Analysis of the indoor thermal environment of traditional residents in the South Sichuan China region in summer and plum rainy seasons—taking Baiyang village in Yibin city as an example

ABSTRACT

In hot summer and cold winter areas, the humid and sultry summer and rainy season problems have long affected the health of residents. Through a questionnaire survey and site measurements, the indoor thermal environment of typical southern Sichuan village houses was investigated. The average indoor humidity of the old traditional dwelling (85–95%) in the rainy season was much greater than that of the new rural house (70–80%), both of which are outside the standard thermal comfort range of the human body. In summer, the situation is slightly better but still uncomfortable. The APMV evaluation results reveal that in the rainy season, the new residential house is more comfortable than the traditional residential house, while in summer, the opposite situation occurs. The measurements show that the thermal inertia index of the old stone masonry house is large, which indicates that the house has a strong ability to resist temperature fluctuations. Therefore, we discussed different performance improvement strategies for these two types of rural residences. This study is conducive to improving the thermal environment of local residential buildings and provides a reference for the future design of rural ecological residential buildings.

GRAPHICAL ABSTRACT

KEYWORDS:

• Southern Sichuan region
• traditional dwellings
• indoor thermal environment
• thermal comfort
• field measurements

1. Introduction

Chinese people in different regions have created traditional dwellings with regional characteristics and adapted to local climate characteristics in the process of development. Even without artificial cold and heat source facilities, traditional dwellings can meet people’s demand for indoor comfort. This traditional ecological construction experience has also been widely recognized and valued. However, with the influence of social development, climate change, the renewal of construction technology and other factors, some performance of traditional dwellings has declined, which cannot meet people’s increasing demand for indoor comfort. Most traditional dwellings still live. Therefore, studying the indoor thermal environment of traditional houses remains a priority in the field of building energy efficiency. Exploring methods for creating green and energy-saving indoor thermal environments for traditional residential buildings in line with the concept of sustainable development has become a hot research topic for scholars in China.

To solve this problem, it is necessary to measure the indoor and outdoor thermal environments of traditional dwellings, investigate the thermal sensation and adaptive behavior of residents living on the spot, and establish a thermal environment evaluation standard that conforms to the local geographical environment and climatic conditions. There have been several achievements in the study of the indoor thermal environment of traditional dwellings in China. Yang Zhenjing et al. studied rammed earth dwellings in Longtang village, Jiangjin District, Chongqing Province; field measurements and analyses showed that rammed earth walls are more beneficial for improving the summer heat environment, and attic space plays a significant role in regulating the indoor thermal environment (Zhenjing and Hanyuan 2015). Taking traditional dwellings in the Guanzhong area of China as an example, Zhang Lei et al. analyzed the passive design method and construction mechanism of local traditional dwellings and obtained an appropriate passive strategy combination for buildings (Lei, Sisi, and Xiaona 2023). Huang Zhijia and others took Huizhou traditional dwellings as the research object and carried out a one-year field test and continuous monitoring of indoor environmental parameters. It is concluded that the thermal performance of Huizhou traditional residential buildings in summer is good, the indoor thermal environment is more suitable, and the indoor thermal environment is more suitable. In winter, cold insulation and airtightness are not effective, and indoor thermal comfort is poor (Zhijia, Mengqi, and Liangji 2018). Zhang Tao et al. studied typical traditional dwellings in the Qinba Mountain area of southern Shaanxi Province, adopted the passive solar building design strategy as the goal, and proposed optimization measures for the summer heat protection of dwellings according to local conditions (Tao, Qiwei, and Yafeng 2021). Peng Mingxi et al. used Chongqing traditional stone houses as the research object. This leads to the conclusion that the organization of nighttime ventilation is an effective way to improve the indoor thermal environment in summer, taking advantage of the thermal delay characteristics of stone masonry dwellings (Mingxi, Zhenjing, and Mingfang 2019). In addition, there are many other types of traditional dwellings where research has yielded rich results in different parts of the country, such as kiln caves on the Loess Plateau, courtyard dwellings in Beijing, tents of nomadic people, dry-fence dwellings in Yunnan, and Tulou buildings in Fujian; additionally, rich results have been achieved (Han et al. 2007; Huang et al. 2017; Li et al. 2018; Ma et al. 2021; Ren and Zhenyu 2006; Soflaei, Shokouhian, and Zhu 2017; F. Wang and Liu 2002; Zhang et al. 2016; Zheng et al. 2021; Zhu et al. 2014). Research on the thermal comfort of traditional dwellings in Spain, Costa-Carrapiço, evaluated the thermal comfort of rural dwellings in Alentejo, Portugal, and established a thermal comfort model (PTC). The results showed that past methods were not suitable for local residences in Alentejo, especially in summer. Although the PTC model considers that heating is insufficient in winter, it can accurately reflect the thermal comfort of traditional dwellings with natural air conditioning in the Alentejo area in summer (Costa-Carrapiço et al. 2022). Albert Malama, a British scholar, conducted field measurements and analyses of traditional houses in Zambia and proposed corresponding strategies to improve the indoor thermal environment based on the local geographic environment and climatic conditions (Malama and Sharples 1997). The British scholar Ben M. Roberts proposed updating building materials in a tropical climate to reduce residential overheating. He explored ways to reduce the overheating of tropical buildings by using recyclable sand-filled plastic bottles to build walls. This approach can not only reduce building energy consumption but also prevent waste from polluting the environment (Roberts et al. 2023). In 2015, Berthold Kaufmann published the book “German Passive House Design and Construction Guide.” Based on the practical experience of Germany and Europe, the technical scheme and quality assurance of passive house design and construction were introduced systematically and in detail from the perspective of theory and practice, and the construction legend method for reducing building energy consumption was provided (Berthold and Wolfgang 2015). The Mexican scholar Mousavi, S. studied several passive technologies under semiarid climate conditions by using building simulation tools. Parametric analysis and testing were carried out, and these energy-saving technologies saved more than 50% of the energy per year and increased the comfort of use by 45% (Mousavi et al. 2022). Many houses in Burkina Faso, Spain, are traditional adobe wall buildings. This kind of local dwelling house technology has a low cost, but its waterproof and moisture proof performance is poor, and it often lacks a comfortable indoor thermal environment. Rincón, L. proposed an alternative low-cost earthen building technology in combination with passive design measures that can significantly improve thermal comfort (Rincón et al. 2019). In summary, China has achieved rich research results on the indoor heat and humidity environment and comfort of traditional dwellings in the countryside in various regions, and there are fewer studies related to the rainy season and summer season in rural areas of Sichuan. However, due to the differences in regional climate and social and economic development conditions, building renovation and thermal comfort objectives are not the same. Currently, there is few sound scientific guidance for improving the indoor thermal environment and conserving energy in rural buildings in Sichuan. Therefore, the research group selected traditional residential buildings in Baiyang village, Junlian County, Yibin city, Sichuan Province, to carry out research and on-site field testing; fully considered the influence of local meteorological conditions and geographical factors; analyzed the influence of thermal characteristics and the envelope structure of two new and old types of residential buildings on buildings; and explored the factors affecting the indoor thermal environment to propose more comprehensive indoor thermal environment optimization measures, optimize the indoor thermal environment and reduce energy consumption, and promote the sustainable development of local traditional residential buildings.

This paper takes old and new dwellings in Baiyang village, Yibin, Sichuan Province, as the research object. Through the combination of field measurements and questionnaire surveys, the advantages and disadvantages of the indoor thermal environment and climate adaptability of new and old traditional dwellings in the plum rainy season and summer are analyzed, and their shortcomings are optimized according to local conditions to improve indoor comfort.

2. Methodology

2.1. Research object

Yibin city is located in southern Sichuan Province. It is located at the intersection of the Jinsha River, Minjiang River and Yangtze River and the core area of the Sichuan-Chongqing-Yunnan-Guizhou junction. It has a subtropical humid monsoon climate. The thermal partitioning of buildings occurs in hot summer and cold winter areas (GB50176-2016. Code for thermal design of civil building). The plum rainy season, also known as Huangmeitian, occurs in the middle and lower reaches of the Yangtze River in China. Between middle and late June and the first half of July each year, the region is characterized by a long period of continuous rainy days, high temperatures and high humidities. Under these special weather conditions, indoor items are prone to mildew. In summer, the area is hot, warm and humid, and the rainfall is concentrated and heavy. Winter is cold, with less sunshine and weak winds. The average annual temperature in Junlian County is approximately 18°C, the average temperature in summer is 29°C, the average temperature in winter is 9°C, and the average annual precipitation is 1050–1618 mm, which mostly occurs in the summer and plum rainy seasons. Considering the strong interaction between humidity and temperature, as well as the high-humidity and high-temperature conditions of this region in summer, we combined the plum rainy season with the summer season as the research period.

The research objects include a local old-fashioned stone dwelling (traditional) and a new type of brick dwelling (modern). The plane shape of the old-fashioned dwelling is a bar, as shown in Figure 1a (the data size unit in the figure is mm). The drawing room is in the middle of the dwelling, and the living room, bedroom and storage room for storing food and sundries are adjacent. The toilet and pig pen are located more hidden north of the kitchen. The dwelling is located mainly in the north‒south direction and somewhat northeast, with no courtyards. The building facade is simple and has a typical style of stone dwelling in southern Sichuan Province (see Figure 1b). The retaining structure is well preserved, and the inner and outer walls are mainly made of 200–400 mm thick stone without additional thermal insulation measures. The roof is a double-slope single-layer structure that uses traditional stone and wood structures, is covered with small green tiles, and is equipped with bright tile lighting; the ground is made of trinity mixture fill. The new modern residence is a two-story building with a brick-concrete structure, as shown in Figure 1c, d. The orientation is the same as that of the traditional residence. On the first floor, there is a kitchen, a drawing room and a master bedroom; on the second floor, there is an extra drawing room and some bedrooms. The wall is mainly made of 240 mm thick bricks, and the floor and roof are paved with coiled waterproof layers. The general building information of the two houses is compared in Table 1.

Figure 1. Current situation of traditional and new dwellings.

Table 1. General building information of the two studied houses.

	Construction year (year)	Floor area (㎡)	Number of floors	External wall material	Window-wall ratio
Traditional dwellings	1999	193.5	1	stone	0.09
New dwellings	2019	226.8	2	brick, concrete	0.45

2.2. Field test method

In this study, field tests of the thermal environment were used to investigate two typical traditional and new dwellings in Baiyang Village, Yibin, during the plum rainy season and summer. Relevant data and residents’ subjective thermal comfort ratings were collected. The test period of the plum rainy season is from 11 June to 14 June 2023, which is an alternating transition season between late spring and early summer. The summer testing period is from 26–31 July 2023.

The measurements are centered on three parameters: the indoor and outdoor air temperature, relative humidity, and wind speed. On-site indoor and outdoor temperature and humidity data were obtained by three instruments automatically recording once every 10 min. The indoor temperature and humidity measuring points of the dwellings were set 1.5 meters from the ground and were arranged in the center of the drawing room and bedroom, respectively. These two main used rooms were chosen to understand the indoor thermal environment of the main use space of the dwellings. To understand the thermal performance of the envelope structure of the research object, thermocouple temperature recorders were used. The wall temperature measuring points were arranged on the inner and outer walls of the drawing room and the bedroom. The measuring points were also set 1.5 meters from the ground, and six measurements were taken a day (at 6:00, 10:00, 14:00, 18:00, 22:00 and 02:00). The indoor and outdoor wind speeds were measured by an anemometer. The wind environment data at three points around the target building were continuously monitored: once an hour, each measuring point was measured for 1–2 minutes, and the instantaneous wind speed was recorded every 3–5 s. At each sampling event, 10–15 measured values (variance and average wind speed can be analyzed) were obtained, and each measuring point recorded 1–2 dominant wind directions each time. The measuring points were measured six times a day (at 6:30, 10:30, 14:30, and 18:30 and at 22:30 and 02:30). The instrument layout was set according to the requirements of the Chinese industry standard “JGJ/T132-2009 Energy-saving Testing Standards for Residential Buildings” (JGJ/T132-2009). The specific positions of the measuring points are shown in Figure 1a. The measuring instruments and their related parameters are shown in Table 2.

Table 2. Test instruments and parameters.

Instrument name	Specification and type	Measuring range	Measuring accuracy	Test mode
High precision temperature and humidity recorder	RC-4H	−40–85°C 0%–100%RH	±0.5°C ±3%RH	Automatic/10 min
Hand-held thermocouple temperature recorder	1310TYPE-K	−20–200°C	±0.2°C	Manual/4 h
Aerovane	Puxicoo/p6-8232	0–360°	±0.5 directions	Manual/4 h
Blackball temperature Recorder	RS-HQ-USB	−40–120°C	±0.2°C	Automatic/10 min

2.3. Residents’ thermal comfort questionnaire survey method

A questionnaire survey was conducted on a random sample of long-term residents in this village to evaluate the different living environments of the traditional buildings and new buildings. A total of 127 and 116 people participated in the plum rainy season and summer, respectively, and 120 and 111 valid questionnaires were returned, which are representative of the data and can more objectively reflect the feelings of local residents about the indoor thermal environment. The interview content involved information about the residents’ thermal comfort feelings of the indoor environment, including the number of permanent residents, gender, age, living habits, building area, and building materials. During the interview, the respondents were asked about their feelings about indoor temperature, humidity, and air quality and whether they felt cold, hot, stuffy, wet and other symptoms. The respondents were also asked whether they used fans, air conditioning or other equipment to adjust their indoor temperature and humidity. In addition, the interviewees were asked about their overall satisfaction with and comfort in the indoor environment. The main content of the questionnaire is listed in Table 3. These questions can help us better understand residents’ feelings about and need for thermal comfort in the indoor environment.

Table 3. Main content of the questionnaire on residents’ thermal comfort.

	Questions
1	Do you live in the countryside?
2	What is refrigeration situation of your hometown residence in summer?
3	How is your current feeling?
4	How comfortable do you think of the current indoor temperature?
5	How do you want the current indoor temperature to change?
6	What do you think of the current indoor humidity?
7	How comfortable do you think of the current indoor humidity?
8	How do you want the current indoor humidity to change?
9	How do you think of the current indoor airflow situation?
10	How do you want to change based on your sense of wind?
11	Do you think the current indoor thermal environment is acceptable?
12	Where do you think of the discomfort of the indoor environment comes from?
13	In the summer, do you need room dehumidification?
14	Do you have any suggestions to improve the comfort level of residential buildings?

2.4. Methodology for establishing the evaluation basis

In 1967, Fanger proposed the Predicted Mean Vote method (PMV) (Omidvar and Kim 2020). Based on available research (Zhao, Lian, and Lai 2021). In this study, a linear relationship between the average temperature during the test period and human comfort was established, and the Adaptive Predicted Mean Vote (APMV) was used as the basis for evaluation. The relationship between the APMV and PMV is expressed as APMV = PMV/(1 + λ*PMV). The measured thermal environment parameters, clothing thermal resistance and metabolic rate data are imported into the MATLAB calculation program to obtain the PMV; λ is the adaptive coefficient. When the PMV ≥ 0 for residential buildings in hot summer and cold winter areas, 0.21 is taken; when PMV < 0, 0.49 is taken to calculate the APMV.

The above method has been proven to be feasible for evaluating the thermal comfort of traditional dwellings in the past, but a basis for evaluating the thermal comfort of the indoor environment of traditional dwellings in Sichuan has not been established. Therefore, this study used regression analysis to establish the relationship between residents’ thermal sensation and operating temperature (TOP) and fitted the two variables. Among them, TOP is the independent variable, the thermal sensation of residents is the dependent variable, and the operating temperature is equal to the average value of air temperature (ta) plus radiation temperature (tr), when the air speed is approximately 0.2 m/s (Z. Wang 2004):

$\mathrm{TOP}=\mathit{\hspace{0.17em}}\left(\mathrm{Ta}+\mathrm{Tr}\right)/2$

3. Test data and analysis

3.1. Analysis of thermal environment data in the rainy season

3.1.1. Indoor and outdoor temperature and humidity

During the plum rainy season, the average outdoor temperature was 23.4°C, and the highest temperature was 30.9°C, which generally occurred between 13:00 and 15:00. The lowest outdoor temperature is 20.1°C, which generally occurs from 5:00–7:00, and the outdoor temperature difference reaches 10.8°C. The relative humidity of the air varies from 48.8% to 87.4%, and the average relative humidity is 76.4%. The outdoor temperature increases with increasing solar radiation, and the air relative humidity decreases. When the outdoor temperature decreases with decreasing solar radiation, the air relative humidity increases.

Figure 2(a) shows the temperature data analysis of the drawing room and the main bedroom in the plum rainy season. During the test period, the mean temperatures of the traditional residential drawing room and the main bedroom were 20.8°C and 21.1°C, respectively, and the fluctuation ranges were between 19.2–24.7°C and 19.3–24.9°C, which are within the indoor comfort temperature range of the transition season (Zhichun and Yanhong 2018). The maximum temperature of the bedroom was 24.9°C, which occurred at approximately 16:00, and the minimum temperature was 19.9°C, which occurred at 5:00–8:00. The temperature difference in the tested bedroom was 5°C throughout the day. The highest temperature in the drawing room was 26°C, which occurred at approximately 16:00, and the lowest temperature was 19.9°C, which occurred from 5:00 to 7:00. The temperature difference in the drawing room was 6.1 throughout the day. The relevant temperature statistics are shown in Tables 4 and 5; the mean temperatures of the new residential drawing room and the first-floor master bedroom are 24.1°C and 23.4°C, respectively. The maximum temperature of the drawing room is 26.3°C, and the minimum temperature is 21.7 at approximately 16:00, which occurs at approximately 7:00. The temperature difference in the drawing room was 4.6°C throughout the day. The maximum temperature of the first floor master bedroom is 25.2°C, which occurs at approximately 21:00, and the minimum temperature is 22.0°C, which occurs from 5:00 to 7:00, with a temperature difference of 3.2°C. The roof of the second-floor bedroom is directly affected by solar radiation. The temperature is the highest of all the test rooms, and the temperature fluctuations are the largest.

Figure 2. Comparison of temperature and humidity between traditional and new houses in the plum rainy season.

Table 4. Indoor and outdoor temperature statistics of traditional dwellings in the plum rainy season.

Position	Mean temperature/℃	Maximum temperature/℃	Minimum temperature/℃	Temperature variation/℃
Outdoor	23.4	30.9	20.1	10.8
Living room	21.0	26.0	19.9	6.1
Bedroom	21.2	24.9	19.9	4.1

Table 5. Indoor and outdoor temperature statistics of new dwellings in the plum rainy season.

Position	Mean temperature/℃	Maximum temperature/℃	Minimum temperature/℃	Temperature variation/℃
Outdoor	23.4	30.9	20.1	10.8
Living room	24.1	26.3	21.7	4.6
First floor Bedroom	23.4	25.2	22.0	3.2
Second floor Bedroom	25.2	30.9	21.7	9.2

Through comparison, it can be seen that the changes in the indoor and outdoor air conditions are basically synchronized, and there is only a delay of 1–2 hours between the highest and lowest temperatures of outdoor and indoor air, indicating that the traditional stone masonry wall has a relatively large thermal inertia index and good thermal insulation performance; at the same time, the fluctuation in the indoor temperature is obviously more moderate than that in the outdoor temperature, which shows that the stone masonry wall has an obvious blocking and delaying effect on the indoor and outdoor temperatures, and the heat insulation effect is obvious in summer. According to the interviews with local residents, traditional stone dwellings are cooler than newly built brick-concrete buildings are, which is consistent with the measured results.

Figure 2(b) shows the relative humidity data collected in the drawing room and master bedroom during the plum rainy season. During the test period, the average indoor relative humidity of the two main rooms in traditional dwellings was 89.2% and 89%, and the fluctuation ranges were between 74.4% and 93.5% and between 79.1% and 92.8%, respectively. The mean humidity of the new residential drawing room is 72.8%, and the average humidity of the bedroom is 78.2%. The indoor humidity in both the new and old dwellings was high, but the mean indoor humidity in the new dwellings was generally lower than that in the traditional dwellings. The reason for the high indoor humidity is that the outdoor humidity is generally high in the plum rainy season, as shown in Tables 6 and 7. The indoor floors of traditional dwellings are paved with triturated soil, which is not good at waterproofing and moisture-proofing. The moisture in the foundation soil will penetrate upward through capillary action, making the floor wet for a long time and leading to high indoor humidity. However, the relative humidity of both the new and traditional dwellings exceeded the comfort range specified in the Chinese national standard “Evaluation Standard for Indoor Thermal and Humid Environment of Civil Buildings” (GBT50785-2012 2012). In addition, from 12:00 to 16:00 every day, the indoor and outdoor relative humidity levels are the lowest, which is also the highest temperature during the day, indicating that the indoor and outdoor air temperature and relative humidity are inversely proportional. Figure 2b shows that the fluctuation in outdoor relative humidity is obvious, and the fluctuation in indoor relative humidity is obviously milder than that in outdoor relative humidity. However, if the indoor environment is in a long state of high relative humidity, the probability of mildew will increase, and the environment will pose a certain hidden danger to the health of people living in the environment, which includes rheumatism, sore throat, asthma and other diseases. Moreover, these processes increase the emission and irritation of harmful gases in building materials and furniture (Hui 2016; Yingxin 2005).

Table 6. Statistics of the indoor and outdoor relative humidity of traditional dwellings in the rainy season.

Position	Medial humidity/%	Maximum humidity/%	Minimum humidity/%	Moisture variation/%
Outdoor	76.4	87.0	49.4	37.6
Living room	89.2	93.5	74.4	19.1
Bedroom	89.0	92.8	79.1	13.7

Table 7. Statistics of the indoor and outdoor relative humidity of new dwellings in the rainy season.

Position	Medial humidity/%	Maximum humidity/%	Minimum humidity/%	Moisture variation/%
Outdoor	76.4	87.0	49.4	37.6
Living room	72.8	79.9	59.0	20.9
First floor Bedroom	78.2	82.7	66.5	16.2
Second floor Bedroom	78.2	82.2	66.2	16.0

This phenomenon can be partially explained by the fact that on nonrainy days outdoors, natural ventilation and lighting are effective, and water in the air is removed and dispersed over time after evaporation, so the relative humidity is lower than that indoors. Moreover, the effective organization of natural ventilation in architectural space layouts can reduce the relative humidity of indoor air, which is an effective strategy for improving the indoor comfort of residential buildings during the transition season. Therefore, the indoor relative humidity of traditional dwellings is high, which can be explained by the architectural design: there is no cross-hall wind between the entrance door of the main drawing room and the adjacent rooms. Second, there is a storage room at the back of the drawing room, and there is no direct window on the outer wall of the drawing room, resulting in poor ventilation and high relative humidity, as shown in Figure 1a. The window of the whole residential facade is small (see Figure 1b); thus, ventilation is insufficient, which is not conducive to the circulation of air, and the water/moisture will remain indoors even if it is evaporated. The relative humidity in traditional stone houses is rather high, resulting in relatively humid indoor environments. Therefore, indoor ventilation and dehumidification should be carried out to obtain better indoor thermal comfort and effectively eliminate the phenomena of ground moisture and wall mildew in traditional dwellings.

3.1.2. Wall thermal performance analysis

The tested south wall of the drawing room was studied. The temperatures of the inner and outer surfaces of the south wall of the new and traditional residential drawing rooms were measured during the test, as shown in Figure 3. During the measurement period, the highest outdoor temperature reached a peak of 24°C at 14:00–16:00 pm. Due to the southward orientation of the wall in the drawing room, the direct radiation from the sun is superimposed when the outdoor temperature is high. The external surface of the south wall is also rapidly heated by solar radiation. This temperature is significantly higher than the outdoor temperature, with a peak temperature of 26.5°C. The temperature of the inner wall changes with the temperature of the outer wall. As the outdoor temperature continues to rise on June 14, at approximately 12:00, the outdoor temperature is higher than the outer surface temperature of the south wall of the drawing room and reaches a peak of 30.7°C at 16:00 p.m. when the outdoor temperature is still higher than the wall temperature outside the wall. Once again, it shows that the thermal resistance and thermal inertia index of traditional stone masonry walls are larger, and they have better thermal insulation performance; in the case of high outdoor air temperature, the stone wall plays a significant role in blocking and delaying the indoor and outdoor temperatures, especially in the case of higher outdoor air temperature, and the heat insulation effect is more obvious.

Figure 3. Temperatures of the southern walls of new and traditional dwelling living rooms during the plum rainy season.

3.2. Summer thermal environment data analysis

3.2.1. Indoor and outdoor air temperature and humidity

The summer test time ranged from July 26 to 31. It was rainy on July 26, cloudy on July 27, and sunny on the other days. The average outdoor temperature is 28.5°C, the temperature fluctuation range is 23.4–38.5°C, the daily maximum temperature is approximately 16:00, and the daily minimum temperature is approximately 06:00. The outdoor relative humidity ranged from 49.6% to 88.4%, with an average of 75.8%. The above results show that this area has a typical summer climate.

Figure 4(a) shows the results of analyzing the temperature inside the living room and master bedroom during the summer in traditional houses. During the investigation period, the average temperatures of the drawing room and the master bedroom of the traditional house were 26°C and 26.5°C, respectively. There was no air conditioning installed in the living room or the master bedroom, and local people used to use traditional fans and natural ventilation to cool the room, resulting in a small range of fluctuations in the indoor temperature. The highest and lowest indoor temperatures occurred at approximately 19:00 and 06:00, respectively. The relevant temperature statistics are shown in Table 8. The mean temperatures of the drawing room, the first floor master bedroom and the second floor master bedroom of the new residential buildings are 28.9°C, 28.4°C and 29.9°C, respectively. The relevant temperature statistics are shown in Table 9. Since the second-floor master bedroom roof of new residential buildings is directly affected by solar radiation, the indoor temperature is high and fluctuates greatly. Although the indoor thermal environment and overall insulation performance of traditional dwellings in summer are better than those of new dwellings, the average temperatures in the living room and master bedroom of both types of dwellings exceed the average summer indoor temperature of 26°C required by China’s standard (GBT50785-2012 2012).

Figure 4. Comparison of summer temperature and relative humidity between traditional and new dwellings.

Table 8. Indoor and outdoor temperature statistics of traditional dwellings in summer.

Position	Mean temperature/℃	Maximum temperature/℃	Minimum temperature/℃	Temperature variation/℃
Outdoor	27.5	38.5	23.4	15.1
Living room	26.0	30.1	24.0	6.1
Bedroom	26.5	30.0	23.9	6.1

Table 9. Indoor and outdoor temperature statistics of new dwellings in summer.

Position	Mean temperature/℃	Maximum temperature/℃	Minimum temperature/℃	Temperature variation/℃
Outdoor	27.5	38.5	23.4	15.1
Living room	28.9	33.3	26.4	6.9
First floor Bedroom	28.4	30.1	27.0	3.1
Second floor Bedroom	29.9	34.1	26.5	7.6

Figure 4(b) shows the analysis of relative humidity data for the research object in summer. During the test period, the mean relative humidities of the Living room and the master bedroom of the traditional dwellings were 87.7% and 85.1%, respectively; the relative humidity statistics are shown in Table 10. The mean relative humidities of the living room and the master bedroom of the new dwellings were 72.5% and 79.3%, respectively; the relative humidity statistics are shown in Table 11. The average indoor relative humidity in traditional homes is generally greater than that in new dwellings. Due to the overall high humidity of the outdoor air in summer and the lack of a moisture-proof layer on the indoor floor of traditional dwellings, the moisture in the foundation soil penetrates upward, while the indoor floor of new dwellings is equipped with a waterproof and moisture-proof layer, and an exhaust fan is provided to improve the efficiency of indoor ventilation, which results in the average indoor relative humidity of traditional dwellings being generally higher than that of new dwellings. Nevertheless, the average relative humidity is greater than the 60% average indoor relative humidity in summer required by China’s Indoor Thermal Environment Evaluation Standard for Civil Buildings (GBT50785-2012 2012).

Table 10. Values of the indoor and outdoor relative humidity of traditional dwellings in summer.

Position	Mean humidity/%	Maximum humidity/%	Minimum humidity/%	Moisture variation/%
Outdoor	75.8	88.4	49.6	38.8
Living room	87.7	93.7	66.6	27.1
Bedroom	85.1	93.4	63.7	29.7

Table 11. Values of the indoor and outdoor relative humidity of new dwellings in summer.

Position	Mean humidity/%	Maximum humidity/%	Minimum humidity/%	Moisture variation/%
Outdoor	75.8	88.4	49.6	38.8
Living room	72.5	81.0	45.0	36.0
First floor Bedroom	79.3	83.8	66.4	17.4
Second floor Bedroom	68.8	78.4	47.8	30.6

3.2.2. Thermal performance analysis of the wall

The tested south wall of the drawing room was studied. The temperatures of the inner and outer surfaces of the south wall of the traditional and new residential drawing rooms were measured during the test period, as shown in Figure 5. During the measurement period, the temperature of the outer wall of the traditional houses was greater than that of the inner wall. The highest outer wall temperature reached a peak of 37.1°C at 20 pm on July 30, and the inner wall temperature changed with the change in the outer wall temperature. The temperature of the inner and outer walls of the new house exhibited the following trend: before 6:00 a.m. to 12:00 a.m., the temperature of the outer wall was lower than that of the inner wall. At 12:00 a.m., the temperature of the outer wall is higher than that of the inner wall until 22:00 p.m. The temperature of the former is lower than that of the latter again. The highest temperature on the outer wall is 37.5°C, which occurred at 18:00 p.m. on July 30. This shows that the thermal resistance and thermal inertia indices of traditional residential stone masonry walls are large and that these walls have good thermal insulation performance.

Figure 5. Temperatures of the southern walls of new and traditional house living rooms in summer.

3.3. Subjective questionnaire analysis

3.3.1. Basic information analysis

The residents of traditional dwellings who completed the questionnaire were mostly 50–70 years old, while the residents of new dwellings were mainly 30–50 years old. Older people prefer to live in traditional homes, while most young and middle-aged people choose to move to new dwellings to live (see Table 12).

Table 12. Basic information of the interviewed residents.

			Age (years)
Season	Dwelling type	Number of people	<30	30–50	50–70	1>70
Rainy season	Traditional dwelling	61	0	2	46	13
	New residential building	59	2	48	2	7
Summer	Traditional dwelling	58	0	4	48	6
	New residential buildings	53	4	46	2	1

3.3.2. Activities and metabolism

Based on the statistical analysis of the residents’ activities measured in the first half an hour before the survey, the residents’ activities in the traditional residential houses in the plum rainy season and summer were mainly standing, sitting and relaxing activities, which do not involve high-intensity physical work, while the residents of the new residential houses in the two seasons had mainly some mild labor. According to the metabolic rate of common activities in China’s Evaluation Standard for Indoor Thermal Environment of Civil Buildings (GBT50785-2012 2012), the metabolic rates of residents in old and new types of dwellings can be calculated to be 0.8 Met and 1.1 Met, respectively.

3.3.3. Thermal sensation and thermal comfort

(1) Thermal sensation and thermal comfort analysis during the plum rainy season

Figure 6(a) shows the residents’ thermal sensation evaluation of the indoor thermal environment in the plum rainy season. Among the residents living in traditional dwellings, 21.31% feel “moderate”, 29.34% feel “very wet and slightly cool”, which accounts for the highest proportion of the residents, and 11.48% feel “cold”; for the new residential dwellers, 35.60% feel “moderate”, while the proportion of residents who feel “cold” is 4.96%, which is much lower than that of traditional dwellings.

Figure 6. Indoor thermal environment analysis of residents in the plum rainy season.

Figure 6 (b) shows that the proportion of residents who feel “acceptable” is the highest for both houses. For the traditional house dwellers, 32.39% of them felt that the indoor comfort was low, and 22.95% felt “comfortable”. For new house dwellers, 13.21% of them feel that the level of indoor comfort is low, far lower than the proportion of traditional house dwellers. This shows that the indoor thermal environment of new residential buildings is better than that of traditional residential buildings during the plum rainy season.

(2) Analysis of thermal sensation and thermal comfort in summer

Figure 7(a) shows the residents’ evaluation of the cold and hot sensations of the indoor thermal environment in summer. Among the traditional houses, 42.62% of the residents felt “moderate”, and 19.92% of the residents felt “hot”; among the new house dwellers, 32.60% of the residents felt “moderate”, which was lower than that of traditional dwellings. In addition, 28.82% of the residents felt “hot”, which was higher than the proportion of traditional houses. Figure 7(b) shows that the proportion of residents who feel “acceptable” is the highest. In traditional dwellings, 20.72% of residents feel that the level of indoor comfort is low, while in new dwellings, this proportion is 39.21%. This leads to the conclusion that the indoor thermal environment of traditional dwellings is better than that of new dwellings during the summer months.

Figure 7. Analysis of the indoor thermal environment evaluation of residents in summer.

4. Residents’ perceptions and thermal comfort ratings

Since the drawing room is the main place where residents perform their activities during the day and the bedroom is the main place where residents rest at night, the thermal comfort index mainly studies the drawing room during the period from 08:00–20:00 and the bedroom from 20:00–08:00. According to the standard (GBT50785-2012 2012), the evaluation of the thermal and humid environmental design of non-artificial heat and cold sources should be based on the calculation method or the graphical method, and when evaluating by the calculation method, the predicted adaptive average thermal sensation index (Adaptive Predicted Mean Vote, APMV) should be taken as the evaluation metric because the APMV is the predicted value after considering the psychological, physiological and behavioral adaptations of the occupants. The APMV should be calculated according to the following formula: APMV = PMV/(1+λ*PMV), where PMV is the predicted mean vote, λ is the adaptive coefficient, and the residential buildings in hot summer and cold winter areas take 0.21 when PMV ≥ 0 and −0.49 when PMV < 0. The APMV is corrected with the adaptive coefficient on the basis of the PMV. The APMV is the adaptive index obtained after correction with the adaptive coefficient on the basis of the PMV, so it is necessary to calculate the PMV index first.

4.1. Thermal comfort evaluation during the plum rainy season

The actual dressing and living habits of local people were investigated according to the measured thermal environment parameters, such as temperature, humidity and wind speed. The PMV was obtained by referring to the standard (GBT50785-2012 2012). According to the PMV-PPD calculation standard GB/T18049-2017, the clothing thermal resistance in the calculation was 0.8 clo, the metabolic rate was 1.1 Met. The PMV of the drawing room of the traditional and new houses in the plum rainy season was obtained, and the corresponding APMV was calculated, as shown in Figure 8. According to the above evaluation criteria, the thermal and humid environments of non-artificial cold and heat sources are divided into three grades, as shown in Table 13.

Figure 8. PMV and APMV of the living rooms of the new and traditional dwellings in the plum rainy season.

Table 13. Evaluation grade of the thermal and humid environments of non-artificial cold and heat sources (GBT50785-2012 2012).

Level	Evaluation Index (APMV)	Satisfaction
GradeI	−0.5≤APMV≤0.5	90% of people can accept the thermal environment
GradeII	−1≤APMVＬ−0.5 or 0.5<APMV≤1	75% of people can accept the thermal environment
GradeIII	APMV<−1 or APMV>1	Less than 75% of people can accept the thermal environment

Through linear regression analysis of the average temperature and PMV, the PMV and operating temperature of the traditional and new houses in the plum rainy season in Baiyang village in Yibin were obtained, as shown in Figure 9. The linear regression equation for the PMV for traditional and new houses is as follows:

$\mathit{PMV}\mathit{\hspace{0.17em}}\mathit{old}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}0.2656\mathit{\hspace{0.17em}}\mathit{TOP}-6.0428,\mathit{\hspace{0.17em}}{R}^2=\mathit{\hspace{0.17em}}0.9979$

$\mathit{PMV}\mathit{\hspace{0.17em}}\mathit{new}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}0.2531\mathit{TOP}-5.8024,\mathit{\hspace{0.17em}}{R}^2=\mathit{\hspace{0.17em}}0.9950$

Figure 9. Fitting analysis of the indoor operating temperature and PMV for two types of dwellings in the plum rainy season.

Assuming that the PMV is equal to 0, the predicted values of the thermal neutrality temperature of the traditional and new houses can be calculated. The thermal neutrality temperature of the traditional house is 22.75°C and that of the new house is 22.93°C. Combined with Figure 9, it can be seen that the traditional dwellings in the plum rainy season are mostly Grade II or Grade III, and a small part of the time is Grade I; in contrast, the satisfaction value of the new residential building is greater than that of the traditional residential building, most of the time Grade I, and a small part of the afternoon is Grade III. Therefore, the comfort of residents of new residential buildings during the plum rainy season is greater than that of residents of traditional residential buildings, which is consistent with the results of the subjective questionnaire.

4.2. Summer thermal comfort evaluation

According to the measured thermal environment parameters, such as temperature, humidity and wind speed, the PMV-PPD calculation standard GB/T 18049-2017 is again used for summer thermal comfort evaluation. In the calculation, the thermal resistance of the clothing is 0.6 clo, the metabolic rate is 1.2 Met. The PMV of the drawn rooms in the two residential houses in summer is obtained, and the corresponding APMV is calculated, as shown in Figure 10.

Figure 10. PMV and APMV of the living rooms of the new and traditional dwellings in summer.

Linear regression analysis of the average temperature and PMV was carried out. The PMV and operating temperature of the traditional and new dwellings in Baiyang Village of Yibin in summer are shown in Figure 11. The linear regression equation of the PMV of traditional and new dwellings is:

$\mathit{PMV}\mathit{\hspace{0.17em}}\mathit{traditional}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}0.2544\mathit{\hspace{0.17em}}\mathit{TOP}-6.1667,\mathit{\hspace{0.17em}}{R}^2=\mathit{\hspace{0.17em}}0.9923$

$\mathit{PMV}\mathit{\hspace{0.17em}}\mathit{new}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}0.2502\mathit{\hspace{0.17em}}\mathit{TOP}-6.0704,\mathit{\hspace{0.17em}}{R}^2=\mathit{\hspace{0.17em}}0.9964$

Figure 11. Fitting analysis of the indoor operating temperature and PMV for two types of residential homes in summer.

The PMV is assigned a value of 0, and the predicted values of the thermal neutral temperature of the traditional and new dwellings can be calculated. The thermal neutrality temperature of traditional dwellings in summer is 24.24°C, and whereas that of new dwellings is 24.26°C. Combined with Figure 11, it can be seen that the traditional dwellings in summer are mostly Grade I or Grade II. Most of the time, the new residential buildings are Grade II or III. Therefore, the comfort of residents in traditional houses in summer is greater than that in new houses, which is consistent with the results of the subjective questionnaire.

5. Discussion

This study illustrates that people living in both old and new types of dwellings have a certain degree of adaptability and tolerance to humid environments in the plum rainy season as well as to extreme temperature environments in summer. Compared with new types of dwellings, traditional dwellings have a greater thermal inertia index and better overall thermal insulation in summer; therefore, they offer a cooler environment in summer. However, traditional dwellings have a higher average humidity indoors in summer and especially in the plum rainy season, which unavoidably causes discomfort.

A comparison of the thermal environments of the two types of buildings yields the following main conclusions:

1. Due to the lack of waterproof and moisture proofing measures for the envelope structure of traditional dwellings and single types of related equipment, the average indoor relative humidities of traditional dwellings in the plum rainy season are 89.2% and 89%, respectively, while those of new dwellings are 72.8% and 78.2%, respectively. In summer, the average humidities of the living room and the main bedroom of the traditional houses were 87.7% and 85.1%, respectively, while those of the new dwellings were 72.5% and 79.3%, respectively. The average indoor humidity in the plum rainy season and summer is greater than that in new houses. The average temperature of the living room and the main bedroom of the traditional dwellings (20.8°C and 21.1°C) is lower than that of the new dwellings (24.1°C and 23.4°C). In summer, the average temperature (26°C and 26.5°C) is also lower than that of the new dwellings (28.9°C and 28.4°C), indicating that the thermal insulation performance of the stone dwellings is better.

2. Compared with new house dwellers, according to the questionnaire results, residents who live in traditional houses for a long time have a greater preference for staying in traditional houses, and these residents are mostly elderly people. Elderly people are more likely to stay in stable neighborhoods and are more likely to reminiscent of the old environment; additionally, they are more likely to consider the benefit of cooler environments in summer and thus have strong adaptability to traditional buildings despite having unsatisfactory temperature and humidity.

3. Compared with those of new dwellers, the residents of traditional dwellings are mostly elderly and have relatively lower incomes, limited methods of self-regulating the indoor thermal environment and comfort and other practical reasons; therefore, they rarely consider renewing traditional homes. Therefore, traditional dwelling environments with higher indoor humidity are acceptable due to the long construction history and poor quality of the envelope structure.

The indoor thermal and humid environment is closely related to the comfort and health of the human body. In hot summer and cold winter areas, summer insulation and winter heating need to be considered, and the dehumidification problem in the plum rainy season needs to be considered due to the high humidity climate in these areas. Some strategies are commonly known to alleviate the dehumidification problem. For example, in the planning of building planes, excessive sunshine should be avoided in the east‒west orientation, the use of a natural ventilation layout should be considered, and the area, position and opening method of doors and windows should be reasonably determined. In the transition season, it should be emphasized that architectural design involves a good natural wind environment, which can effectively improve the indoor thermal environment of buildings, improve indoor thermal comfort, and reflect people-oriented design ideas. The construction technical measures should increase the moisture-proof treatment of building details and adopt a closed air interlayer to the overhead floor and hollow wall to facilitate dehumidification and heat dissipation. This measure has achieved good results through the moisture proof design of the overhead ground of rammed earth dwellings in Sichuan and Chongqing, as studied by Nan Yanli (Yanli, Ya, and Huizhi 2015).

To eliminate moisture in a timely manner, some modifications and optimizations have been made to the rooms in the traditional residential floor plan, as shown in Figure 12. The building should be oriented toward the dominant wind direction during the transition season, and the angle between the orientation of the building and the dominant wind direction should not exceed 30°. The building floor plan adopts L-shaped and concave shapes, forming a semi enclosed space and wind field dam and striving for more dominant wind to reach the indoor space for heat dissipation and dehumidification. Additionally, direct lighting and ventilation in the living room are provided, with the dining room adjacent to the living room. The positions of all rooms and the layouts of doors and windows should be adjusted to promote air circulation, dynamic and static zoning, and bedroom privacy within the building. To improve the ventilation and lighting conditions of the traditional houses studied, organized natural ventilation and lighting are the most effective measures for heat dissipation and dehumidification in most rural residential areas.

Figure 12. Plan renovation of traditional and new dwellings.

On the building facade, the keys to traditional residential renovation can be to add skylights to the roof, increase the lighting area, accelerate the air circulation speed, and improve dehumidification efficiency without damaging the original cultural texture of the old traditional residential exterior walls. The focus of the renovation of new residential buildings is to create a rooftop garden on the roof using green plants to enhance the thermal insulation performance of the house in summer. However, the characteristics of southern Sichuan residential buildings, such as the combination of wood and stone and the dry railing structure, are still reflected in the exterior facade style of the building, as shown in Figure 13.

Figure 13. Facade renovation of traditional and new dwellings.

With the development of the rural social economy, the proportion of air conditioning equipment installed in residential buildings is increasing. How to utilize the high thermal inertia index of traditional stone masonry buildings, improve the indoor humidity environment and improve the new brick-concrete building indoor thermal environment are other topics worth discussing. Over time, the lifestyles of rural residents and the means of indoor thermal environment control have changed. In the study of new and traditional rural houses, how to combine these new changes, analyze traditional construction technology and materials, and construct a low-energy rural residential construction technology system that meets the needs of the contemporary era still needs to be discussed in depth.

The content of the building optimization is only presented at the theoretical level, and the effective feasibility of these optimization measures will be discussed in further research through environmental simulation and data analysis.

6. Conclusions and outlook

Through the study of the indoor thermal environment of old and new houses in Yibin city, southern Sichuan Province, during the plum rainy season and summer, through a subjective questionnaire survey and on-site measurements, the thermal neutrality temperature and APMV index were used to investigate and analyze the indoor thermal and humid environment and thermal comfort of the houses and propose targeted countermeasures according to the problems found. The specific conclusions are as follows:

1. According to the thermal neutral temperature and PMV evaluation, the thermal neutral temperature of old-style dwellings and new-style dwellings is similar in summer. Considering the influence of other factors in the environment, such as wind speed and humidity, the actual thermal sensation is less sensitive to temperature changes than is the predicted value. In the plum rainy season, the new residential buildings are mostly in comfort levels I and II, and the traditional residential buildings are mostly in comfort levels II and III. The comfort level of new residential buildings during the plum rainy season is greater than that of traditional residential buildings. In summer, traditional dwellings are usually classified as comfort level I or II, while new dwellings are usually classified as comfort level II or III. The comfort of old dwellings is higher than that of new dwellings in summer.

2. According to the measured results, the changes in the indoor and outdoor air quality of the old-style stone houses are basically consistent, and there is only a delay of 1–2 hours between the highest and lowest temperatures of the outdoor and indoor air. The thermal resistance and thermal inertia of the stone walls of the old-style houses are high, and the houses exhibit good thermal insulation performance, especially in the case of the outdoor air temperature. The higher the temperature is, the more obvious the heat insulation effect is. The climate adaptability of new residential buildings represented by brick-concrete structures is not obvious, and the indoor thermal environment quality is relatively poor in summer. The mean temperature of the living room and the main bedroom of the new residential buildings in summer (28.9°C, 28.4°C) was higher than that of the old residential buildings (26.0°C, 26.5°C).

3. According to the measurement data, the average indoor relative humidity in old residential buildings is greater than 85%, which is greater than that in new residential buildings. Moreover, this value is higher than the 60% average indoor humidity requirement in summer in China’s “Indoor Thermal Environment Evaluation Standards for Civil Buildings” (GBT50785-2012 2012).

In summary, indoor humidity is the most important factor affecting the comfort of residents. In addition, opening skylights on roofs can promote natural ventilation and improve the indoor lighting environment. Moisture absorbing materials or waterproof and moisture proof layers can also be laid inside walls and floors to reduce indoor humidity and improve the comfort of humid and hot environments. The traditional and new houses in Baiyang Village, Yibin, have different thermal performances during the plum rainy season and summer. Traditional residential buildings can create cooler and more humid indoor environments than new residential buildings. Although residents of traditional residential buildings exhibit strong adaptability, the indoor humid environment, insufficient lighting, and poor ventilation still need to be improved.

In addition, due to time limitations, this paper only analyses the indoor thermal environment of two village buildings during the rainy and summer seasons. To comprehensively evaluate and improve the thermal environment of traditional and new dwellings in hot summer and cold winter areas, it is better to carry out relevant tests and analyses in winter. However, this paper focuses on hot and humid climate seasons and compares traditional and new village buildings during these seasons. The findings are expected to provide reference and theoretical guidance for the renewal design of later ecological village dwellings.

Author contributions

Conceptualization, Yan Zhang; Methodology, Yan Zhang and Biao Wang; Validation, Yan Zhang and Biao Wang; Investigation, Luting Xu; Writing – Original Draft Preparation, Yan Zhang.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability statement

The data used to support the findings of this study are included within the article.

Additional information

Funding

The work was supported by the Humanities and Social Sciences Fund of Ministry of Education [21XJCZH005]; Sichuan Housing and Urban‒Rural Development Science and Technology Innovation Project [SCJSKJ2022-01].

Notes on contributors

Yan Zhang

Yan Zhang is an associate professor from School of Architecture and Civil Engineering, Chengdu University, Chengdu, China. Her research interest is architecture design, green building technology.

Luting Xu

Luting Xu is an associate professor from School of Architecture and Civil Engineering, Chengdu University, Chengdu, China. Her research interest is green building technology.

Biao Wang

Biao Wang is an associate professor from School of Architecture, Soochow University, Suzhou, China. His research interest is green building technology and building natural ventilation.