Thermal Comfort Evaluation for the Rural Elderly Based on the Spatiotemporal Differentiation of Daily Activities During Summer in Xi’an, China

Abstract

To meet the comfort and health needs of the elderly in daily activity environments, a refined temporal and zonal thermal environment design across diverse spaces must align with dynamic changes in their daily activity spatiotemporal trajectories. This constitutes a research gap in the existing literature. This study focused on elderly individuals in rural Xi’an, integrating on-site subjective daily activity questionnaires, thermal comfort field surveys, and continuous thermal environment monitoring to evaluate summer thermal environments based on spatiotemporal activity differentiation. The key conclusions are as follows: (1) Elderly people primarily engage in activities in indoor and outdoor spaces, with considerably fewer activities occurring in semi-outdoor areas. Summer outdoor activities occur between 6:00 and 9:00 and 17:00–21:00, while indoor activities dominate other times. (2) The established adaptive thermal response models indicate indoor and outdoor neutral temperatures are 23.8 °C (Operative temperature) and 28.8 °C (UTCI). Indoor 80% acceptability upper limit is 27.5 °C and outdoor 80% acceptability upper limit is 34.1 °C. These results exhibit distinct differences from those observed in alternative climate zones and urban areas in the same climate zone. (3) The thermal environment of outdoor shaded areas remains within the acceptable range for a longer duration than that of indoors, and kitchens have the worst indoor thermal quality. This evaluation provides supplementary insights into current spatiotemporal thermal environment research.

1. Introduction

In the context of global demographic transition, China is experiencing an unprecedentedly rapid and profound aging process, with its severity intensifying steadily over the past few decades. During the “14th Five-Year Plan” period, the proportion of the population aged 60 and above of the total population will exceed 20%, entering into a moderately aged society [1]. With the intensification of China’s population aging trend, ensuring a healthy and comfortable life for the elderly has become one of the important themes of the era. To better address residential environment challenges arising from population aging in China, such as the inadequate age-friendly renovation of living spaces and the unbalanced development of age-friendly residential environments between urban and rural areas, the National Health Commission of the People’s Republic of China and other relevant ministries and commissions have successively issued a series of pertinent policies and plans since 2017. Among them, the “13th Five-Year Plan for Healthy Aging” explicitly puts forward the goals of “promoting the construction of elderly-friendly environments, advancing the age-appropriate renovation of elderly residences, and supporting the construction of age-appropriate housing” to “create a safe, convenient, comfortable, and barrier-free system of elderly-friendly environments” for the broad elderly group [2], and the “14th Five-Year Plan for Healthy Aging” issued in 2022 also points out the need to accelerate the age-appropriate renovation of residences [1]. According to the results of China’s seventh national population census, nearly 60% of elderly people live in rural areas [3], and their living environments are generally inferior to those in urban areas in terms of thermal conditions, air quality, and other aspects of quality of life [4,5]. With the acceleration of urbanization, the majority of young people in rural areas have migrated to urban areas for employment, leading to the phenomena of hollowing-out and aging in rural areas. To meet their spiritual needs, rural elderly people often carry out various daily social activities with peers of the same age, and the scope of these activities mainly centers on their own houses and extends to neighborhood spaces and outdoor sites. To ensure the comprehensive, healthy and comfortable quality of life for rural elderly people, it is necessary to optimize and improve the thermal environments of various spaces where they live all year round.

Existing thermal comfort studies have not fully incorporated human activity trajectories into their analytical frameworks, thereby failing to address the precise “temporal—zonal” requirements for thermal environment comfort. However, by leveraging core theories of time geography to delineate the spatiotemporal differentiation patterns of elderly people’s daily activities, and further establishing a comprehensive evaluation benchmark that covers all activity spaces and time periods, it becomes feasible to provide refined guarantees for the comfort and health of elderly people’s residential environments. Compared with other age groups, elderly people have more flexible activity times, and the content and scope of their activities may vary due to the changes in their physical functions [6]. Currently, most studies on the daily activities of elderly people focus on urban areas [7,8]. On this basis, some scholars have also conducted research on design strategies for architectural spaces and urban spaces based on daily activities [9,10,11,12,13]. However, rural elderly people have developed a strong adaptability to the environment where they have lived for a long time, and there are significant differences in the temporal and spatial laws of their daily activity trajectories compared with urban elderly people. The daily activity scope of rural elderly people mainly centers on their own houses, extending to neighborhood spaces and open areas. Due to psychological and social needs and the survival needs they bear in the family, elderly people’s activity spaces at different times of the day are not limited to a single space, and their daily activities exhibit certain temporal and spatial differentiation laws. At present, most studies on the daily activities of rural elderly people focus on aspects such as rural living circle planning and environmental facilities [14,15]. The specific research focuses and findings of each literature are shown in Table 1.

Table 1. Findings of studies related to daily activities.

Authors	Location	Research Focus	Findings	Year of Publication	Reference
Li and Jiang	Urban Nanjing, China	Temporal and spatial characteristics	Elderly people maintain a highly regular daily schedule, and the spaces where various daily activities take place are relatively fixed.	2024	[7]
Xiong	Urban Xi’an, China	Community life circle	From the perspective of the spatiotemporal behavior of the elderly, a research framework for the behavior-space coupling mechanism is proposed, and based on this, it is evaluated that there exists a mismatch between the spatiotemporal configuration of some facilities in Nanyuamen Subdistrict and the time utilization of the elderly.	2021	[8]
Sun et al.	Urban Beijing, China	Temporal and spatial characteristics	The distribution of the elderly’s outdoor activity time periods greatly affects the utilization efficiency of urban leisure spaces and facilities. Seasonal variations exert a significant impact on the content, modes and outdoor activity radius of their activities.	2001	[9]
Yu et al.	Urban Nanjing, China	The association of outdoor environment on outdoor daily activities	Different types of activities exhibit varying degrees of correlation with the road accessibility, anti-slip measures, greening, layout in the urban physical environment, as well as air quality and noise levels.	2021	[10]
Zou et al.	Urban Nanjing, China	Spatiotemporal behavior and the use of community public service facilities	Residents’ daily activity spaces exhibit hierarchical, shared, and directional characteristics, and there are significant differences in daily activity spaces among different age groups. Based on this, a differentiated supply strategy of public service facilities is proposed.	2021	[11]
Chai et al.	Urban Beijing, China	The Impact of Macroeconomic Factors on Individuals’ Daily Life Experiences	Studied the impact of urban spatial layout on the temporal–spatial behavior of individuals and their behavioral decision-making, exploring the interactive relationship between individual behavioral activities and the spatial environment	2011	[12]
Ta and Chai	Urban Beijing, China	Neighborhood rhythm and living circle	Residents in the study area have insufficient utilization of community time, and their community life time exhibits significant spatiotemporal differentiation, activity type differentiation, and group differentiation. The planning of community living circles needs to analyze the characteristics of life-time utilization and address the spatiotemporal mismatch between behavioral space and physical space.	2023	[13]
Guan	Rural Chongqing, China	Rural life circle	Villagers‘ travel purposes, travel capabilities, and travel frequencies respectively, affect the distribution, scope, and demand differences in rural daily activity spaces.	2022	[14]
Wu et al.	Rural Ningbo, China	The Degree of Influence of Environmental Factors	The degree of influence of environmental support, site safety, spatial convenience, landscape aesthetics, and facility diversity on the outdoor sports activities of the elderly decreases in sequence.	2022	[15]

Among the existing studies on the daily activities of rural elderly people, there is still a lack of interdisciplinary research related to thermal comfort. Nevertheless, the adaptability of rural elderly people to the living environment is also reflected in the fact that they have different laws from urban elderly people in terms of psychological, physiological, and behavioral adjustments [16]. There are significant differences between urban and rural residential buildings in aspects such as architectural layout, thermal performance of envelope structures, building operation modes, and indoor thermal environments [17]. The income of rural residents is quite different from that of urban residents; rural residences are built by farmers with their own funds. To save construction and operation costs, the thermal performance of envelope structures is poor, and these residences are in a state of natural ventilation for most of the year. This leads to poor indoor thermal environment quality, which can hardly meet the high-quality needs of rural elderly people for home living and elderly care in the future [18]. In addition, outdoor spaces are also important places for rural elderly people to engage in social activities. However, with economic development, village roads have been hardened with cement, and the courtyards of most families are also treated in the same way, resulting in the quality of the outdoor microclimate environment and comfort level being significantly lower than that of before. Therefore, how to conduct a refined evaluation on the thermal comfort of various daily living spaces based on the climate adaptation characteristics of elderly people in rural areas and the transition trajectory of their activity spaces is a topic worthy of exploration.

Xi’an is a typical representative city of China’s cold climate. Currently, most of the existing studies on the thermal comfort of elderly people in cold regions have been conducted in residential buildings and elderly care buildings in urban areas [19,20,21,22,23,24,25]. The detailed information is presented in Table 2. However, research on the thermal comfort of elderly people in rural areas remains insufficient. The human thermal adaptation model is not a unified linear relationship on an annual scale [26]. Summer is the warmest season of the year, and given the impaired environmental adaptability and diminished thermal sensitivity among the elderly [27], prolonged exposure to extreme high-temperature conditions exerts a significant adverse impact on their physical health. Adults aged 65 and above, particularly those with cardiovascular and cerebrovascular comorbidities, are identified as the most susceptible population group [28]. However, due to psychological and social needs, elderly people still have a demand for activities in outdoor spaces. Studying the laws and demands of elderly people’s thermal comfort in summer is of great research value for ensuring their health [29], and can provide a theoretical basis for the establishment of a healthier age-friendly living environment.

Table 2. Findings of studies related to the thermal comfort of elderly people.

Authors	Location	Findings	Year of Publication	Reference
Ge	Urban Xi’an, Rural Zhoukou, China	Obtained the laws of the elderly‘s thermal sensation and thermal acceptability changing with the cold and heat variations in the indoor thermal environment in different seasons, and derived the comfortable temperature range.	2024	[19]
Wang and Wang	Rural Renqiu, China	The actual indoor temperature of rural elderly care buildings in winter was higher than the indoor thermal environment temperature expected by the elderly, and there was a significant deviation between the comfort level expected by the elderly and their actual comfort perception.	2019	[20]
Zheng et al.	Urban Baoding, China	Established various subjective response models, obtained the comfortable ranges of temperature, relative humidity and air velocity, and provided a scientific basis for the improvement of the thermal environment.	2023	[21]
An et al.	Urban Beijing, Xi’an, Hami, Chima	Studied the outdoor thermal comfort of urban parks in three cold-region cities, and found that the neutral UTCI (Universal Thermal Climate Index) of residents in Xi’an was the highest, and its comfortable range was slightly wider than that of the other two cities.	2021	[22]
Ma	Urban Xi’an, China	Investigated the temporal and spatial distribution characteristics of the elderly and their thermal perception (including thermal sensation, thermal comfort and thermal acceptability) in the open space of an urban park in Xi’an, and quantitatively determined the outdoor thermal comfort benchmark for the elderly in Xi’an.	2022	[23]
Xu et al.	Urban Xi’an, China	Used the methods of climate monitoring and thermal comfort questionnaire survey to study the winter thermal comfort of different parks in Xi’an, and obtained the neutral UTCI.	2022	[24]
Wang et al.	Urban Baoding, China	Studied the indoor thermal environment of institutional elderly care facilities in Baoding by means of field investigation, questionnaire survey and on-site measurement, and obtained the neutral temperature and thermal demand of the elderly in summer.	2022	[25]

To sum up, the existing studies on the thermal comfort of rural elderly people’s daily activity spaces in China are still in the initial stage, which mainly focus on clarifying the laws of thermal comfort and demand benchmarks. Meanwhile, the daily activity contents of elderly people and the laws of their temporal and spatial differentiation remain unclear, resulting in the lack of a “time-segmented and zone-specific” refined evaluation method for the thermal comfort of daily living spaces based on these factors. Taking this as an opportunity, this paper conducts an in-depth study, selecting the rural areas of Xi’an, Shaanxi Province, in the summer as the research area. The research objectives of this study are delineated as follows:

• To identify the daily activity contents of elderly residents in rural Xi’an and characterize the spatiotemporal differentiation patterns of their daily activities;
• To analyze thermal responses and adaptive thermal demands of elderly people across distinct activity spaces;
• To conduct a “time-segmented and zone-specific” refined evaluation of thermal comfort levels for elderly people.

2. Materials and Methods

2.1. Overview of the Technical Route

Figure 1 illustrates the overall research framework of this paper. First, a survey on daily activities of elderly people was conducted to analyze the spatiotemporal differentiation of activity content and summarize the corresponding spatiotemporal differentiation laws, which provides a methodological basis for subsequent research on thermal comfort and thermal environments. Based on the findings of the aforementioned investigation, a survey on thermal comfort was carried out in the spaces where elderly people engage in daily activities most frequently, aiming to develop a field-measured thermal comfort model for elderly people. In addition, continuous monitoring of the thermal environments was implemented in typical daily activity spaces to obtain the actual residential thermal environment conditions of elderly people in rural Xi’an. Finally, the field-measured thermal comfort model was applied to evaluate the all-day thermal environment conditions of the various measured spaces. In the discussion section, the results obtained from the above steps will be compared with the findings of relevant studies. The specific research methods are elaborated in the following sections.

Figure 1. Technical route map.

2.2. Overview of the Surveyed Area

Xi’an is the capital city of Shaanxi Province, located between 107°40′ and 109°49′ E longitude and 33°42′–34°45′ N latitude. It has a warm, temperate, semi-humid continental monsoon climate and belongs to the cold region of the building thermal climate zones [30] (Figure 2), featuring distinct four seasons, mild climate and moderate rainfall. The annual average temperature ranges from 13.5 °C to 14.9 °C, the annual precipitation is between 506.1 mm and 690.3 mm, and the annual sunshine duration varies from 1679.9 h to 2062.6 h. Xi’an has hot and rainy summers, with prominent summer droughts and frequent thunderstorms and gales.

Figure 2. Building climate zoning map and the location of Xi’an.

The survey was conducted in seven villages in Chang’an District of Xi’an, including Nanqiang Village, Yejiazhai Village, Shangcao Village, Xiacao Village, Guonan Village, Xiacao Village and Song Village. The locations of these villages are shown in Figure 3.

Figure 3. The location of the surveyed villages.

2.3. Field Survey

2.3.1. Survey on Daily Activities

The investigation of daily activities was conducted using the interview method and questionnaire method (Figure 4). Due to the large number of professional terms in the questionnaire and the difficulty for elderly people in using electronic devices, interviewers described the questionnaire in more colloquial language and conducted one-on-one interviews with respondents to understand their answers and improve the accuracy of the responses. The questionnaire consists of two parts: basic information and daily activities. The first part, basic information, covers the respondents’ fundamental status, including details such as gender, age, height, weight, and presence of chronic diseases. The second part, daily activities, includes the content, time, and location of the respondents’ daily activities. The time of daily activities was recorded at one-hour intervals. The locations of daily activities are divided into indoor spaces, outdoor spaces, and semi-outdoor spaces; among these, indoor spaces include bedrooms, living rooms, kitchens, and other similar areas, outdoor spaces include courtyards, doorways, alleys, squares, and other similar areas, and semi-outdoor spaces include hallway spaces or other areas partially shaded by buildings. The specific questionnaire content is presented in Appendix A.

Figure 4. Photos from the research site of the survey on daily activities.

Certain underlying diseases may impair the mobility of elderly people, and the relevant questionnaires have been excluded from subsequent analyses. A total of 93 valid questionnaires were collected during this stage of the investigation, including 47 from females and 46 from males. The basic information of the respondents is shown in Table 3.

Table 3. Basic information about the respondents of the survey on daily activities.

Gender		Age/yrs	Height/cm	Weight/kg
Female (47)	Minimum value	60	147	42
	Maximum value	88	171	80
	Average value	70.36	158.34	60.19
	Standard deviation	7.13	5.19	8.45
Male (46)	Minimum value	60	160	60
	Maximum value	86	176	88
	Average value	70.43	168.28	68.20
	Standard deviation	6.48	4.53	7.67

2.3.2. Survey on the Thermal Comfort

The investigation on the thermal comfort of the living environment was conducted through on-site thermal comfort surveys, which included objective thermal environment testing and subjective thermal comfort questionnaire surveys. For the objective thermal environment testing, a Delta HD32.3 thermal comfort meter (Figure 5, Table 4) was used to measure relative humidity (RH), air velocity (v_a), air temperature (t_a), and globe temperature (t_g). During the testing, the instrument was placed within 1 m of the subject and at the same height as the subject’s head. The subjective thermal comfort questionnaire survey covered three parts: the subjects’ basic information, subjective sensations, and thermal adaptation behaviors. The subjects’ basic information included gender, age, height, weight, presence of chronic diseases, activity content (metabolic rate) within 20 min before the survey, and clothing status (clothing insulation). The subjective sensations included thermal sensation, thermal acceptability and thermal preference. A 7-point scale was used for thermal sensation voting, namely “Cold” (−3), “Cool” (−2), “Slightly Cool” (−1), “Neutral” (0), “Slightly Warm” (+1), “Warm” (+2), and “Hot” (+3). A 4-point scale was adopted for thermal acceptability evaluation, namely “Completely Unacceptable” (−2), “Just Unacceptable” (−1), “Just Acceptable” (+1), and “Completely Acceptable” (+2). A 3-point scale was used for thermal preference voting, namely “Cooler” (−1), “No change” (0), “Warmer” (+1). The thermal adaptation behaviors included three parts: the space where the respondents were located, the opening/closing status of doors and windows, and thermal environment improvement measures. The spatial locations of the respondents were divided into three categories: indoor, semi-outdoor, and outdoor. The opening/closing status of doors and windows included three situations: “Fully Open”, “Partially Open”, and “Fully Closed”. The thermal environment improvement measures included the use of air conditioners, fans, electric blankets, Chinese-Kang (a common heating facility in northern China, typically constructed with bricks and earthen materials with a built-in flue), and other devices [29]. The specific questionnaire content is presented in Appendix B.

Figure 5. Monitoring instruments and a photo from the research site during survey on the thermal comfort: (a) Thermal comfort meter; (b) outdoor temperature-humidity recorder; (c) globe temperature recorder; (d) portable weather station; and (e) photo from the research site.

Table 4. Parameters and accuracy of the instruments.

Instrument Type	Model	Measured Parameters	Accuracy Specifications
Thermal Comfort Meter	Delta HD32.3	Relative Humidity: 0–100%	Relative Humidity: ±1.5% (0–90%)
		Globe Temperature: −10–100 °C	Globe Temperature: 0.1 °C
		Air Velocity: 0.1–5 m/s	Air Velocity: ±0.2 m/s (0–1 m/s); ±0.3 m/s (1–5 m/s)
Outdoor Temperature-Humidity Recorder	HOBO MX2301	Temperature: −40–70 °C	Temperature: ±0.25 °C (−40–0 °C); ±0.2 °C (0–70 °C)
		Humidity: 0–100%	Humidity: ±2.5%
Globe Temperature Recorder	HQZY-1	Globe Temperature: −20–80 °C	±0.3 °C
Portable Weather Station	Kestrel 5500	Air Velocity: 0–40 m/s	0.1 m/s

A total of 430 valid questionnaires were collected during this stage of the investigation. Among them, 298 questionnaires were from respondents in indoor spaces (107 from males and 191 from females); 113 questionnaires were from respondents in outdoor spaces (49 from males and 64 from females); and 19 questionnaires were from respondents in semi-outdoor spaces (9 from males and 10 from females). The basic information of the respondents is shown in Table 5. According to the findings of the previous investigation stage, the proportion of the elderly’s activities occurring in semi-outdoor spaces is relatively low, accounting for only about 1% of their daily activities. Additionally, the number of samples obtained for semi-outdoor spaces in the survey is limited. Therefore, only the valid questionnaires corresponding to indoor and outdoor spaces were used in the subsequent analysis.

Table 5. Basic information about the respondents of the survey on thermal comfort.

Space		Age/yrs	Height/cm	Weight/kg
Indoor (298)	Minimum value	60	140	40
	Maximum value	90	183	90
	Average value	69.08	162.21	62.60
	Standard deviation	6.66	7.05	9.57
Outdoor (113)	Minimum value	60	145	42
	Maximum value	89	180	90
	Average value	68.56	162.37	62.46
	Standard deviation	7.25	7.07	9.72

2.3.3. Monitoring of the Thermal Environments

For the monitoring of the thermal environment in daily activity spaces, instruments including a thermal comfort meter, an outdoor temperature-humidity recorder, a globe temperature recorder, and a portable weather station were placed at fixed measuring points and mounted on tripods at a height of 1.1 m above the ground to conduct continuous testing of the physical environment parameters in the typical spaces where elderly people engage in daily activities. The test was carried out in a residential house in Song Village. This dwelling is typical of rural residences in Xi’an: it generally consists of a front hall, a central courtyard and a rear room, with a long, narrow rectangular plan, four-sided enclosure and a south-facing orientation; it is equipped with basic electrical appliances such as electric fans; and the permanent residents are usually elderly couples. The physical environment parameters measured included relative humidity (RH), air velocity (v_a), air temperature (t_a), and globe temperature (t_g). A Delta HD32.3 thermal comfort meter was used for testing in indoor spaces; for outdoor spaces, a HOBO MX2301 outdoor temperature-humidity recorder was employed to measure air temperature and relative humidity, a HQZY-1 globe temperature recorder was used to test globe temperature, and a Kestrel 5500 portable weather station was applied to measure air velocity (Figure 5). The parameters and accuracy of the instruments are shown in Table 4.

Two elderly people over 60 years old live in the tested residence, which is a typical rural residential house in Xi’an. The house faces south with its back to the north, and the courtyard is narrow and long, approximately 24 m in length and 10 m in width. Typical activity spaces for elderly people: living room, bedroom, kitchen, courtyard, and the doorway of the north-side residence (shaded area where the elderly are more active) were selected as measurement points. The floor plan of the residence and the selected measurement points are shown in Figure 6.

Figure 6. Floor plan and photos of the tested residence with the location of the measurement points.

3. Results

3.1. Daily Activity Patterns

3.1.1. Spatiotemporal Trajectory of Daily Activities

According to the investigation, the daily activity time of the elderly starts between 5:00 and 6:00 and ends between 23:00 and 24:00. During 6:00–9:00 and 17:00–21:00, the proportion of elderly people staying outdoors exceeds 40%; in other time periods, the elderly mainly conduct indoor activities. The proportion of time spent on activities in semi-outdoor spaces is very small and relatively scattered, among which the time periods with relatively more activities in semi-outdoor spaces are from 8:00 to 9:00 and 15:00–17:00, accounting for approximately 7% of each respective time period (Figure 7).

Figure 7. Distribution of indoor and outdoor daily activities in summer.

Through the above analysis, it can be concluded that the daily activities of elderly people in summer have temporal characteristics: their activities in outdoor spaces are mainly concentrated between 6:00 and 9:00 and 17:00–21:00, and they stay indoors for activities during all other time periods.

3.1.2. Content and Classification of Daily Activities

According to the daily activity questionnaire survey, the daily activity contents of the rural elderly in Xi’an are shown in Table 6. To facilitate subsequent statistical analysis, these activities were categorized into nine types based on the characteristics of the activity contents, including getting up, dining, family activities, leisure activities, siesta (i.e., afternoon nap, a common daily rest behavior among rural elderly in northern China), entertainment activities, labors, wellness activities, and sleeping, as detailed in Table 6.

Table 6. Classification of daily activity contents.

Content	Type
/	Getting up
Breakfast, lunch, dinner	Dining
Doing housework, looking after children, caring for family members	Family activities
Sunbathing, lying at rest, sitting at rest, walking and chatting	Leisure activities
/	Siesta
Playing mahjong, playing chess, watching TV, listening to the radio, using mobile phones, reading	Entertainment
Going to work, agricultural production activities, gardening activities, animal rearing	Labors
Dancing square dances, exercising, massaging	Wellness activities
	Sleeping

3.1.3. Spatiotemporal Differentiation Patterns of Daily Activities

With time periods as the independent variable, the distribution law of elderly people’s activity contents in the summer outdoor spaces within each time period was statistically analyzed, as shown in Figure 8 and Figure 9.

Figure 8. Heat map of the distribution of daily indoor activities.

Figure 9. Heat map of the distribution of daily outdoor activities.

The elderly’s daily activities in summer exhibit significant temporal and spatial characteristics: In terms of sleep, the peak period for getting up is 6:00–7:00 (with the highest proportion); the participation rate in siesta reaches 80% (12:00–15:00); and going to bed is concentrated between 21:00 and 23:00. All dining activities occur indoors and show the characteristics of typical three-meal time periods (7:00–8:00, 11:00–13:00, 18:00–19:00). Family activities are mainly conducted indoors (8:00–11:00), while outdoor family activities are distributed between 8:00 and 10:00 and 15:00–18:00. Leisure activities are mainly concentrated outdoors (16:00–21:00). Entertainment activities are mostly carried out indoors in scattered time periods (9:00–12:00, 14:00–18:00, 20:00–22:00). All physical labor takes place outdoors and shows a double-peak pattern in the morning and evening (7:00–10:00 and 16:00–19:00). Wellness activities are mainly conducted outdoors during dawn and dusk (6:00–8:00 and 19:00–21:00).

3.2. Thermal Environments and the Elderly’s Thermal Responses

3.2.1. Thermal Environments

Based on the results of the temporal and spatial trajectory of elderly people’s daily activities, indoor spaces and outdoor spaces are the main types of activity spaces for the elderly. Therefore, this section mainly statistically analyzes the distribution characteristics of four key parameters of the indoor and outdoor thermal environment, including air temperature (t_a), relative humidity (RH), globe temperature (t_g), and air velocity (v_a), as shown in Table 7. From the statistical results, it can be observed that the average indoor temperature in summer is lower than the outdoors; the average indoor relative humidity is higher than the outdoors; the average outdoor globe temperature is higher than the indoors; and the average outdoor air velocity is slightly greater than the indoors.

Table 7. Distribution characteristics of thermal environment parameters.

Space		ta/°C	RH/%	tg/°C	va/m/s
Indoor	Minimum value	22.10	30.60	22.00	0.00
	Maximum value	35.80	101.40	39.40	1.28
	Average value	27.34	67.89	27.93	0.19
	Standard deviation	3.96	20.64	4.50	0.20
Outdoor	Minimum value	27.00	28.40	28.70	0.00
	Maximum value	36.30	74.60	38.80	1.43
	Average value	31.87	55.42	32.29	0.37
	Standard deviation	2.08	11.79	1.91	0.26

3.2.2. Thermal Sensation and Thermal Acceptability

The thermal sensation and thermal acceptability votes of elderly people in indoor and outdoor activity spaces were statistically analyzed, and the statistical results are shown in Figure 10 and Figure 11.

Figure 10. Indoor thermal sensation and thermal acceptability voting statistics.

Figure 11. Outdoor thermal sensation and thermal acceptability voting statistics.

For indoor thermal sensation voting, 47% of respondents chose “Neutral”; the total proportion of respondents who selected “Slightly Warm”, “Warm”, and “Hot” was 41.6%; and only 7.4% of respondents chose “Slightly Cool”. In the thermal acceptability voting, 77.2% of respondents in total selected “Just Acceptable” and “Completely Acceptable”, while the proportion of those who chose “Just Unacceptable” and “Completely Unacceptable” was 22.8%. In the thermal preference voting, 48% of respondents desired a slightly cooler environment, 44.6% preferred no change, and 7.4% wished for a slightly warmer environment.

For outdoor thermal sensation voting, 32.7% of respondents chose “Neutral”; the total proportion of respondents who selected “Slightly Warm”, “Warm”, and “Hot” reached 65.5%; and only 1.8% of respondents chose “Slightly Cool”. In the thermal acceptability voting, 79.7% of respondents in total selected “Just Acceptable” and “Completely Acceptable”, while the proportion of those who chose “Just Unacceptable” and “Completely Unacceptable” was 20.3%. In the thermal preference voting, 87.6% of respondents desired a slightly cooler thermal environment, while 12.4% preferred no change to the current conditions.

Based on the above analysis, it can be concluded that for both indoor and outdoor spaces, elderly people have a low level of unacceptability toward the thermal environment. However, most elderly people still hope the environment could be cooler. Having lived in the area for a long time, elderly people have already adapted to the current ambient temperature, but they still expect further improvements to the residential thermal environment.

3.2.3. Thermal Response Models

For the indoor environment, the operative temperature (T_op) was selected to evaluate the thermal comfort of the indoor spaces. The calculation of T_op was performed in accordance with the Standard GB/T 50785 [31]. For the outdoor environment, the Universal Thermal Climate Index (UTCI) was adopted to assess the thermal comfort of the outdoor spaces for elderly people, as it is recognized as the most accurate indicator for studying the outdoor thermal environment in Xi’an [24]. The calculation of UTCI was conducted using the Rayman software. Detailed operations are provided in Section 4.2 of the Rayman manual. There are differences between the two indices in terms of capturing and omitting thermal environment parameters: T_op mainly focuses on air temperature and mean radiant temperature in a stable indoor environment, ignoring the impact of outdoor dynamic factors such as wind speed and solar radiation; although UTCI covers four core thermal environment parameters, there are essential differences in the treatment of radiation and wind speed compared with T_op—the mean radiant temperature (T_mrt) in UTCI is calculated by integrating solar radiation, surface long-wave radiation, and atmospheric long-wave radiation, while the mean radiant temperature in T_op is mainly derived based on the surface temperature of indoor envelope structures.

In the thermal acceptability voting, the ratings of “Completely Unacceptable” (−2) and “Just Unacceptable” (−1) were categorized as “dissatisfied”, the “Just Acceptable” (+1) and “Completely Acceptable” (+2) were categorized as “satisfied”, based on which the percentage of dissatisfaction was calculated (Equation (1)).

PD = {[N(−2) + N(−1)]/[N(−2) + N(−1) + N(+1) + N(+2)]} × 100%,

(1)

where N (−2) denotes the number of votes for “Completely Unacceptable”, N (−1) denotes the number of votes for “Just Unacceptable”, N (+1) denotes the number of votes for “Just Acceptable”, N (+2) denotes the number of votes for “Completely Acceptable”.

The temperature frequency method (BIN method) was adopted to segment the indoor operative temperature (T_op) and UTCI at intervals of 1 °C. Intervals with fewer than five votes were excluded. For each remaining interval, the average temperature (average T_op for indoor, average UTCI for outdoor), average thermal sensation, and average percentage of dissatisfaction were calculated. Subsequently, four correlation relationships were established: correlations between indoor T_op and thermal sensation, outdoor UTCI and thermal sensation, indoor T_op and percentage of dissatisfaction, and outdoor UTCI and percentage of dissatisfaction. The resulting fitting graphs are shown in Figure 12a,b, while the obtained regression equations and their corresponding R² values are presented in Table 8 and Table 9.

Figure 12. Indoor thermal comfort-related graphs: (a) Relationships between indoor T_op/outdoor UTCI and thermal sensation; and (b) relationships between indoor T_op/outdoor UTCI and PD.

Table 8. Regression equations of indoor operative temperature/outdoor UTCI and thermal sensation.

	Regression Equation	R2	Neutral Temperature/°C
Indoor	MTSV = 0.018 Top − 4.28	0.83	23.8
Outdoor	MTSV = 0.33 UTCI − 9.4	0.76	28.8

Table 9. Regression equations of indoor operative temperature/outdoor UTCI and percentage of dissatisfaction.

	Regression Equation	R2	Upper limit Temperature of 80% Acceptance/°C
Indoor	PD = 0.002 Top2 − 0.1 Top + 1.24	0.85	27.5
Outdoor	PD = 0.05 UTCI2 − 3.03 UTCI + 48.9	0.87	34.1

Based on the regression coefficients of the regression equations, elderly people show greater thermal sensation sensitivity in outdoor spaces than in indoor spaces during summer. In the fitting equations, setting the mean thermal sensation vote (MTSV) to zero enables the derivation of the estimated neutral temperature; using the linear regression equations, the measured neutral temperature for elderly people in rural Xi’an in summer is calculated as 23.8 °C for indoor spaces and 28.8 °C for outdoor spaces. It should be noted that the outdoor measured neutral temperature is obtained through calculation based on the fitting equation and does not fall within the range of actual measured data, so this value is for reference only.

According to the standards in ASHRAE 55, under normal conditions, at least 80% of respondents should feel thermally comfortable. Setting the percentage of dissatisfaction (PD) to 20%, the 80% acceptable temperature range for elderly people was calculated as ≤27.5 °C indoors and ≤34.1 °C outdoors.

3.3. Thermal Comfort Evaluations

3.3.1. Thermal Environment Conditions of Daily Activity Spaces

China’s national standard GB/T 50785 [31] explicitly stipulates that: “The measurement period for thermal environment shall be 24 to 48 h”. To accurately characterize the thermal environment conditions of various spaces where the elderly engage in their daily activities, continuous monitoring of indoor and outdoor thermal environment parameters was conducted for seven consecutive days in July, a typical summer month (from 22 to 28 July 2024). On this basis, continuous 24 h data that is representative of the typical summer weather in Xi’an was extracted for subsequent analysis. By comparing with the average temperatures recorded in Xi’an during July in previous summers, the data of a typical sunny day from 00:00 to 24:00 on 26 July 2024 was ultimately selected for analysis. For indoor thermal environment monitoring, continuous tests were carried out at 1 min intervals; for outdoor monitoring, the temperature-humidity recorder and globe temperature recorder operated at 1 min intervals, while the anemometer ran at 5 s intervals. This high-frequency testing was intended to collect sufficient data and ensure data accuracy.

During the thermal environment evaluation, the temperature frequency method (BIN method) was adopted: indoor and outdoor thermal environment parameters were segmented into 30 min intervals. For each interval, the average values of four parameters—air temperature (t_a), globe temperature (t_g), relative humidity (RH), and air velocity (v_a)—were calculated. These average values were then used to further compute the indoor operative temperature (T_op) and outdoor UTCI. Figure 13 and Figure 14 show the thermal environment test results of three indoor measurement point spaces and two outdoor measurement point spaces in summer.

Figure 13. Test results of four thermal environment parameters across different indoor spaces: (a) Air temperature; (b) globe temperature; (c) relative humidity; and (d) air velocity.

Figure 14. Test results of four thermal environment parameters across different outdoor spaces: (a) Air temperature; (b) globe temperature; (c) relative humidity; and (d) air velocity.

The test results of the indoor thermal environment show that the air temperature (t_a) in the kitchen is significantly higher than that in the bedroom and living room, with a maximum of 35 °C, which may be related to the kitchen facing south and receiving direct sunlight; the temperatures of the bedroom and living room are similar, fluctuating in the range of 28–30.5 °C. The temperatures of the three spaces all reach their peaks at 16:00, but the kitchen starts to heat up earlier (from 7:00). The variation trend of globe temperature (t_g) in the three spaces is basically consistent with that of air temperature (t_a). The relative humidity (RH) shows the characteristic of living room > bedroom > kitchen: the humidity is lower during the daytime (10:00–18:00) and gradually increases at night before stabilizing at approximately 70%. In terms of air velocity (v_a), the bedroom has the highest value (with a peak of 0.06 m/s, and remains relatively high from 9:30 to 23:00), which is presumably related to the use of electric fans; the kitchen ranks second (0–0.05 m/s); and the living room has the lowest value (0–0.02 m/s). The living room and bedroom are connected by a door. However, field measurements revealed that the temperature and relative humidity of the two spaces were similar, even when the air velocity in the bedroom was significantly higher than that in the living room. This is presumed to be due to residents typically keeping the bedroom door closed or installing a door curtain for shielding.

The test results of the outdoor thermal environment show that the air temperature (t_a) in the courtyard is significantly higher than that at the doorway (facing north), with a peak of approximately 41 °C (11:30–15:30), while the temperature at the doorway fluctuates slightly (26–36 °C). This may be due to the large solar altitude angle in summer: the measurement point in the courtyard is exposed to direct sunlight, resulting in higher temperature and greater fluctuations, whereas the measurement point at the north-facing doorway is shaded by the building and not exposed to direct sunlight. The variation trend of globe temperature (t_g) in the two spaces is consistent with that of air temperature (t_a): the courtyard has a globe temperature (t_g) peak of 56 °C from 12:30 to 14:30, while the maximum globe temperature (t_g) at the doorway is only 36 °C. The relative humidity (RH) at the doorway (35–60%) is consistently higher than that in the courtyard (15–34%), but their variation trends are synchronized. The difference in air velocity (v_a) between the two spaces is small (0–0.55 m/s), and the air velocity (v_a) at the doorway is slightly higher than that in the courtyard.

3.3.2. Evaluations of Thermal Comfort in Daily Activity Spaces

Using the measured data, the indoor operative temperature (T_op) for each indoor measurement point and the outdoor Universal Thermal Climate Index (UTCI) for each outdoor measurement point were calculated respectively. Based on the 80% thermal acceptability range obtained from previous studies, the measured indoor and outdoor spaces were evaluated separately, and the resulting evaluation results are presented in Figure 15 and Figure 16.

Figure 15. Evaluation of indoor thermal environment: (a) Bedroom; (b) kitchen; and (c) living room.

Figure 16. Evaluation of outdoor thermal environment: (a) Courtyard; and (b) doorway.

For the three indoor measurement point spaces (bedroom, living room, and kitchen), the operative temperature (T_op) in all spaces over 24 h was higher than the 80% thermal acceptability range (27.5 °C), indicating an overall poor thermal environment quality. The minimum operative temperature (T_op) of the bedroom was 28 °C, which was 1.5 °C higher than the upper limit of the 80% thermal acceptability range, and the maximum was 30.5 °C, which was 3 °C higher than the upper limit of the 80% thermal acceptability range. The minimum operative temperature (T_op) of the living room was 28 °C, 0.5 °C higher than the upper limit of the 80% thermal acceptability range, and the maximum was 30.3 °C, 3.2 °C higher than the upper limit of the 80% thermal acceptability range. The minimum operative temperature (T_op) of the kitchen was 28.9 °C, 1.4 °C higher than the upper limit of the 80% thermal acceptability range, and the maximum was 35 °C, 7.5 °C higher than the upper limit of the 80% thermal acceptability range. The thermal environment conditions of the bedroom and living room were relatively similar; however, the kitchen faces south, is directly exposed to sunlight, and has no fan implemented, unlike the bedroom and living room. As a result, the operative temperature (T_op) of the kitchen was higher than that of the other two indoor spaces at the same time, leading to an even worse thermal environment quality in the kitchen.

For the two outdoor measurement point spaces (courtyard and doorway), the UTCI of the courtyard was below the upper limit of the 80% thermal acceptability range (34.1 °C) for approximately 79.2% of the day, while the UTCI of the north-facing doorway was below this upper limit for about 70.8% of the day. The maximum UTCI of the courtyard was approximately 44.8 °C, 10.7 °C higher than the upper limit of the 80% thermal acceptability range, and its thermal environment fell within the 80% thermal acceptability range before around 11:30 and after 16:30. The maximum UTCI of the doorway was approximately 36.2 °C, 2.1 °C higher than the upper limit of the 80% thermal acceptability range, and its thermal environment was within the 80% thermal acceptability range before around 11:20 and after 18:30. The overall UTCI of the doorway was lower than that of the courtyard, which may be attributed to the fact that the doorway is covered by a large area of building shade, whereas the courtyard is more exposed to solar radiation.

Overall, the quality of the outdoor shaded areas’ thermal environment is significantly better than that of the indoor thermal environment. Outdoor shaded areas reduce the mean radiant temperature and UTCI value by blocking solar radiation, thereby improving the thermal comfort level; while indoor thermal discomfort is mainly driven by the high operative temperature. The indoor thermal environment quality is relatively poor, with the kitchen having the worst thermal environment quality among all indoor spaces. Accordingly, the courtyard is a relatively comfortable area for elderly people to carry out daily activities except during the time period of 11:30–16:30, while the doorway is a relatively comfortable area except during 11:30–18:30.

4. Discussion

4.1. Daily Activity Patterns of the Elderly

Table 10 analyzes the classification of daily activity contents and the temporal–spatial differentiation patterns of the elderly across different thermal comfort studies. Compared with the daily activity contents of urban elderly people [7,8,9,10,32], rural elderly people [14,15] engage in fewer shopping activities and medical care activities; in addition, rural elderly people participate in physical labor such as field work, which is an activity not observed among urban elderly people. Regarding the temporal–spatial differentiation patterns, the wake-up and bedtime of rural elderly people [15] are relatively similar to those of urban elderly people [7,8,9,32], and most of them have the habit of taking a noon break, which is related to the physiological acclimatization of the elderly. Both urban and rural elderly people conduct more outdoor activities in the morning and afternoon, and overall, indoor activities occur more frequently than outdoor activities. However, rural elderly people still engage in a large number of outdoor activities such as health maintenance activities and leisure activities, after 19:00, while urban elderly people mostly stay at home for activities after 19:00. This difference may be attributed to the distinct building forms between rural residences and urban residential buildings, as well as the different public space forms between rural and urban areas. Furthermore, due to the income gap between urban and rural elderly people and their spending habits, rural elderly people rarely use electrical appliances such as air conditioners for indoor thermal environment regulation. Therefore, the difference in outdoor activities in the evening may be because rural elderly people tend to go to more comfortable outdoor spaces rather than adjust the uncomfortable indoor spaces.

Table 10. Classification of daily activity contents and patterns of temporal–spatial differentiation of elderly people in different studies.

City	Climate Zone	Region	Time & Space	Contents	Patterns of Temporal–Spatial Differentiation	Reference
Xi’an	Cold	Rural	Indoor & Outdoor (Summer)	Waking up; Dining; Family activities; Leisure activities; Siesta; Entertainment; Labors; Wellness activities; Sleeping	Waking up at 6:00–7:00, sleeping at 21:00–23:00; approximately 80% of the elderly take a noon break. Family activities are mainly indoor (8:00–11:00); leisure activities are mainly concentrated outdoors (16:00–21:00); entertainment activities are mostly conducted indoors with scattered time; physical labor is all outdoor, distributed at 7:00–10:00 and 16:00–19:00; wellness activities are mainly outdoor at 6:00–8:00 and 19:00–21:00.	This study
Chong-qing	Hot-summer & Cold-winter	Rural	Outdoor (Whole year)	Living-related activities; Commuting-related activities; Occasional activities	/	[14]
Ningbo	Hot-summer & Cold-winter	Rural	Outdoor (Spring & Summer)	Health-related activities; Social-related activities; Living-related activities; Entertainment-related activities	Peak outdoor activities occur at 9:00, 15:00 and 19:00	[15]
Nanjing	Hot-summer & Cold-winter	Urban	Indoor & Outdoor (Whole Year)	Sleeping or napping; Housework; Personal affairs; Leisure; Work; Shopping	Wake-up time is concentrated around 6:00; noon break time is concentrated at 12:00–14:00; sleeping time is concentrated at 20:00–22:00. Time spent on housework is long, mainly at home; shopping activities are concentrated at 8:00–10:00; leisure activities also last for a long time, with peak periods at 8:00–10:00, 14:00–17:00 and 19:00–21:00 (mostly outdoor in the morning and afternoon, mostly at home in the evening).	[7]
Xi’an	Cold	Urban	Outdoor (Autumn)	Family responsibility-based activities; Living-based activities; Survival-based activities	Activities with strong purposes are mostly conducted when going out in the morning, noon and evening; activities with strong entertainment, elderly service activities and family responsibility-based activities are mostly conducted in the morning and afternoon.	[8]
Beijing	Cold	Urban	Indoor & Outdoor (Whole Year)	Work; Housework; Shopping; Personal affairs; Sleeping or napping; Leisure & entertainment (puzzle-solving, mood-soothing, health-building, communication, public welfare)	Waking up before 7:00, sleeping before 22:00, with the habit of noon break. Outdoor leisure activities are concentrated at 6:00–10:00 and 16:00–18:00; activities after 19:00 are mainly indoor.	[9]
Nanjing	Hot-summer & Cold-winter	Urban	Outdoor (Spring & Summer)	Social activities; Leisure activities; Daily living activities; Natural exposure activities	/	[10]
Shen-yang	Severe cold	Urban	Indoor & Outdoor (Winter)	Life-essential activities; Health-building & fitness activities; Cultural & entertainment activities; Social interaction activities; Medical care activities	Wake-up time is concentrated at 5:00–6:00; noon break time is at 12:00–14:00; most fall asleep after 19:00. Health-building activities mainly occur at 6:00–7:00, 8:00–10:00 and 14:00–16:00; cultural & entertainment activities mainly occur at 7:00–11:00, 13:00–17:00 and 18:00–19:00; social interaction activities mainly occur at 8:00–12:00 and 14:00–16:00. The proportion of indoor activities is significantly higher than that of outdoor activities.	[32]

4.2. Adaptive Thermal Comfort of the Elderly in Different Daily Activity Spaces

Table 11 analyzes the indoor and outdoor neutral temperatures of the elderly in summer across different thermal comfort studies. In cold regions, the indoor neutral temperature (23.8 °C) for rural elderly individuals in Xi’an is lower than those in urban areas of Baoding (27.25 °C) [25] and Taiyuan (27.4 °C) [33], and is slightly lower than that of Xi’an (24.1 °C) [34]; in contrast, their outdoor neutral temperature (28.8 °C) is significantly higher than those in urban areas of Xi’an (22.1 °C) [23], Lhasa (19.6 °C) [35], and Dalian (22.6 °C) [36]. It can be concluded that, compared with urban elderly people, rural elderly people in cold regions generally have higher thermal tolerance; their thermal comfort requirements for the indoor thermal environment are relatively similar, yet their thermal tolerance for the outdoor environment is much higher. This may be related to their higher frequency of outdoor activities and habits of physical labor; moreover, the elderly have lived in the local area for a long time, leading to high adaptability to the local thermal environment.

Table 11. The neutral indoor and outdoor temperatures of elderly people in summer in different thermal comfort studies.

City	Climate Zone	Region	Indoor Neutral Temperature/°C	Outdoor Neutral Temperature/°C	Reference
Xi’an	Cold	Rural	23.8	28.8 (UTCI)	This study
Xi’an	Cold	Urban	24.1	/	[34]
Baoding	Cold	Urban	27.25	/	[25]
Taiyuan	Cold	Urban	27.4	/	[33]
Shanghai	Hot-summer & Cold-winter	Rural	29.8	/	[37]
Changsha	Hot-summer & Cold-winter	Urban	26.3	/	[38]
Chengdu	Hot-summer & Cold-winter	Urban	25.5	/	[39]
Taiwan	Hot-summer & Warm-winter	Urban	25.2	/	[40]
Northern Guangxi	Hot-summer & Warm-winter	Rural	27.06	/	[41]
Xi’an	Cold	Urban	/	22.1 (UTCI)	[23]
Lhasa	Cold	Urban	/	19.6 (PET)	[35]
Dalian	Cold	Urban	/	22.6 (PET)	[36]
Changchun	Severe cold	Urban	/	5.8 (UTCI)	[42]
Shanghai	Hot-summer & Cold-winter	Urban	/	24.29 (PET)	[43]
Chengdu	Hot-summer & Cold-winter	Urban	/	27.01 (PET)	[44]
Guangzhou	Hot-summer & Warm-winter	Urban	/	25.96 (UTCI)	[45]

In cross-regional comparisons, it is found that the summer indoor neutral temperature in Xi’an is generally lower than the indoor neutral temperatures in the hot-summer and cold-winter zone as well as the hot-summer and warm-winter zone [37,38,39,40,41], while the outdoor neutral temperature in Xi’an is much higher than that in the severe cold zone [42] and slightly higher than those in the hot-summer and cold-winter zone and the hot-summer and warm-winter zone [43,44,45]. Thus, it can be inferred that, as a representative city of cold regions, elderly people in Xi’an have relatively weaker adaptability to high indoor temperatures in summer but higher thermal tolerance for the outdoor environment. It should be noted that the prediction accuracy of PET (Physiological Equivalent Temperature) and UTCI is better in warm and relatively hot thermal environments [46], and UTCI is a more suitable research indicator for the hot-summer and cold-winter zone as well as the hot-summer and warm-winter zone [47]. Therefore, the comparison of outdoor neutral temperatures may be affected by the differences in evaluation indicators adopted in various studies.

Table 12 analyzes the 80% acceptable temperature ranges for elderly people in indoor and outdoor environments during summer across different thermal comfort studies. In cold regions, the upper limit of the 80% indoor thermal acceptable temperature range (27.5 °C) for rural elderly individuals in Xi’an is lower than that in urban areas of Xi’an (30.3 °C) [34], Baoding (28.97 °C) [25], and Taiyuan (28.8 °C) [33]. This indicates that the rural elderly people have a slightly lower acceptance of high indoor temperatures compared to their urban counterparts. However, the upper limit of the 80% outdoor acceptable temperature range (34.1 °C) for the rural elderly in Xi’an is higher than those in urban areas of Xi’an (30 °C) [23], Lhasa (25 °C) [35], and Dalian (27.08 °C) [36], suggesting that the rural elderly people exhibit a higher outdoor thermal tolerance.

Table 12. The upper limit of 80% acceptable indoor and outdoor temperatures of elderly people in summer in different thermal comfort studies.

City	Climate Zone	Region	Indoor Upper Limit of 80% Acceptable Temperature/°C	Outdoor Upper Limit of 80% Acceptable Temperature/°C	Reference
Xi’an	Cold	Rural	27.5	34.1	This study
Xi’an	Cold	Urban	30.3	/	[34]
Baoding	Cold	Urban	28.97	/	[25]
Taiyuan	Cold	Urban	28.8	/	[33]
Shanghai	Hot-summer & Cold-winter	Rural	31	/	[37]
Changsha	Hot-summer & Cold-winter	Urban	29.9	/	[38]
Chengdu	Hot-summer & Cold-winter	Urban	27.2 (90%)	/	[39]
Taiwan	Hot-summer & Warm-winter	Urban	27.1	/	[40]
Northern Guangxi	Hot-summer & Warm-winter	Rural	29.58	/	[41]
Xi’an	Cold	Urban	/	30 (UTCI) (Yearly) (90%)	[23]
Lhasa	Cold	Urban	/	25 (PET)	[35]
Dalian	Cold	Urban	/	27.08 (PET)	[36]
Changchun	Severe cold	Urban	/	21.4 (UTCI) (Yearly)	[42]
Shanghai	Hot-summer & Cold-winter	Urban	/	29.64 (PET) (90%)	[43]
Chengdu	Hot-summer & Cold-winter	Urban	/	29.32 (PET) (90%)	[44]
Guangzhou	Hot-summer & Warm-winter	Urban	/	29.89 (UTCI)	[45]

In cross-regional comparisons, the upper limit of the 80% indoor thermal acceptable temperature range (27.5 °C) for elderly people in Xi’an is slightly higher than the upper limits in the hot-summer and cold-winter zone as well as the hot-summer and warm-winter zone [37,38,39,40,41]. Meanwhile, the upper limit of the 80% outdoor acceptable temperature range (34.1 °C) is much higher than that in the severe cold zone [42], and slightly higher than those in the hot-summer and cold-winter zone and the hot-summer and warm-winter zone [43,44,45]. These results demonstrate that elderly people in cold regions have relatively weaker adaptability to high indoor temperatures in summer, while their outdoor thermal tolerance is slightly higher. The comparison results of the upper limits of the comfortable temperature ranges are basically consistent with those of the neutral temperatures, and the inferred reasons for this consistency are also similar to those for the neutral temperatures: rural elderly people often engage in physical labor outdoors, which enhances their adaptability to the outdoor thermal environment.

4.3. Evaluations of Thermal Comfort Based on Spatiotemporal Differentiation of Daily Activities

In the existing evaluation studies on the indoor thermal environment, most studies have not directly applied the measured thermal comfort models for the elderly to evaluate the measured thermal environment data after developing these models. For instance, in 2024, Wu conducted a study on the thermal comfort of the elderly in rural residences in northern Guangxi. First, the measured data of the summer indoor thermal environment of buildings with different structures were evaluated in accordance with the standard GB/T 50785 [31], and the main factors causing the unsatisfactory thermal environment were identified. However, no comparative evaluation was performed with the thermal sensation model for the elderly obtained subsequently [41]. Additionally, in 2023, Jiang carried out a study on the indoor thermal comfort evaluation of elderly care buildings in Changsha. Initially, the measured data of typical elderly care buildings were evaluated based on the standard GB/T 18883. For the thermal comfort model for the elderly obtained later, building modeling and simulation were conducted using modeling and simulation software such as Design Builder, and then the model was applied for evaluation [38].

In existing evaluations of outdoor thermal comfort, the ENVI-met simulation software is mostly used to study the impacts of different plants, pavements, and other factors on thermal comfort, while evaluations based on all-day measured data are relatively scarce. For example, in 2024, Wang conducted an outdoor thermal comfort evaluation of the Lilong blocks in Wuhan. Based on the established relationship between thermal sensation and PET, the ENVI-met simulation software was used to simulate and determine that the period from 12:00 to 16:00 was the main thermal discomfort period in the blocks [48]. In the same year, Song carried out a study on age-friendly outdoor thermal comfort in urban parks of Jinan. First, the neutral PET and 90% acceptable PET range for the elderly were obtained through surveys, and then the ENVI-met simulation software was applied to verify the advantages and disadvantages of various optimization ideas [49]. In 2025, Li [50] selected one residential area each in Xi’an and Zhenjiang for outdoor thermal environment testing, and also used ENVI-met simulation to calculate the health and comfort of outdoor thermal environments created by different plant configurations, underlying surfaces, and water bodies. In the same year, in a study on the thermal comfort evaluation of riverside spaces in Tianjin conducted by Wei et al., the winter and summer thermal neutral temperatures of mixed populations were studied and compared with measured data, and the time periods within the thermal neutral range throughout the day were evaluated [51].

In this study, the calculated indoor and outdoor thermal comfort models for elderly people were directly compared with the on-site continuous monitoring data to evaluate the actual thermal environment. The time periods during which different indoor and outdoor spaces were within the thermal comfort range and the time periods when they were insufficiently comfortable throughout the day were obtained. These results can provide guidance for elderly people to engage in more comfortable and healthier daily activities, offer more accurate references for creating age-friendly residential environments, and supplement different perspectives in the current research on spatial thermal environment evaluation.

4.4. Limitations and Future Work

In this study’s research on daily activities and thermal comfort, samples only included healthy elderly people, excluding those with mobility impairments or illnesses, which prevents comparing their thermal comfort differences. In addition, this paper only analyzes the thermal environment data within a limited time period, and rural residential samples lack diversity.

In future research, supplementary studies should expand samples to include elderly people with mobility impairments or illnesses (≥30% proportion) for comparative analysis of thermal comfort with healthy groups, improving the results’ generalizability. Extension of spatial and temporal monitoring is also requested to cover 3–5 different types of rural residences and prolong the monitoring period, including various weather types. Improvements in these two aspects can help provide a more comprehensive scientific basis for age-friendly building design.

Furthermore, current research on the refined “time-segmented and zone-specific” thermal comfort of urban elderly populations remains incomplete and requires supplementation. Given that the regular patterns of daily activities vary across seasons, the scope of relevant research could be expanded to cover diverse regions and multiple seasons.

5. Conclusions

This study centers on adults aged 60 and over residing in rural areas of Xi’an, Shaanxi Province, China, and investigates adaptive thermal comfort and the evaluation of summer thermal environments by examining the spatiotemporal variation characteristics of their daily activities. Key conclusions derived from the study are as follows:

• During summer, the daily activities of rural elderly people in Xi’an primarily occur in indoor and outdoor spaces, with relatively few activities in semi-outdoor areas, and these activities exhibit a distinct spatiotemporal differentiation pattern. The content of rural elderly people’s daily activities differs slightly from that of urban elderly people: they engage in fewer shopping and medical care activities, but more physical labor. In terms of time, the daily schedules of rural and urban elderly people are largely similar: indoor activities are more frequent than outdoor ones, and both groups conduct more outdoor activities in the morning (6:00–9:00) and afternoon (17:00–19:00). However, rural elderly people participate in more outdoor activities after 19:00 compared to urban elderly people. This difference may be attributed to factors including distinct building forms between rural residences and urban residential buildings, different public space forms in rural and urban areas, and elderly people’s electrical appliance usage habits driven by the urban–rural income gap.
• Based on the indoor and outdoor adaptive thermal comfort models for elderly people established in this study, the measured neutral temperature for indoor operative temperature is 23.8 °C, with the upper limit of 80% thermal acceptability at 27.5 °C; the measured neutral temperature for outdoor UTCI (Universal Thermal Climate Index) is 28.8 °C, and the upper limit of 80% thermal acceptability is 34.1 °C. Compared with urban elderly people in the same cold region, rural elderly people generally exhibit higher thermal tolerance: their thermal comfort requirements for the indoor thermal environment are relatively similar, whereas their thermal tolerance for the outdoor environment is significantly higher. This may be attributed to two factors: rural elderly people have relatively fewer indoor thermal environment adjustment means compared with urban elderly people, and rural elderly people exhibit higher outdoor activity frequency and physical labor habits. Furthermore, compared with other thermal zones, elderly people in Xi’an, a representative area in the cold zone, exhibit relatively weaker adaptability to high indoor temperatures in summer.
• In elderly people’s summer residential environment, the outdoor shaded areas’ environment remains within the 80% thermal acceptability range for a longer duration than that of indoors. The courtyard (except 11:30–16:30) and doorway (except 11:30–18:30) are relatively comfortable areas for elderly people’s daily activities. Indoor operative temperature exceeds the upper limit of the 80% thermal acceptability range throughout the day, with the kitchen exhibiting the worst thermal environment. These results can guide elderly people in conducting more comfortable and healthy daily activities, provide more accurate references for developing age-friendly residential environments, and address gaps in current spatial thermal environment evaluation research, including the lack of direct evaluation using measured data and precise “temporal—zonal” evaluation.