Study on Comprehensive Benefit Evaluation of Rural Houses with an Additional Sunroom in Cold Areas—A Case Study of Hebei Province, China

Abstract

To address the issues of poor thermal performance and high energy consumption in rural dwellings in cold regions of China, this study investigates multi-type energy-efficient retrofitting strategies for rural houses in the Hebei–Tianjin region. By utilizing a two-step cluster analysis method, 458 rural dwellings from 32 villages were classified based on household demographics, architectural features, and energy consumption patterns, identifying three typical categories: pre-1980s adobe dwellings, 1980s–1990s brick–wood structures, and post-1990s brick–concrete houses. Tailored sunspace design strategies were proposed through simulation: low-cost plastic film sunspaces for adobe dwellings (dynamic payback period: 2.8 years; net present value: CNY 2343), 10 mm hollow polycarbonate (PC) panels for brick–wood structures (cost–benefit ratio: 1.72), and high-efficiency broken bridge aluminum Low-e sunspaces for brick–concrete houses (annual natural gas savings: 345.24 m³). Economic analysis confirmed the feasibility of the selected strategies, with positive net present values and cost–benefit ratios exceeding 1. The findings demonstrate that classification-based retrofitting strategies effectively balance energy-saving benefits with economic costs, providing a scientific hierarchical implementation framework for rural residential energy efficiency improvements in cold regions.

1. Introduction

1.1. Overview

With the rapid development of social and economic conditions, the expectations of rural residents regarding the comfort of residential environments are constantly increasing [1]. However, most existing rural houses in China were built before 2010 and generally feature an envelope structure with poor thermal performance. In addition, energy consumption is constantly increasing [2]. Against this background, research on rural residential energy-saving transformation in China is imperative.

An additional sunlight room is a simple, direct, and efficient way of using solar energy resources and passive solar technology, and it can effectively improve the winter indoor thermal environment of rural houses in cold areas [3]. An additional sunlight room not only increases heat collection components and supplies the room with solar heat energy but also acts as an indoor and outdoor buffer zone [4], reduces the room’s cold air penetration heat loss, and greatly improves the indoor thermal comfort of the room. It is worth noting that the design and operational efficiency of sunspaces largely depend on the selection of key design parameters: A suitable orientation can maximize the capture of solar radiation. An appropriate geometric form (depth, height, window ratio, and the connection mode with the main building) should be chosen to balance the relationship between heat collection, heat storage, heat dissipation, and usable space. Material selection (especially the type and thermal insulation performance of transparent envelopes, as well as the material and layout of internal heat storage bodies) also directly affects the efficiency of solar energy acquisition, preservation, and transfer to the interior. Therefore, optimizing these elements is key to achieving efficient and stable heating effects in sunspaces.

1.2. Literature Review

Since the beginning of the 21st century, with the rapid development of computer technology, researchers have been optimizing sunlight rooms through simulation, with a large number of studies focusing on the impact of sunlight room design on building energy consumption. Bataineh et al. [5] evaluated the actual energy-saving effect of a sunroom located in Amman, Jordan, and explored the influence of sunroom design factors on the building’s energy-saving effect. The simulation results show that the energy-saving efficiency is highest when the sunroom is oriented south and lowest when the sunroom is oriented north. A double-glazed sunroom that uses ventilation technology at night in summer, curtains during the day, and insulation curtains at night in winter can reduce the annual heating and cooling load by 42 percent. Aelenei et al. [6] conducted a study of Portuguese sunrooms using EnergyPlus software. Their results showed that sunrooms reduced heating energy consumption by 48% in winter but increased cooling energy consumption by 10% in summer. The increased energy consumption in summer was mitigated by installing adjustable shading and ventilation systems in sunrooms. Ulpiani et al. [7] used the dynamic simulation software DesignBuilder and EnergyPlus to compare the effects of different sunroom design factors on building energy consumption in the Mediterranean climate. The results show that the optimal depth of the sunroom in this region is 1.5 m, and the energy saving for the double-glazed sunroom is 1 kW·h/d compared to the single-glazed sunroom. Chiesa et al. [8] selected 50 sunrooms located in Southern and Central Europe to evaluate and analyze their energy-saving potential. By comparing different technology options, an energy consumption dynamics simulation was carried out on the building model to calculate the potential of sunrooms to reduce heating energy consumption. The results show that the heating energy consumption of buildings without insulation measures can be significantly reduced in any single-, double-, or triple-glazed sunroom.

In addition, some scholars have also carried out related research on the economic evaluation of sunrooms. Lopez et al. [9] found that the annual heat collected by the sunshine room in a practical project in Spain ranged from 2.2 MW·h to 4.6 MW·h, and the cost reduced by the energy-saving effects of the sunshine room was EUR 325–836/year, and the investment payback period was 41 years. Kottia et al. [10] analyzed the economic benefits of the energy-saving renovation of Greek residential buildings and found that using solar rooms could save 10% of energy use in residential buildings, and the investment payback period was 15 years. In 2008, Jin Hong et al. [11] investigated and measured the traditional rural houses in Jilin, calculating and comparing the combined solar house of “direct benefit + sun room” with the traditional rural house, and the results showed that the combined solar house could achieve an energy-saving rate of more than 70%. In 2015, Yang Yanxia [12] analyzed the heat action mechanism of sunrooms on buildings, established the heat balance equation of sunrooms by using the dynamic heat network method, conducted experimental research on the heat transfer process of sunrooms in winter in southern Liaoning, and obtained the calculation method for the sunroom convective heat transfer coefficient and air circulation heat flow rate suitable for southern Liaoning, as well as the empirical value of the convective heat transfer coefficient in this region. Wu Haojun [13] used DeST-h to simulate and analyze the passive solar house prediction model based on the measured data of rural energy-saving houses in Beijing and the traditional rural house model, and the results showed that the energy-saving rate for solar house heating reached 69%. Existing studies have rarely explicitly quantified the ecological benefits of sunspaces. The team led by Zhao Jing [14] was the first to quantitatively verify the eco-energy synergistic benefits of sunspaces in rural houses in cold regions through a combination of empirical and simulation methods. Compared with the baseline scenario, this strategy reduced carbon emissions during the heating season by 51.73%. Aiming to target the common thermal performance defects of building envelopes (high energy consumption and poor thermal environment) in rural houses in cold regions of China, Cao Ping’s [15] team pointed out that existing studies mostly focus on single-technique improvements, lacking multi-objective collaborative optimization. Taking rural houses in Tongchuan, Shaanxi, as empirical cases, they filled the quantitative gap regarding the comprehensive benefits of envelope renovations through a full-chain method of “energy consumption simulation-multi-index decision-making-scheme verification”, proving that the contribution of carbon emission reduction (with a weight of 17.55%) can serve as a quantitative basis for government subsidies.

To further clarify the positioning of this study, a direct comparison between it and previous studies on sunspaces is presented in Table A1. There are many studies on the energy-saving renovation of the building envelope, and there are quite a lot of studies on optimizing solar rooms. The studies on the winter thermal performance and thermal environment optimization of solar room are gradually becoming mature, especially for optimizing solar rooms in different regions, and different scholars have presented different optimization strategies [16]. However, at present, the following problems in research on the energy-saving transformation of sunshine rooms and the enclosure structures of agricultural houses remain:

①

Applicability of the research results of the energy-saving renovation of agricultural houses: Most existing studies only focus on the optimization of a typical rural house in a certain area, but in fact, rural houses in a region are multi-type in nature due to the differences in their construction ages, their envelope structure performances, the income levels of the residents, and other factors, leading to optimizations of the design of rural house sunshine rooms and energy-saving changes to the envelope structure. Each house’s energy-saving demands and economic demands are different [17] and therefore, research on the optimal design of sunrooms for existing rural houses and energy-saving changes to the enclosure structure needs to be conducted according to the different types and different remaining lifecycles of the building. This will meet the actual needs of rural residents and be suitable for classification research [18].

②

Limitations of existing sunshine room research in China: At present, research on the optimal design of the transparent envelope structure of sunshine rooms in China is only focused on glass material, and there are few studies that have comprehensively researched and analyzed the energy-saving potential, cost savings, suitability, and convenience of other transparent envelope materials for sunshine rooms in the Chinese building materials market.

At present, conventional energy-saving transformation research is mostly based on the enclosure structure [19]. However, this is a more comprehensive problem. The existing building’s operating status, the family’s economic level, user awareness of energy savings, energy-saving demand, and other factors are key constraints of energy-saving renovations for rural houses.

Based on an in-depth investigation of rural houses in Hebei Province (through interviews, questionnaires, and mapping), this study focuses on solving the actual energy consumption problems of rural buildings. Using the two-stage clustering method and principal induction method, three typical types of rural houses were identified: adobe houses from the 1980s, brick–wood structures from the 1980s to 1990s, and brick–concrete houses from after the 1990s. Considering the different remaining service lives of the main bodies of these three types of rural houses, this study focuses on analyzing the cost- and energy-saving potential of optimal sunspace designs.

Through simulation analysis and other methods, this study matched the optimal sunspace renovation strategies for each type of rural house (such as short payback period schemes, broken bridge aluminum alloy double-glass schemes and high-energy-saving long-life schemes), aiming to effectively reduce their winter heating energy consumption and provide classified guidance for energy-saving renovations of rural houses in similar areas.

Innovations of the Thesis

①

Construction of a Typical Rural House Classification Model in Hebei Province Based on Subjective–Objective Integration

Research Approach: Through a literature review and field surveys of existing rural houses in Hebei Province, this study analyzes and summarizes the basic information of residents, architectural characteristics, and current energy consumption status. Using IBM SPSS Statistics v22.0 (IBM Corporation, Armonk, NY, USA) software for a cluster analysis of statistical data, a typical rural house classification model is constructed by integrating subjective judgment with objective data support.

Practical Validation: Field tests are conducted on the sunspaces of existing rural houses. By analyzing the thermal environment data inside and outside the houses, this study identifies problems in using local rural house sunspaces, providing ideas and basic data for further optimizing their design.

②

Comprehensive Benefit Evaluation and Optimal Selection of Transparent Materials for Rural House Sunspaces Based on Full Lifecycle

Model Innovation: For the first time, a comprehensive benefit evaluation model for sunspace optimization strategies is proposed and constructed based on the remaining lifecycles of existing rural houses, focusing on both economic (such as energy-saving-related cost reductions and renovation cost recovery) and ecological benefits.

Decision-making Tool: Taking energy-saving optimization parameters as the benchmark, this study systematically evaluates the comprehensive benefits of different transparent envelope materials for rural house sunspaces. Finally, an innovative “Optimization Scheme Priority Menu for Sunspaces Adapted to Different Typical Rural Houses” is proposed, providing a basis for differentiated and implementable optimization paths for farmers and decision-makers.

2. Materials and Methods

According to the standard GB/T 42074-2022 “Classification of Climate Seasons” [20], Hebei Province belongs to a temperate continental monsoon climate zone. To comprehensively represent the research objects and considering the feasibility of the research, this study adopted a stratified sampling method, finally selecting 17 villages in this region in which to carry out surveys on more than 600 rural households. After data collation, 458 valid questionnaires were recovered (with a validity rate of 77%). To control for the non-response bias, 30 non-respondents were randomly selected for telephone interviews based on key variables (age, architectural characteristics, heating methods, etc.), and the statistical analysis showed that there was no significant difference between them and the response group (p > 0.05). The research scope and sample size ensured that the data effectively reflected the basic status of rural dwellings in cold regions. The main research methods are shown in Table 1.

Table 1. Research framework.

Serial Number	Survey Category	Research Method	Specific Research Content
1	Basic information of residents	Questionnaire	Number of permanent residents of the household, major industries, annual household income
2	Information on the construction of rural houses	Questionnaire Field mapping	Age of Construction, Yard Form, Number of Building Floors, Building Area, Functional Layout, Form of Enclosure Structure
3	Indoor thermal environment of rural house	Experimental	Outdoor air temperature, relative humidity, wind speed, solar radiation intensity of rural house
4	Habits and wishes of residents	Questionnaire	Heating area, heating cost, room rate, energy-saving renovation intention

①

Field mapping

We used a field surveying and mapping method to understand the courtyard layout, building size, envelope structure form, etc. The design configuration of sunrooms added to rural houses was thoroughly investigated, providing points of reference and benchmarks for further research on the optimized design of rural house sunrooms and adaptive models for the energy-saving retrofitting of rural houses.

②

Interview and questionnaire

Through interviews and questionnaires on the living feelings of rural residents, we could gain a general understanding of rural residents’ living habits, energy consumption habits, subjective thermal environment feelings, acceptance of sunshine room and energy-saving transformation, etc., and lay foundations for the next stage of classification research on existing rural houses.

③

Temperature test

A total of 50 rural houses of 2–5 households in each village were selected as typical rural houses for 48 h of temperature measurement, and the change in room temperature was observed.

2.1. Classification of Existing Rural Houses

Through qualitative analysis of the survey data, the household population characteristics, building characteristics of rural houses, and some aspects of users’ energy consumption habits that can affect rural houses’ energy consumption were used as classification variables [21], using the second-order clustering method to classify the characteristics of rural houses in Hebei and Tianjin. From the factors that have an important impact on the energy-saving transformation of rural houses, the general and easily accessible rural house feature identification factors were selected, and classification research was carried out on the sunroom optimization of rural houses via the second-order clustering method. The specific selected cluster analysis indicators are shown in Table 2.

Table 2. Classification index of rural houses.

Variable Attributes	Variable Name	Variable Encoding	Variable Description	References
Continuous variables	Number of permanent residents	Number of permanent residents	Unit: People	Diao et al. [22]: Modeling energy consumption in residential buildings: a bottom-up analysis based on occupant behavior pattern clustering and stochastic simulation.
	Annual household income	Annual household income	Unit: Yuan/year	Barthelmes et al. [23]: Profiling occupant behaviour in Danish dwellings using time use survey data.
	Floor area	Floor area	Unit: m	Liu Jing [24]: Investigation on the Current Situation and Energy-saving Approaches of Rural Residential Buildings in Severe Cold and Cold Regions
	Width of main room face	Width of the main room	Unit: m	Zhu Kedi [25]: Research on Energy Consumption Benchmark in Energy—saving Renovation of Existing Residential Buildings in Chongqing from the Typological Perspective.
	Heating area	Heating area	Unit: m	Zhou Heming [26]: Statistical and Evaluation Study on Heating Energy Consumption of Rural Houses in Cold Regions.
Categorical variable	Age of construction	1970 1985 2000 2010	Before 1980 1980–1990 1990–2010 2010 onwards	Zhu Kedi [25]: Research on Energy Consumption Benchmark in Energy—saving Renovation of Existing Residential Buildings in Chongqing from the Typological Perspective.
	Number of building floors	1 2	1 floor 2 floors	Sun Ruijun et al. [27]: Investigation and Analysis of Energy Consumption of Rural Residences in Northeast China.
	Cubicles	3 4 5	3 cubicles 4 cubicles 5 rooms	Qian Zhijun [28]: Research on the Application Strategy of Thermal Buffer Space in the Renovation of Rural Dwellings in Zhangbei Region Oriented towards Near—Zero Energy Consumption.
	Rural house construction	1 2 3	Adobe walls Brick and wood construction Brick–concrete construction	Liu Yaqi [29]: Analysis of Carbon Emissions of Vernacular Rural Dwellings in the Regions along the Yellow River in Inner Mongolia and Research on Carbon Reduction Strategies.
	Exterior wall thickness	240 370 490	Wall thickness 240 mm Wall thickness 370 mm Wall thickness 490 mm	Giusti et al. [30]: Impact of building characteristics and occupants’ behaviour on the electricity consumption of households in Abu Dhabi (UAE).
	Materials for doors and windows	0 2 4 6	Wood-framed single glass Windows Aluminum alloy single-glass window Aluminum alloy double-glass window Plastic steel window	Chang Wentao [31]: Research and Testing on the Energy-saving Performance of External Windows.

2.2. Rural House Clustering Results

According to an analysis of the survey data, the optimal number of agricultural residence classifications was determined, and 4 was determined to be the clustering number. As shown in Figure 1, the measured value of the clustering contour was 0.4, indicating that the clustering quality was close to the excellent value, thus indicating that the classification was effective.

Figure 1. Existing rural residence feature clustering quality map.

The specific characteristic information of 4 types of rural houses is shown in Figure 2. According to the size of the cluster, from left to right, there are brick and concrete rural houses from the 1990s, brick and wood rural houses from the 1980s and 1990s, brick and concrete rural houses built 10 years ago, and adobe rural houses built 80 years ago, accounting for 43.8%, 30.6%, 14.6%, and 11.0%, respectively. The importance of the cluster decreases from top to bottom, and the darker the color, the more important it is. Among the input clustering indicators, the age of construction, the structure of rural houses, and the material of doors and windows are the most important. The characteristics of these three types of rural houses play a decisive role in the classification of rural houses.

Figure 2. Summary diagram of characteristic clustering for existing rural houses.

To analyze the characteristics of each typical rural house after classification, the field investigation information and the results of second-order clustering are summarized, and the characteristics of 4 types of typical rural houses in Hebei and Tianjin are shown in Table 3. As the brick–concrete rural houses from the 1990s and 10 years later were both brick–concrete rural houses with little change in architectural geometric characteristics, they were classified into one type for analysis. Finally, the typical rural houses in Hebei and Tianjin were divided into adobe rural houses from 80 years ago, brick–wood rural houses from the 1980s, and brick–concrete rural houses from the 1990s.

Table 3. Results of rural residence classification.

Residence Feature	Adobe Rural House from 80 Years Ago	1980s–1990s Brick-and-Wood Rural House	1990–2010s Brick Rural House	Brick Rural House from 10 Years Ago
Number of permanent residents	2–3 people	2–3 people	3–4 people	3–4 people
Annual household income	About CNY 45,000 Medium annual income	Around CNY 30,000 Lower annual income	About CNY 50,000 Medium annual income	About CNY 60,000 Higher annual income
Years of construction	Before 1980	1980–1990	1990–2010	2010 onwards
Floor area	About 110 m2	About 110 m2	About 160 m2	Around 200 m2
Number of layers	1 layer	Floor 1	1 layer, very few 2 layers	1 layer, very few 2 layers
Kaina	3 bays	3 bays, 4 bays	3 bays, 4 bays	4 bays, 5 bays
Rural house construction	Civil construction Stone and wood construction	Brick and wood construction	Few brick-and-wood structures Most masonry	Brick-mix
Exterior wall thickness	490 mm	370 mm	240 mm, 370 mm	240 mm, 370 mm
Material for doors and Windows	Wooden frame single glass	Wooden frame single glass	Mostly aluminum alloy single glass A few aluminum alloy double glass	Aluminum alloy windows Plastic steel windows
Heating method	Earth kang and stove Gas wall hanging stove	Earth kang, stove Gas wall hanging stove	Earth kang, stove Gas wall hanging stove	Stove Gas wall-mounted stove
Heating area	About 60 m2	Around 70 m2	Around 80 m2	About 90 m2
Heating costs	About CNY 1500	Around CNY 2000	Around CNY 3000	More than CNY 3000
Photograph
Make Up	Less	Medium	More	Less

2.3. Parameters of the Rural House Building Model

Adobe rural houses from 80 years ago and brick–wood farmhouses from 90 years ago both feature gable roofs and L-shaped courtyard layouts, with the main houses having three bays and an interior net height of 3 m, as shown in Figure 3. The adobe rural houses mainly use unfired adobe (made by pressing clay mixed with fibrous materials such as straw and wheat stalks) for walls, and some areas have rammed earth walls. Adobe materials have high thermal conductivity, have poor insulation, and are vulnerable to rain erosion. The thick adobe walls (usually 50–60 cm), as the main load-bearing structure, rely on their own weight for stability, resulting in weak seismic resistance. For brick–wood rural houses, the exterior walls were mostly built with solid red clay bricks, while the interior walls used adobe or blue bricks, with brick joints filled with mixed mortar (cement, lime, and sand). The typical wall thickness is about 24–37 cm, with improved thermal insulation compared to adobe houses, but still lacking insulation layers. The brick walls bear the load, supplemented by wooden beams and columns (partially) to support the roof, enhance structural stability, though the wooden components are prone to dampness and decay. After the 1990s, brick–concrete farmhouses also had gable roofs and L-shaped courtyard layouts, but the main houses had four bays and an interior net height of 3 m, as depicted in Figure 4. Their walls were constructed with clay bricks or concrete blocks (such as hollow bricks), and most exterior walls were 24 cm thick. In some areas, thermal insulation mortar or external insulation layers (such as polystyrene boards) began to be used, significantly improving energy efficiency. A “brick-concrete structural system” composed of reinforced concrete ring beams and construction columns was formed, with walls and concrete components jointly bearing the load, increasing the seismic resistance to grade 6–7. The specific parameters of the building envelopes of various farmhouses are shown in Table 4.

Figure 3. Floor plans of adobe and brick–wood rural houses from before the 1990s.

Figure 4. Floor plans of brick–concrete rural houses from the 1990s.

Table 4. The thermal parameters of the envelope structures of rural houses at various stages.

Type of Rural Building	Structure Type	Heat Transfer Coefficient	Materials	Thickness
		W·(m2·K)		mm
Pre-1980s Adobe rural house	Exterior wall	1.46	Solid clay bricks	240
			Adobe	240
			Mixed mortar	10
	Roofing	0.96	Roof shingles	10
			Grass mud	80
			Grass mat	50
			Wooden frame construction	-
			Wooden keel	-
			Suspended ceiling layer	10
	Interior walls	1.82	Mixed mortar	20
			Solid clay brick	240
			Mixed mortar	20
	Ground	2.74	Cement mortar	20
			Concrete bedding	60
			Tamped plain earth	100
	Doors and Windows	4.75	Wood frame +6 mm single glass
1980s–1990s Brick and wood rural homes	Exterior wall	1.54	Solid clay brick	370
			Mixed mortar	20
	Roofing	0.96	Roof shingles	10
			Grass mud	80
			Grass mat	100
			Wooden house frame structure	-
			Wooden keel	-
			Suspended ceiling layer	10
	Interior walls	1.82	Mixed mortar	20
			Solid clay brick	240
			Mixed mortar	20
	Ground	2.74	Cement mortar	20
			Concrete bedding	60
			The plain soil is compacted	100
	Doors and Windows	4.75	Wood frame +6 mm single glass
Post-1990s Brick-and-mix rural homes	Exterior wall	1.49	Cement mortar	20
			Solid clay brick	370
			Mixed mortar	20
	Roofing	2.87	Roof shingles	10
			Grass mud	50
			Reinforced concrete hollow floor slabs	130
			Light steel keel	-
			Suspended ceiling layer	10
	Interior walls	1.82	Mixed mortar	20
			Solid clay brick	240
			Mixed mortar	20
	Ground	2.74	Cement mortar	20
			Concrete bedding	60
			Tamped plain earth	100
	Doors and Windows	5.7	Aluminum frame +6 mm single glass

2.4. Additional Sunshine Room Status

The feedback from the farmers’ questionnaires in the survey is shown in Table 5. In northern Hebei Province, additional sunrooms can provide significant heat storage and insulation in winter, and there are no obvious adverse effects in summer. In southern central Hebei Province, additional temporary sunshade measures are needed in summer to avoid an indoor temperature increase caused by overheated sunrooms. The biggest factor restricting farmers’ choice of this transformation method is cost [32].

Table 5. Cost-benefit comparison of commonly used materials for self-built sunrooms by farmers.

Warming Gallery Form	Single Pane	Double Glazing	Plastic Sheeting	Sun Panels
Geographic location	Luoxiaoying Village, Hengshui City	Duan Zhuang Village, Shijiazhuang City	Houshuangtuo Village, Qinhuangdao city	Yaozhuangzi Village, Cangzhou City
House features
SIZE	Depth 1.5 M	Depth 1.68 M	2.6 M depth	Depth 1.7 M
Spending	USD 1739.46	USD 1739.46	USD 217.43	USD 1739.46
Interior temperature before renovation	11–16 °C	17 °C	10–15 °C	17–20 °C
Indoor temperature after renovation	12–19 °C	18–20 °C	14–19 °C	19–22 °C

The survey found that to reduce the cost, some farmers have built their own sunrooms using PC sun panels as the transparent material. With data based on market research, the cost is shown in Table 6 (Cost data were collected in Chinese Yuan (CNY) and converted to US Dollars (USD) based on the average annual exchange rate of the People’s Bank of China in 2023 (USD 1 = CNY 6.8985)). Using PC sun panels as a transparent material has great cost advantages, but based on the material characteristics, the service life of the common sunlight boards available on the market is 12–15 years and there is a large difference compared with the conventional glass curtain wall [33]. The material expenses shown in Table 6 are based on field surveys conducted from July to September 2023 in 17 villages of Hebei Province, obtained through face-to-face interviews with local building material suppliers, construction teams, and rural households. All prices were cross-verified using three data sources (farmers’ purchase receipts, suppliers’ quotation sheets, and price records filed by village committees) and included transportation costs and taxes. In the case of price differences for the same material, weighted average values were adopted.

Table 6. Cost details of common materials used in farmers’ self-built sun house.

Material	Unit Price (CNY)	Unit Price (USD)
Regular hollow glass window	280/m2	40.59/m2
Low-e hollow glass window	370B/m2	53.63/m2
Square steel tube skeleton	23/m	3.33/m2
Round steel skeleton	12/m	1.74/m2
3 mm solid PC board	70/m2	10.15/m2
6 mm single-layer broken bridge aluminum glass window	140/m2	20.29/m2
10 mm hollow PC board	30/m2	4.35/m2

Therefore, multi-objective optimization was performed on the incremental cost and energy-saving benefit of using different types of materials for the sunroom, seeking the best scheme for the energy-saving renovation of rural housing. Because non-standard products are mainstream in the rural housing market, prices used in this study were based on market research.

2.5. Climate and Parameter Setting

Typical meteorological data in the Standard Meteorological database (CSWD) collected by China Meteorological Administration from Hebei City were used for simulation analysis. The heating period was from 15 November to 15 March of the following year, and the cooling period was from 1 June to 31 August. According to our research, the heating temperature of the main function room was set to 18 °C; the cooling temperature was set to 28 °C. The bathroom, storage room, and other secondary function rooms were set to not heat or cool, and other operating parameters were set according to GB/T 50824-2013 “Energy saving design standards for Rural Residential Buildings” [34]. The specific relevant parameters are shown in Table 4 and Table 7.

Table 7. The basic parameters of a typical rural house.

Parameter Name	Parameter Setting
Meteorological parameters	Typical annual Meteorological hourly data of Tianjin (CSWD)
Main function rooms	Winter 18 °C (Heating period: “15 November–15 March”)
Interior design temperature and time	28 °C in summer, (Cooling period: “1 June–31 August”) [35]
Number of air changes	0.5 h

2.6. Benefit Evaluation of Agricultural Houses with an Additional Sunshine Room

For the sunshine room to meet the functional needs of farmers, the depth of the sunshine room design scheme was set as 1.2 m. Taking into account the lighting needs of the main room of the rural house, the roof of the sunshine room used transparent materials as the enclosure structure [36]. Other design factors of the sunroom included the optimal energy consumption simulation value obtained in the optimization analysis of the sunroom, namely the window/wall ratio of the south facade of the sunroom, which was 100%. The maximum angle of the roof was selected to meet the requirements. Based on this, the benefits of applying different transparent envelope materials in the sunlight room of rural houses were comprehensively analyzed.

This study concentrated on the winter energy consumption characteristics of sunspaces in cold regions. Houses in Tianjin, located in a temperate continental monsoon climate zone, exhibit significantly higher energy consumption for heating in winter compared to that used for cooling in summer. Given that the increased cooling energy consumption attributable to sunspaces during summer is substantially lower than the reduced heating energy consumption facilitated during winter, this research primarily analyzed their impact on heating energy consumption.

• Selection of Evaluation Period

For existing buildings that have already gone through the pre-construction preparation and materialization stages of the building lifecycle and have been in use for a certain period, the analysis of energy-saving renovation projects should focus on their remaining lifecycles. The remaining lifecycle of an existing building refers to stages such as operation and maintenance, demolition, and disposal within its remaining service life. The service lives of various building types in China are specified in Table 8 according to GB 50068-2018 “Code for Reliability Design of Building Structures” [37]. Rural houses belong to the category of ordinary houses and structures, with a designed service life of 50 years. Therefore, the remaining service lives of adobe rural houses built before the 1980s, brick–wood rural houses built in the 1980s–1990s, and brick–concrete rural houses built after the 1990s are 10 years, 20 years, and 30 years, respectively. A follow-up evaluation of the benefits of energy-saving renovations for different rural houses was carried out based on these remaining service lives.

Table 8. Table of designed service lives for various building types.

Category	Designed Service Life (Years)
Temporary building structures	5
Easily replaceable structural components	25
Ordinary houses and structures	50
Landmark buildings and particularly important building structures	100

2.

Cost composition

For energy efficiency retrofit projects for existing rural residential buildings, the cost primarily consists of two aspects: economic and environmental costs. Both types of cost can be divided into three components: retrofit, maintenance, and demolition costs. Since the subject of study is rural housing, there are typically no management, inspection, or service fees arising from the energy efficiency retrofit during the operational phase. Therefore, the maintenance cost referred to in this study for existing rural residential building energy efficiency retrofit projects is solely the cost incurred due to the replacement of retrofit materials. During the demolition stage, the retrofitted components are demolished along with the building structure. Compared with non-retrofitted rural residences, the additional demolition cost for retrofitted residences is negligible. Consequently, this study excluded the demolition cost from the overall incremental cost.

①

Economic cost

To further evaluate the economic benefits of the rural residential building energy efficiency retrofit project, this study discounted the economic cost of the retrofit back to the point of retrofit completion. Therefore, the present value of the incremental cost of the retrofit ${NPV}_{cost}$ is:

$${NPV}_{cost}=\sum _{i=1}^n{\displaystyle \frac{C_{retrofit}+C_{maintenance}}{({1+i_c)}^t}}$$

where $C_{retrofit}$ represents the initial investment cost for the energy efficiency retrofit of the rural residence within its remaining lifecycle; $C_{maintenance}$ represents the material replacement cost for the energy efficiency retrofit of the rural residence, with an annual growth rate of 0.4% assumed for building material prices; t represents the operational time of the rural residence after the retrofit (in years); and $i_c$ represents the discount rate, taken as 5%.

②

Environmental cost

While rural residential building energy efficiency retrofit projects yield environmental benefits, they also contribute to environmental pollution during the construction phase. This primarily refers to the pollutant emissions potentially arising from the embodied stage of various building materials. Therefore, the environmental cost $C_{environment}$ of the retrofit within the remaining lifecycle is:

$$C_{environment}=C_p+C_E$$

where $C_p$ represents the environmental cost during the construction phase of the energy efficiency retrofit within the remaining lifecycle, and $C_E$ represents the environmental cost during the maintenance phase of the energy efficiency retrofit within the remaining lifecycle.

3.

Benefit composition

In the lifecycle of energy-saving renovation projects for existing rural houses, incremental benefits mainly include economic benefits and environmental benefits.

①

Economic benefits

The economic benefits during the lifecycle of energy-saving renovation projects for existing rural houses mainly include the costs of primary energy reduced due to energy-saving renovations in the operation stage of rural houses. Therefore, the present value of incremental economic benefits from energy-saving renovations of rural houses is:

$${NPV}_{benefits}=\sum _{i=1}^Y{\displaystyle \frac{S_{economy}}{{(1+i_c)}^t}}$$

where Y is the remaining service life of the existing rural house, $S_{economy}$ is the annual economic benefit of the energy-saving renovation of the rural house, t is the operation time (in years) of the rural house after the energy-saving renovation, and the discount rate is 5%.

②

Environmental benefits

The environmental benefits during the lifecycle of energy-saving renovation projects for existing rural houses mainly refer to the reduction in pollutants indirectly caused by the combustion of primary energy, as well as the decrease in pollutant emission costs resulting from such reductions. This includes emissions of air pollutants such as carbon dioxide, sulfur dioxide, nitrogen oxides, and soot generated via the use of non-renewable energy. Therefore, the annual environmental value of energy-saving renovations for rural houses is

$$S_{environment}=\sum _{i=1}^n∆Q{C}_i\times Y_i$$

where $∆Q{C}_i$ is the difference in energy-saving emissions of the i pollutant before and after renovation, and $Y_i$ is the emission charging price of the i pollutant.

To provide a reliable decision-making basis for optimizing energy-saving renovation design schemes for rural houses, we constructed a comprehensive benefit evaluation model for rural house energy-saving renovations. From the perspective of cost/benefit analysis, we comparatively analyzed the effects of applying different energy-saving renovation schemes. The economic benefits of energy-saving renovations for existing rural houses were evaluated using three economic indicators—net present value, dynamic payback period, and benefit/cost ratio—to determine whether the renovation project could generate good economic benefits and was economically feasible. The environmental benefits of energy-saving renovations for existing rural houses were evaluated using indicators such as emission reductions and the monetized value of ecological benefits. The specific method of calculation for the evaluation model is as follows:

$$NPV={NPV}_{benefits}-{NPV}_{cost}$$

wherein NPV is the net present value of energy-saving renovations for rural houses, $N{PV}_{benefits}$ is the present value of incremental benefits from energy-saving renovations, and $N{PV}_{cost}$ is the present value of incremental costs from energy-saving renovations.

$$P_t=n-1+{\displaystyle \frac{\left|N{PV}_t\right|}{N{PV}_n}}$$

where $P_t$ is the dynamic payback period of the energy-saving renovation investment for rural houses, n is the year when the cumulative net cash flow turns positive, $N{PV}_t$ is the present value of the cumulative net cash flow in the n − 1 year, and $N{PV}_n$ is the discounted present value of the net cash flow in the n year.

$$R_{E/C}={\displaystyle \frac{E}{C}}$$

where $R_{E/C}$ is the benefit/cost ratio of energy-saving renovations for rural houses, E is the present value of incremental benefits from energy-saving renovations, and C is the present value of incremental costs from energy-saving renovations. If the net present value (NPV) is less than 0, this indicates that the renovation project cannot generate favorable economic benefits and is not economically feasible. If the NPV is greater than 0, the project’s investment payback period is shorter than the remaining service life of the building, and if the benefit/cost ratio (BCR) is greater than 1, the renovation project is considered economically feasible.

2.6.1. Environmental Benefit Evaluation

We evaluated the environmental benefits of sunlight rooms for different types of rural houses. Firstly, according to the emission coefficients of major emissions from energy types used for heating and cooling (as shown in Table 9), the emission reduction costs and benefits of the six sunroom design schemes were calculated for each typical rural house. The costs and benefits of various sunroom pollutants are shown in Table 10 and Table 11. According to the discharge charge standard of each pollutant in China (see Table 12), the monetization value of the ecological benefit of each sunroom was then calculated, and the results are shown in Table 13.

Table 9. Emission coefficient of major energy emissions.

	Natural GAS	Unit	Electricity	Unit
CO2	55.54 [38]	tCO2/TJ	1.10 [39]	kgCO2e/kWh
SO2	0.1 [38]	g/m3	0.117 [40]	g/kWh
NOx	0.63 [41]	g/m3	0.168 [40]	g/kWh
Soot	0.24 [41]	g/m3	0.017 [40]	g/kWh

Table 10. Carbon emissions and carbon reduction in six sunspace design schemes for each typical rural residence.

Type of Rural Homes	Sunroom Category	Building Material Category	Amount Used	Unit	Carbon Emission Factor	Carbon Emission	Carbon Reduction
					kgCO2e/Units	kgCO2e	kgCO2e
Pre-1980s adobe rural house	Plastic sheathed sunroom	PE film	101.8	m2	0.31 [38]	101.26	3268.71
		Round steel skeleton	0.034	t	2050 [38]
	3 mm PC board Sunshine room	3 mm PC board	50.9	m2	4.93 [42]	462.09	2667.36
		Square steel tube skeleton	0.103	t	2050 [38]
	10 mm hollow PC board sunroom	10 mm hollow PC board	50.9	m2	1.97 [42]	311.42	3392.90
		Square steel tube skeleton	0.103	t	2050
	Broken bridge aluminum alloy single glass sunroom	Broken bridge aluminum alloy single-glass window	50.9	m2	40 [41]	2066.54	2733.28
	Broken bridge aluminum alloy double glass sunshine room	Broken bridge aluminum alloy hollow window	50.9	m2	65.9 [39]	3456.11	3594.59
	Broken bridge Aluminum Low-e Sunroom	Broken bridge aluminum Low-e window	50.9	m2	77.3 [39]	4036.37	3912.72
1980s–90s brick-and-wood rural homes	Plastic sheeting sunroom	PE film	197.6	m2	0.31	200.66	7115.36
		Circular steel skeleton	0.068	t	2050
	3 mm PC board Sunshine room	3 mm PC board	98.8	m2	4.93	909.38	5361.23
		Square steel tube skeleton	0.206	t	2050
	10 mm hollow PC board sunroom	10 mm hollow PC board	98.8	m2	1.97	616.94	6863.73
		Square steel tube skeleton	0.206	t	2050
	Broken bridge aluminum alloy single glass sunroom	Broken bridge aluminum alloy single-glass window	49.4	m2	40	2005.64	5492.46
	Broken bridge aluminum alloy double glass sunshine room	Broken bridge aluminum alloy hollow window	49.4	m2	65.9	3354.26	7276.17
	Broken bridge aluminum Low-e Sunroom	Broken bridge aluminum Low-e window	49.4	m2	77.3	3917.42	7921.28
After 1990s brick-and-mix rural homes	Plastic sheeting sunroom	PE film	378	m2	0.31	354.57	13,701.14
		Round steel skeleton	0.116	t	2050
	3 mm PC board sunshine room	3 mm PC board	126	m2	4.93	1125.48	11,072.14
		Square steel tube skeleton	0.246	t	2050
	10 mm hollow PC board sunroom	10 mm hollow PC board	126	m2	1.97	752.52	14,189.03
		Square steel tube skeleton	0.246	t	2050
	Broken bridge aluminum alloy single glass sunroom	Broken bridge aluminum alloy single-glass window	63	m2	40	2557.80	11,352.28
	Broken bridge aluminum alloy double glass sunshine room	Broken bridge aluminum alloy hollow window	63	m2	65.9	4277.70	15,023.15
	Broken bridge aluminum Low-e Sunroom	Broken bridge aluminum Low-e window	63	m2	77.3	4995.90	16,332.20

Table 11. Carbon emissions and carbon reduction amounts of the 6 sunspace design schemes for each typical rural residence.

Type of Rural Residence	Sunroom Category	SO2	NOx	Soot/g
		g	g	g
Pre-1980s Adobe rural house	Plastic wrap sunroom	163.48	1029.93	392.36
	3 mm PC board sunshine room	89.85	965.96	401.02
	10 mm hollow PC board sunshine room	120.63	1210.44	498.32
	Broken bridge aluminum alloy single-glass sunshine room	91.66	991.00	411.68
	Broken bridge aluminum alloy double-glass sunshine room	126.78	1285.32	529.83
	Broken bridge aluminum Low-e sunroom	138.01	1399.04	576.70
1980s–90s Brick and wood rural homes	Plastic sheeting sunroom	355.87	2241.97	854.08
	3 mm PC board sunshine room	148.95	2032.67	864.72
	10 mm hollow PC board sunshine room	212.93	2538.28	1065.81
1980s–90s Brick and wood rural homes	Broken bridge aluminum alloy single-glass sunshine room	152.48	2082.77	886.11
	Broken bridge aluminum alloy double-glass sunshine room	225.57	2691.22	1130.12
	Broken bridge Aluminum Low-e sunroom	248.48	2921.45	1224.92
Post-1990s Brick-and-mix rural homes	Plastic sheeting sunroom	685.25	4317.07	1644.60
	3 mm PC board sunroom	362.68	4039.27	1683.66
	10 mm hollow PC board sunroom	498.14	5080.22	2095.70
	Broken bridge aluminum alloy single-glass sunshine room	369.26	4148.95	1731.07
	Broken bridge aluminum alloy double-glass sunshine room	522.88	5391.96	2227.33
	Broken bridge aluminum Low-e sunroom	570.23	5856.64	2418.09

Table 12. Charge standards for the discharge of air pollutants [43].

	CO2	SO2	NOx	Dust
	RMB/kg	RMB/kg	RMB/kg	RMB/kg
Charging standard	0.06	4.8	4.8	0.03

Table 13. Environmental value estimation of six sunspace design schemes for each typical rural residences.

Type of Rural Homes	Sunroom Category	CO2	SO2	NOx	Soot	Total
		RMB	RMB	RMB	RMB	RMB
Pre-1980s Adobe rural house	Plastic sheathed sunroom	190.04	0.78	4.94	0.012	195.77
	3 mm PC plate sunroom	132.32	0.43	4.64	0.012	137.40
	10 mm hollow PC board sunshine room	184.89	0.58	5.81	0.015	191.30
	Broken bridge aluminum alloy single-glass sunshine room	40.00	0.44	4.76	0.012	45.21
	Broken bridge aluminum alloy double-glass sunshine room	8.31	0.61	6.17	0.016	15.11
	Broken bridge aluminum Low-e sunshine room	−7.42	0.66	6.72	0.017	−0.02
1980s–1990s brick-and-wood rural homes	Plastic sheeting sunroom	414.88	1.71	10.76	0.026	427.38
	3 mm PC plate sunroom	267.11	0.71	9.76	0.026	277.61
	10 mm hollow PC board sunshine room	374.81	1.02	12.18	0.032	388.04
	Broken bridge aluminum alloy single-glass sunshine room	209.21	0.73	10.00	0.027	219.97
	Broken bridge aluminum alloy double-glass sunshine room	235.31	1.08	12.92	0.034	249.34
	Broken bridge aluminum Low-e sunshine room	240.23	1.19	14.02	0.037	255.48
Post-1990s brick-and-mix rural homes	Plastic sheeting sunroom	800.79	3.29	20.72	0.049	824.85
	3 mm PC plate sunroom	596.80	1.74	19.39	0.051	617.98
	10 mm hollow PC board sunshine room	806.19	2.39	24.39	0.063	833.03
	Broken bridge aluminum alloy single-glass sunshine room	527.67	1.77	19.91	0.052	549.40
	Broken bridge aluminum alloy double-glass sunshine room	644.73	2.51	25.88	0.067	673.19
	Broken bridge aluminum Low-e sunshine room	680.18	2.74	28.11	0.073	711.10

It can be seen from the above calculation results that the longer the remaining service life of a typical rural house, the greater the emission reduction benefit provided by the sunlight room. For all typical rural houses, the plastic film sunlight room and the 10 mm hollow PC board sunlight room have higher environmental value, and the maximum environmental emission reduction that can be obtained is CNY 833.03. For adobe rural houses built before the 1980s with a short remaining service life, the ecological benefits of a glass sunlight room are low, and its construction value is small from the point of view of environmental protection.

2.6.2. Evaluation of Economic Benefit

(1)

Incremental Cost

As for the economic evaluation of sunrooms of different typical rural houses, we first calculate the costs of sunrooms made from different transparent envelope materials for each typical rural house [44]. The sunroom types include plastic film sunlight rooms, 3 mm PC board sunlight rooms, 10 mm hollow PC board sunlight rooms, broken bridge aluminum alloy single-glass sunlight rooms, broken bridge aluminum alloy double-glass sunlight rooms, and broken bridge aluminum Low-e sunlight rooms.

The material costs of different sunroom enclosure structures are shown in Table 14. The initial investment cost and the present value of the total incremental cost can be calculated according to the material used and the number of required maintenance operations for each sunroom program, and the specific calculation results are shown in Table 15.

Table 14. Sunspace material initial investment unit price.

Material	Unit Price	Materials	Unit Price
PO plastic wrap	4 RMB/m	Regular hollow glass window	CNY 280/m
3 mm solid PC board	70 RMB/m	Low-e hollow glass window	CNY 370/m
6 mm broken bridge aluminum single-glass window	140 RMB/m	Square steel tube skeleton	CNY 23/m
10 mm hollow PC board	30 RMB/m	Round steel skeleton	CNY 12/m

Table 15. The incremental cost of the design scheme of each typical rural residence with different sunspaces.

Type of Rural Houses	Sunroom Type	Number of Repairs	Single Repair Cost	Initial Investment Cost	Incremental Cost Present Value
		Repairs	CNY	CNY	CNY
Pre-1980s Adobe rural house	Plastic sheathed sunroom	1	1283.62	1283.62	2446.14
	3 mm PC board sunshine room	0	0	5758.28	5484.08
	10 mm hollow PC board sunroom	0	0	3672.12	3497.26
	Broken bridge aluminum alloy single-glass sunshine room	0	0	7826.56	7453.87
	Broken bridge aluminum alloy double-glass sunshine room	0	0	14,953.12	14,241.07
	Broken bridge aluminum Low-e sunroom	0	0	19,534.48	18,604.27
1980s–1990s brick-and-wood rural house	Plastic sheeting sunroom	3	1277.74	1277.74	4702.95
	3 mm PC board sunroom	1	5655.38	5655.38	11,638.80
	10 mm hollow PC board sunroom	1	3628.02	3628.02	7466.48
	Broken bridge aluminum alloy single-glass sunroom	0	0	7620.76	7257.87
	Broken bridge aluminum alloy double-glass sunshine room	0	0	14,541.52	13,849.07
	Broken bridge aluminum Low-e sunroom	0	0	18,990.58	18,086.27
Post-1990s brick-and-mix rural homes	Plastic sheeting sunroom	5	1488.00	1488.00	7877.06
	3 mm PC board sunroom	1	6904.00	6904.00	12,556.07
	10 mm hollow PC board sunshine room	1	4334.00	4334.00	7882.10
	Broken bridge aluminum alloy single-glass sunshine room	0	0	9520.00	9066.67
	Broken bridge aluminum alloy double-glass sunshine room	0	0	18,340.00	17,466.67
	Broken Bridge Aluminum Low-e sunroom	0	0	24,010.00	22,866.67

The analysis and calculation results show that for the three typical rural houses, the sunroom with the largest incremental cost at present is the broken bridge aluminum Low-e sunroom. The plastic film sunshine room and PC board sunshine room have 5 years and 15 years of use, respectively, because of their limited service life. For a typical rural house with a long remaining life cycle, the maintenance cost of these two types of sunshine room also increases with the increase in the service life of the rural house, resulting in the present value of its total incremental cost also increasing

(2)

Incremental benefit

The differences in annual heating energy consumption and cooling energy consumption between the six sunroom design schemes and the non-sunrooms of each typical rural house can be obtained via simulation when the heating design temperature is 18 °C and the cooling design temperature is 26 °C. According to the calculation formula, the annual saving amounts for natural gas and electricity and the present total incremental benefit can be calculated. The specific calculation results are shown in Table 16.

Table 16. Incremental benefit of different design schemes between typical rural residences and sunspaces.

Type of Rural Residences	Sunroom Type	Annual Saving for Heating Energy Consumption	Annual Saving for Cooling Energy Consumption	Annual Gas Saving	Annual Energy Saving	Total Incremental Benefit Present Value
		/kW·h	/kW·h	/m3	/kW·h	CNY
Pre-1980s Adobe rural house	Plastic wrap sunroom	1634.81	0.00	163.48	0.00	4789.77
	3 mm PC board sunroom	1720.68	−224.88	172.07	−70.28	4641.39
	10 mm hollow PC board sunroom	2132.43	−253.31	213.24	−79.16	5797.20
	Broken bridge aluminum alloy single-glass sunroom	1766.79	−232.53	176.68	−72.66	4762.90
	Broken bridge aluminum alloy double-glass sunroom	2268.19	−273.60	226.82	−85.50	6158.88
	Broken bridge aluminum Low-e sunroom	2468.83	−297.76	246.88	−93.05	6703.76
1980s–1990s brick-and-wood rural homes	Plastic sheeting sunroom	1779.34	0.00	177.93	0.00	10,944.74
	3 mm PC board sunroom	1869.60	−307.65	186.96	−96.14	10,351.27
	10 mm hollow PC board sunroom	2294.91	−336.49	229.49	−105.15	12,859.69
	Broken bridge aluminum alloy single-glass sunroom	1915.90	−315.49	191.59	−98.59	10,606.78
	Broken bridge aluminum alloy double-glass sunshine room	2433.46	−357.08	243.35	−111.59	13,635.01
	Broken bridge aluminum Low-e sunroom	2636.30	−381.23	263.63	−119.13	14,792.51
Post-1990s brick-and-mix rural homes	Plastic sheeting sunroom	2284.16	0.00	228.42	0.00	22,139.13
	3 mm PC board sunroom	2411.20	−328.83	241.12	−102.76	21,435.80
	10 mm hollow PC board sunroom	2991.26	−363.98	299.13	−113.74	26,851.21
	Broken bridge aluminum alloy single-glass sunroom	2479.89	−341.61	247.99	−106.75	22,026.32
	Broken bridge aluminum alloy double-glass sunshine room	3180.55	−393.19	318.06	−122.87	28,513.98
	Broken bridge aluminum Low-e sunroom	3452.40	−424.38	345.24	−132.62	30,965.40

As shown in Table 16, after adding a sunshine room in each typical rural house, the heating energy consumption of the rural houses in winter significantly decreased. The better the thermal performance of the transparent enclosure structure material in the sunshine room, the more the heating energy consumption of the rural house was reduced, and the change trend for the cooling energy consumption of the rural house in summer was the opposite. The increased energy consumption for cooling rural houses in summer has little impact on the reduction in energy consumption for heating in winter, and the winter income is much larger than the summer loss. According to the calculation results of the present value of the total incremental benefit, the longer the remaining service life of the rural house, the greater the incremental benefit of the sunroom.

(3)

Benefit evaluation

According to the calculation formula for the cost/benefit evaluation model, the net present value, dynamic payback period, and transformation efficiency cost ratio of the six sunroom design schemes for each typical rural house are calculated. The calculation data are shown in Table 17.

Table 17. An evaluation of the economic benefits of the design schemes for different sunspaces for each typical rural residence.

Rural Home Type	Sunroom Type	Net Present Value	Dynamic Payback Period	Cost-Effectiveness Ratio
		CNY	Year
pre-80s Adobe rural house	Plastic sheathed sunroom	2343.63	2.80	1.96
	3 mm PC board sunroom	−842.69	12.38	0.85
	10 mm hollow PC board sunshine room	2299.94	6.50	1.66
	Broken bridge aluminum alloy single-glass sunshine room	−2690.97	16.11	0.64
	Broken bridge aluminum alloy double-glass sunshine room	−8082.19	23.01	0.43
	Broken bridge aluminum Low-e sunroom	−11,900.51	27.06	0.36
1980s–1990s brick-and-wood rural homes	Plastic sheeting sunroom	6241.79	2.56	2.33
	3 mm PC board sunroom	−1287.53	11.49	0.89
	10 mm hollow PC board sunroom	5393.21	6.09	1.72
	Broken bridge aluminum alloy single-glass sunshine room	3348.91	14.87	1.46
	Broken bridge aluminum alloy double-glass sunshine room	−214.06	21.39	0.98
	Broken bridge aluminum Low-e sunshine room	−3293.76	25.26	0.82
Post-1990s brick-and-mix rural homes	Plastic wrap sunroom	14,262.07	2.33	2.81
	3 mm PC board sunroom	8879.73	10.72	1.71
	10 mm hollow PC board sunroom	18,969.11	5.51	3.41
	Broken bridge aluminum alloy single-glass sunroom	12,959.65	14.15	2.43
	Broken bridge aluminum alloy double-glass sunshine room	11,047.31	20.42	1.63
	Broken bridge aluminum Low-e Sunroom	8098.73	24.17	1.35

As shown in the calculation results, for the six sunroom design schemes of adobe rural houses from before the 1980s, only the plastic film sunroom and 10 mm hollow PC board sunroom are economically feasible, the net present value is greater than 0, the efficiency ratio is greater than 1, and the dynamic recovery period is less than the remaining service life of the rural house, while the other sunroom design schemes have low economic benefits and are not economically feasible. The net present value of the plastic film sunshine room, 10 mm hollow PC board sunshine room, and broken bridge aluminum alloy single-glass sunshine room are all greater than 0; the efficiency ratio is greater than 1; and the dynamic recovery period is less than the remaining service life of the rural house, which is economically feasible. Among these, the dynamic recovery period of the sunshine room made from 3 mm PC board is also less than the remaining service life of the rural house. However, due to its high maintenance cost, if it is replaced by a more economical plastic film material during maintenance after the end of its service life, it may also be economically feasible. For the six kinds of sunroom design schemes for brick concrete rural houses from after the 1990s, the net present value is greater than 0, the efficiency-to-cost ratio is greater than 1, and the dynamic recovery period is less than the remaining service life of such as rural house, so all the schemes are economically feasible.

2.7. Analysis of Adaptation Between Sunshine Room and Different Rural Houses

To meet the design requirements of rural families with different incomes, three different sunroom design schemes were provided for each typical rural house. The control program was suitable for families with average incomes, and the sunroom design program with low renovation cost and certain energy-saving effect was selected. The general program was suitable for families with a relatively good economic level, and the sunroom design scheme with a moderate cost, no frequent maintenance, and a good energy-saving effect was selected. The preferred item was suitable for higher-income families, and the sunroom design scheme with a relatively high cost and significant energy saving was selected.

For those from before the 1980s, the remaining service life of the adobe rural house main body is shorter, which is more suitable for the dynamic recovery period of the short sunshine room design scheme. The net present value of the economically feasible sunroom scheme used for this type of rural house is plastic film sunroom >10 mm hollow PC board sunroom; in the remaining 4 kinds of sunroom design schemes, the dynamic recovery period of the 3 mm PC board sunroom is the shortest, its appearance is more beautiful, and there will be no maintenance in the remaining service life of the rural house [45], making it one of the schemes worth choosing. The dynamic recovery period of the glass sunshine room is longer and not suitable for this type of rural house, and the recommended options for a sunshine room design scheme for an adobe rural house from before the 1980s is shown in Table 18.

Table 18. Adobe rural residence before the 1980s: sunspace design scheme’s recommended options.

Option	Sunroom Scheme
Economic option	Plastic sheathed sunroom
Standard option	10 mm hollow PC board sunroom
Preferential option	3 mm PC board sunroom

The remaining service life of the main body of brick-and-wood rural houses from the 1980–1990s is moderate, and their energy-saving effect is better. The design schemes of the broken bridge aluminum alloy double-glass and the broken bridge aluminum Low-e sunlight rooms can provide more energy-saving benefits during their service cycles. Since the energy-saving efficiencies of these two kinds of sunshine are similar but the broken bridge aluminum alloy double-glass sunshine room is more economical, the latter is the preferred option. The economic benefits and general sunroom design scheme make these the top two schemes in the net present value ranking of the economic feasibility of sunroom schemes (plastic film sunroom > 10 mm hollow PC board sunroom > broken bridge aluminum alloy single-glass sunroom. The recommended options for sunroom designs for brick-and-wood rural houses from the 1980s and 1990s are shown in Table 19.

Table 19. Recommended options for a sunspace design scheme for a 1980s–90s brick rural residence.

Option	Sunroom Scheme
Economic option	Plastic sheathed sunroom
Standard option	10 mm hollow PC board sunroom
Preferential option	Broken bridge aluminum alloy double-glass sunshine room

The remaining service life of the main body of the brick–concrete rural houses built in the 1990s is longer, making them more suited to sunlight rooms with a higher energy-saving rate and a longer service life. The net present value ranking of the sunshine room schemes based on economic feasibility for this kind of rural house is as follows: 10 mm hollow PC board sunshine room > plastic film sunshine room > broken bridge aluminum alloy single-glass sunshine room > broken bridge aluminum alloy double-glass sunshine room > 3 mm PC board sunshine room > broken bridge aluminum Low-e sunshine room. The recommended options for a sunroom design for brick–concrete rural houses from the 1990s are shown in Table 20.

Table 20. Recommended options for the sunroom design scheme of brick–concrete rural houses from the 1990s.

Option	Sunroom Scheme
Economic option	10 mm hollow PC board sunroom
Standard option	Broken bridge aluminum alloy double-glass sunshine room
Preferential option	Broken bridge aluminum Low-e sunlight room

3. Results

In this study, the existing rural houses in Hebei Province are taken as the research object to investigate their operation status and the actual living and energy-saving needs of residents in depth through interview questionnaires, field surveys, and mapping. Using the two-stage clustering method and the main body induction method, a classification model for typical rural houses in this region is constructed. This study shows that optimizing the sunspace design of rural houses in Hebei can significantly reduce their winter heating energy consumption. The main conclusions are as follows:

• Classification of rural houses:

Three types of typical rural houses are identified: ① adobe rural houses from the 1980s, ② brick–wood rural houses from the 1980s–1990s, and ③ brick–concrete rural houses from the 1990s.

2.

Suggestions to optimize the benefits include the following:

①

Adobe rural houses in the 1980s: Due to the short remaining service lives of the main body, a sunspace design scheme with a shorter dynamic payback period (that is, faster investment recovery) is mor suitable.

②

Brick–wood structure rural houses in the 1980s–1990s: The main body has a moderate remaining service life, and a broken bridge aluminum alloy double-glass sunspace scheme is preferred.

③

Brick–concrete structure rural houses in the 1990s: The main body has a long remaining service life and design schemes with a higher energy-saving rate and longer service life of the sunspace itself are more applicable.

However, this study still has certain limitations. First, all building energy consumption simulations are based on meteorological parameters for a single year (2023), failing to fully reflect the dynamic impacts of extreme weather events (such as persistent cold waves and abnormal high temperatures) on passive heating systems. Second, despite the current background of global warming, long-term climate trends have not been taken into account, which may lead to deviations in evaluations of the thermal performance of building envelopes and in the efficiency of renewable energy utilization. Future research should combine multi-year meteorological data with climate prediction models to further quantify the potential risks of climate change sensitivity to the design of rural passive buildings.