Research on the Design and Application of Rural Houses in Northern China Based on Energy-saving Retrofit Technology

. The poor thermal performance of rural buildings and the severe pollution caused by coal combustion in northern China are urgent problems that need to be addressed. To solve these two issues, the current status of building envelopes in rural houses in northern China and the commonly used heating methods by rural households were analyzed. The study found that most rural houses lack insulation measures in their building envelopes and require energy-saving renovations. To improve the thermal performance of rural houses, the thermal performance of a rural household's building envelope was measured and energy-saving renovations were conducted. The measurement results showed that the heat transfer coefficients of the walls, windows, and roofs decreased by 72.7%, 61.59%, and 64.99%, respectively, after the energy-saving renovations, compared to the national limit values. The thermal performance of all building envelope components met the national requirements after the energy-saving renovations. The actual statistics showed a 66.74% reduction in biomass fuel consumption per unit temperature difference after the renovations, indicating a significant improvement in energy efficiency.


Introduction
There are still numerous issues surrounding heating systems in rural areas of northern China. Most rural houses in the north lack insulation measures, and the thermal transfer coefficient of their building materials exceeds the specified requirements. The heavily polluted coal-fired boiler heating system remains the primary heating method in rural areas. Some economically disadvantaged farmers use low-efficiency and highly polluting inferior coal to save costs, exacerbating the environmental pollution caused by coal combustion. In order to accelerate the development of the new rural areas and address the issues in rural heating, the 14th meeting of the Central Leading Group for Finance and Economics proposed to reduce the pressure of transforming rural coal-fired heating systems towards cleanliness and actively promote the development and utilization of clean energy. The promotion of clean energy sources such as electricity, geothermal, and solar energy as alternatives to coal, as well as the implementation of multiple policies to popularize clean energy sources like biomass fuels and solar energy, are actively advocated.

Literature Review
S.H. Zhu (2022) conducted a survey and analysis of rural houses in North China and found that the building envelope structures of rural houses were relatively weak, with the majority lacking insulation measures altogether [1][2][3]. Due to the excessive height of rural buildings and outdated heating facilities, the thermal efficiency is low, resulting in significant building energy consumption. The study provided some recommendations regarding the current status of building envelope structures, offering theoretical data support for energy-saving in the construction of new rural areas, but lacked empirical data. L.H. Zhang (2022) studied the thermal performance of building envelopes in Tangyin County, Henan Province, and conducted optimization analysis. They proposed energy-saving renovation schemes such as adding an attic floor for external wall and roof insulation, replacing single-layer aluminum alloy glass with insulated glass or double-layer glass windows, but lacked actual data support [4][5].
L.P. Zhou (2022) conducted a research analysis on the total energy consumption of heating in households in Hebei Province. The study results showed that the proportion of households consuming coal annually between 1 ton and 2.5 tons was 63%, while the proportion of households consuming more than 2.5 tons was 15%. Additionally, there were some households with coal consumption exceeding 4 tons annually, while the proportion of households with coal consumption below 1 ton was very small [6]. L. Guo (2022) conducted a survey in five villages in Baoding, Hebei Province in 2014, visiting a total of 543 households. Survey statistics showed that the usage rate of scattered coal reached 97% in that region [7]. Therefore, scattered coal remains the primary energy source in rural areas of northern China. It was estimated that during the heating season from winter 2013 to spring 2014, the scattered coal consumption in rural Baoding exceeded 5 million tons, with emissions of particulate matter and sulfur dioxide reaching 54,000 tons and 112,000 tons respectively. The emissions from coal combustion in scattered coal have a severe impact on the environment due to the lack of relevant environmental treatment measures and the low emission height [8][9][10].
In summary, the building envelope structures of rural households in northern China have poor thermal performance, with low adoption rates of insulation measures for walls, windows, roofs, and other components. While there have been some improvements in the indoor thermal environment of rural houses, they still fall short of the required standards. Meanwhile, the energy consumption of coal continues to rise, with the total energy consumption in rural areas of Northeast China reaching 65.87 million tons in 2014 alone. Therefore, energy-saving renovations and effective utilization of clean energy in rural houses in northern China become crucial research topics.

Internal Insulation of External Walls
During the renovation process, insulation measures need to be taken for the external walls of the building, with the most common approach being the addition of insulation layers. Common insulation materials include rock wool, extruded polystyrene (XPS) board, and polystyrene board. Rock wool has the highest fire resistance rating but is relatively expensive and has poorer waterproofing performance. Polystyrene board, on the other hand, offers a better balance of fire resistance, thermal insulation performance, and adhesive properties, making it a cost-effective and highly flameretardant choice. To determine the specific thickness of the economically reasonable polystyrene board insulation layer, numerical simulations using energy analysis software such as EnergyPlus are needed.

Addition of an Extra Window Layer
Typically, windows can be in the form of sliding windows or casement windows. Sliding windows, although equipped with sealing strips, may have certain gaps during the sliding process. Over time, the wear of the sealing strips can lead to the gradual widening of these gaps, resulting in increased energy wastage. In contrast, casement windows, due to their good soundproofing and sealing properties, have become the advocated window type for energy-saving purposes. Therefore, it is recommended to use casement windows during the window renovation process to achieve energy-saving effects.

External Roof Insulation
The existing rural houses have a low proportion of insulation measures applied to the roofs, making it a key area for energy-saving renovation. Similar to the principles of external wall renovation, we choose to add a polystyrene board insulation layer on the roof. When laying the polystyrene board on the roof, adhesive properties do not need to be heavily considered. Even if the adhesive properties of the anti-cracking mortar decrease due to low outdoor temperatures during the construction process, it has little impact on the insulation effect of the polystyrene board. Therefore, we choose to insulate the exterior of the roof. To address the issue of potential water leakage at the junction of the roof and external walls, we recommend installing a layer of waterproof colored steel tiles on top of the roof. The colored steel tiles should be laid above the polystyrene board insulation layer, and an additional layer of bricks should be laid on top of the colored steel tiles to prevent the polystyrene board from being blown away by strong winds. In summary, our renovation measures include internal insulation with polystyrene board on the external walls, external insulation with polystyrene board on the roof with ceiling treatment, and the addition of an extra layer of windows on the existing windows.

Energy Consumption Analysis
Based on the energy consumption analysis results before renovation as a reference, this study conducted an energy consumption analysis on the typical farmhouse using the energy simulation software Energyplus to determine the thickness of expanded polystyrene (EPS) insulation to be added during energy-saving retrofitting. The main change made in this study was the modification of the building envelope materials, including the addition of EPS insulation with varying thicknesses to the exterior walls and roof, as well as the addition of an additional layer of aluminum alloy windows. For specific details, please refer to Table 1. In this energy consumption analysis, apart from the variations in the building envelope materials, all other factors were kept constant, including the building structure, weather conditions, simulated indoor design temperature setpoint, heating method, and supply/return water temperatures. Four different thicknesses of expanded polystyrene (EPS) insulation were considered: 30mm, 50mm, 75mm, and 100mm. Table 2 presents a comparative analysis of energy consumption results obtained from the energy simulation software EnergyPlus for the selected retrofitting solutions with different thicknesses of expanded polystyrene (EPS) insulation. In the retrofitting solutions where a single-layer aluminum alloy window was added outside the original wooden window, and the exterior walls and roof were enhanced with varying thicknesses of EPS insulation, significant changes were observed in the total energy consumption and thermal load index required to maintain an indoor temperature of 14℃ throughout the entire heating season (approximately from November 1st of the first year to March 1st of the following year). As the thickness of the EPS insulation increased from 0mm to 30mm, 50mm, 75mm, and 100mm, the heating total energy consumption decreased by 64.15%, 73.42%, 79.67%, and 83.21%, respectively. The energy efficiency rate reached 60% in all cases, indicating a substantial significance of energy-saving retrofitting. From Figure 1, it can be observed that an increase in the thickness of the EPS insulation layer leads to a reduction in the total energy consumption during the heating season. However, the rate of change becomes smaller, indicating a decreasing energy-saving efficiency with the increase in insulation layer thickness. Combining the simulation results from Table 2 and Figure 1 with market prices, it can be concluded that using a 50mm thickness of EPS insulation layer provides the highest cost-effectiveness in energy-saving retrofitting.  After the energy-saving retrofitting in the farmhouse, a continuous 7-day measurement of the heat transfer coefficient was conducted on the exterior walls, windows, and roof using the same method. The results are shown in Table 3. Comparing the pre and postretrofitting values, the heat transfer coefficient of the exterior walls decreased by 72.7% from 1.492 W/(m²·K) to 0.406 W/(m²·K), meeting the required limit of 0.45 W/(m²·K) specified by the standards. The heat transfer coefficient of the roof decreased by 64.99% from 1.057 W/(m²·K) to 0.370 W/(m²·K), satisfying the required limit of 0.5 W/(m²·K). The heat transfer coefficient of the windows decreased by 64.99% from 3.997 W/(m²·K) to 1.535 W/(m²·K), meeting the required limit of 2.8 W/(m²·K). The post-retrofitting heat transfer coefficients improved by over 60% in all cases, complying with the energy-saving standards set by China. The energy-saving retrofitting in the farmhouse achieved satisfactory results. To clarify the improvement effect of indoor thermal comfort before and after the renovation, temperature measurements were conducted using an ETH-P model environmental test equipment intelligent multi-channel inspection instrument (Figure 3). Both before and after the renovation, radiators were utilized as end terminals (Figure 2), with a biomass solid fuel stove as the heat source. The temperature measurement equipment selected was capable of ±0.2°C temperature accuracy and ±1.5% humidity accuracy. Figures 4 and 5 depict the arrangement of sensors for measuring indoor temperature before the renovation.   Figure 6 presents the measurement results comparing indoor and outdoor temperatures before and after the renovation. Although the outdoor temperature noticeably decreased after the renovation compared to before, the indoor temperature did not decrease accordingly and instead showed an increase. After the renovation, the indoor-outdoor temperature difference increased from 11.9°C to 16.6°C, and the temperature fluctuation indoors increased from 2.82°C to 4.4°C, representing an increment of 4.7°C and 1.58°C, respectively. This indicates an improvement in the thermal performance of the building envelope after the renovation, with increased thermal inertia and reduced heat transfer coefficient. The impact of outdoor temperature variations on indoor temperature has decreased, contributing to maintaining a stable indoor temperature. Furthermore, the energy-saving renovation has significantly enhanced indoor thermal comfort.  Table 4 presents the measured energy consumption values before and after the energy-saving renovation. Before the renovation, maintaining an indoor temperature of 14.370°C required approximately 60 kg of biomass solid fuel per day. However, after the renovation, maintaining a temperature of 17.580°C only required 30 kg of fuel per day. Calculations show that prior to the renovation, the household consumed 5.20 kg of biomass fuel per unit temperature difference, whereas after the renovation, only 1.73 kg of fuel was needed. The actual energy-saving rate achieved was 66.74%, which is in close proximity to the simulated value of 75.69% obtained from EnergyPlus analysis. This demonstrates the reliability of using EnergyPlus for energy consumption analysis and highlights the significant energy-saving effect of the renovation.

Effect of energy-saving renovation
The thermal transmittance of the building envelope, indoor thermal comfort, and actual energy consumption all indicate a successful outcome of the energy-saving renovation. To gain acceptance from farmers and promote the renovation in rural areas, it is crucial to ensure that the cost of the energy-saving measures is affordable for them. For this particular renovation, the prices were set according to the local market rates as follows: 200 yuan/m² for the addition of an extra layer to doors and windows, 120 yuan/m² for external wall renovation, and 120 yuan/m² for roof renovation. The total cost amounted to 30,000 yuan, which falls within the economic capacity of the households.

Conclusion
A study analyzing the energy consumption of households after energy-saving renovations using EnergyPlus revealed that there is a potential for over 60% energy savings. The retrofit plan involving the addition of 50mm thick polystyrene boards was found to be economically reasonable for households in the Weifang area of Shandong province, China. The thermal transmittance of each building envelope component was measured using the heat flow meter method before and after the energy-saving renovation. The results showed that the thermal transmittance of the external walls, windows, and roof of the building envelope decreased by over 60% after the renovation. Additionally, there was a reduction in the fluctuation range of indoor temperature, and the biomass fuel consumption per unit temperature difference between indoor and outdoor environments was reduced by 66.74%. The energy-saving effect of the renovation is significant.It is worth noting that for this particular energy-saving renovation, sliding windows were chosen instead of energy-efficient casement windows. However, it should be noted that using energy-efficient casement windows would have resulted in even greater energy savings.