The Impact of Traditional Raw Earth Dwellings’ Envelope Retrofitting on Energy Saving: A Case Study from Zhushan Village, in West of Hunan, China

Abstract

This study presents the CO₂ emissions and energy performance of traditional raw earth dwellings’ envelope retrofitting located in the Zhushan Village, western Hunan Province, China. The numerical simulations of heating energy consumption on the building models were performed using DesignBuilder, an energy simulation program. The energy performance was evaluated using the indexes (including energy consumption, CO₂ emissions, heat balance analysis, and air temperature profiles). The detailed evaluation process of the energy performance is presented as follows. First, the current situation was analyzed through the field research, and two typical building models were built. Second, all schemes were simulated using the DesignBuilder software. Subsequently, the four main retrofit measures (replacing the external insulation windows, setting the external wall insulation layer, setting the roof insulation layer, and setting the ceiling insulation layer) were analyzed, respectively. The optimal parameters of the respective retrofit measure were calculated. Lastly, a multi-objective optimization analysis was conducted on all retrofit plans using the coupling method. In the winter, the results indicated that the “I-shape” dwelling heat consumption of the enclosure structure was reduced by 12.8 kW·h/m², and the CO₂ emissions were reduced by 882.8 kg. While in the benchmark building, the results showed that the “L-shape” dwelling heat consumption of the enclosure structure was decreased by 13.27 kW·h/m², and the CO₂ emissions were reduced by 894.4 kg. As the renewal scheme has been progressively implemented, the whole Zhushan Village will save energy by 11.2 × 10⁴ kW·h after the insulation renewal of the envelope structure is completed.

1. Introduction

The majority of research and analyses have suggested that building-related energy consumption accounts for over 30% of the total energy consumption worldwide, which has reached 35–40% in developed nations [1]. To be specific, 30–60% of the energy has been utilized to optimize the indoor thermal environment of buildings [2]. China’s whole process of building carbon emissions has expanded 2.24 times over the past 15 years, with an average annual growth of 5.92%, from 2.234 billion tons of CO₂ to 4997 million tons of CO₂ [3]. With the rapid growth of China’s economy, it is imperative for the Chinese government to control the output of internal sources and limit building energy consumption to no more than 1.1 billion tons of standard coal (23%), as well as to ensure a healthy and comfortable indoor environment [4,5,6].

Increasing the energy efficiency of buildings is one of the most effective ways to address energy shortage and global warming to achieve sustainable development [7,8]. In China, numerous traditional dwellings are warm in winter and cool in summer since local building craftsmen have gained a lot of experience in building with the local natural environment through centuries of summing up and experimentation, and traditional dwellings have gradually developed and evolved in the process of developing and adapting to the climate environment [9]. Traditional dwellings have gradually developed a strategy of mutual adaptation to the climate environment in the process of development and evolution [10]. However, there are considerable design and construction irrationalities since theoretical guidance and norms are lacking in the construction of traditional dwellings [11]. Accordingly, coupling modern science and technology with traditional construction methods will bring breakthrough development and changes to the balance between energy consumption and thermal comfort [12]; the determinants of energy use in dwellings are complex and include occupants, equipment, climate, and specially the building design and envelope. The choice of materials used to construct the building envelope will therefore have a major impact on thermal performance. In order to prevent excessive heat loss and reduce the CO₂ emissions in a temperate cold climate, a building should be well insulated [13].

In accordance with the China Climate Change Info-Net, China accounts for nearly 30% of CO₂ emissions, one of the largest proportions from a single nation. Accordingly, reducing CO₂ emissions in China will take on a critical significance in global CO₂ emissions reduction.

It is generally known that there are two strategies for reducing atmospheric concentrations of CO₂. The first strategy involves most efforts, and it focuses on reducing CO₂ emissions to the atmosphere, including increasing energy efficiency or switching to low- or zero-carbon fuel sources. The other strategy is to remove carbon from the atmosphere and sequester it using the deployment of negative emissions technologies. This study puts forwards an optimal retrofitting for traditional dwellings’ envelope in western Hunan of China to save energy and reduce CO₂ emissions.

The buildings have high energy demands due in part to the indoor thermal comfort requirements of buildings. A continuous and cheap supply of energy is desired for economic and social development [14]. Using nonrenewable energy can result in environmental problems, global warming, and a reduced quality of life in humans. The efficient use of fossil fuel sources is one of the most effective ways of reducing the energy use [15]. Thermal insulation in a building envelope is capable of decreasing the heat loss or gain through the building envelopes in the winter [16]. Design and construction with optimal insulation thickness should be a prerequisite and a top priority for energy savings in buildings [17]. The definition of retrofitting for traditional dwellings’ envelope has been elucidated by numerous researchers worldwide. The degree-day (DD) method has been widely adopted to obtain the energy needs of buildings. Moreover, the methods of calculating the retrofitting are proposed in accordance with the DD method [18].

In general, the housing issue of Chinese farmers has been solved, whereas there is a significant difference between the urban and rural indoor living environment [19]. How to let farmers live a comfortable and environment-friendly life has been considered a vital issue affecting the harmonious development of society and as an essential task for rural development at the current stage [20,21]. The heating and air conditioning equipment has a very low utilization rate in poor rural areas [22]. Natural ventilation is the major operation mode of buildings, and residents primarily respond to changes in the indoor thermal environment in winter through adaptive behavior [23]. It is imperative to carry out energy-saving renovation of rural residential buildings to optimize the indoor thermal environment in winter and numerous researchers worldwide have conducted extensive relevant research [24,25,26,27,28]. Thus, the characteristics of traditional Hunan native dwellings were investigated in this study using several methods, including the field mapping of 70 native dwellings in Zhushan village, the on-site temperature and humidity data, the questionnaire survey, and the simulation.

Accordingly, the aim of this study was to investigate the characteristics of traditional Hunan houses and quantitatively analyze the winter thermal performance of the building envelope to guide the design and updating of the winter thermal insulation of Hunan houses [29]. On that basis, this study attempted to enhance the winter thermal comfort and reduce the heating energy consumption in the area [30].

2. Methods

2.1. Location and Climate

The raw earth dwelling is located in Zhushan Village, western Hunan Province, China. The geographic location is between 109°10′–110°23′ East longitude and 27°44′–29°38′ North latitude, with a humid subtropical monsoon climate. This area shows a mountainous landform in the connection of river and hills. Figure 1 shows the location, layout, and the surrounding environment of the dwellings. The Wuling Mountains slope from northeast to southwest across the whole territory. The original landscape of this area comprises the Dishan Mountains and the central and southeastern low hills and plains landscape area.

It is characterized by the topography of the Wuling Mountains: the winter is relatively cold, the summer is warm and wet, the spring and summer are bright wet and rainy, the weather is changeable, the weather turns cool in autumn, the temperature drops, the monsoon is obvious, and the four seasons are distinct. The average temperature for years ranges from 16.0 to 17.0 °C. To be specific, the average temperature in winter ranges from 6.5 to 7.2 °C, the average maximum temperature and the average minimum temperature reach 9 °C and 3 °C in January. In addition, the average rainfall, the historical maximum temperature, and the historical minimum temperature in January are 44 mm, 23 °C, and −6 °C, respectively. In February, the average maximum temperature is 11 °C, the average minimum temperature is 4 °C, the average rainfall is 55 mm, the historical maximum temperature is 30 °C, and the historical minimum temperature −3 °C. In March, the average maximum temperature is 16 °C, the average minimum temperature is 8 °C, the average rainfall is 80mm, the historical maximum temperature 36 °C, and the historical minimum temperature is 2 °C (

Table 1. Local winter climate information.

Month	Mean Maximum Temperature	Mean Minimum Temperature	Record Temperature	Record Low Temperature
January	9 °C	3 °C	23 °C	−6 °C
February	11 °C	4 °C	30 °C	−3 °C
March	16 °C	8 °C	36 °C	2 °C

).

2.2. Questionnaire

A questionnaire survey was carried out to study residents’ satisfaction with indoor environment in Zhushan Village in January, winter 2022. A total of 70 valid questionnaires were collected.

Table 2. Indoor environment survey.

Type of Indoor Environment	Questions	Remarks	Scale/Options
Thermal	A: Are you satisfied with the indoor thermal condition in winter?	+1: satisfied; +0: improving; −1: dissatisfied
	B: What is your thermal sensation in winter?	+1: warm; 0: neutral; −1: cool; −2: cold;−3: very cold
	C: Do you feel comfortable with the indoor thermal condition in winter?	0: comfortable; 1: slightly uncomfortable; 2: uncomfortable; 3: very uncomfortable; 4: limited tolerance
	D: What is your expectation for indoor thermal condition in winter?	–	A: cooler; B: warmer; C: maintain

lists the specific questions and their corresponding scales and remarks. Residents have marked certain positions on the scale to represent their subjective perceptions [31]. The options selected by the residents indicate their expectations of the indoor environment [32].

As depicted in Figure 2, in winter, most villagers are very dissatisfied with the indoor thermal environment, some find it acceptable and only a small number of residents are satisfied with their living environment.

2.3. Analysis of Residential Characteristics

Zhushan Village is in the northwest of the ancient city of Fenghuang, Hunan Province. It is nearly 3.5 km wide from north to south and 3.8 km long from east to west. In general, the construction materials of the village dwellings include adobe bricks, stone, and wood, with hard hill purlins, single-layer tile roofs, as well as a square structure, as presented in Figure 3a. A total of 57% of the dwellings are “I-shaped”, 39% of the dwellings are “L-shaped”, and the remaining 4% are “U-shaped”. The statistical data are presented in Figure 3b. In terms of the “I-shaped” dwelling, the average area is 119.9 m², the average depth reaches 8.2 m, the average width is 14.8 m, the average shape coefficient of building reaches 0.5 m⁻¹, and the average window to floor ratio is 0.031. In terms of the “L-shaped” dwelling, the average area is 138.1 m², the average depth reaches 7.7 m, the average width is 16.3 m, the average shape coefficient of building is 0.63 m⁻¹, and the average window-to-floor ratio is 0.021. Thus, the Long F.Y. house was selected as a typical “I-shaped” dwelling for winter temperature and humidity measurement and simulation study (with the main house area of 120.29 m², the depth of 8.9 m, the width of 12.2 m, the shape coefficient of building is 0.63 m⁻¹, as well as the window to floor ratio of 0.044). The house of Wu J.L., a villager, was employed as a typical “L”-shaped house for winter temperature and humidity measurement and simulation study (with the main house area of 145.29 m², the depth of 7.3 m, the width of 18.5 m, shape coefficient of building is 0.57 m⁻¹, as well as the window-to-floor ratio of 0.018).

The living space on the first floor of the dwelling is primarily three open rooms with internal partitions of wooden boards, so the indoor airtightness is poor. To be specific, the doors are primarily ordinary wooden doors, a few windows are ordinary single-layer glass, and the rest are plastic films or without any shade. The local traditional dwellings are not likely to meet the modern living needs of residents due to the backwardness of living facilities and the low comfort of indoor environment (

Table 3. Typical floor plan type of traditional vernacular dwelling in Zhushan Village.

Plan Type	Photos	Vertical and Sectional View
“I-shaped” dwelling
“L-shaped” dwelling
“U-shaped” dwelling

).

2.4. Winter Temperature and Humidity

The indoor and outdoor temperature field tests were performed for 10 consecutive days in three traditional dwellings and a farmer’s self-built house.

Table 4. Winter temperature detection.

Plan Type	Measurement Point	Measured Data
“I-shaped” dwelling
“L-shaped” dwelling
“U-shaped” dwelling
Villager Self-built houses

lists the measurement points and temperature fluctuations.

As depicted in Figure 4e, we hung the measuring instruments on the earthen brick walls and wooden posts, 1.5–1.8 m from the ground.

As depicted in Figure 4a,b, the outdoor air temperature in Zhushan Village fluctuates significantly in winter. To be specific, the average daily difference is 10.5 °C, the minimum temperature is 2.4 °C, the maximum temperature is 13.8 °C, and the average temperature is 5.6 °C. In terms of the “I-shaped” dwelling, the average temperature indoor reaches 6.6 °C. For the “L-shaped” dwelling, the average temperature indoor reaches 6.3 °C. The average indoor temperature of traditional houses of 6.4 °C is higher than that of farmer-built houses (5.9 °C). However, in accordance with the Green Building Design Standard for Hot Summer and Cold Winter Areas, the indoor comfortable temperature in winter in this area should be 18 °C [33], so it is necessary to update the indoor thermal environment of Zhushan villagers’ houses in winter to meet the demand of improving thermal comfort.

The result of the research of the whole village suggests that the doors of traditional houses are closed at 00:00–06:20, and the doors open at 06:20–20:00; the windows are always open due to damage, the doors of living rooms of new houses are closed at 00:00–05:41 and 20:10–24:00, while opening at 05:41–20:10; the window of the new residential house is closed at 00:00–05:45 and 20:10–24:00, while opening at 05:45–20:10, approximately a 70% opening rate of windows and doors throughout the house, as depicted in Figure 4c, the average indoor temperature of traditional dwellings is 1.4 °C higher than that of new dwellings. Moreover, the highest value of outdoor air temperature occurs at nearly 14:00, the highest value of indoor air temperature of novel dwellings occurs at nearly 15:00 when the windows are opened, while the highest value of indoor air temperature of traditional dwellings occurs at approximately 15:30 when the windows are opened [34]. Traditional houses have a better indoor thermal environment than those of new houses in winter for their better heat storage and temperature delay [35].

Accordingly, the following indoor thermal environment simulations were conducted to identify the weak points of heat loss in the envelope of local houses to preserve the characteristics of traditional houses and their construction techniques, and to make them meet the requirements of energy-saving standards. On that basis, the traditional houses can be efficiently renovated [36].

2.5. Simulation work

2.5.1. Software

In this study, the energy consumption of raw earth dwellings in Zhushan Village was predicted using the building energy simulation program DesignBuilder. DesignBuilder calculates the heat load of the building based on the heat balance method. This method considers all heat balances on outdoor and indoor surfaces and transient heat transfer throughout the building. In accordance with the user descriptions of the building and the associated mechanical and other systems, DesignBuilder was adopted to calculate the heating and cooling loads required to maintain thermal control set points and conditions based on secondary HVAC (heating, ventilation, and air conditioning) systems and coil loads, as well as the energy consumption of major plant equipment, thus facilitating the energy analysis of different residential homes [30].

2.5.2. Modeling

As depicted in Figure 5a–d, to accurately investigate the winter thermal performance of the entire village habitat, the typical dwelling of villager F. y. Long was simulated as a “I-shaped” typical dwelling with an area of 121.36 m², a depth of 8.2 m, a width of 14.8 m, a shape coefficient of building is 0.5, as well as a window-to-floor ratio of 0.03. The villager J. l. Wu’s house was taken as the “L-shaped” typical dwelling for simulation study, the dwelling area of this typical dwelling was 142 m², the depth was 7.3 m, the width as 17.5 m, the shape coefficient of building is 1.74, and the window to floor ratio is 0.042.

As depicted in Figure 5a,d, The interior of the “I-shaped” dwelling has almost no internal wall partitions. The “L-shaped” dwelling with internal wall partitions but no internal doors. All the data fell in the average range of the “I-shaped” and “L-shaped” dwellings in the village, as presented in Figure 3b.

2.6. Winter Heat Load Simulation

Two typical residential houses were selected for the indoor thermal environment simulation under winter conditions, as presented in

Table 5. Material settings of the traditional dwelling envelope.

Type of Building Envelop		Material Thermal Index	Thickness (mm) Density (kg/m³)	Specific Heat KJ/(kg.K)	Heat Conductivity W (m.k)
External Wall (Outside-to-In)	300 mm clay brick	610	1820	1060	0.81
Partition wall	10 mm wooden board	30	510	2520	0.17
Roof (Outside-to-In)	6 mm gray tile roofs 2 mm wooden board	6 3.9	2045 515	1055 2512	0.96 0.18
External Window	Single-glass window	K = 4.71 (W/m2)	-	-	-
External Door	10 mm wooden door	40	500	2515	0.175
Inner Door	10 mm wooden door	22	520	2520	0.175
Floor	5 mm rammed earth	35	1795.5	882	0.71

. The thermal equilibrium simulation analysis of two typical residential buildings was conducted using DesignBuilder to further investigate the thermal performance of the winter envelope of traditional residential buildings in Zhushan Village. Figure 6 presents the heat balance analysis of the dwelling.

As depicted in

Table 6. Energy consumption.

“I-Shaped” Heat Balance (kW·h)				“L-Shaped” Heat Balance (kW·h)
Energy Outputs			Total	Energy Outputs			Total
Envelope	Wall	567.49 (18.9%)	2461.41	Envelope	Wall	646.73 (17.7%)	2885.37
	Ceilings	686.27 (22.8%)			Ceilings	565.86 (15.5%)
	Roofs	652.46 (21.7%)			Roofs	848.82 (23.3%)
	Glazing	13.36 (0.04%)			Glazing	62.24 (1.7%)
External air		541.83 (22%)		External air		761.72 (26%)

, the heat transfer of the envelope structure and the “I-shaped” dwelling were the most significant factors causing the indoor heat loss of the residential building, and the total heat dissipation during the winter was 2461.41 kW·h. The building heat loss of the envelope structure accounted for 78% of the total heat loss. The heat dissipation of external air was 541.83 kW·h, accounting for 18% of the total heat dissipation (heat loss caused by cracks, holes, and window gaps in the building structure). The heat gains due to lighting, occupancy, and sensible heating represent the sensible heating introduced into the zone through the HVAC system.

In typical “L-shaped” residential buildings, the heat transfer from the envelope and building ventilation was found as the largest factor of heat loss in the building, and the total heat dissipation during the winter was 2885.37 kW·h. The heat loss of the building envelope accounted for 74% of the total heat loss. The heat dissipation of external air was 761.72 kW·h, accounting for 26% of the total heat dissipation (heat loss caused by cracks, holes, and window gaps in the building structure). Furthermore, the heat gains due to lighting, occupancy, and sensible heating.

2.7. Building Energy-Saving Retrofit Methods

As depicted in Table 6, the indoor heat loss of the typical residential house in Zhushan Village in winter primarily comprised the heat dissipation from the envelope structure and air infiltration.

Thus, analyzing in the simulation study of (1) exterior wall insulation renewal, (2) roof insulation renewal, (3) ceiling thermal insulation renewal, and (4) door and window renewal, were taken as the focus to increase the indoor thermal insulation efficiency in winter.

3. Result

The basic heat transfer equation for the envelope is written as [37]:

Q = K · F · Δt = F · Δt/(R + R′) = F · Δt/(R + δ/λ)

(1)

where Q denotes the heat transfer through the enclosure; Δt represents the temperature difference between indoor and outdoor; R expresses the thermal resistance other than the insulation layer (including the thermal resistance of convective heat transfer between indoor and outdoor); R′ denotes the thermal resistance of the insulation layer, R′ = δ/λ; δ is the thickness of the insulation layer; and λ represents the thermal conductivity of the insulation layer [38].

The derivation of Equation (1) twice yields the relationship between the incremental heat transfer through the envelope and the thickness of the insulation layer as expressed in Equation (2) [38].

Q^″ = 2F · Δt/λ²(R + δ/Λ)⁻³

(2)

Accordingly, the Q increment decreases with the increase in δ. The above analysis suggests that increasing the insulation layer of the envelope is an effective energy-saving measure. However, when the insulation layer is increased, the contribution of its unit thickness to energy saving will be smaller with the increase in the thickness. Thus, the thickness of the insulation layer should be reasonably obtained. The thickness of the insulation layer is considered an optimized thickness when the reduction in energy consumption of the building is small, i.e., the contribution of energy saving is small. To investigate the relationship between the change of the thickness of the insulation layer of the envelope structure and the influence of the building heating heat consumption energy saving, the ratio of the change of heating energy saving and the change of the thickness of the insulation layer was obtained and called the rate of change of heating energy saving. This ratio was adopted to characterize the relationship between the change of the thickness of the insulation layer and the change of the building heating heat consumption. The ratio is expressed in Equation (3) [37].

φ = Q − Q _i/δ _i

(3)

where φ denotes the rate of change of building heating energy saving; Q represents the heating heat consumption of the building without insulation; and Q _i expresses the heating heat consumption of the building when the thickness of the insulation layer is δ _i; δ _i is the thickness of insulation layer [34].

3.1. Exterior Wall Renewal

The renovation method of the external wall is to increase the external insulation layer of a certain thickness and reasonably use the construction design to increase the airtightness of the external wall. Polyurethane was selected as the building exterior wall insulation material according to the specific climate of the Western HuNan region and local construction conditions. The parameters of polyurethane are listed in

Table 7. Material settings of the exterior wall retrofitting.

External Wall (Outside-to-In)	Material Thermal Index
	Thickness (mm)	Density (kg/m³)	Specific Heat KJ/(kg·K)	Heat Conductivity W(m·k)
Earth wall	600	1800	1050	0.81
Polyurethane	10–80 mm	30	1300	0.03
Woodern board	5	500	2510	0.17

, including the thermal conductivity of 0.03 W/(m·K), the density of 30 kg/m³, and the specific heat capacity of 1300 KJ/(kg·K). The external wall insulation layer was used in the simulation calculation, the thickness of the external wall insulation layer increased from 10 mm, marking an increase of 5 mm, and the maximum thickness was set to 80 mm.

In the “I-shaped” dwelling, the solution of energy-saving renewal of the external wall of buildings is to increase the internal insulation layer, and the rate of change of heat consumption of the external wall φ was negatively correlated with the thickness of the insulation layer δ_i, as presented in Figure 7a. The fitting relationship between φ and δ_i is expressed in Equation (4).

φ = 21.786 − 0.586x + 0.008x² − 4.121 × 10 − 5x³, R² = 0.999

(4)

when δ_i = 64.709 mm, the change of the heat consumption change rate φ of the external wall tends to be stable, and the polyurethane insulation layer with a thickness of 65 mm is the optimal insulation layer thickness, and the heat loss of the external wall is reduced by 3.344 kW·h/m² based on the design state of this thickness insulation layer, and the energy-saving rate is 70.7%.

In the typical “L-shaped” residential building, the solution of energy-saving renewal of the external wall of the building is to increase the internal insulation layer, and the rate of change of heat consumption of the external wall φ was negatively correlated with the thickness of the insulation layer δ_i, as presented in Figure 7b. The fitting relationship between φ and δ_i is expressed in Equation (5).

φ = 24.946 − 0.708x + 0.1x² − 5.328 × 10 − 5x³, R² = 0.991

(5)

when δ_i = 63.547 mm, the change of the heat consumption changed rate φ of the external wall tended to be stable, and the polyurethane insulation layer with a thickness of 64 mm was the optimal insulation layer thickness, and the heat loss of the external wall was reduced by 3.263 kW·h/m² under the design state of this thickness insulation layer, and the energy-saving rate was 69.6%.

3.2. Roof Renewal

As presented in Figure 8, the roof was renewed by introducing a layer of roofing panels under the original roof and by filling the two layers of roofing panels with foamed polyurethane to form an insulation layer with the parameters (

Table 8. Material settings of the roof retrofitting.

External Wall (Outside-to-In)	Material Thermal Index
	Thickness (mm)	Density (kg/m³)	Specific Heat KJ/(kg·K)	Heat Conductivity W(m·k)
Earth wall	600	1800	1050	0.81
Polyurethane	10–60 mm	30	1300	0.03
Woodern board	5	500	2510	0.17

), including the thermal conductivity of 0.03 W/(m·K), the density of 30 kg/m³, and the specific heat capacity of 1300 KJ/(kg·K). The thickness of the roof insulation layer was set from 10 mm in increments of 5 mm in the simulation calculation, and the maximum thickness was set to 60 mm.

For the “I-shaped” dwelling, the change rate of the heat consumption φ of the building roof energy-saving renewal is negatively correlated with the thickness of the insulation layer δ_i, as presented in Figure 9a. The fitting relationship between φ and δ_i is expressed in Equation (6)

φ = 38.503 − 1.483x + 0.028x² − 1.922 × 10 − 4x³, R² = 0.999

(6)

when δ_i = 48.56 mm, the change of the roof heat consumption rate φ tended to be stable, and the polyurethane insulation layer with a thickness of 49 mm was the optimal insulation layer thickness, and the roof heat loss was reduced by 4.077 kW·h/m² under the design state of this thickness insulation layer, and the energy-saving rate accounted for 74.9%.

For a typical “L-shaped” residential building, the rate of the change of heat consumption of the building roof energy-saving renewal φ was negatively correlated with the thickness of the insulation layer δ_i, as presented in Figure 9b. The fitted relationship between φ and δ_i is written in Equation (7).

φ = 27.093 − 1.020x + 0.020x² − 1.435 × 10 − 4x³, R² = 0.999

(7)

when δ_i = 46.46 mm, the change of the roof heat consumption rate φ gradually became stable, the polyurethane insulation layer with a thickness of 46 mm was the optimal insulation layer thickness, and the roof heat loss was reduced by 2.61 kW·h/m² based on the design state of this thickness insulation layer, and the energy-saving rate accounted for 42.4% [39,40].

3.3. Ceiling Renewal

As presented in Figure 10, ceilings, the enclosure structure between the attic space and the first floor of the residence, play a role in beautifying the indoor space and bear the function of heat insulation. The update method adopted in this study is to achieve the role of a fixed ceiling by setting the keel on the original floor, and the material of the ceiling from top to bottom is listed in

Table 9. Material settings of the ceiling retrofitting.

Ceiling (Top-to-Bottom)		Material Thermal Index
		Thickness (mm)	Density (kg/m³)	Specific Heat KJ/(kg·K)	Heat Conductivity W(m·k)
Wooden Floor	10 mm wooden board	10	500	2510	0.17
Fiber Cotton Rock	10 mm	10–60 mm	120	980	0.04
Plaster board	5 mm	5	500	2510	0.17
Wood veneer	5 mm	10	500	2510	0.17

. The density was 30 kg/m³, and the specific heat capacity was 1300 KJ/(kg·K). In the simulation calculation of the ceiling insulation layer, the initial thickness of the insulation layer was 10 mm and was increased by 5 mm, and the maximum thickness was set to 60 mm.

For the “I-shaped” dwelling, the rate of the change of heat consumption of the ceiling energy-saving update φ was negatively correlated with the thickness of the insulation layer δ_i, as presented in Figure 11a. The fitted relationship between φ and δ_i is expressed in Equation (8)

φ = 39.608 − 1.567x + 0.03x² − 2.104 × 10 − 4x³, R² = 0.998

(8)

when δ_i = 47.53mm, the change of the heat consumption rate φ of the roof gradually became stable, and the thickness of 48 mm was the optimal insulation layer thickness. Under the design state of this thickness, the heat loss of the roof was reduced by 4.15 kW·h/m², and the energy-saving rate accounted for 72.6%.

For a typical “L-shaped” residential building, the change rate of the heat consumption φ of the ceiling energy-saving update was negatively correlated with the thickness of the insulation layer δ_i, as presented in Figure 11b. The fitting relationship between φ and δ_i is written in Equation (9)

φ = 30.128 − 1.26x + 0.025x² − 1.767 × 10 − 4x³, R² = 0.997

(9)

when δ_i = 47.16 mm, the change of the roof heat consumption rate φ tended to be stable, the thickness of 47 mm of the fiber wool rock insulation layer was the optimal insulation layer thickness, the roof heat loss was reduced by 2.56 kW·h/m² under the design state of this thickness insulation layer, and the energy-saving rate accounted for 62.4%.

3.4. External Window Renewal

The equations proposed in reference [28] were adopted to evaluate the airflow rates through an opening via natural convection:

$$\mathrm{G}=\frac{2}{3}B{C}_d\sqrt{\frac{g{H}^3}{2}}•\sqrt{\frac{\Delta t}{T_0}}$$

(10)

B = b₁h₁ + b₂h₂ +···+ b_nh_n/H

(11)

where G denotes the air flow rate in m³/s; B represents the equivalent width of the windows and doors that can be obtained by Equation (11); b₁.b₂ − b_n,h₁.h₂ − h_n are the sizes of the windows and doors on the partition wall of the sunspace; Cd denotes the discharge coefficient obtained by Equation (12); g is the acceleration of gravity in m/s; H is the height of the stack effect; ∆t is the temperature difference inside and outside of the openings in °C; T₀ represents the average temperature inside and outside of the openings in K; and T_i is the air temperature of the sunspace in K.

Cd = 0.40 + 0.0045/T_i − T0_Ι/

(12)

Once the flow volumes are obtained, the convective heat transfer can be quantified by Equation (13).

Q = ρ · c · G · Δt

(13)

where Q denotes the convective heat transfer in W; ρ is the air density in kg/m³; and c represents the air mass specific heat with a constant value of 1.01 kJ/(kg °C).

This study focuses on increasing the air tightness and thermal insulation efficiency of the exterior windows as an update solution due to the considerable amount of damage to the exterior windows of the local residential houses in Zhushan Village that results in the reduction of air tightness.

The exterior window walls of local residential houses in Zhushan Village were relatively small (Figure 12), the heat loss caused by heat exchange through the glass was very small (

Table 10. Heat transfer coefficients of external windows.

Type of the Window	ΔExternal Air (kW·h)	Glazing (kW·h)
6 mm single layer glass	228.6437	1.21
5 + 6 A + 5 mm double insulating glass	228.4937	9.43
Low-e6 + 12 A + 6 mm double insulating glass	228.4037	13.96
Low-e6 + 12 A+6 + 12 A + 6 mm double insulating glass	228.4637	10.89
6 + 12 A + 6 mm three-layer insulating glass	228.3937	13.98
6 + 12 A + 6 + 12 A + 6 mm three-layer insulating glass	228.3737	15.63

), and the energy-saving efficiency brought by the improvement of the airtightness of the replacement window frames in the renewal process was significantly high.

Thus, the renewal practice of 6mm single-layer glass and ordinary wooden window frames with pressed glue filling is confirmed as the optimal solution for the energy-saving renewal of exterior windows from the perspective of cost-effectiveness (

Table 11. Heat transfer coefficients and retrofit costs of external windows.

Type of the Window	Material Thermal Index
	Uw (W/m2 K)	C (¥/m2)
6 mm single layer glass	5.746	24.79
5 + 6 A + 5mm double insulating glass	3.125	41.265
Low-e6 + 12 A + 6 mm double insulating glass	1.761	75.529
Lowe6 + 12 A + 6 + 12 A + 6 mm double insulating glass	2.665	61.402
6 + 12 A + 6 mm three-layer insulating glass	1.754	77.574
6 + 12 A + 6 + 12 A + 6 mm three-layer insulating glass	1.307	92.320

).

3.5. Envelope Renewal

Comparing measured temperature data and the simulated temperature data from the 9 January to the 18 January, it can be found the deviation between the measured data and the simulated data was less than 0.5 °C, approaching 4.1% of the temperature change over the entire simulation [28,41].

Figure 13a shows that the value of the “I-shaped” dwelling indoor temperature increases by 2.6 °C after improvement. As we can see in Figure 13b, the air temperature of the “L-shaped” dwelling increased by 2.4 °C. After renewal, the indoor thermal comfort of the raw earth dwelling is improved compared with that before renovation. Through the simulation of the building energy-saving scheme, the improved raw earth dwelling has a prominent performance in the thermal environment as the indoor mean temperature increased.

As depicted in

Table 12. The energy consumption after renewal.

“I-Shaped” Heat Balance (kW·h)				“L-Shape” Heat Balance (kW·h)
Envelope Energy Outputs			Total	Envelope Energy Outputs			Total
Envelope	Wall	192.95 (24.2%)	795.7	Envelope	Wall	220.81 (18.4%)	1197.7
	Ceilings	161.62 (20.3%)			Ceilings	194.03 (16.2%)
	Roofs	99.02 (12.4%)			Roofs	456.03 (38.1%)
	Glazing	4.23 (0.5%)			Glazing	6.17 (0.5%)
External air		333.89 (42%)		External air		285.71 (23.9%)

, in the winter, the energy consumption of the external wall insulation in the “I-shaped” dwelling was reduced to 192.95 kW·h, marking a decrease of 374.54 kW·h; the energy consumption of ceilings was reduced to 161.62 kW·h, decreased 524.65 kW·h; the energy consumption of roofs was reduced to 99.02 kW·h, decreased 553.44 kW·h; the energy consumption of glazing was reduced to 4.23 kW·h, marking decreased 9.13 kW·h; the energy consumption of external air was reduced to 333.89 kW·h, decreased 553.44 kW·h; and the total reduction of the “I-shaped” dwelling was 1665.7 kW·h. (The energy consumption before the renewal was analysed in Section 2.6).

As depicted in Table 12, in the winter, the energy consumption of the external wall insulation in the “L-shape” dwelling was decreased to 220.81 kW·h, marking a decrease of 425.92 kW·h; the energy consumption of the ceilings was decreased to 194.03 kW·h, decreased 371.83 kW·h; the energy consumption of the roofs was decreased to 456.03 kW·h, decreased 392.79 kW·h; the energy consumption of the glazing was reduced to 6.17 kW·h, decreased 56.07 kW·h; the energy consumption of the external air was reduced to 285.71 kW·h, decreased 476.01 kW·h; the total reduction of the “L-shape” dwelling was 1687.59 kW·h. (The energy consumption before the renewal was analysed in Section 2.6).

4. Disscussion

The indexes below were adopted for the synthetical evaluation of the energy consumption of the building investigated in this study. E_heating represents the annual total energy consumption per unit building area. E_CO2 measures the CO₂ emissions per unit building area, which is expressed in Equation (14) [28].

E_CO2 = E_fuel · C_con

(14)

where E_CO2 denotes the carbon dioxide emissions per unit building area (kg); E_fuel represents the total fuel consumption per unit building area from the heating or cooling system obtained using the simulation software (kW·h); C_con denotes the carbon dioxide conversion factor that defines the amount of carbon dioxide emitted per unit of energy consumption. The Chinese Ministry of Housing and Urban-Rural Development has divided China into seven different regions, and Hunan Province is part of the southern regional grid, which has an emission factor of 0.53 [42].

The heat balance analysis of a building is conducive to explaining the occurrence of heating energy [8]. Thus, a statistical analysis heat transfer via the building envelopes was conducted to evaluate the heat flow out of the buildings. The above indexes, in conjunction with the CO₂ emission, are applicable to the evaluation of the energy consumption performance of the respective building.

As depicted in Figure 14a,b, the winter CO₂ emissions of “I-shaped” dwelling roofs were 32.3–62.9 kg/Day 57% of the time, and after renewal were 1.7–5.1 kg/Day 52% of the time. The CO₂ emissions of ceilings were 32.3–52.7 kg/Day 62% of the time, and after renewal were 1.7–8.5 kg/Day 66% of the time. The CO₂ emissions of walls were 32.3–52.7 kg/Day 55% of the time, and after renewal were 5.1–8.5 kg/Day 62% of the time; a total reduction of 882.8 kg CO₂ (which is expressed in Equation (14) and the energy consumption E_fuel was analysed in Section 3.5).

As depicted in Figure 14c,d, the winter CO₂ emissions of “L-shape” dwelling roofs were 10.2–23.8 kg/Day 57% of the time, and after renewal were 9.86–15.9 kg/Day 56% of the time. The CO₂ emissions of ceilings were 3.4–17 kg/Day, and after renewal were 5.78–7.8 kg/Day. The CO₂ emissions of walls were 3.4–17 kg/Day, and after renewal were 5.78–7.8 kg/Day; a total reduction of 894.42 kg CO₂ (which is expressed in Equation (14) and the energy consumption E_fuel was analysed in Section 3.5).

5. Conclusions

In this study, the indoor thermal environment and energy-saving mode of the traditional raw earth dwellings in Zhushan village were simulated and analyzed by DesignBuilder. The thickness of the thermal insulation materials was determined by comparing the energy-saving rate. Furthermore, the data change relationship was analyzed by fitting the function to obtain the optimal energy-saving renewal plan. The major conclusion is drawn in the following.

(1) In terms of energy consumption, the selection of energy-saving renovation measures follows an order of external window, ceiling, roof, and external wall.

(2) For the winter indoor thermal environment renewal of the “I-shaped” dwelling, polyurethane with a thickness of 65 mm was selected for the exterior wall; polyurethane with a thickness of 65 mm was selected for the roof; 48mm of polyurethane was employed for the ceiling; the original wooden window frames were replaced with 6 mm single under the above scheme, the heat consumption of the enclosure structure was reduced by 11.2 kW·h/m², and the heat consumption of infiltration was reduced by 1.6 kW·h/m²; the total reduction was 12.8 kW·h/m², and the CO₂ emissions reduction was 882.8 kg during the winter.

(3) The typical “L-shaped” residential house was updated with indoor thermal environment in winter, and the thickness of polyurethane of 64 mm was selected as the thickness of the insulation layer for the external wall; 46 mm polyurethane was selected as the insulation layer thickness for the roof; 47 mm polyurethane was employed as the insulation layer thickness for the ceiling; the original wooden window frames were replaced with 6mm single under the above scheme, the heat consumption of the enclosure structure was reduced by 10.3 kW·h/m², and the heat consumption of infiltration was reduced by 2.97 kW·h/m²; the total reduction was 13.27 kW·h/m², and the CO₂ emissions reduction was 894.4 kg during the winter.

(4) With the promotion and implementation of the renewal scheme, the whole Zhushan Village will save energy of 11.2 × 10⁴ kw·h after the completion of the insulation renewal of the envelope structure.

Low-energy structures are designed to reduce the energy consumption of a building and to increase the energy efficiency of the whole structure. This novel construction evolution has been considered as the future of sustainable buildings by the scientific community. The concept is currently in various stages of development that are subject to the country considered. The essential reasons for this are presented as follows. The lack of incentives and policies has led to more investors entering the industry. Some novel techniques have been proposed, so further rigorous research in this area is needed. The public has a perception of the cost of low-energy buildings, and a volatile public is sensitive to low-energy structures. Thus, local requirements and the attitudes and activities of residents should be considered in the design of criteria and recommendations for low-energy structures. Likewise, it is important to study from previous lessons and use emerging technology that has been adopted in most developed countries to sustain their companies’ capacities for technical advancement, encourage manufacturing improvements, and eventually form a structured and commercialized low-energy building development structure. It is still necessary to increase the effect of low-energy buildings by raising public consciousness of energy efficiency and encouraging emerging technology and goods that can conserve energy. Furthermore, the sustainability of low-energy buildings should be enhanced through the formulation of appropriate incentive policies and enhanced financial support. Lastly, it is imperative to provide scientific and technological guidance for low-energy traditional dwelling growth by increasing the investment in basic low-energy building science.