A Review on Adaptive Thermal Comfort of Ofﬁce Building for Energy-Saving Building Design

: The thermal environment quality of ofﬁce buildings has an important role because thermal comfort is directly related to human productivity. Thermal comfort conditions are inﬂuenced by climate, location, and the built environment; hence, comfort standards are required to assist building designers in creating a comfortable indoor environment for building occupants. In this context, the present study analyzes the adaptive thermal comfort studies conducted in ofﬁce buildings from various countries. A large number of research articles selected from the Scopus database were considered for this study. Based on the analysis, outdoor climatic conditions have a greater inﬂuence on indoor thermal conditions in naturally ventilated than in air-conditioned ofﬁce buildings. The temperature required for comfort is as low as 17.6 ◦ C and as high as 31.2 ◦ C in naturally ventilated buildings. An adaptive comfort equation for naturally ventilated and air-conditioned ofﬁce buildings has also been proposed to predict the indoor comfort temperature. Various studies show that a substantial amount of energy can be saved by changing the set point and natural ventilation. Furthermore, this study successfully provides hearty evidence that there is a need for climate-speciﬁc standards on thermal comfort for energy-efﬁcient design development because existing comfort standards might not be applicable to all climates.


Overview
People spend eighty-seven percent of their time in indoor climates [1]. As a result, research on indoor thermal comfort has dramatically increased. Thermal comfort is defined as "a condition of mind that expresses satisfaction with the thermal environment" [2], which indicates that thermal comfort is subjective. The most common method for defining thermal comfort conditions is through subjective evaluation, which is performed by monitoring each subject's thermal feelings, preferences, and physical and personal comfort factors over a group of subjects in a field or a lab [2]. Thermal comfort studies conducted over the years suggest that the thermal comfort condition is influenced by climate, geographical location, and the built environment [3][4][5][6]. Thermal comfort standards are required to assist building designers in providing a thermally comfortable indoor environment for building occupants [7,8]. Thermal discomfort reduces human productivity [9][10][11], so thermal sensation may play an important role, particularly in office buildings. This has prompted scientists and researchers to conduct thermal comfort studies in office buildings around the world, covering various climates and built environments and proposing an adaptive model [12][13][14][15][16][17][18][19][20][21][22][23][24].

Adaptive Thermal Comfort
The adaptive approach to thermal comfort considers how individuals interact with their thermal environment [3,6,25]. A model that relates indoor design temperatures or acceptable temperature ranges to outdoor temperature is known as an adaptive model [2]. According to the adaptive method, if a change causes discomfort, people will respond in a way that tends to make them feel more comfortable [3,6,7,[26][27][28][29][30]. The adaptive approach to thermal comfort was developed from field studies of people in daily life and emphasizes to the interaction between people and their thermal environment [3,6,25,31]. This approach considers how people interact with their thermal surroundings. The adaptable approach, particularly in free-running buildings, allows building designers to predict the indoor temperature that building occupants are most likely to find comfortable [7]. Office workers have limited adaptation opportunities compared to those in other building types because of the shared use of space and organizational culture [32,33]. As a result, the type of building, in terms of providing adaptive opportunities to users, can have a significant contextual influence on occupants' thermal experiences. However, it is claimed that the thermal comfort recommendations provided by current international standards, such as ASHRAE Standard 55 [2] and EN 15251 [34], are applicable to a variety of building types. Given that the adaptive comfort recommendations used in the current standards, such as ASHRAE RP-884 [35] and SCATS [27], were created from field data almost acquired from offices. the following three categories of adaptation [5,6,36] determine the flexibility and scope of adaptation. a) Behavioral adaptation: Conscious and unconscious actions taken by subjects, which in turn change the body's mass and heat fluxes. It functions on three levels: personal, technological, and cultural. b) Physiological adaptation: Long-term exposure causes changes in occupants, such as the prevailing climate having an impact on the occupants themselves. Again, it comes in two types: general and acclimation. c) Psychological adaptation: It is a result of the subject's prior experiences and is influenced by one's socioeconomic and cultural environment. It is largely driven by expectation and perception.
It is essential to note that all three of the aforementioned adaptations occur simultaneously, making it challenging to predict how each would affect the situation separately [36][37][38]. Several adaptive comfort equations have been developed, some as part of international comfort standards [2,31,34] and some for specific climatic regions [17,21,39,40]. However, we do not know the difference and similarity in the trend of the slope of adaptive models from different countries. It is imperative to study the different literature to know how the comfort temperature is influenced by the outdoor temperature and the clothing level.

Energy Saving
In most countries, buildings account for at least 40% of total energy consumption [41]. Office buildings use a lot of energy during construction and operation, which has a negative impact on the environment as well as the health and comfort of users [42,43]. In order to provide a secure and comfortable living environment, the building is essential. The heating, ventilation, and air-conditioning (HVAC) systems use around 50% of the energy in a building to create a more comfortable indoor thermal environment [44]. Aside from thermal comfort, energy consumption is an important factor to consider in HVAC systems because it determines the system's operating costs as well as its environmental impact [8,26,45,46]. The indoor temperature set points have the greatest influence on thermal comfort. In terms of energy demand, consumption, and occupant thermal comfort, the indoor temperature set point is the most important parameter [43,45,[47][48][49][50][51][52]. If people can remain comfortable over a wider range of conditions, significant energy can be saved by relaxing thermal comfort standards [53,54]. Thus, it is imperative to study energy saving by application of adaptive models from different studies to see the potential of an indoor set point on energy saving in air-conditioning buildings.

Objectives of the Study
The main objective of this study is to analyze the adaptive thermal comfort studies conducted in office buildings across the world. Specific objectives are:

•
To analyze the comfort temperature by regression equation; • To analyze the relation of comfort temperature (seasonal differences) with indoor or outdoor temperature; • To analyze the relation between clothing insulation and outdoor air temperature; • To analyze the energy-saving potential by thermal adaptation.

Methodology
To carry out this study, the keyword "thermal comfort study in office building" is used to search the Scopus database. The present study gives detailed insight into the thermal comfort studies conducted in office buildings across the world. The following procedure has been adopted in this research.

Paper Collection
A large number of papers were collected from the Scopus database for a systematic review. Related articles were searched using keywords from the article abstract, title, and keyword list, as shown in Table 1. Then, tables were created by extracting information from the research articles such as sample size, survey time, geographic location, climate, office operating style, comfort temperature, average clothing level, average outdoor temperature, etc. It was found that several research articles did not provide the seasonal comfort temperature in the text. As a result, the given figure was used to extract the comfort temperature.

Research Area Keywords
Adaptive thermal comfort Thermal comfort; Thermal environment; Thermal adaptation; Thermal perception; Adaptive approach; Productivity loss; Comfort temperature; Seasonal differences; Regional differences Energy issue Energy saving; Energy conservation; Energy performance; Adaptive control algorithm; Natural ventilation; Thermostat management; Set points The review process identified 120 papers suitable for a systematic review related to the topic. The line graph in Figure 1 displays the research that has been reviewed chronologically in this article. The considerable recent publication reveals significant importance of the topic.

Outdoor Climatic Data Collection
Outdoor temperature data were not given in some of the research papers and thus an online weather data source was used to extract the data from the studied location [55].

Mode Definition
Different studies have different modes of office, as defined below.

•
Naturally Ventilated (NV): An office is designed to function in free-running (FR) mode throughout the year. • Free running (FR): An office building which is naturally ventilated or has an HVAC system, but during the study period, either the heating (HT) or cooling (CL) systems were turned off. • Heating, ventilation, and air conditioning (HVAC): A heating and cooling system is installed in an office, and either system was turned on during the study period. • Mixed Mode (MM): MM building which is classified as; "concurrent" (where natural ventilation and HVAC take place simultaneously), "zoned" (when natural ventilation and HVAC occur in different zones of buildings), and "change-over" (natural ventilation and HVAC take place in the same location at various times) [20,24].
Energies 2023, 16, x FOR PEER REVIEW 4 of Figure 1. Number of articles used in this research.

Outdoor Climatic Data Collection
Outdoor temperature data were not given in some of the research papers and th an online weather data source was used to extract the data from the studied location [5

Mode Definition
Different studies have different modes of office, as defined below.

•
Naturally Ventilated (NV): An office is designed to function in free-running (F mode throughout the year. • Free running (FR): An office building which is naturally ventilated or has an HVA system, but during the study period, either the heating (HT) or cooling (CL) system were turned off.

•
Heating, ventilation, and air conditioning (HVAC): A heating and cooling system installed in an office, and either system was turned on during the study period.

•
Mixed Mode (MM): MM building which is classified as ; "concurrent" (where natur ventilation and HVAC take place simultaneously), "zoned" (when natural ventil tion and HVAC occur in different zones of buildings), and "change-over" (natur ventilation and HVAC take place in the same location at various times) [20,24].

Data Analysis
The collected data from various papers were analyzed by making a table and scatt plot. This study examined the regression equations and adaptive thermal comfort mode in various office operation modes. Very few studies provided adaptive thermal comfo models. The relation of comfort temperature with indoor and outdoor temperature w analyzed by making the scatter plot. The relation of comfort with the clothing level w analyzed, as clothing is a major variable for people in order to maintain their therm comfort. Besides thermal comfort, energy consumption is an important factor to consid in HVAC systems because it determines the system's operating costs as well as its env ronmental impact [8,26,45,46]. The invention of air conditioning enabled modern buil ings to regulate their indoor climate regardless of the outside conditions [32]. So, the e ergy-saving potential through the application of the adaptive model was analyzed in va

Data Analysis
The collected data from various papers were analyzed by making a table and scatter plot. This study examined the regression equations and adaptive thermal comfort models in various office operation modes. Very few studies provided adaptive thermal comfort models. The relation of comfort temperature with indoor and outdoor temperature was analyzed by making the scatter plot. The relation of comfort with the clothing level was analyzed, as clothing is a major variable for people in order to maintain their thermal comfort. Besides thermal comfort, energy consumption is an important factor to consider in HVAC systems because it determines the system's operating costs as well as its environmental impact [8,26,45,46]. The invention of air conditioning enabled modern buildings to regulate their indoor climate regardless of the outside conditions [32]. So, the energy-saving potential through the application of the adaptive model was analyzed in various studies. Since natural ventilation is another efficient way to reduce the amount of energy used by buildings, the potential of energy saving through the application of natural ventilation was analyzed through different studies.

Outdoor and Indoor Thermal Conditions
In heated (or cooled) mode, the closed skin of the structure and the functioning of the heating and cooling systems inside it decouple the indoor temperature from the exterior temperature. Whereas, in free-running mode, the indoor temperature is connected to the outdoor temperature through the building's fabric. The comfort temperature, indoor or globe temperature, outdoor temperature, and number of respondents from different field studies from all over the world has been tabulated in Table 2. The relation of the globe or indoor temperature and the outdoor temperature in naturally ventilated or free-running and air-conditioned office buildings is plotted in Figure 2. Studies revealed a strong correlation (R 2 = 0.72) between indoor or globe temperature and outdoor temperature in naturally ventilated buildings, as indicated in Figure 2; however, the correlation is much weaker in air-conditioned buildings. This indicates that outdoor temperature is likely to affect the thermal sensation and comfort temperature in NV buildings, as mentioned by Dhaka et al. [56]. Whereas, in air-conditioning buildings, the indoor climate regulates irrespective of outdoor conditions [32]. The range of outdoor temperature was 6.7 to 38 • C and indoor temperature was 21.8 to 30.2 • C in other types (HT, CL, EC, and MM) of buildings. Whereas, in FR or NV buildings, the range of outdoor temperature was 13.3 to 34 • C and the indoor temperature was 16.3 to 31.9 • C. This suggests that the mean indoor temperature in naturally ventilated buildings varies more from winter to summer. Subjects in naturally ventilated buildings are more responsive to their environments in order to maintain thermal comfort by changing their clothing level, opening windows and doors, and using fans. and air-conditioned office buildings is plotted in Figure 2. Studies revealed a strong correlation (R 2 = 0.72) between indoor or globe temperature and outdoor temperature in naturally ventilated buildings, as indicated in Figure 2; however, the correlation is much weaker in air-conditioned buildings. This indicates that outdoor temperature is likely to affect the thermal sensation and comfort temperature in NV buildings, as mentioned by Dhaka et al. [56]. Whereas, in air-conditioning buildings, the indoor climate regulates irrespective of outdoor conditions [32]. The range of outdoor temperature was 6.7 to 38 °C and indoor temperature was 21.8 to 30.2 °C in other types (HT, CL, EC, and MM) of buildings. Whereas, in FR or NV buildings, the range of outdoor temperature was 13.3 to 34 °C and the indoor temperature was 16.3 to 31.9 °C. This suggests that the mean indoor temperature in naturally ventilated buildings varies more from winter to summer. Subjects in naturally ventilated buildings are more responsive to their environments in order to maintain thermal comfort by changing their clothing level, opening windows and doors, and using fans.

Relation between Thermal Sensation and Indoor Temperature
A thermal sensation vote (TSV) based on the occupant's subjective rating of the occupied thermal environment using the ASHRAE 7-point scale can be used to determine their thermal comfort. Many studies have used the regression method to determine the comfort or neutral temperature [56,62,75]. The point at which the thermal sensation vote is zero or four is used to determine the comfort temperature. The regression method has traditionally been used to determine the comfort temperature by relating TSV and indoor temperature. In a climate chamber study, Fanger [84] found a regression coefficient of 0.33; thus, the temperature change required to shift one thermal sensation vote (T req ) would be 3 • C (=1/0.33). Regression equations from different field studies around the world have been tabulated in Table 3. The regression coefficients of 0.25 to 0.33 in NV buildings and 0.2 to 0.3 in HVAC buildings are often obtained in field surveys (Table 3), i.e., 3 to 5 • C is required to shift one thermal sensation vote. However, few field surveys have less than a 0.2 regression coefficient, i.e., greater than 5 • C is needed to shift one thermal sensation vote, which seems unreliable to calculate the comfort temperature including thermal comfort zone (center three categories). This indicates that we need to be very careful while calculating the comfort temperature by the regression method. Thus, recently many people have been adopting Griffiths' method to calculate the comfort temperature [13,14,17,18,21,58,62]. In Table 3, many researchers have binned the data [23,24,62,85] to increase the R 2 value. It has been found that the value of R 2 of binned data is much higher than that of raw data but the regression coefficient is similar in both analyses [38,[86][87][88].

Comfort Temperature from Various Field Studies
The comfort temperature from various studies has been tabulated as shown in Table 2. The temperature required for comfort is as low as 17.6 • C and as high as 31.2 • C. Humphreys [83] also found the comfort range from 17 to 30 • C in 1978. We found the range of comfort temperature in the naturally ventilated building or free-running mode is 17.6 to 31.2 • C and the range in the air-conditioned building is 20.3 to 27.5 • C. This indicates that naturally ventilated buildings experience a higher fluctuation in comfort temperature, from winter to summer, as the occupants in these buildings are more responsive to various adaptive actions.

Seasonal Differences in Comfort Temperature
Seasonal differences in comfort temperature in office buildings from different parts of the world have been listed in Table 4. A seasonal difference of 0.3 to 5.4 K has been recorded in different field studies. However, in dwellings, the seasonal difference is much higher than in office buildings ( opportunities than in dwellings. These seasonal differences in comfort temperature indicate that there is a need for a climate-specific adaptive model in both naturally ventilated and air-conditioned buildings for energy saving.

Relation between the Comfort Temperature and Indoor Temperature
From Table 2, the relation between the globe or indoor temperature of office buildings and the comfort temperature of the occupants in naturally ventilated and air-conditioned buildings has been plotted in Figure 3. The strength of correlation between the comfort temperature and globe or indoor air temperature in the naturally ventilated building is higher than in the air-conditioned building. This indicates that occupants are more adaptive to the indoor climate in the NV building. At indoor globe or air temperatures below 24.5 • C, the comfort temperature is always higher than the indoor air temperature, and above 24.5 • C, the comfort temperature is always lower than the indoor air temperature. The relation between comfort and indoor temperature from various studies has been tabulated in Table 6. Where the R 2 in all studies has a higher value except from Japan which used raw data for regression analysis.

Relation between the Comfort Temperature and Outdoor Temperature
Indoor comfort temperature and outside temperature are strongly correlated [83]. Because people adapt well in their offices using various behavioral, physiological, and psychological adaptations [3,6], the comfort temperature indoors has been found to vary with the outdoor temperature. Field thermal comfort studies have revealed that indoor comfort temperatures are a function of the mean outdoor temperature [83,94]. This means we can relate the climate and season to the indoor comfort temperature. Standards based on such a relationship would not only improve adaptive thermal comfort but would also save energy [95]. Comfortable buildings can be designed using the adaptive relationship between comfort temperature and outdoor temperature [7].
The relationship shown in Figure 4 from Table 2 can explain the difference observed between comfort temperatures in buildings in the naturally ventilated and air-conditioned modes. People indoors in a naturally ventilated building adapt to the outdoor temperature mediated by the building's walls, operable windows, roofs, and floors. The thermostat, often set by the building manager, controls the temperature in a heated or cooled building, and its occupants adapt to this pre-selected temperature. The given Figure 4 shows that occupants of the naturally ventilated building have higher comfort temperatures and are more correlated to the outdoor temperature than in air-conditioned buildings. When we conducted regression analysis, we found the following equations.

Relation between the Comfort Temperature and Outdoor Temperature
Indoor comfort temperature and outside temperature are strongly correlated [83]. Because people adapt well in their offices using various behavioral, physiological, and psychological adaptations [3,6], the comfort temperature indoors has been found to vary with the outdoor temperature. Field thermal comfort studies have revealed that indoor comfort temperatures are a function of the mean outdoor temperature [83,94]. This means we can relate the climate and season to the indoor comfort temperature. Standards based on such a relationship would not only improve adaptive thermal comfort but would also save energy [95]. Comfortable buildings can be designed using the adaptive relationship between comfort temperature and outdoor temperature [7].
The relationship shown in Figure 4 from Table 2 can explain the difference observed between comfort temperatures in buildings in the naturally ventilated and air-conditioned modes. People indoors in a naturally ventilated building adapt to the outdoor temperature mediated by the building's walls, operable windows, roofs, and floors. The thermostat, often set by the building manager, controls the temperature in a heated or cooled building, and its occupants adapt to this pre-selected temperature. The given Figure 4 shows that occupants of the naturally ventilated building have higher comfort temperatures and are more correlated to the outdoor temperature than in air-conditioned buildings. When we conducted regression analysis, we found the following equations.
Other type buildings (HT, CL, EC and MM) Tc = 0.21To + 19.66 (R 2 = 0.54) The above Equations (1)-(4) for NV, HVAC, MM, and Other type buildings, respectively, will be helpful for the thermal design of building. We obtained the coefficient of 0.43 for the NV building, which is steeper than the ASHRAE standard. Similarly, we obtained a 0.22 coefficient for the HVAC building, which is also steeper in slope than the CIBSE guide. This might be because of a wider range of outdoor climatic variation that  The above Equations (1)-(4) for NV, HVAC, MM, and Other type buildings, respectively, will be helpful for the thermal design of building. We obtained the coefficient of 0.43 for the NV building, which is steeper than the ASHRAE standard. Similarly, we obtained a 0.22 coefficient for the HVAC building, which is also steeper in slope than the CIBSE guide. This might be because of a wider range of outdoor climatic variation that has been covered by this study. Whereas, in the MM building, we obtained the coefficient of 0.18, which is more similar as found by Tewari et al. [61] in a field study. Figure 4 has a regression line for heating, ventilation, and air-conditioning (HVAC) buildings that can be used to assess the need for heating or cooling in buildings. Because the comfort indoor temperature is always higher than the outdoor mean temperature in air-conditioned buildings, it should be possible to achieve the desired indoor temperature without the use of a heating plant in many cases. In certain climates where the outdoor mean temperature exceeds 25 • C, the comfort indoor temperature is always lower than the outdoor mean temperature, so it should be possible to achieve the desired indoor temperature without using a cooling plant in many cases through careful design.
Adaptive thermal comfort models from different field studies have been shown in Table 7 and illustrated in Figures 5 and 6. The length and slope of the line, respectively, show the temperatures at which they are valid and the degree to which the individuals were sensitive to a change in temperature. For this, the term "Comfort threads" was used in a previous study [96], which illustrates how the comfort temperature varies according to different ranges of outdoor air temperatures. Different countries have their own adaptive thermal comfort model, and the trend is similar. The length of the line indicates the range of comfort temperature in a particular location. Because of the different ranges of outdoor temperatures, the line has various lengths. The comfort temperature increases with an increase in outdoor temperature. However, the coefficient is different in different countries. Adaptive thermal comfort is known to be dependent on behavioral, physiological, and psychological adaptations [3,6]. countries. Adaptive thermal comfort is known to be dependent on behavioral, physiological, and psychological adaptations [3,6].    [2,17,21,34,39,40,56,63,66,85,97].

Relation between Clothing Insulation and Outdoor Air Temperature
People can change personal variables to improve their thermal comfort. One major variable for people is clothing insulation. Linear regression analysis is commonly used to predict clothing insulation that varies with the outdoor air temperature [5]. The regression equation of clothing insulation from various studies has been compiled in Table 8, and these relations of clothing insulation with outdoor air temperature are illustrated in Figure 7. The clothing insulation and outdoor air temperature have a negative correlation; whereas, comfort temperature increases with an increase in temperature (see Figures 5 and 6). The negative correlation demonstrates that the respondents added layers of clothes in response to a drop in temperature and removed layers in response to an increase in temperature as a measure of adaptation. Because of the adaptive nature of the occupant in naturally ventilated buildings, the slope in NV buildings are steeper than the AC buildings except in Kumar et al. [60]. Thapa et al. [63] studied in the cold climate of India and they found a much steeper slope with high clothing insulation compared to other studies. A steeper slope indicates that the subjects respond quickly to changes in outdoor environmental conditions and are thus considered as highly adaptive [56].

Energy Saving by Thermal Adaptation
The building is important in creating a safe and comfortable living environment. In terms of physiological adaptation, psychological adaptation, and behavioral adaptation, occupants are considered as not only passive objects in the thermal environment, but also as active factors [6,91]. Human thermal adaptability is closely related to thermal environments in which they are exposed to large amounts of energy consumed by air-conditioning systems in order to maintain tight control of room air temperatures [51]. An indoor temperature standard that varies with outdoor temperature can reduce energy use in airconditioned buildings by reducing the difference between indoor and outdoor temperatures [91].

Energy Saving by Changing Temperature Settings Found in Various Studies
Several field studies show that occupants are willing to accept a much wider temperature range than is typically used in practice [22,23,43,91,[98][99][100][101]. Furthermore, occupants with access to a personal environmental control (PEC) system can extend the acceptable

Energy Saving by Thermal Adaptation
The building is important in creating a safe and comfortable living environment. In terms of physiological adaptation, psychological adaptation, and behavioral adaptation, occupants are considered as not only passive objects in the thermal environment, but also as active factors [6,91]. Human thermal adaptability is closely related to thermal environments in which they are exposed to large amounts of energy consumed by airconditioning systems in order to maintain tight control of room air temperatures [51]. An indoor temperature standard that varies with outdoor temperature can reduce energy use in air-conditioned buildings by reducing the difference between indoor and outdoor temperatures [91].

Energy Saving by Changing Temperature Settings Found in Various Studies
Several field studies show that occupants are willing to accept a much wider temperature range than is typically used in practice [22,23,43,91,[98][99][100][101]. Furthermore, occupants with access to a personal environmental control (PEC) system can extend the acceptable ambient temperature range to as much as 18-30 • C [101]. Previous research has shown that when occupants are actively involved in the thermal control process, including thermostat use, their satisfaction increases [3,102]. Office workers have fewer opportunities to adapt to changing thermal environments than do homeowners [103]. However, even when subjected to the same thermal environments, clothing, and activity levels, occupants with varying degrees of personal control had significantly different thermal responses [54].
In general, changing the indoor temperature by 1 K can save about 10% of the energy used for heating or cooling [104]. Table 9 summarizes the energy-saving potential found in various studies by changing temperature settings. The results showed that by lowering the temperature setting for heating and increasing the temperature setting for cooling, significant amounts of heating and cooling energy could be saved. It is clear that significant amounts of energy saving are possible, ranging from a 37% decrease in overall energy costs in small office buildings (one floor) in the USA [99] to a 6% reduction in energy costs in Australian office buildings by lowering the 1 • C set point temperature [105]. Similar review conducted by Rijal et al. [106] and a literature analysis conducted by Yang et al. [107] also support these findings. Additionally, by using a strong passive building design and permitting different adaptation measures, we can reduce the energy used for heating and cooling.

Energy Saving by Natural Ventilation and Adaptive Model in Various Studies
Natural ventilation refers to a system in which an occupied space's interior air is continuously replenished by the relatively fresh exterior air through vents, windows, doors, and other openings [109]. Natural ventilation is a good way to reduce the amount of energy used by buildings. Natural ventilation has a tremendous potential to lower energy consumption and the cost of the HVAC system by supplying and removing air from an indoor space without the usage of mechanical devices [110,111]. Advanced NV technologies, such as wind towers, solar chimneys, and automated window controls, are already receiving a lot of attention in Europe and North America [6,112,113], and they show a significant reduction in cooling energy usage of up to 40-50% in some places [114]. Natural ventilation does not require a mechanical system and does not use electricity [115]. Natural ventilation can be used to reduce building energy consumption, achieve thermal comfort, and maintain a healthy indoor environment [116]. Table 10 summarizes the energy-saving potential by natural ventilation and adaptive models in various studies. By implementing adaptive models in air-conditioned buildings, significant amounts of heating and cooling energy can be saved. It has been observed that up to 27.5% of cooling energy has been saved by an adaptive control algorithm, and up to 78% of cooling energy has been saved through the natural ventilation, having an impact on climate and ambient air quality. Adaptive models would be important control strategies for energy-saving building designs because of different adaptive measures in adaptive models such as window opening and clothing adjustments.

Overall Discussion
The indoor operative temperature at which the average subject votes neutral on the thermal sensation scale is referred to as the comfort temperature. As shown in Figure 3, studies revealed a strong correlation (R 2 = 0.72) between indoor or globe temperature and outdoor temperature in naturally ventilated buildings; whereas, the correlation between indoor or globe temperature and outdoor temperature in an air-conditioned building is much weaker (R 2 = 0.37). This suggests that outdoor temperature is likely to affect thermal sensation and comfort temperature, and that indoor temperature fluctuates more from winter to summer in NV buildings, as mentioned by Dhaka et al. [56].
The adaptive approach to thermal comfort [7] was developed based on the results of field studies of thermal comfort. Previously, the regression method was used in many studies to calculate comfort temperatures. Few field surveys [24,39,56,58,60] have less than a 0.2 regression coefficient, i.e., greater than 5 • C is needed to shift one thermal sensation vote. More than 5 • C to shift one thermal sensation vote seems impractical. The linear regression approach can sometimes provide an extraneous number when the TSV is located away from the neutral point, as previously noted by Thapa et al. [63] and Indraganti [119]. Therefore, we need to be careful while calculating comfort temperature from regression analysis. The Griffiths' method, which has been used by various researchers [14,18,19,39,57,60,63,104], is another method for calculating the comfort temperature.
Adaptive thermal comfort in office buildings studies from around the world revealed that comfort temperatures [60] in tropical areas were higher than those in cold climatic zones, and comfort temperatures in winter were lower than in summer in the same region [13,23,39,56,60,75], indicating that people might adapt to the climate. The temperature required for comfort can range between 17.6 • C and 31.2 • C. Humphreys [83] also found the comfort range to be from 17 • C to 30 • C in 1978. However, different field studies found that the seasonable difference in comfort temperature in an office building is lower (0.3-5.4 K) than in dwellings (4.9-13.8 K) (Tables 4 and 5).
Furthermore, as discovered by [38,92,93], there is a strong relationship between comfort temperature and indoor air or operative temperature. A naturally ventilated building has a stronger correlation than an air-conditioned building. Because people adapt well in their offices using various behavioral, physiological, and psychological mechanisms, the comfort temperature indoors was found to vary with the outdoor temperature [3,6].
Clothing is a major variable for people in order to maintain their thermal comfort. The clothing level always varies with the outdoor temperature. Because of different adaptive measures in naturally ventilated buildings, the correlation of clothing insulation of occupants to the outdoor air temperature is higher than in air-conditioned buildings.
According to various thermal comfort field studies, indoor comfort temperature is related to outdoor climate. As a result, standards based on this relationship would not only improve comfort but would also result in energy saving [95]. Data from several large field studies (Table 8) show that occupants are willing to accept a much wider temperature range than is typically used in practice [22,23,43,91,[98][99][100]. Furthermore, natural ventilation has the potential to significantly reduce energy consumption and the cost of the HVAC system, which supplies and removes air from an indoor space without the use of mechanical systems [110,111].
In addition to thermal comfort, energy consumption in HVAC systems is vital to consider. Human thermal adaptation is directly connected to thermal environments where people are subjected to high energy consumption from air-conditioning systems to maintain tight control over the temperature of the room [51]. Besides thermal adaptation, the recent studies [120,121] show that energy saving from accurate occupancy detection by implementing artificial intelligence in the building also has a significant role.
Comfort temperatures proposed by different studies from naturally ventilated or free-running buildings are plotted on the ASHRAE 55 comfort band, as shown in Figure 8 from Table 2. The regression line shows that the slope of this study is steeper as it has a higher coefficient than in ASHRAE 55. This might be because of a wider range of outdoor climatic variation that has been covered by this study. It has been observed that most of the comfort temperature lies within the ASHRAE comfort band, and comfort temperature is increasing with the increase in outdoor temperature. As we know, the adaptive behavior is inherent in culture and climate.
ior is inherent in culture and climate.
Comfort temperatures proposed by different studies from air-conditioned buildings are plotted on the CIBSE guide, as shown in Figure 9 from Table 2. The regression line shows that the slope of this study is steeper as it has a higher coefficient than in the CIBSE guide. This might be because the CIBSE guide was derived from the European context while this study covers all studies conducted in the world. Therefore, it has been observed that more than one-third of the comfort temperature is outside the CIBSE comfort band. Figure 9 clearly shows that the equation derived from the CIBSE guide in air-conditioned buildings might not be applicable in the other context. Comfort temperatures proposed by different studies from air-conditioned buildings are plotted on the CIBSE guide, as shown in Figure 9 from Table 2. The regression line shows that the slope of this study is steeper as it has a higher coefficient than in the CIBSE guide. This might be because the CIBSE guide was derived from the European context while this study covers all studies conducted in the world. Therefore, it has been observed that more than one-third of the comfort temperature is outside the CIBSE comfort band. Figure 9 clearly shows that the equation derived from the CIBSE guide in air-conditioned buildings might not be applicable in the other context.

Conclusions
This study on adaptive thermal comfort in office buildings based on a literature review found the following conclusions: • Studies revealed a strong correlation between indoor or globe temperature and outdoor temperature in naturally ventilated buildings. Whereas, the correlation is much

Conclusions
This study on adaptive thermal comfort in office buildings based on a literature review found the following conclusions:

•
Studies revealed a strong correlation between indoor or globe temperature and outdoor temperature in naturally ventilated buildings. Whereas, the correlation is much weaker in air-conditioned buildings. • While calculating comfort temperature through the regression method we have to be careful as it may require more than 5 • C to shift one thermal sensation vote, which is inappropriate.

•
The temperature required for comfort is as low as 17.6 • C and as high as 31.2 • C. • Different field studies found that the seasonable difference in comfort temperature in office buildings is 0.3-5.4 K, which is much lower than in dwellings. • A strong relation of comfort temperature and indoor temperature was observed from various field studies.

•
The new adaptive thermal comfort Equations (1)-(4) were proposed on the basis of different field studies for NV, HVAC, MM, and other types of office buildings, which will be helpful for the thermal design of buildings.

•
The correlation of clothing insulation of occupants to the outdoor air temperature in naturally ventilated buildings is higher than in air-conditioned buildings.

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Various studies show that substantial amounts of energy can be saved by changing the set point and natural ventilation. It was observed that up to 37% of cooling energy was saved by raising set point temperature based on an adaptive model. In addition, up to 27.5% of cooling energy was saved by the adaptive control algorithm and up to 78% cooling energy was saved through natural ventilation.
This study was conducted covering all climate through a single glass. The most significant limitation of field studies is the difficulty in obtaining reliable data. Because it is difficult to cover all aspects of comfort in a single study, this limitation can be overcome by increasing the number of studies and thus increasing the possibility of covering the most aspects of thermal comfort. These variations may be due to different climate types and dynamic natures, such as different behavioral patterns, study periods, number of observations, methods of analysis, and indices used. Adaptive thermal comfort standards based on climatic adaptations for desired indoor design temperatures will pave the way for energy-efficient design development. Additionally, this study successfully demonstrates the necessity for climate-specific thermal comfort standards for the development of energyefficient designs, as existing comfort standards might not be applicable in all regions.
Author Contributions: As a primary author, P.L. contributed to the data collection, analysis, review writing, and preparing the final draft of the manuscript. S.B.B. edited the content of the paper. H.B.R. revised and edited the paper's content, contributed to structuring the paper, and advised on the review analysis. All authors have read and agreed to the published version of the manuscript.