where the combustion of fossil fuels refers to the generation of greenhouse gases.

Considering the differing statistical practices across various stages of engineering, no uniform calculation formula has been established. Therefore, each stage is computed separately. The carbon emissions during the material acquisition stage of the unit area curtain wall can be represented as follows:

$$GH{G}_{\mathrm{gain},j}={\displaystyle \sum _{i=1}^n{G}_{i,j}{M}_i}.$$

(3)

In the equation, $G_{i,j}$ denotes the consumption per unit area of a specific material i for a given curtain wall j, in kg/m² or m²/m², while $M_i$ represents the carbon emission factor of material i, in kgCO₂e/kg or kgCO₂e/m². Building curtain wall materials encompass profiles, glass, stone, aluminum panels, fireproof insulation materials, sealing materials, fasteners, embedded parts, hardware materials, packaging materials, etc. Typically, the manufacturers provide the carbon emission factors of materials.

The processing and production stage of curtain walls mainly includes the processing of profiles, assembly of unit modules, assembly of opening fans, assembly of sun shading louvers, glass gluing, and storage packaging processes. During the processing and production process, materials such as packaging, cleaning, and auxiliary materials are involved, as well as energy consumption from equipment operation and on-site transportation. The carbon emission intensity of curtain wall processing and production per unit area can be expressed as follows:

$$GH{G}_{\mathrm{proc},j}={\displaystyle \sum _{i=1}^n{G}_{i,j}{M}_i}+{\displaystyle \sum _{i=1}^n{P}_{i,j}{E}_i}+GH{G}_{\mathrm{comb},j}.$$

(4)

In the equation, $P_{i,j}$ represents the consumption per unit area of a specific energy i for curtain wall j, in (kW·h)/m² or L/m² or kg/m², where energy types may include electricity, petroleum, coal, etc.; $E_i$ represents the carbon emission factor of energy i, measured in kgCO₂e/(kW·h), kgCO₂e/L, or kgCO₂e/kg; $GH{G}_{\mathrm{comb},j}$ represents the carbon emissions per unit area generated by the combustion of fossil energy i for curtain wall j, which can be calculated using the following formula:

$$GH{G}_{\mathrm{comb},j}={\displaystyle \sum _{i=1}^n\left(F{C}_{i,j}\times NC{V}_i\times C{C}_i\times O{F}_i\times \frac{44}{12}\right)}.$$

(5)

In the equation, $F{C}_{i,j}$ represents the consumption per unit area of a specific type of fossil energy i for curtain wall j, in L/m² or kg/m²; $NC{V}_i$ represents the average lower heating value of fossil energy type i, in GJ/L or GJ/kg; $C{C}_i$ represents the carbon content per unit heat value of fossil energy type i, in kgC/GJ; $O{F}_i$ represents the carbon oxidation rate of fossil energy type i, in %.

The transportation stage of building curtain walls primarily involves two parts: transporting the acquired materials to the processing workshop and then transporting them to the construction site after processing. Transportation within the workshop or site, as well as the external transportation of waste during the dismantling phase, are not included. Unlike other stages, carbon emissions during transportation are typically calculated based on different transportation methods and distances. The carbon emissions of curtain wall transportation per unit area can be expressed as follows:

$$GH{G}_{\mathrm{tran},j}={\displaystyle \sum _{k=1}^m{\displaystyle \sum _{i=1}^n{Q}_{k,i,j}{D}_{k,i,j}{F}_k}},$$

(6)

In the equation, $Q_{k,i,j}$ denotes the quantity per unit area of the material i for transportation mode k of the curtain wall j, in kg/m²; $D_{k,i,j}$ represents the transportation distance of the material i for transportation mode k of the curtain wall j, in km; $F_k$ signifies the carbon emission factor of the transportation mode k, in kgCO₂e/(kg·km).

The calculation of carbon emissions $GH{G}_{\mathrm{cons},j}$ during the installation and construction phase of unit-area curtain wall construction relies on Equations (4) and (5), encompassing measures such as the on-site storage of components, on-site transportation, auxiliary installation of scaffolding, installation processes, and curtain wall cleaning.

Similarly, the calculation of carbon emissions $GH{G}_{\mathrm{use},j}$ during the use and maintenance phase of the unit-area curtain wall is also based on Equations (4) and (5). The stage involves activities such as replacing aged materials, repairing faulty components, daily cleaning of curtain walls, and energy consumption of control systems like electric sunshades. When the photovoltaic curtain wall is applied, $P_{i,j}$ in Equation (4) should include the electricity generation resulting from the photovoltaic, with a negative value indicating carbon reduction due to photovoltaic capacity.

The calculation of carbon emissions $GH{G}_{\mathrm{aban},j}$ during the dismantling phase of a unit-area curtain wall is likewise based on Equations (4) and (5), primarily including the material and energy consumption from dismantling and auxiliary measures during the dismantling process, as well as carbon emissions from waste transportation.

3.3. Case Study

A specific aluminum-glass curtain wall is considered, comprising both framed and photovoltaic curtain walls. The building is designed to endure for 50 years. The area of the framed glass curtain wall is 8000 m², with a designated service life of 25 years, while the photovoltaic curtain wall spans 1000 m², similarly engineered for a 25-year period. The design drawing for the curtain wall section is shown in Figure 2.

The material quantities and carbon emission factors during the material acquisition phase of the curtain wall are outlined in

Table 2. Activity level data and carbon emission factors during the material acquisition stage.

Materials	Technical Specifications	Material Quantity		Carbon Emission Factors
		Values	Units	Values	Units
Aluminum alloy profiles	Thermal break, powder coating	10	kg/m2	20.3	kgCO2e/kg
Hollow glass 1	Low-E float glass 10 mm + 12Air + Tempered glass 10 mm, homogenization treatment	0.36	m2/m2	57.9	kgCO2e/m2
Hollow glass 2	Low-E float glass 6 mm + 12Air + Tempered glass 6 mm, homogenization treatment	0.64	m2/m2	34.7	kgCO2e/m2
Aluminum plate	2 mm, powder coating	0.29	m2/m2	159.6	kgCO2e/m2
Fireproof insulation	100 mm thick rock wool insulation, density 100 kg/m3	2.90	kg/m2	1.98	kgCO2e/kg
Steel components	Hot-dip galvanized steel	0.9	kg/m2	2.4	kgCO2e/kg
Sealant material 1	Ethylene Propylene Diene Monomer	1.6	kg/m2	2.67	kgCO2e/kg
Sealant material 2	Silicone sealant	1.8	kg/m2	2.91	kgCO2e/kg
Fasteners	Stainless steel fasteners	0.15	kg/m2	6.8	kgCO2e/kg
Auxiliary materials	Polyethylene foam rods	0.04	kg/m2	2.81	kgCO2e/kg

. When calculating the consumption of materials, material loss should be considered and converted based on the service life of the materials and the projects. Utilizing Equation (3), the carbon emissions $GH{G}_{\mathrm{gain},j}$ during the material acquisition phase of the curtain wall were calculated to be 310.9 kgCO₂e/m².

The material and energy consumption, as well as the carbon emission factors during the processing and production stage of the curtain wall, are delineated in

Table 3. Activity level data and carbon emission factors during the processing and production stage.

Materials and Energy	Purpose	Material Quantity		Carbon Emission Factors
		Values	Units	Values	Units
Electricity	Operation of machinery, on-site transportation, etc.	0.9	(kW·h)/m2	0.9419	kgCO2e/(kW·h)
Corrugated paper	Packaging materials, etc.	0.2	kg/m2	1.41	kgCO2e/kg

. These factors primarily encompass the electricity consumption of processing machinery and packaging materials. Based on Equations (4) and (5), the carbon emissions $GH{G}_{\mathrm{proc},j}$ during this stage were calculated to be 1.13 kgCO₂e/m².

Table 4. Activity level data and carbon emission factors during the transport stage.

Product	Purpose	Values (kg/m2)	Distance (km)	Transportation Mode	Carbon Emission Factors (kgCO2e/(kg·km))
Materials	Aluminum alloy profiles	10	500	Medium-sized diesel trucks (Load capacity of 8 tons)	0.179 × 10−3
	glass	50	400	Medium-sized diesel trucks (Load capacity of 8 tons)	0.179 × 10−3
	Steel components	0.9	300	Small-sized diesel trucks (Load capacity of 2 tons)	0.286 × 10−3
	Other materials	10	300	Small-sized diesel trucks (Load capacity of 2 tons)	0.286 × 10−3
Semi-finished products	-	60	80	Large-sized diesel trucks (Load capacity of 10 tons)	0.162 × 10−3

presents the materials, energy consumption, and carbon emission factors for the transportation phase of the curtain wall. Usually, aluminum alloy profiles and glass materials are sourced from manufacturing companies, resulting in longer transportation distances. Steel components can be purchased from the market, allowing for sourcing from nearby locations. Semi-finished products typically come from processing plants near the construction site, thus minimizing transportation distances.

Considering the carbon emissions during the return journey of the transportation vehicle, the transportation distance needs to be calculated twice. Consequently, employing Equation (6) yielded the carbon emissions $GH{G}_{\mathrm{tran},j}$ for the transportation phase as 12.38 kgCO₂e/m².

The materials, energy consumption, and carbon emission factors during the installation and construction stage of the curtain wall are detailed in

Table 5. Activity level data and carbon emission factors during the installation and construction stage.

Materials and Energy	Purpose	Material Quantity		Carbon Emission Factors
		Values	Units	Values	Units
Electricity	Operation of machinery, on-site transportation, etc.	2	(kW·h)/m2	0.9419	kgCO2e/(kW·h)
Diesel fuel	On-site transportation, etc.	0.36	kg/m2	0.3383	kgCO2e/kg
Carbon steel	Carbon structural steel, Q235B, Hot-dip galvanizing	0.6	kg/m2	2.05	kgCO2e/kg
Tap water	Building water supply	0.03	kg/m2	0.168 × 10−3	kgCO2e/kg

. The carbon emissions primarily encompass the energy consumption during the construction process and the utilization of non-recyclable auxiliary materials. The average low-heat $NC{V}_i$ of diesel, essential for calculation purposes, stands at 42.652 × 10⁻³ GJ/kg. Correspondingly, the carbon content per unit calorific $C{C}_i$ is estimated to be 20.2 kgC/GJ, with a carbon oxidation rate of 99%. Consequently, the carbon emissions $GH{G}_{\mathrm{cons},j}$ of the curtain wall installation and construction stage were derived through the application of Equations (4) and (5), resulting in 4.37 kgCO₂e/m².

Table 6. Activity level data and carbon emission factors during the using stage (annually).

Materials and Energy	Purpose	Material Quantity		Carbon Emission Factors
		Values	Units	Values	Units
Electricity	Maintenance and upkeep	0.5	(kW·h)/m2	0.9419	kgCO2e/(kW·h)
	Power consumption of control systems	0.01
Tap water	Building water supply	1.5	kg/m2	0.168	kgCO2e/kg
Cleaning agent	Glass cleaner	0.1	kg/m2	2.0	kgCO2e/kg

presents the annual material and energy consumption, along with carbon emission factors during the usage phase of the curtain wall. With a service life of 25 years, the carbon emissions $GH{G}_{\mathrm{use},j}$ resulting from the consumption of materials or energy during this stage amounted to 17.02 kgCO₂e/m², calculated using Equations (4) and (5).

The material and energy consumption, as well as the carbon emission factors during the dismantling phase of the curtain wall, are presented in

Table 7. Activity level data and carbon emission factors during the dismantling stage.

Materials and Energy	Purpose	Material Quantity		Carbon Emission Factors
		Values	Units	Values	Units
Electricity	Dismantling machinery	1.8	(kW·h)/m2	0.9419	kgCO2e/(kW·h)
Diesel fuel	Dismantling machinery	0.35	kg/m2	0.3383	kgCO2e/kg
Mild carbon steel	Carbon structural steel, Q235B, Hot-dip galvanizing	0.3	kg/m2	2.05	kgCO2e/kg

. The activity level data and carbon emission factors during the transportation process are shown in

Table 8. Activity level data and carbon emission factors during garbage transportation.

Product	Purpose	Values (kg/m2)	Distance (km)	Transportation Mode	Carbon Emission Factors (kgCO2e/(kg·km))
Demolition waste	From the demolition site to the waste disposal facility	61	80	Medium-sized diesel trucks (Load capacity of 8 tons)	0.179 × 10−3
	From the waste disposal facility to the recycling center	14	20	Small-sized diesel trucks (Load capacity of 2 tons)	0.286 × 10−3

. Based on Equations (4)–(6), the carbon emissions $GH{G}_{\mathrm{aban},j}$ during the dismantling phase of the curtain wall were calculated to be 5.43 kgCO₂e/m².

Based on the cumulative analysis, the carbon emissions per unit area of the framed glass curtain wall for this building project amounted to 351.23 kgCO₂e/m². The photovoltaic curtain wall of the project generates approximately 100 kWh/m² of electricity annually. The design lifespan realistic with photovoltaic products is 25 years. Utilizing the electricity carbon emission factor from Table 6, the carbon reduction during its usage phase was calculated to be −100 × 25 × 0.9419 = −2354.75 kgCO₂e/m². Apart from the usage phase of electricity generation, other phases’ configurations were similar to the framed glass curtain wall, with negligible differences. Referring to the carbon emission calculation results of the framed glass curtain wall, the cumulative carbon emissions per unit area of the photovoltaic curtain wall in this building project were −2354.75 + 351.23 = −2003.52 kgCO₂e/m².

The total carbon emissions $GH{G}_{\mathrm{cw},\mathrm{all}}$ of the building curtain wall over its lifecycle amounted to 1612.640 tCO₂e, calculated according to Equation (1). The carbon emissions data for different stages are illustrated in Figure 3. Insights from the case study reveal the following: (1) Excluding photovoltaic components, the material acquisition stage contributes nearly 90% of carbon emissions, serving as the primary source of emissions for glass curtain walls. Consequently, mitigating emissions should primarily focus on material reduction; (2) The scientific application of photovoltaic curtain walls can significantly reduce the carbon emissions associated with building curtain walls.

4. Carbon Reduction Pathways for Building Curtain Walls

Carbon emissions in building curtain walls primarily encompass embodied carbon emissions from the materials and operation stage. The reduction of this in the operation stage can be facilitated through the application of photovoltaics. As for the materials, the methods for carbon reduction can be categorized into three main aspects:

(1)

Optimizing structures to reduce material usage: For instance, curtain wall frame and panel strength designs are considered unfavorable positions, leading to similar construction and material usage for both lower and upper levels in buildings, which may lead to redundancy on the low floor of the building. Therefore, by improving and optimizing the structural forms of curtain walls while ensuring no reduction in performance, the carbon emissions of these structures can be reasonably reduced. Actually, through the analysis of the frame mechanical performance, the optimization of wall thickness and dimensions based on different application positions, such as using thicker and larger profiles for high-rise and high-wind pressure areas while using the thinner and smaller profiles for low-rise and low-wind pressure areas, can effectively reduce the carbon emissions of frame materials. In addition, developing energy-saving and material-saving panel materials and promoting new insulation materials, such as vacuum glass, low-emissivity glass, and aerogel-filled materials, are equally effective;

(2)

Material recycling and substitution: With the advancement of carbon emission assessment for building curtain walls, there is a growing emphasis on materials with low implicit carbon emissions, high recyclability, and long lifespans, such as bamboo, wood, and plant-based framing materials which inherently store carbon, offering significant carbon reduction advantages. Traditional frame materials like aluminum alloy and steel profiles, with similar lifespans and recyclability as buildings, can fully leverage their advantages through enhanced surface anti-corrosion treatment techniques. As for glass, addressing the improvement of aging resistance in sealing materials for assembled hollow glass and refining the recycling processes for raw glass are necessary;

(3)

Reducing carbon emissions during the production process: Considering the technological characteristics of curtain wall materials and the energy structure of factories, the technical pathways for reducing carbon emissions may include: (1) Optimizing product production processes by prioritizing the use of state-encouraged advanced technology processes and advocating for green production concepts; (2) Constructing distributed photovoltaic systems using unused roofs of factories for photovoltaic power generation to improve the sustainability of enterprise operations and reduce the use of non-renewable energy sources; (3) Applying air-source heat pumps and ground-source heat pumps to replace externally purchased thermal energy and split air conditioning systems; (4) Establishing smart energy island systems by enhancing factory energy data collection, upgrading control systems.

In fact, in addition to reducing carbon emissions from the perspective of building curtain wall engineering, it is equally crucial to establish a unified standard and evaluation method for calculating carbon emission levels from policy and market perspectives. This is also the objective of the work presented in this paper. Relevant standards can regulate the carbon emission levels of curtain walls, while the evaluation system can incentivize enterprises to reduce this through market competition.

5. Current Existence of Problems

While the assessment and calculation of carbon emissions have become a focal point in the curtain wall industry, there are still many pressing practical issues that need to be addressed, as follows:

(1)

Inadequate database of carbon emission factors: The building curtain wall sector lacks a comprehensive database for carbon emission factors associated with key materials, including glass, profiles, hardware, and sealing materials. This deficiency hinders the selection of materials based on accurate data, leading to uncertainty in assessments. Additionally, considering that recycling and reuse have a significant impact on balancing the carbon emission levels of raw materials, the lack of sufficient data on recycling and reuse rates can lead to overestimated results in the final evaluation;

(2)

Lack of unified assessment methodology for carbon emissions: There is no standardized system specifically for assessing carbon emissions in building curtain walls. The diverse construction forms of building curtain walls in real projects introduce complexities to carbon emission assessments. In the absence of a unified evaluation technique, the evaluation results provided by different evaluation agencies and personnel may lack scientific data collection, be incomplete in the evaluation process, or be based on different sources of calculation. These issues result in evaluation outcomes that lack objective comparability;

(3)

Lack of correlation research between curtain wall performance and carbon emissions of buildings during the operation phase: Building envelope structures are significant pathways for building energy consumption, with doors and windows accounting for over 40% of total energy consumption and building curtain walls sometimes exceeding 80%. Therefore, the question of how to consider the carbon emissions caused by the operational stage of buildings attributed to curtain walls is a question that requires further research.

6. Conclusions

Based on the analysis of the current research status of carbon emissions from curtain walls, this paper proposes a carbon emission calculation method based on the entire life cycle. Through the analysis of a glass curtain wall case study, the following conclusions are drawn:

(1)

High-carbon-emission materials such as aluminum profiles, glass panels, and sealing agents are commonly utilized in architectural curtain walls, significantly contributing to the overall carbon emissions of buildings. At the same time, the impact of curtain wall energy efficiency on the operational carbon emissions of buildings cannot be overlooked;

(2)

Findings from a case study on a glass curtain wall project indicate that the material acquisition stage constitutes nearly 90% of the total carbon emissions associated with glass curtain walls. In addition, the scientific application of photovoltaics presents a viable approach to substantially reducing the carbon footprint of architectural curtain walls.

Overall, the calculation method for curtain wall carbon emissions plays a crucial role in promoting the application of green and low-carbon building materials and advancing the construction of carbon emission standard systems. However, challenges persist in the current evaluation of carbon emissions in the curtain wall field, including severe deficiencies in the database of material carbon emission factors, a lack of uniformity in carbon emission level calculation and evaluation methods, and the need for further research on the correlation between curtain wall performance and carbon emissions of buildings during the operation phase.