Study on Carbon Emissions from the Renovation of Old Residential Areas in Cold Regions of China

Abstract

With the implementation of dual-carbon and new human-centric urbanization strategies, the renovation of old buildings in China was inevitable. In this study, we establish the carbon emission values of retrofitting building from the perspective of carbon emissions, and propose a carbon accounting calculation method. Meanwhile, according to an economic viewpoint, we propose carbon emission evaluation indexes, including carbon increments, carbon emission intensity, carbon saving during the operation phase, and the static payback period of carbon increments. We retrofitted a building in an old residential area in Jinan, which both extended the building’s life and met the energy consumption needs of modern buildings. Through the case study, the annual carbon emissions during the use phase were reduced by 80.64% after retrofitting, and the building materials generated carbon emissions during the materialization phase of 11.04 t CO₂/a. Considering the carbon increment factor, the comprehensive carbon emission reduction was 71.43%. The carbon increment per unit of building area was 110.32 kg CO₂/m², of which the carbon emission during the materialization stage accounted for 96.04%. Promoting low-carbon building materials and improving the energy efficiency would be an important means to reduce the carbon increments during building renovations. The static payback period for the carbon increment was 2.05 years, indicating that retrofitting measures were effective. Our work is informative for the development and quantitative assessment of low-carbon retrofitting programs for older buildings.

1. Introduction

Greenhouse gas emissions have become a significant global environmental concern in recent years. The Chinese government has undertaken various emission-reduction measures in response to global climate change [1]. In the 75th United Nations General Assembly [2], China pledged to peak its carbon emissions by 2030 and reach carbon neutrality by 2060.

The building sector has faced high-energy-consumption and environmental pollution issues. The statistics [3] show that the total carbon emissions from China’s building industry in all processes amounted to 4.997 billion t CO₂ in 2019, accounting for 50.6% of the national carbon emissions, including 28% in the production stage of building materials, 1% in the construction stage, and 21.6% in the operation stage. Therefore, reducing building carbon emissions has become an essential strategy for improving environmental quality and achieving sustainability. Meanwhile, the report [3] shows over 40 billion m² of residential buildings, some of which were built over 20–30 years. According to the estimated data, China has about 160,000 old residential areas because of early construction. These buildings have problems, such as disrepair, poor living conditions, and high-energy-consumption activity.

The Chinese government highly prioritizes the renovation of old residential areas. With the in-depth promotion of new human-centric urbanization strategies [4], China’s urban construction has developed in the direction of equal emphasis on the stock upgrading and quality transformation of old neighborhoods, and the optimization and adjustment of incremental structures [5]. Old neighborhoods [6] are residential neighborhoods built before 2000, with poor public facilities and are not included in the shantytown renovation plan. The State Council included the renovation of old neighborhoods into the scope of urban renewal [7] and promulgated the “Guidance on Comprehensively Promoting the Renovation of Old Urban Neighborhoods” in 2020, the renovation of old urban neighborhoods built before the end of 2000 that need to be renovated will be completed “by the end of the 14th Five-Year Plan period”. The cold regions cover about 42% of the national population and are the primary building renovation areas in China’s old neighborhoods. According to the Ministry of Housing and Construction statistics, the number of old neighborhoods in cold regions accounts for more than 30% of the overall urban stock, such as Jinan and Beijing, accounting for 49% and 47%, respectively.

Therefore, considering the carbon emissions of buildings and the carbon emission reduction potential of old residential areas, it is significant to enforce a carbon assessment of renovating old residential areas with a high amount of emissions in China.

There are significant differences in the climate between the south and north of China, and the focus on building energy use is different [8], among which building operation energy consumption in the northern heating zone accounts for about 54% of the national building operation energy consumption [9]. Therefore, the work to improve old districts in cold climate zones mainly focuses on building energy efficiency renovation, including other measures involving people’s livelihoods.

A high number of “Khrushchyovka” buildings were built in the 1960s in the former Soviet Union [10] to encounter people’s housing problems. Due to the construction conditions at that time, there was a relatively large gap in modern building energy requirements. At present, about 40% of the total number of Khrushchyovka buildings in the Russian region have been built. Since 1997, the Russian government has been retrofitting “Khrushchyovka” buildings to meet modern building energy requirements; Arena [11] uses fiber cement and cellulose insulation to improve energy efficiency; Astakhova [12] shows two types of designs for the first generation of industrialized residential buildings that address the social problems of settlement in a timely manner. Absimetov [13] presented a solution to the problem of modernizing the thermal energy of the facade and improving hygienic living conditions. The buildings of the 1980s and 1990s in northern China are very similar to the Khrushchyovka buildings in terms of construction methods. Therefore, the “Khrushchyovka” building renovation model provides a reference for the renovation of old buildings in northern China. After entering the 21st century, Europe, Canada, the United States, Japan, and other countries also introduced a series of policies for the renovation of old buildings. Scholars have conducted the relevant research studies based on the cases. Shady Attia [14] developed an energy performance dataset and built a performance simulation benchmark model for studying near-zero energy consumption after renovations or new constructions. Gon Kim [15] et al. targeted old apartments for double building skins to manage the interaction between outdoor and indoor environments. Robert Shipley [16] et al. proposed building renovation options from a return-on-investment perspective.

In recent years, Chinese scholars have also conducted research on the renovation of old buildings with Chinese characteristics. Lei et al. [17] explored the effects of external walls, external windows, and roofs on building energy consumption rates. Han et al. [18] explored the reasonable value of building exterior-wall-insulation thickness and the impact on air-conditioning load. Liu et al. [19] established a retrofit model for the energy efficiency of non-energy-efficient residential buildings in typical cities in severely cold regions. Huang [20] established a building envelope thermal performance optimization model for the energy efficiency retrofit of existing residential buildings based on the life cycle cost analysis, and proposed an economical insulation thickness of exterior walls formula to calculate the optimal relationship between building thermal envelope performance. Zhang [21] proposed a TNM-based calculation model for winter heating and domestic-hot-water-supply energy consumption before and after the retrofitting of old buildings in Beijing.

In the context of carbon neutrality, building retrofitting has to meet the requirements of modern buildings’ energy demands and architectural designs. Carbon emissions from building retrofitting has also become a key concern. The input–output model was applied to study the ecological footprint in 1998 [22]; Jonas [23] used the input–output model to perform a top–down analysis of the Swedish building. Luo [24] proposed a carbon footprint calculation method in “building materialized” based on BIM, and proposed a list of different accounting objective analysis models; Luo [25] calculated the carbon emissions of office buildings at the embodied stage based on this method. Wang [26] established a carbon emissions model for external-wall-insulation systems in cold areas. Vanessa [27] explored how to measure building facades’ life cycle CO₂ emissions based on LCA. Tettey [28] analyzed the impact of carbon emissions of different insulation materials in a multi-story residential building. Georgios [29] compared the embodied CO₂ emissions of four typical construction materials. Seyed [30] conducted a multi-objective simulation to optimize building renovation in terms of energy consumption and life cycle cost. Zhang [31] analyzed the energy-carbon-investment return of a prefabricated envelope system. Manuela [32] studied the relationship between the cost of building retrofit energy efficiency measures and energy efficiency benefits and optimizing building renovation measures.

Building renovation is a systematic project, and different combinations of solutions and retrofit strategies can be designed according to different scenarios and requirements. There are few studies on carbon increments in retrofitting solutions, carbon reduction in building operations after retrofitting, and carbon increment recovery period. This paper references a whole life cycle management approach, uses a building retrofitting case in Ji’nan as the research object, quantifies and analyzes the impact of renovation measures on the carbon emissions of buildings in materialization and operation stages, accounts for the carbon increments in multiple renovation measures and the carbon emission intensity of building unit area retrofitting practices, analyzes the importance and impact of multi-objective renovation measures on building operation carbon emissions and carbon saving, and constructs a method for calculating building retrofit carbon accounting, operation carbon reduction, and carbon increments’ static recovery period. From a carbon perspective, the renovation of old buildings means extending the life of the buildings and meeting the housing needs of modern residents.

2. Methodology

2.1. System Boundary

Based on LCA, building carbon emissions are the sum of carbon emissions from the materialization, operation, and disposal stages [33]. In this paper, we studied the carbon increments generated by building renovation measures and the carbon saving of building operations before and after an energy-saving retrofit. These renovation measures are integral parts of the original building and are not considered for separate demolition. Meanwhile, the carbon emissions in the demolition stage have a smll proportion, accounting for 0.7–1.2% [25]. Based on the IPCC’s principle [34] of “sustainability and need for precise measurement” in carbon accounting, we did not include carbon emissions of the demolition phase in establishing the study boundary. The technology roadmap is shown in Figure 1.

2.2. Basic Calculation Method

Building carbon emissions are the sum of the greenhouse gas emissions produced in each phase, expressed in carbon dioxide equivalents. The emission factor is a widely used and accepted method of carbon accounting in the building sector. It works by multiplying energy consumption and carbon emission factors [35]. In our work, we also used carbon emission factors for carbon accounting. Carbon emission factors of relevant construction materials and construction machinery quantities referred to the Chinese national standard “Standard for building carbon emission calculation” (GB/T 51366—2019). The accounting Formula (1) is as follows:

$$E={\displaystyle \sum A{D}_i\times E{F}_i}$$

(1)

where $E$ is CO₂ emissions, $A{D}_i$ is activity level data. $E{F}_i$ is the emission factor, the level of CO₂ emissions per unit of activity level.

2.3. Carbon Emissions Calculation

According to the Kyoto Protocol’s Clean Development Mechanism (CDM) [36], the calculation of carbon emissions should be realistic and measurable. However, it is difficult to accurately measure carbon emissions from the demolition phase of the building renovation process, especially since there was only one building in the project. At the same time, carbon emissions from the demolition phase account for a small percentage of the life cycle carbon emissions of buildings [9]. In this study, carbon emissions calculations included two aspects: the materialization and use stages. The Formula (2) is as follows:

$$E_{r\mathrm{e}}=E_{ma}+E_{op}$$

(2)

where $E_{re}$ is the renovation process resulting in carbon emissions from the residential area. $E_{ma}$ is the carbon emissions 6 during the materialization stage. $E_{op}$ is the carbon emissions during the operation stage.

2.3.1. Carbon Emissions during the Materialization Stage

Carbon emissions during the materialization stage mainly include energy consumption and processes releasing CO₂ during the production of building materials, CO₂ from fuel combustion during material transportation, and carbon emissions generated by the energy consumption levels of mechanical equipment in the construction stage. The Formula (3) is as follows:

$$E_{ma}=E_p+E_t+E_c$$

(3)

where $E_{ma}$ is the carbon emissions during the materialization stage, kg CO₂; $E_p$ is the carbon emissions from material production, kg CO₂; $E_t$ is the carbon emissions from material transportation, kg CO₂; and $E_c$ is the carbon emissions during the construction stage, kg CO₂.

• It generates CO₂ when building materials are produced and processed because of energy consumption and raw material mining. These values should be accounted for by carbon emission factors from the National Standards [37]. The Formula (4) is as follows:

$$E_p={\displaystyle \sum M_i\times C{F}_i}$$

(4)

where $M_{}$ is material consumption, which would be obtained or calculated from the projects list; $i$ is the type of building materials involved in the renovation; and $C{F}_{}$ is the carbon emissions factor for the materials.

2.

Carbon emissions released from material transportation are associated with the mode of transportation and distance. The calculating Formula (5) is as follows:

$$E_t={\displaystyle \sum M_i}\times D_i\times T_i$$

(5)

where $D_i$ is the transport distance; the default transportation distance for concrete is 40 km, and for other building materials it is 500 km [37]. $T_i$ is the carbon emissions factor for the transportation type.

3.

The type of machinery and equipment calculates carbon emissions and shifts during the retrofitting process.

$$E_c={\displaystyle \sum E_{c,i}}\times E{F^{\prime }}_i$$

(6)

where $E_{c,i}$ is the sum of energy consumption; $i$ is the type of energy; and $E{F^{\prime }}_i$ is the carbon emissions factor of energy.

2.3.2. Carbon Emissions during the Operation Stage

This study focused on the carbon reduction effect of retrofit measures during the operation stage, ignoring the carbon source/sink. The energy consumption of buildings includes electricity, heating, and cooling. The Formula (7) used to calculate carbon emissions during the use stage is as follows:

$$E_{op}=({{\displaystyle \sum E_{op,i}}}_i\times E{F^{\prime }}_i)\times y$$

(7)

where $E_{op,i}$ is the sum of energy consumption in the building operation; $i$ is the type of energy; and y is running time in years.

2.4. Carbon Emission Assessment Indicators of Renovation

Building renovations would increase production activities, which would increase carbon emissions. Following the economics input recovery perspective, we proposed the concept of carbon recovery. By quantifying the carbon emission indicators of building retrofitting, we were able to guide the low-carbon retrofitting process and optimize the impact of each retrofitting measure on the carbon emissions from building operations.

2.4.1. Carbon Increments

During the renovation process, building materials are used for the existing building to improve residential conditions. Meanwhile, production, transportation, and construction would increase the carbon emissions in the life cycle of the building, which is called carbon increments, numerically equal to carbon emissions at the materialization stage.

2.4.2. Carbon Saving

During the operation phase, the retrofitted building reduces building energy consumption for cooling, heating, and lighting compared to the existing building. The building retrofit operation’s carbon saving value is calculated according to Equation (8):

$$E_{sa}=\left|E_{op}-{E^{\prime }}_{op}\right|$$

(8)

where $E_{sa}$ is carbon saving; $E_{op}$ is the sum of energy consumption in an existing building operation; and ${E^{\prime }}_{op}$ is the energy consumption sum in a retrofitted building operation.

2.4.3. Carbon Emission Intensity of Building Renovation per Area

Carbon emission intensity of building renovation per area is a parameter measuring the carbon indicator used for retrofitting measures in buildings, communities, or old residential neighborhoods, which can be calculated according to Equation (9):

$$C_{re}=E_{ma}/(A\times y^{\prime })$$

(9)

where $C_{re}$ is the carbon emission intensity of building renovations per unit area spread over the expected years of use remaining; $A$ is the building area; and $y^{\prime }$ is the expected years of use remaining.

2.4.4. Building Renovation Carbon Increment’s Static Payback Period

Regarding an economic point of view, the static payback period T is calculated for the carbon increments, see Equation (10):

$$T=E_{in}/E_{sa}$$

(10)

3. Case Study

3.1. Case Selection

The study researched a building in China’s cold region in Ji’nan city, Shandong Province. The building was built in 1990 and had a service life of 50 years in architectural design. It was a five-story, typical residential building, with a height of 16 m, a flat roof, a brick-wall structure with a total construction area of 1800.72 m², 500.45 m² of roof area, 1427.87 m² of exterior wall area, and 371.25 m² of exterior window area. The building included three dwelling units, but the stairwell was exposed to the outside environment without doors and windows. These buildings were developed in the 1980s to meet the rapid growth and wide distribution of the population. Central heating is used for heating in the winter; air conditioners are used for cooling in every household. Because of the houses’ age, the lack of insulation and comfortable living conditions means that it has not been easy to satisfy good living requirements. The renovation was completed in April 2021 and the the building will remain in use for 18 years.

The floor plan of the case study building is shown in Figure 2.

3.2. Building Model

Based on the basic design parameters of the building envelope and the specific data in

Table 1. Basic design parameters of the existing building envelope.

Envelope	Materials and Construction	U-Value
External Wall	Lime Mortar 20 mm + Clay Solid Brick 370 mm + Cement 20 mm	2.24 W/(m2·K)
Interior Walls	Lime Mortar 20 mm + Clay Solid Brick 240 mm + Cement 20 mm	1.94 W/(m2·K)
Roof	Cement 20 mm + Cement Perlite 120 mm + Cement Coke 30 mm + Reinforced Concrete 130 mm	1.14 W/(m2·K)
External Window	Single-pane Aluminum Windows	4.70 W/(m2·K)

, the building model was set in DesignBuilder to simulate energy consumption values in the building. In this building model, 26 °C and 18 °C were set as set-point values for summer and winter, respectively.

Efficiency had a direct impact on the primary energy consumption and carbon emissions values. For room air conditioners, energy efficiency was selected at 4.00 (energy efficiency grade III) [38]. Because heat-supply networks have long operation times with poor insulation conditions and heat loss rates of up to 25% and higher [39], the heat loss of a heat-supply network was set at 25% in this study.

The annual load of the building was simulated at 57,499.85 kWh converted into 31.93 kWh/m² (cooling), 759,214.73 MJ converted into 117.12 kWh/m² (heating), and 492.75 kWh (public lighting in the stairwell). The primary energy consumption was converted from the building load. According to Formula (1), the carbon emissions in the use stage of the existing building will be calculated at 119.99 t CO₂/a, which equals 66.63 kg CO₂/(m²·a). The carbon emissions factor for heating is 0.11 t CO₂/GJ; the electricity emissions factor in Shandong province is 0.5810 t CO₂/MWh.

Table 2. Annual carbon emissions from refrigeration, heating, and public lighting before the transformation.

Project	Load	Efficiency	Energy Consumption	Carbon Emissions Factor	Annual Carbon Emissions/(t CO2/a)
Cooling	57,499.85 kWh	4.00	14,374.96 kWh, electricity	0.5810 t CO2/MWh	8.35
Heating	759,214.73 MJ	75%	1,012,286.31 MJ, heating	0.11 t CO2/GJ	111.35
Public Lighting	492.75 kWh	——	492.75 kWh, electricity	0.5810 t CO2/MWh	0.29
Sum					119.99

shows the annual carbon emissions in the use stage.

3.3. Renovation Measures

According to the Design Standard for Energy Efficiency of Residential Buildings [40], the energy saving rate should stay at 75%, which means renovated buildings have to reduce 75% of their energy use. Hence, these energy-saving measures were designed to attain an energy saving level and are shown in Figure 3. The parameters of the renovated building envelope are shown in

Table 3. Parameters of the existing building envelope after renovation.

Envelope	Materials and Construction	U-Value
External Wall	Lime Mortar 20 mm + Clay Solid Brick 370 mm + Cement 20 mm + EPS 80 mm	0.40 W/(m2·K)
Roof	Cement 20 mm + Cement Perlite 120 mm + Cement Coke 30 mm + Reinforced Concrete 130 mm + XPS 30 mm + Slope 30°	0.35 W/(m2·K)
External Window	Aluminum–Plastic Co-Extruded Double-Glazed Windows	2.30 W/(m2·K)

.

3.3.1. External Wall

The outer layer of the exterior wall of the original building fell off, and there were no insulation materials in the walls. In the renovation project, the EPS external insulation system covered the existing walls to improve the thermal insulation’s performance. The thickness of the EPS insulation board was calculated at 80 mm.

Construction: powder coatings and insulation mortar were used to fix the external insulation system (manpower and machinery).

The U-value of a new and renovated external wall was 0.40 W/(m²·K).

3.3.2. Roof

The roof of the original building was severely damaged, including the insulation and waterproof layers. During the renovation project, the insulation and waterproof layer needed to be repaired and light-galvanized-color steel needed to be installed on the four-sided sloped roof.

The thermal insulation material used was an XPS thermal insulation board, with a thermal conductivity value of 0.03 W/m·K. In order to satisfy the energy saving requirements, the thickness of the XPS board was calculated at 35 mm.

An SBS-modified asphalt waterproof coil was used for the waterproofing project.

Under the condition that the parapet was not damaged on the original roof, the light steel roof truss was set by a two-point support, and the slope was 30°.

Construction details (manpower and machinery):

• We used 120 mm × 2.5 mm square steel with 3.5 m spacing as the frame;
• We used 60 mm × 2.0 mm square steel as bollards;
• We used C-shaped steel C140 × 50 mm × 20 mm × 2.5 mm as roof purlins;
• We used 120 mm × 2.5 mm steel beams.

The roof was pitched on all sides and laid with galvanized-colored steel tiles.

The U-value of the new and renovated roof was 0.35 W/(m²·K).

3.3.3. External Window

There were single-layer glass aluminum alloy windows in the old residential building. In the renovation project, the aluminum–plastic co-extruded double-glazed windows replaced the original windows.

Construction: demolish the existing windows, then install the prefabricated window (manpower and machinery).

The U-value of the new and renovated windows was 2.3 W/(m²·K).

3.3.4. Stairwell

Due to disrepair, there were no main doors and windows in the public stairwell, with poor insulation and air tightness. During the renovation project, the main doors and double-glazed windows were installed to improve comfort.

Construction: install the prefabricated window (manpower).

3.3.5. Public Lighting

There was incandescent light (45 w) in the stairwell, during the renovation project, under the same luminous flux; LED lights (7 w) with an acousto-optic time-delay control (the 60 s) were used as alternative lighting. The sum of lights was 15.

Construction: install LED lights (manpower).

3.3.6. Heat-Supply Network

During the renovation project, the heating pipe network was repaired with insulation, and the pipes in poor condition were replaced. Meanwhile, a heat-supply network upgrade requires the installation of a compensator and heat metering device.

Construction: demolish the existing insulation in poor condition, then place high-performance insulation on the heating pipe (manpower and machinery).

4. Results

4.1. Carbon Emissions during the Materialization Stage

Carbon emissions during the materialization stage were divided into production, transportation, and construction factors. Carbon emissions during this stage depended on the number of building materials collected and the construction methods. The data are from the field research, budget list, reference statistics, and standard value. The details are shown in

Table 4. List of building materials’ production stages and carbon emissions.

Renovation Projects	Materials	Quantities	Carbon Emissions Factor [37]	Carbon Emissions /(t CO2)	Carbon Emissions per Unit Area/(kg CO2/m2)
Roof Insulation	Brick, 30 mm	500.45 m2	19.5 kg CO2 e/m2	9.76	5.42
	Cement Mortar Screed, 20 mm	13.01 t	735 kg CO2e/t	9.56	5.31
	XPS, 35 mm	17.52 m3	669 kg CO2e/m3	11.72	6.51
Subtotal				31.04	17.24
Roof Waterproofing	SBS Modified Asphalt Waterproof Coil [41]	500.45 m2	4.01 kg CO2/m2	2.01	1.12
Subtotal				2.01	1.12
Slope Roof	Square Steel, 120 mm × 2.5 mm	0.62 t	2190 kg CO2e/t	1.36	0.76
	Square Steel, 60 mm × 2.0 mm	0.26 t	2190 kg CO2e/t	0.57	0.32
	Shaped Steel, C140 × 50 × 20 × 2.5 mm	3.99 t	2190 kg CO2e/t	8.74	4.85
	Roof Tiles	4.98 t	2190 kg CO2e/t	10.91	6.06
Subtotal				21.58	11.98
EPS External Insulation System	Cement Mortar, 8 mm	12.85 t	274 kg CO2e/t	3.52	1.95
	EPS Board, 80 mm	114.23 m3	534 kg CO2e/m2	61.00	33.88
	Mortar, 8 mm	12.85 t	242 kg CO2e/t	3.11	1.73
	Anchor Bolt	0.26 t	2190 kg CO2e/t	0.57	0.32
	Glass Fiber Mesh	0.28 t	574 kg CO2e/t	0.16	0.09
	Coating	0.28 t	2410 kg CO2e/t	0.67	0.37
Subtotal				69.03	38.33
External Window	Aluminum–Plastic Co-Extruded Double-Glazed Windows	371.25 m2	129.5 kg CO2e/m2	48.08	26.70
Subtotal				48.08	26.70
Stairwell	Double-Glazed Windows	18.96 m2	129.5 kg CO2e/m2	2.46	1.37
Subtotal				2.46	1.37
Public Lights	LED, 7 W	105 W	0.067 kg CO2e/W [42]	0.01	0.056
Subtotal				0.01	0.0056
Heating Pipe	Steel Pipe, φ159, 100 m	3.67 t	3150 kg CO2e/t	11.56	6.42
	Polyurethane, 32.5 mm,100 m	0.40 t	5220 kg CO2e/t	2.10	1.17
Subtotal				13.66	7.59
Sum				187.86	104.32

,

Table 5. Carbon emissions during building materials’ transportation.

Materials	Quantities /t	Carbon Emissions Factor /[kge/(t·km)]	Type of Transportation	Distance	Carbon Emissions /t CO2
Cement	38.71	0.057	Truck (46 t)	40 km	0.09
Steel	10.39	0.129	Truck (18 t)	500 km	0.67
Coating	0.28	0.344	Truck (2 t)		0.05
Brick	10	0.162	Truck (10 t)		0.81
Heat-Preservation Material	3.95	0.344	Truck (2 t)		0.68
Waterproof Material	1	0.344	Truck (2 t)		0.17
Doors and Windows	4	0.344	Truck (2 t)		0.69
Heating Pipes	4.07	0.179	Truck (8 t)		0.36
Sum					3.52

,

Table 6. Carbon emissions during construction.

Machine	Shifts [43]	Energy Consumption	Carbon Emissions Factor	Carbon Emissions /t CO2
Automobile Crane (8 t)	5	Diesel, 28.43 kg	72.59 t CO2/TJ	0.45
Semi-Automatic Cutting Machine	30	Electricity, 98.00 kWh	0.6101 t CO2/MWh	0.003
Electroslag Welder (1000 A)	30	Electricity, 147 kWh		0.004
Truck (5 t)	30	Gasoline, 31.34 kg	67.91 t CO2/TJ	6.82
Sum				7.28

and

Table 7. Sum of carbon emissions during materialization stage.

Projects	Production Stage	Transportation Stage	Construction Stage	Carbon Emissions/t CO2	Rate
Roof Insulation	31.04	0.58	1.20	32.82	16.52%
Roof Waterproofing	2.01	0.04	0.08	2.13	1.07%
Slope Roof	21.58	0.40	0.84	22.82	11.49%
EPS External Insulation System	69.03	1.29	2.68	73.00	36.75%
External Window	48.08	0.90	1.86	50.84	25.59%
Stairwell	2.46	0.05	0.10	2.60	1.31%
Public lights	0.01	0.00	0.00	0.01	0.01%
Heating Pipe	13.66	0.26	0.53	14.45	7.27%
Sum				198.66	100%

. According to Formula (3), the carbon emission value during the materialization stage is 198.66 t CO₂.

4.2. Carbon Emissions from the Operation Stage

Following the renovation of the building, the building’s remaining useful life is 18 years. Annual carbon emissions during the operation stage can be simulated and calculated by energy loads of retrofitted buildings, which are 43,157.97 kWh, 138,451.70 MJ, and 76.65 kWh, respectively, for cooling, heating, and lighting from DesignBuilder. When the pipe insulation was improved, the heat network efficiency was enhanced to 90% [44], which meant that the heat lost from the heat-supply network was reduced to 10%. Based on Formula (7), carbon emissions can be obtained at 23.24 t CO₂/a and 12.90 kg CO₂/(m²·a) in

Table 8. Annual carbon emissions during cooling, heating, and public lighting after transformation.

Project	Load	Efficiency	Energy Consumption	Carbon Emissions Factor	Annual Carbon Emissions/(t CO2/a)
Cooling	43,157.97 kWh	4.00	10,789.49 kWh, Electricity	0.5810 t CO2/MWh	5.89
Heating	138,451.70 MJ	90%	153,835.22 MJ, Heating	0.11 t CO2/GJ	16.92
Public Lighting	76.65 kWh	——	76.65 kWh, Electricity	0.5810 t CO2/MWh	0.04
Sum					23.24

.

The performance of the building envelope was improved by retrofitting measures, which improved heating and cooling energy consumption rates. According to the relevant research, Dodoo [45] obtained energy renovation measures for a typical 1970s multi-family building and saved 35–43% of energy in this project.

4.3. The Life Cycle

As shown in Figure 4, the carbon emissions from the original building during the operation stage is 119.99 t CO₂/a; following the transformation, it was reduced to 23.24 t CO₂/a and the rate of reduction was up to 80.64%. However, the renovation process generated carbon emissions of 198.66 t CO₂ in total, which was converted to 11.04 t CO₂/a; considering this carbon emissions value, the carbon reduction rate still remains at 71.43%.

Overall,

Table 9. Carbon emissions during the life cycle after the transformation.

Stages	Carbon Emissions during Life Cycle/t CO2	Carbon Emissions per Unit Area during Life Cycle/ (kg CO2/m2)	Rates
Materialization Stage	198.66	110.32	32.20%
Operation Stage	418.32	232.31	67.80%
Sum	616.98	342.63	100%

shows the carbon emissions of this study. Regardless of the attenuation, the carbon emissions during the use stage were accumulated over 18 years to attain 418.32 t CO₂ taking up 67.80% of the whole carbon emissions during life cycle.

5. Discussion

5.1. Impact of Renovation Measures on Carbon Increments

In this project, the additional carbon emission values were from the use of renovation material, which equaled 198.66 t CO₂.

Table 10. List of carbon increments.

Stages	Carbon Emissions/t CO2	Carbon Increments per Unit Area/(kg CO2/m2)	Rate
Production	187.86	104.32	96.04%
Transportation	3.52	1.95	1.77%
Construction	7.28	4.04	3.66%
Sum	198.66	110.32	100%

and Figure 5a shows the rates of carbon emissions during the materialization stage. The material production generated 187.86 t CO₂, accounting for 96.04% of the carbon increments. Meanwhile, Figure 5b shows the carbon emissions during the renovation measures. Envelope conversion generates high carbon emissions and performs well to improve the thermal insulation’s performance, including the external wall, windows, and roof, and reduce energy consumption rates. In terms of quality-of-life improvements, such as repairing heat pipes and installing main doors, windows, and LED lights in the stairwell, their carbon emissions take up 8.59%.

Based on the results, it is necessary to focus on carbon increments and their proportion. Ignoring the carbon increments produced by renovations leads to an overestimation of the carbon-reduction effects of some actions. In this case, the carbon-reduction effect would be overestimated by 9.18%. Some studies have achieved similar conclusions. Anderson’s [46] analysis determined that the materials used for renovation projects increased the buildings’ embodied emissions by 7 and 14%. In particular, the use of materials affects the carbon increments as they occupy a high percentage. The retrofitting measures of the EPS external system, external window, sloped roof, and roof insulation are the main cause of carbon increments. Cang [47] observed that steel, thermal-insulation materials, doors and windows, and water-based paint were major sources of carbon emissions during the materialization phase. The results of these studies suggest that carbon increments are taken into account and focus on the choice of building materials.

5.2. Impact of Renovation Measures on Carbon Saving

Following the renovation, the carbon emissions from cooling, heating, and public lighting in the use phase were reduced and exceeded our expectations at 12.90 kg CO₂/(m²·a). Compared to the original building, the annual carbon savings were 96.75 t CO₂/a, which represents 53.73 kg of CO₂ per unit area and would decrease during the use stage each year. There is a significant decline in heating parts, with the greatest carbon reduction at 94.43 t CO₂/a. Energy use for public lighting remarkably declined by 84.45% after switching to LED bulbs. Considering the life cycle factor, the retrofitted building can reduce emissions to 1741.58 t CO₂ during the use stage.

The performance of the building envelope was improved by retrofitting measures, which provide benefits for heating and cooling energy consumption rates. According to the relevant research, Dodoo [45] obtained energy renovation measures for a typical 1970s multi-family building and saved 35–43% of the energy during this project (

Table 11. Carbon saving during the life cycle.

Project	Annual Carbon Saving/(t CO2/a)	Annual Carbon Saving per Unit Area/[kg CO2/(m2·a)]	Carbon Saving Rate	Carbon Saving during Life Cycle/t CO2
Cooing	2.08	1.16	24.94%	37.50
Heating	94.43	52.44	84.80%	1699.73
Public Lighting	0.24	0.13	84.45%	4.35
Sum	96.75	53.73	80.64%	1741.58

).

5.3. Carbon Increments’ Static Payback Period

In this case study, the carbon increment was 198.66 t CO₂, equal to 110.32 kg CO₂/m². Following the renovation, carbon emissions during the use stage will be reduced by 53.73 kg CO₂/m² annually. According to Formula (10), the carbon payback of all renovation measures is 2.05 years. Figure 6 shows the static payback period of each measure to compare and analyze that measures that should be promoted, preferentially.

There are correlational studies about carbon payback time. In Anderson’s study [46], he simulated the improved thermal performance of residential buildings’ envelope structures and accounted for the payback time for CO₂ emissions of renovations ranging from 1 to 3 years. Bull [48] observed that the carbon payback time for energy-saving renovations of a school building in the UK was 3.9 years. The results of these studies suggest that the renovations are beneficial and should be considered.

6. Conclusions

In our work, we proposed a carbon accounting boundary and calculation method for retrofitting buildings from the perspective of carbon emissions. We analyzed the carbon savings in the use phase of the buildings and quantified the carbon emissions and carbon increments of various retrofitting processes. Meanwhile, for retrofitting buildings, we established carbon emission assessment indexes, including carbon increments, carbon saving, carbon emission intensity, and static payback period of carbon increments. These indicators provide a reference calculation method for quantitatively assessing building retrofitting practices from a carbon perspective.

We conducted the assessment for our case study. The carbon emissions from the building during the operation stage before and after renovations were 119.99 t CO₂/a and 23.24 t CO₂/a, respectively. The carbon increment was 198.66 t of CO₂ from the transformation during the materialization stage. Based on these results, the main conclusions are as follows:

• In this case, renovation measures would positively influence carbon emission reductions during the use stage. A total of 96.75 t of CO₂ would be reduced every year, and the rate of carbon saving would be about 80%. Nevertheless, renovation measures must rebuild the existing building with new materials, increasing the life cycle of carbon emissions. Significantly, the renovation involved in the building envelope (EPS external insulation, roof insulation, sloped roof, and external windows) would comprise a 91.41% carbon increase, improve common areas (stairwell, lighting), and replace heat pipes occupying 1.32% and 7.27%, respectively. In sum, the carbon increments per unit area of the combination of retrofitting measures are 110.32 kg CO₂/m². The static payback period of carbon increments is 2.05 years.
• In this case, the production of materials during the materialization stage was 96.04% of the carbon increments. Hence, promoting low-carbon, lightweight, high-performance building materials with low coats is essential. In other words, there is an effective way to reduce payback time, which reduced carbon increments and saves more energy to decrease carbon emissions.
• The carbon emissions from the operation occupies the highest proportion, close to 70%. Electricity and heating are the primary sources of carbon emissions. If energy sectors were expedited to shift to green energy and fossil energy was substituted by renewable energy in the building sector, carbon emissions would be decreased further.