Towards Rural Revitalization Strategy for Housing in Gully Regions of the Loess Plateau: Environmental Considerations

Under the background of Chinese Rural Revitalization Strategy, how to improve rural regional environment and living quality is very important and urgent. At present, residential buildings in gully regions of the Loess Plateau have poor insulation and high-energy consumption. Thus, better ecological design can largely save energy and improve living comfort. The findings of this paper provide an insight into the ecological design potentials for reducing energy demand across rural regions in China. In this paper, we select three main types of residential buildings in gully regions and build energy demand models based on the Life Cycle Assessment (LCA) method. The results show that the energy demand in the building use stage is extremely high in all three typical buildings, which account for around 90% of the whole life cycle. The energy demand of the traditional adobe residential building is lower than the brick-concrete structure buildings. The LCA method used in this paper can quantify the energy demand in each stage of life cycle, which helps to put forward the corresponding ecological design strategy. The research results can be used as a reference in the future development of this region and other rural regions in China.


Regional Issues
The Loess Plateau is located in the mid-west of China, which is among the earliest human settlements and most fragile ecological areas, covered with 30-300 m thick calcareous yellow soil. It is about 530 thousand square kilometers, accounting for 1/l8 of total Chinese territory [1]. The gully region, located in the southern part of the Loess Plateau, mainly refers to the gully regions of Wei Bei Loess Plateau, Shan Bei Loess Plateau and Long Dong Loess Plateau, including 7 cities, 18 counties, a population of about 4.3 million, and a total area of about 14.8 thousand square kilometers [2], as shown in Figure 1 and Table 1.  In the history of the gully regions of the Loess Plateau, in order to adapt to the arid climate and make the best use of limited natural resources, local people created unique traditional vernacular dwellings forms: underground cave dwellings and adobe residential buildings. Underground cave dwelling is characterized by low cost, low energy consumption and low pollution. It takes full advantage of the characteristics of the loess and merges harmoniously into nature [3]. Adobe residential building characterized by "soil and wood" structures, proven techniques, simple construction, cheap and easily available materials, energy saving, and ecological/environmental protection. It is widespread in the gully region of the Loess Plateau [4]. However, because of the backward productivity and economic conditions, only local materials can be used, and the building construction methods are very simple. Nowadays, with the development of the social productivity and industrialization, as well as bricks and concrete, these "modern" architectural materials are gradually used in the construction of residential buildings, leading to the "brick and concrete" structure dwelling becoming popular in gully regions. In 2010, the local government initiated a project named "Three farewells"-all the people living in cave dwellings were ordered to be moved out within three years, and the cave dwellings to be demolished for farmland. During the field investigation, only a few people were found to still live in cave dwellings, and in 2014, almost all the cave dwellings disappeared. In 2002, the underground cave dwelling made up over 50% of vernacular dwellings, while the number decreased to less than 30% in 2013. In 2014, almost all the underground cave dwellings had disappeared. The traditional cave dwellings are out of the historical stage and cannot be established in large amounts as a vernacular dwelling form in the future of countryside construction in gully regions. Therefore, the most common residential buildings-adobe residential buildings, one-story brick-concrete structure residential buildings, and two-story brick-concrete residential buildings-are regarded here as the main research objects.
In terms of current countryside construction, due to the lack of ecological design as a theoretical guideline, new construction uses simple methods-not understanding, but copying urban houses, including in using high-pollution building materials. On the surface, the village appearance is improved, while in fact, it is a huge waste of resources and energy consumption. As the gully regions of the Loess Plateau belong to the Cold Region in China [5], the thermal comfort conditions in winter Energies 2020, 13, 3109 3 of 27 are extremely poor. Figure 2 shows the annual hourly temperature and mean monthly temperature of the typical cities in each gully region. As shown elsewhere [6], the low temperature indoors in winter is the common in both "Brick-concrete" structure residential buildings and adobe residential buildings. Therefore, the application of energy-efficient design is needed.
Energies 2020, 13, x FOR PEER REVIEW 3 of 27 including in using high-pollution building materials. On the surface, the village appearance is improved, while in fact, it is a huge waste of resources and energy consumption. As the gully regions of the Loess Plateau belong to the Cold Region in China [5], the thermal comfort conditions in winter are extremely poor. Figure 2 shows the annual hourly temperature and mean monthly temperature of the typical cities in each gully region. As shown elsewhere [6], the low temperature indoors in winter is the common in both "Brick-concrete" structure residential buildings and adobe residential buildings. Therefore, the application of energy-efficient design is needed. In the long history of this region, there has been no thermal insulation in residential buildings. Even nowadays, in most traditional residential buildings, there is still no interior and exterior thermal insulation. On the one hand, for historical reasons, local residents have no awareness of it. On the other hand, it is due to the backward economic situation-the poverty makes few villagers consider thermal insulation which would result in greater construction costs. Due to the poor thermal storage and exterior insulation, the low winter temperature makes the indoor environment very uncomfortable, and the energy demand in winter is extremely high. In order to improve residents' living quality and take energy saving into consideration, the design and adoption of interior and exterior thermal insulation is very necessary.
The findings of this paper hope to provide an insight into the ecological design potentials for reducing energy demand in the Loess Plateau, even across rural regions in China. The research results of this paper will lay the foundation for future research on how to implement ecological design and improve residential living quality, which is another important aspect of the entire body of research on Chinese Rural Revitalization Strategy in this region.

Research Background
Scholars have studied the impact of climate change on areas and the climate adaptation of buildings to reduce energy consumption. Dastgerdi et al. [7] have analyzed the impact of climate change on world heritage sites and proposed improvements to the conservation framework to protect cultural heritage in the context of climate change [8]. Soutullo et al. [9] compared the impact of three different climates on the energy performance of a Madrid house to arrive at potential design strategies. Kalm et al. [10] have demonstrated experimentally that the use of advanced personalized ventilation systems can provide adequate thermal comfort without reducing daylight performance. Oh et al. [11] proposed five new pre-screening analysis methods, using interval data recorded by smart meters to analyze the energy saved by installing home automation devices. Rodríguez et al. [12] introduced the commissioning of the Controlled and Automatized Testing Facility for Human Behavior (CASITA), which will facilitate the study of human behavior in a controlled space and can be used to study human energy use aspects. In the long history of this region, there has been no thermal insulation in residential buildings. Even nowadays, in most traditional residential buildings, there is still no interior and exterior thermal insulation. On the one hand, for historical reasons, local residents have no awareness of it. On the other hand, it is due to the backward economic situation-the poverty makes few villagers consider thermal insulation which would result in greater construction costs. Due to the poor thermal storage and exterior insulation, the low winter temperature makes the indoor environment very uncomfortable, and the energy demand in winter is extremely high. In order to improve residents' living quality and take energy saving into consideration, the design and adoption of interior and exterior thermal insulation is very necessary.
The findings of this paper hope to provide an insight into the ecological design potentials for reducing energy demand in the Loess Plateau, even across rural regions in China. The research results of this paper will lay the foundation for future research on how to implement ecological design and improve residential living quality, which is another important aspect of the entire body of research on Chinese Rural Revitalization Strategy in this region.

Research Background
Scholars have studied the impact of climate change on areas and the climate adaptation of buildings to reduce energy consumption. Dastgerdi et al. [7] have analyzed the impact of climate change on world heritage sites and proposed improvements to the conservation framework to protect cultural heritage in the context of climate change [8]. Soutullo et al. [9] compared the impact of three different climates on the energy performance of a Madrid house to arrive at potential design strategies. Kalm et al. [10] have demonstrated experimentally that the use of advanced personalized ventilation systems can provide adequate thermal comfort without reducing daylight performance. Oh et al. [11] proposed five new pre-screening analysis methods, using interval data recorded by smart meters to analyze the energy saved by installing home automation devices. Rodríguez et al. [12] introduced the commissioning of the Controlled and Automatized Testing Facility for Human Behavior (CASITA), which will facilitate the study of human behavior in a controlled space and can be used to study human energy use aspects. For a long time, the research on ecological and energy saving of vernacular dwellings in China had been maintained at a qualitative or half-quantitative level, mainly focusing on the design strategy of energy conservation to adapt to the climate, indoor thermal environment testing and improvement [13]. According to the environmental characteristics of the western region of China, Liu et al. [14,15] studied the climate and ecological experience of the traditional residence in this region, and proposed a new type of immature soil house. Yang et al. [16,17] analyzed the layout of traditional villages and the ecological strategies of residential buildings in Jiangnan, and put forward appropriate low-energy technical transformation strategies for the defects of traditional residential buildings near water. Wang et al. [18,19] and others analyzed the form, color and material of the traditional residential buildings in Zhejiang province, and analyzed the indoor environment of the residential buildings through field investigation and measurement. In view of the current situation of rural housing and passive energy-saving technology, Jin et al. [20,21] have carried out relevant research, and built a series of demonstration projects.
Most of the studies on the gully regions of the Loess Plateau in China focus on the spatial distribution and morphological features. Yu et al. [2] studied the ecological theory and planning methods of human settlements in the gully regions of the Loess Plateau. Huang et al. [22] analyzed the main problems of the strip spatial form in the gully regions of the Loess Plateau, and determined the ring spatial form of the future urban spatial expansion. Through the systematic analysis of the regional and morphological characteristics of the spatial distribution of small towns, Liu et al. [23] put forward the idea of the regional adaptive development model of the organic growth of urban space. Wu et al. [24] improved the original ecological experience of the village houses in the gully regions of the Loess Plateau, and summarized the ecological design method of the local village houses.
This quantitative research on vernacular dwellings for the energy demand in building materials manufacture, building erection, living, demolition and disposal are not enough, especially the total energy demand in the full architectural life cycle for ensuring good thermal functions. Establishing an assessment system on architectural energy demand and to conduct analyses on energy demand for the full life cycle for residential buildings will act as basic research for the future "green" assessment.
The LCA method is an assessment method on a product or an activity from raw material mining and manufacture to the final disposal. There are two main types of LCA research [25]. One is the study of the combination of building materials and building components, which is the analysis of the environmental impact of a single product and the comparison of the environmental impact between different products. Takano et al. [26] demonstrated the influence of material selection on the life cycle energy balance of a building in Finland; Wong et al. [27] analyzed the life cycle cost of rooftop gardens in Singapore; Eleftheriadis et al. [28] surveyed the current developments in the energy efficiency of structural systems by brought the theory of Life Cycle Assessment and the capabilities of BIM together. The other is the study of the whole development, including residential buildings and public buildings. Bastos et al. [29] introduced a life cycle energy and greenhouse gas analysis of three representative houses in Lisbon; Wu et al. [30] provided an LCA of the energy consumption and CO2 emission of a typical office building in China; Kofoworola et al. [31] analyzed a typical office building in Thailand using the LCEA method; Kua et al. [32] analyzed the life cycle greenhouse gas emission and energy consumption of a multi-storied commercial building in Singapore.
In this paper, residential buildings are regarded as product, and five processes were fixed by LCA; they are building materials manufacturing, building construction, building use, building demolition and waste disposal. Using these five steps establishes an energy demand model and shows the total energy demand for residential buildings in the full life cycle. Finally, comparative analyses should be performed for the data in different processes of vernacular dwellings and used as bases for future ecological design. This research can not only guide the construction of residential buildings in gully regions of the Loess Plateau in China, but can also have reference significance for other regions of China.

Application of Life Cycle Assessment (LCA) in Architecture
The architectural full life analysis means to use the life cycle in biology and combine it with the theory of social organism and system, and regard architecture as a special industrial product, then perform the analyses on its full life cycle [6]. Architectural standardization and industrialization provide a good platform for the application of LCA in the architectural full life cycle, as well as for the assessment of material conversion. Previous research on architectural energy saving and emission reduction only focuses on the primary process of architectural design and building usage and maintenance, while ignoring the building material manufacturing, building demolition and building material disposal. LCA requires highly efficient energy usage and saving in the full life cycle, including the process of building materials manufacturing, building erection, building use, building demolition, and waste disposal. Among them, the energy and materials input, the solid waste and the emission of waste gas, as well as their both positive and negative influence on the environment, should all be taken into consideration for each process.
In this paper, the most common residential buildings-adobe residential, one-story brick-concrete residential buildings, and two-story brick-concrete residential buildings-are the research object regarded as products. Five processes were fixed by LCA, and they are building materials manufacturing, building construction, building use, building demolition, and waste disposal-as shown in Figure 3.

Application of Life Cycle Assessment (LCA) in Architecture
The architectural full life analysis means to use the life cycle in biology and combine it with the theory of social organism and system, and regard architecture as a special industrial product, then perform the analyses on its full life cycle [6]. Architectural standardization and industrialization provide a good platform for the application of LCA in the architectural full life cycle, as well as for the assessment of material conversion. Previous research on architectural energy saving and emission reduction only focuses on the primary process of architectural design and building usage and maintenance, while ignoring the building material manufacturing, building demolition and building material disposal. LCA requires highly efficient energy usage and saving in the full life cycle, including the process of building materials manufacturing, building erection, building use, building demolition, and waste disposal. Among them, the energy and materials input, the solid waste and the emission of waste gas, as well as their both positive and negative influence on the environment, should all be taken into consideration for each process.
In this paper, the most common residential buildings-adobe residential, one-story brickconcrete residential buildings, and two-story brick-concrete residential buildings-are the research object regarded as products. Five processes were fixed by LCA, and they are building materials manufacturing, building construction, building use, building demolition, and waste disposal-as shown in Figure 3. In this research, there are significant differences in the energy demand of the building materials, methods and building use, and even in the demolition in the different residential buildings. This paper focus on the energy demand in the full life cycle; the resource consumption-such as land, mineral, and water resources-are not calculated in detailed. The purpose of this research is to establish a list for each energy demand, then use LCA to figure out the difference among these residential buildings and their influence on the environment.

Full Life Circle Energy Demand Model for Residential Buildings
In this paper, the energy units used as functional units in this paper are energy demand: MJ, energy demand per unit area: MJ/m 2 and energy demand per unit area per year: MJ/m 2 ·Y [33,34]. All the meanings of the equation symbols are shown in Appendix  In this research, there are significant differences in the energy demand of the building materials, methods and building use, and even in the demolition in the different residential buildings. This paper focus on the energy demand in the full life cycle; the resource consumption-such as land, mineral, and water resources-are not calculated in detailed. The purpose of this research is to establish a list for each energy demand, then use LCA to figure out the difference among these residential buildings and their influence on the environment.

Full Life Circle Energy Demand Model for Residential Buildings
In this paper, the energy units used as functional units in this paper are energy demand: MJ, energy demand per unit area: MJ/m 2 and energy demand per unit area per year: MJ/m 2 ·Y [33,34]. All the meanings of the equation symbols are shown in Appendix Table A1. The total energy demand equation in the architectural full life cycle is [35][36][37]:

Total Energy Demand in the Process of Building Materials Manufacture
In the process of building materials manufacturing, the total energy demand includes the total demand of materials used in construction and in the later renovation.
Energies 2020, 13, 3109 6 of 27 Because of the different functions, the lifetime for each building material is different. The following equation can be used to calculate the total energy demand.

Total Energy Demand in the Process of Building Erection
The total energy demand in the process of building erecting can be divided into construction energy demand and building material transportation. Different construction methods have different energy demand, and the transportation methods and distance have a huge influence on the total energy demand. It can be calculated by the following equation: The equation of energy demand in the process of construction is: Energy demand calculation equation for building material transportation:

Total Energy Demand in the Process of Building Use
In terms of residential buildings' usage, the calculated energy demand is the energy demand for providing a comfortable inside environment, but not the real energy consumption for the buildings. During the field investigation and the research on the local climate, the cooling load in summer can be ignored in this research. The energy demand for heating in winter is the most energy demanding part of this process.
Meanwhile, in the gully regions of the Loess Plateau, inner heat gained by solar radiation in winter is very important. We used the auxiliary energy demand coefficient, which refers to the energy supplied by some indoor heating equipment in unit time, and the construction area for keeping indoor temperature under the condition of average outdoor temperature. The calculation methods are showed as follows [38]: (1) Calculation for the heat load coefficient NLC [13]: heat load coefficient is the sum of heat loss in the enclosure structure and the heat loss in the cold wind penetration structure. The equation is: (2) Calculation for the solar effective heat S M of directly influence houses; the equation is: Energies 2020, 13, 3109 7 of 27 (3) Calculation for ratio of monthly solar heat load SLR M ; the equation is: (4) The calculation of the monthly solar heating rate SHF M . According to the above equation, the previous SLR M can be calculated. Then, by checking Figure A1, the SHF-SLR curve chart, the SHF M can be obtained. (5) The calculation of monthly auxiliary energy demand Q aux,M (KJ/M) and the yearly auxiliary energy demand Q aux,q (KJ/Y).

Total Energy Demand in the Process of Building Demolition
The energy demand for the transportation of materials can be calculated by the construction area, the average speed of soil cover, the average proportion of materials, and the average transportation distance. For traditional residential buildings, the demolition methods are usually manual demolition on site. Due to the difficulty of calculating the energy demand of manual works and the lack of related literature, this is not calculated in this paper. Even for the steel and concrete architecture, the energy demand for demolition takes just 0.18% of the full life cycle [39]. In this research, the energy demand for demolition is calculated as 90% of energy demand in erecting, combined with the energy demand for soil fulfilling. The average depth for the soil fulfill is 1.5 m, the average proportion for recovering materials is 2, and the unit energy demand for a short transportation distance is 1.836 MJ/Ton·km [40]. Therefore, the energy demand of this process can be calculated using the following equation:

Total Energy Demand in the Process of Waste Disposal
Waste building materials can be divided into two parts: the recyclable and the non-recyclable. When the remanufacture materials are reused in buildings, this part of energy demand should be subtracted from the total energy demand [41]. Therefore, the energy demand can be calculated using the following equation:

Introduction of Typical Residential Buildings
This research selects three typical residential buildings in the gully regions: adobe residential buildings, one-story brick-concrete residential buildings, and two-story brick-concrete residential buildings. Because the data are influenced by several factors and conditions, in order to obtain relatively accurate data to perform the comparative analyses, this paper selects three kinds of residential buildings in a similar area-all located in Nan Zhai Town, Qian Yang County, Figure 4. Thus, the meteorological Energies 2020, 13, 3109 8 of 27 geographical information is ensured to be similar. Nan Zhai town is located 5.6 km northeast of Qian Yang county center, including 15 villages. There are 5547 families, and the total population is 21,097 [42]. These residential buildings are all used for living, as shown in Figure 5. The detailed information about their types, distribution and building area is shown in Table 2.
Energies 2020, 13, x FOR PEER REVIEW 8 of 27 total population is 21,097 [42]. These residential buildings are all used for living, as shown in Figure  5. The detailed information about their types, distribution and building area is shown in Table 2.   The adobe residential building was selected in Yang Po village, and the building area was 44.10 m 2 . The one-story brick-concrete residential building was selected in Zhao Yang village, and the building area was 46.21 m 2 . The two-story brick-concrete residential building was selected in Zhao Yang village, and the building area was 46.44 m 2 . The architectural drawings are shown in Figure A2.

Calculation of the Energy Demand in Building Materials Manufacture
In this process, the energy demand for manufacturing unit construction material is the measurement criteria. The energy demand for manufacturing unit building materials and the waste proportion are shown in Table A2.
The waste proportion of building materials also has an influence on the final calculated results (P.S.32.5 cement is the most commonly used in residential buildings). Table 3 shows the construction material list for these three residential buildings and their energy demand. According to the data in total population is 21,097 [42]. These residential buildings are all used for living, as shown in Figure  5. The detailed information about their types, distribution and building area is shown in Table 2.   The adobe residential building was selected in Yang Po village, and the building area was 44.10 m 2 . The one-story brick-concrete residential building was selected in Zhao Yang village, and the building area was 46.21 m 2 . The two-story brick-concrete residential building was selected in Zhao Yang village, and the building area was 46.44 m 2 . The architectural drawings are shown in Figure A2.

Calculation of the Energy Demand in Building Materials Manufacture
In this process, the energy demand for manufacturing unit construction material is the measurement criteria. The energy demand for manufacturing unit building materials and the waste proportion are shown in Table A2.
The waste proportion of building materials also has an influence on the final calculated results (P.S.32.5 cement is the most commonly used in residential buildings). Table 3 shows the construction material list for these three residential buildings and their energy demand. According to the data in  The adobe residential building was selected in Yang Po village, and the building area was 44.10 m 2 . The one-story brick-concrete residential building was selected in Zhao Yang village, and the building area was 46.21 m 2 . The two-story brick-concrete residential building was selected in Zhao Yang village, and the building area was 46.44 m 2 . The architectural drawings are shown in Figure A2.

Calculation of the Energy Demand in Building Materials Manufacture
In this process, the energy demand for manufacturing unit construction material is the measurement criteria. The energy demand for manufacturing unit building materials and the waste proportion are shown in Table A2.
The waste proportion of building materials also has an influence on the final calculated results (P.S.32.5 cement is the most commonly used in residential buildings). Table 3 shows the construction Energies 2020, 13, 3109 9 of 27 material list for these three residential buildings and their energy demand. According to the data in Table 3, Equation (3) can be used to calculate the energy demand in the materials manufacture process. The results are shown in Table 4.

Calculation of the Energy Demand in Building Erection
According to the investigation, materials used in adobe residential buildings and brick-concrete residential buildings-such as clay bricks, cement, steel, and some other materials-can be bought within Qian Yang County, and the average transportation distance is 10.6 km.
Using to Equations (6) and (7), construction and transportation energy demand can be respectively calculated. Then, using Equation (5), the total energy demand of construction and transportation can be calculated. The construction steps and methods should be classified and the energy demand list can be calculated in Table A3. The construction steps, methods and construction energy demand list of different residential buildings are shown in Table 5. The calculated energy demand for the transportation of building materials is shown in Table 6. The T value in the equation can be seen in Table A4.
The values in Table A3 are just suitable for the transportation energy demand in plain regions. According to the field investigation, these values should be multiplied by weighted coefficient 1.15 when the transportation is in the gully region, ensuring that the results are more accurate. The total energy demand for three different residential buildings in this process can be calculated by adding the energy demand of both construction erection and construction materials transportation, and the unit energy demand can also be calculated. The results are shown in Table 7.

Calculation of the Energy Demand in Building Use
Besides in winter, there are no requirements for indoor temperature in other seasons for the residential buildings in this region, and the auxiliary heating is zero. In this paper, in order to better compare the three different residential buildings, when calculating Q aux , the basic indoor temperature is 14 • C [36]. This basic indoor temperature means the lowest temperature by the heating standard design; once the temperature is lower than i, auxiliary energy should be used for heating. The detailed calculation method and results are shown as follows: (1) Model simplification In order to make the calculation simpler and easier for the following data calculation and to conduct the comparative study, the model simplification data for the three residential buildings are shown in Table 8. (2) Calculation of the heat load coefficient NLC for three different residential buildings The calculation results of the heat load coefficient of cold wind penetration are shown in Table 9. The closure structure areas A i for three different residential buildings are shown in Table 10:  Calculation results of the thermal conductivity coefficient K i in three different residential buildings are shown in Tables 11 and 12.  According to the above data, the A i K i in three residential buildings can be calculated, as shown in Table 13. The NLC value of each residential building can be calculated by Equation (8), and is shown in Table 14. (3) Calculation of annual auxiliary energy demand The heating period in the gully regions starts in November and ends in the following March. The value S ot in Equation (9) is shown in Table A5.
Simplified formula for monthly solar efficient heat S M for each residential building is shown in Table 15.  (10), the specific months and days DD M can be figured out in reference. The solar heating rate SHF M can be obtained using Figure A1. Using Equations (11) and (12), the monthly and yearly auxiliary energy demand can be respectively calculated. Suppose there are four people living in one house-due to daily indoor movement, the average heat radiation is about 500 KJ per person per day; the time people stay indoors is about 12 h; thus, the indoor heating from human body radiation is about 500 × 4 × 12= 24,000 KJ. According to the investigation, the commonly used domestic electrical appliances in these residential buildings are lights, televisions, washing machines, and refrigerators, and the average power for theses appliances in 600 W. If using these appliances for 12 h, the average heating should be 600 × 60 × 60 × 12 = 25,920 KJ; thus, the total indoor manmade heat is 49,920 KJ. All these parameters should be put into the equation to calculate the auxiliary energy demand for each month in the heating period, as shown in Tables 16-18. The above calculation is for the yearly energy demand in the building use process, the ratio of this calculated value and construction area is the yearly energy demand of unit construction area for the building use process, and the detailed list is shown in Table 19.

Calculation of the Energy Demand in Building Demolition
Energy demand in the process of demolition can be calculated using Equation (13). The recovery area for flatting land should be calculated by the excavation boundary. Energy demand for demolition takes 90% of erection, as shown in Table 20.

Calculation on the Energy Demand in the Waste Disposal
Among the researched materials in this paper, glass, wood, and steel belong to common recyclable materials, and the recyclable rate is shown in Table A6. For the demolished bricks and stones, local residents directly reuse them, and the most common use is for fulfilling road bases. In this process, the disposal of non-constructional material is through open and loop recyclable treatment. The three construction materials researched in this paper-glass, wood, and steel-are all recovery and recyclable materials. However, the glass cannot be used in architecture after being recycled, and the wood is hard to use directly in architecture. Thus, the energy demand for remanufacture is very different. In this paper, these materials are regarded as recyclable materials, but these make no contribution to energy saving. Among the three materials, only steel is an open and loop material, and the energy demand for remanufacture is 20-50% of the manufacturing energy demand-in this paper it is calculated by 40%, and the recovery coefficient is 0.50 [44,45].
For the recyclable materials, the average distance from the construction site to the treatment area is about 56.6 km, and for the non-recyclable material, the average distance is 10.6 km. The unit transportation energy demand is 1.836 MJ/Ton·km [44]. According to Equations (14)-(16), the energy demand in the recyclable process of the three residential buildings can be calculated and is shown in Table 21.

Calculation of the Total Energy Demand During the Full Life Cycle
For all architectures, energy demand in the full life cycle can be changed through time. In each different process of the full life cycle, the proportion in total energy demand is different. According to the field investigation, most of local residential buildings in this region are used for around 30 years. The energy demand in each process is shown in Table 22.

Analyses of the Energy Demand in Building Materials Manufacture
The analysis results can be showed in Figures 6 and 7. According to the analysis results, among the three kinds of residential buildings, adobe residential buildings have the lowest energy demand for unit construction area, and the lowest total energy demand in the process of building material manufacture. The highest total energy demand is for the two-story brick-concrete residential building, and the highest energy demand for unit construction area is for the one-story brick-concrete residential building. For the brick-concrete houses, high energy demand construction materials are widely used, such as tiles, clay bricks, cement, and concrete. Among them, clay bricks are the largest construction materials used, which means that the energy demand is more than three times higher than for traditional immature soil houses. Therefore, reducing the use of these high energy demand materials is a better way to realize low energy demand residential buildings in the future. Energies 2020, 13, x FOR PEER REVIEW 16 of 27

Analyses of the Energy Demand in Building Erection
The calculated results are shown in Figures 8 and 9. According to the calculated results, the building areas and total energy demand are both similar for the adobe residential building and onestory brick-concrete building, while for the two-story brick-concrete building, the building area is twice as large and the total energy demand is also twice as large as the others. Thus, the building area has a direct influence on the calculation of energy demand in this process: the larger the building area, the larger the energy demand for building erection. Beside the building area, the construction method and distance for construction material transportation are also important factors which have an impact on energy demand. The design principle of simple construction methods and the usage of local construction materials can largely reduce energy demand.

Analyses of the Energy Demand in Building Erection
The calculated results are shown in Figures 8 and 9. According to the calculated results, the building areas and total energy demand are both similar for the adobe residential building and one-story brick-concrete building, while for the two-story brick-concrete building, the building area is twice as large and the total energy demand is also twice as large as the others. Thus, the building area has a direct influence on the calculation of energy demand in this process: the larger the building area, the larger the energy demand for building erection. Beside the building area, the construction method and distance for construction material transportation are also important factors which have an impact on energy demand. The design principle of simple construction methods and the usage of local construction materials can largely reduce energy demand.

Analyses of the Energy Demand in Building Erection
The calculated results are shown in Figures 8 and 9. According to the calculated results, the building areas and total energy demand are both similar for the adobe residential building and onestory brick-concrete building, while for the two-story brick-concrete building, the building area is twice as large and the total energy demand is also twice as large as the others. Thus, the building area has a direct influence on the calculation of energy demand in this process: the larger the building area, the larger the energy demand for building erection. Beside the building area, the construction method and distance for construction material transportation are also important factors which have an impact on energy demand. The design principle of simple construction methods and the usage of local construction materials can largely reduce energy demand.

Analyses of the Energy Demand in Building Use
The calculated results in Figures 10 and 11 show that the energy demand in the process of building an adobe residential building and a one-story brick-concrete building are almost the same. The unit energy demand for the two-story brick-concrete building is 30% lower than the other two, but the total energy demand is 25% higher than both of them. All in all, the energy demand in the building use process is high for all three residential buildings.

Analyses of the Energy Demand in Building Use
The calculated results in Figures 10 and 11 show that the energy demand in the process of building an adobe residential building and a one-story brick-concrete building are almost the same. The unit energy demand for the two-story brick-concrete building is 30% lower than the other two, but the total energy demand is 25% higher than both of them. All in all, the energy demand in the building use process is high for all three residential buildings.

Analyses of the Energy Demand in Building Use
The calculated results in Figures 10 and 11 show that the energy demand in the process of building an adobe residential building and a one-story brick-concrete building are almost the same. The unit energy demand for the two-story brick-concrete building is 30% lower than the other two, but the total energy demand is 25% higher than both of them. All in all, the energy demand in the building use process is high for all three residential buildings.

Analyses of the Energy Demand in Building Demolition
The calculated results in Figures 12 and 13 show that the unit area energy demand is almost the same for the demolition of the three residential buildings. While for the total energy demand in the process of building demolition, the adobe residential building and one-story brick-concrete building are almost the same, the two-story brick-concrete building is twice as high as the other two-the main reason being the large area of the two-story brick-concrete structure building.

Analyses of the Energy Demand in Building Use
The calculated results in Figures 10 and 11 show that the energy demand in the process of building an adobe residential building and a one-story brick-concrete building are almost the same. The unit energy demand for the two-story brick-concrete building is 30% lower than the other two, but the total energy demand is 25% higher than both of them. All in all, the energy demand in the building use process is high for all three residential buildings.

Analyses of the Energy Demand in Building Demolition
The calculated results in Figures 12 and 13 show that the unit area energy demand is almost the same for the demolition of the three residential buildings. While for the total energy demand in the process of building demolition, the adobe residential building and one-story brick-concrete building are almost the same, the two-story brick-concrete building is twice as high as the other two-the main reason being the large area of the two-story brick-concrete structure building.  Figure 11. Energy demand of unit construction area in the process of building use.

Analyses of the Energy Demand in Building Demolition
The calculated results in Figures 12 and 13 show that the unit area energy demand is almost the same for the demolition of the three residential buildings. While for the total energy demand in the process of building demolition, the adobe residential building and one-story brick-concrete building are almost the same, the two-story brick-concrete building is twice as high as the other two-the main reason being the large area of the two-story brick-concrete structure building.

Analyses of the Energy Demand in Waste Disposal
The calculated results in Figures 14 and 15 show that the total energy demand and unit construction area energy demand for waste disposal in the adobe residential building are the lowest, the construction area of the two-story brick-concrete building is twice as high as the one-story brick-concrete building, and the total energy demand is 1.5 times higher than one-story brick-concrete building.

Analyses of the Energy Demand in Waste Disposal
The calculated results in Figures 14 and 15 show that the total energy demand and unit construction area energy demand for waste disposal in the adobe residential building are the lowest, the construction area of the two-story brick-concrete building is twice as high as the one-story brickconcrete building, and the total energy demand is 1.5 times higher than one-story brick-concrete building.

Analyses of the Energy Demand in Waste Disposal
The calculated results in Figures 14 and 15 show that the total energy demand and unit construction area energy demand for waste disposal in the adobe residential building are the lowest, the construction area of the two-story brick-concrete building is twice as high as the one-story brickconcrete building, and the total energy demand is 1.5 times higher than one-story brick-concrete building.

Analyses of the Total Energy Demand during the Full Life Cycle of Residential Buildings
(1) According to the results in Table 22, the total energy demand in the three residential buildings and the total energy demand in unit area in the full life cycle can be seen in Figures 16 and 17, which can be summarized as follows: For energy demand: from both energy demand in each process of full life cycle and the final total energy demand, the lowest is the adobe residential building, then the onestory brick-concrete building, and the highest is the two-story brick-concrete building. Regardless of viewing either the analysis of material selection or that of residential building use, it shows that the change of life-style makes the energy demand greater and greater.

Analyses of the Total Energy Demand during the Full Life Cycle of Residential Buildings
(1) According to the results in Table 22, the total energy demand in the three residential buildings and the total energy demand in unit area in the full life cycle can be seen in Figures 16 and 17, which can be summarized as follows: For energy demand: from both energy demand in each process of full life cycle and the final total energy demand, the lowest is the adobe residential building, then the one-story brick-concrete building, and the highest is the two-story brick-concrete building. Regardless of viewing either the analysis of material selection or that of residential building use, it shows that the change of life-style makes the energy demand greater and greater. and the total energy demand in unit area in the full life cycle can be seen in Figures 16 and 17, which can be summarized as follows: For energy demand: from both energy demand in each process of full life cycle and the final total energy demand, the lowest is the adobe residential building, then the onestory brick-concrete building, and the highest is the two-story brick-concrete building. Regardless of viewing either the analysis of material selection or that of residential building use, it shows that the change of life-style makes the energy demand greater and greater.  (2) The two-story brick-concrete structure building supplies a larger living space with relative smaller unit energy demand. Due to the close relationship between total energy demand in unit area and construction area, the energy demand for the two-story brick-concrete building is much higher than for the other two.
(3) According to Table 22, the energy demand in each process of the full life cycle of three residential buildings can be seen in Figure 18. and the total energy demand in unit area in the full life cycle can be seen in Figures 16 and 17, which can be summarized as follows: For energy demand: from both energy demand in each process of full life cycle and the final total energy demand, the lowest is the adobe residential building, then the onestory brick-concrete building, and the highest is the two-story brick-concrete building. Regardless of viewing either the analysis of material selection or that of residential building use, it shows that the change of life-style makes the energy demand greater and greater.  (2) The two-story brick-concrete structure building supplies a larger living space with relative smaller unit energy demand. Due to the close relationship between total energy demand in unit area and construction area, the energy demand for the two-story brick-concrete building is much higher than for the other two.
(3) According to Table 22, the energy demand in each process of the full life cycle of three residential buildings can be seen in Figure 18. (2) The two-story brick-concrete structure building supplies a larger living space with relative smaller unit energy demand. Due to the close relationship between total energy demand in unit area and construction area, the energy demand for the two-story brick-concrete building is much higher than for the other two.
(3) According to Table 22, the energy demand in each process of the full life cycle of three residential buildings can be seen in Figure 18.

Discussion
The calculated results show that: (1) In the three residential buildings, the energy demand for building use is greatest, taking 90% of the total energy demand. Due to the 30 years usage time, the proportion of energy demand in this process is the highest, and will be much higher as time passes. (2) In the full life cycle, energy demand for demolition is a little bit lower than for the erecting 2.08% 6.97% 7.97%

Discussion
The calculated results show that: (1) In the three residential buildings, the energy demand for building use is greatest, taking 90% of the total energy demand. Due to the 30 years usage time, the proportion of energy demand in this process is the highest, and will be much higher as time passes. (2) In the full life cycle, energy demand for demolition is a little bit lower than for the erecting process; thus, these two processes can seem reversible. If the energy demand in the erecting process can be reduced, then the energy demand for demolition also is reduced. (3) The proportion for the energy demand of waste construction materials disposal is the smallest.
The energy demand in this process for each residential building is far less than 1%, but it does not mean that this process can be neglected.
According to these results, several suggestions can be submitted for future residential buildings construction in the research regions: (1) Low carbon materials principle: in the full life cycle of residential buildings, the choice of construction materials plays an important role in reducing energy demand in both the manufacturing and demolition processes. Traditional low carbon materials-including stone, wood, and immature soil-are usually easily manufactured and recycled with small energy consumption. Moreover, they can also keep the special regional characteristics.
(2) Low carbon construction engineering: According to the theory of LCA, energy and resource consumption in the building erecting process are high, though the unnecessary energy consumption in this process can be reduced through reasonable design and construction methods. Low carbon construction engineering mainly includes architectural structure, decoration and construction. Through the improvement of architectural structure design, the change of decoration material choice, and different construction methods, the energy consumption in this process can be largely reduced. The reduction in this consumption also means that the energy consumption in demolition can be reduced. (3) Low carbon energy use: the residential building use process consumes around 90% of the energy demand of the total energy consumption; thus, through the selection of different energy, the consumption will be very different. Low carbon energy consumption entails the reasonable use of current energy and recyclable new energy. Residents' living activities can produce certain heat, and so, the use of this energy can reasonably reduce energy consumption. For example, smoke and heat from cooking can be used for heat in winter-thus a suitable design for the chimney can improve the usage of this kind of energy. The adoption of suitable thermal insulation materials and construction for both interior and exterior, such as the introduction of expanded polystyrene for exterior insulation, and the introduction of the brick cladding thermal insulation concrete. These thermal insulation designs can largely improve the inner comfort, and also reduce energy consumption in the building use process. Moreover, the adoption of clean and renewable energy can largely reduce energy consumption in residential buildings. Biogas and solar energy can be largely adopted.
In order to ensure good thermal functions and lower energy demand in residential buildings, the previous quantitative research mainly focuses on the energy demand in buildings, while for the full life cycle energy demand, this research is not enough, and it is hard to establish an assessment system for the future "green" assessment. In this paper, the life cycle energy demand of residential buildings in the gully regions of the Loess Plateau is calculated using the LCA method. The results suggest that the energy demand can be quantitative analyzed in each phase of the building's life by establishing the energy demand model and improving the architectural design method. Such kinds of energy demand model analyses are much more systematic, and the results are more credible and valuable for use as a reference during the ecological design of residential buildings in the research regions.
LCA can also be widely used in the study of the whole development, including residential buildings, public/administrative buildings, stadiums, and so on. The research object can be analyzed as a product, and five processes can be fixed as materials manufacturing, construction, building use, demolition, and waste disposal. When this method is adopted to analyze energy demand in different buildings, the relevant building laws and regulations should be taken into consideration. Furthermore, the location of those buildings, the relevant culture, and location environment should also be researched.
Due to some objective reasons, the LCA in this paper still has some limitations and needs to be improved. The domestic statistics and data cannot supply detailed basic data; the hypothesis is made by the shortage of current resource and data. If there are detailed data, the model should reduce the process of hypothesis, calculate the respective data, and then obtain the total energy demand, thus providing a more accurate assessment of the energy demand in the full life cycle. Therefore, it is very necessary to conduct the relative energy statistics work, and to establish a complete database for the energy demand in the full life cycle.
During the theoretical exploration and practice of the life cycle energy demand of residential buildings, there are still lots of problems which need to be explored. For various reasons, the conclusions of this paper have not yet been applied in the design of the project. In the future, if this method can be applied in real design works, more practical and suitable improvements can be added to this method. The paper hopes that through the results of this research, more researchers in the fields of environment, ecology, architecture, and other related fields can be stimulated to conduct more research on the human settlement planning in the gully regions of the Loess Plateau, and to make contributions to the construction of ecological civilization and economic construction.

Conclusions
This research has assessed the life cycle energy demand of three forms of residential buildings in the gully regions of the Loess Plateau in China, including adobe residential buildings, one-story brick-concrete residential buildings, and two-story brick-concrete residential buildings, to further determine the environmental benefits of these different building forms. For life cycle energy, compared with brick-and-concrete buildings, the energy demand of traditional adobe buildings is less. All three types of buildings have an extremely large energy demand in the building use stage, which accounts for around 90% of the whole life cycle. In terms of the convenience of life, the traditional adobe building is inferior to the brick-concrete building. Therefore, different improvement measures should be taken for different building forms to reduce the energy demand of rural residential areas in this region.
Whether the design is successful or not has a direct influence on the total energy demand in the architectural full life cycle. The primary design process should foresee the influence of energy demand in each process of the full life cycle. If the renovation improves the living environment after being accomplished, it will cause a larger amount of unnecessary energy waste. Therefore, although there are some defects in this paper, the statistics and calculated data can still reflect the current situation of energy demand, and can be used as basic data to guide future research analyses and methods, as well as showing great promise for the use of energy demand in the architectural full life cycle. The research results can also be used as a reference in other rural constructions in China.   (Date Source: [40]). Table A1. Meanings for each character in Equations (MJ).

Characters Meanings ETot
The total energy demand in full life cycle Emanu The total energy demand in building materials manufacturing Eerect The total energy demand in building erecting Euse The total energy demand in building use Edemo The total energy demand in building demolition Edis The total energy demand in building waste disposal Emanu,prod Total energy demand for manufacturing building materials used in construction Emanu,renov Total energy demand in the process of renovation n Numbers of different building materials used in the construction (types) mi Usage amount of construction building material "i" (Ton) wi Disposal proportion of building materials "i" in manufacturing (%)

Mi
Unit energy demand of manufacturing building material "i" (MJ) Figure A1. Functional relationships between solar heating rate SHF and solar heat load. (Date Source: [40]). Table A1. Meanings for each character in Equations (MJ).

E Tot
The total energy demand in full life cycle E manu The total energy demand in building materials manufacturing E erect The total energy demand in building erecting E use The total energy demand in building use E demo The total energy demand in building demolition E dis The total energy demand in building waste disposal E manu,prod Total energy demand for manufacturing building materials used in construction E manu,renov Total energy demand in the process of renovation n Numbers of different building materials used in the construction (types) m i Usage amount of construction building material "i" (Ton) w i Disposal proportion of building materials "i" in manufacturing (%) M i Unit energy demand of manufacturing building material "i" (MJ) Y bui Architectural service life (year), refers to the main structure Y mat Building material service life (year) (b) Drawings of one-story brick-concrete structure building.
(c) The first floor ( d) The second floor Figure A2. Drawings of three typical residential buildings.  Figure A2. Drawings of three typical residential buildings.