Field studies on human thermal comfort — An overview

Building and Environment 64 (2013) 94e106

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Field studies on human thermal comfort d An overview Asit Kumar Mishra*, Maddali Ramgopal Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, West Bengal 721302, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 December 2012 Received in revised form 26 February 2013 Accepted 27 February 2013

This paper presents an exhaustive overview of the field studies carried out in the past few decades on human thermal comfort. To get a better grasp of patterns in observed data and to facilitate comparison across investigations, the thermal comfort field studies are grouped using the KöppeneGeiger climatic classification of their locations. Effects of relevant environmental, physiological, and other aspects that can have an effect on thermal comfort are reviewed and discussed. Field studies across the board show that people have considerable capacity to adapt to their surroundings provided they have sufficient adaptive opportunities. This observation holds good for both air-conditioned as well as free running buildings. However, studies show that conditioned spaces have narrower comfort zones compared to free running buildings. Across climatic zones, most popular means of adaptation are related to the modification of air movement and clothing. The ease, economy, and effectiveness (the 3 ‘E’s) of adaptive opportunities play a major role in occupants’ adaptation to the surroundings. Studies show that individuals are likely to perceive the same thermal environment differently and environments lacking adaptive avenues normally receive poor comfort ratings. Studies also indicate that for adaptive comfort equations, the running mean temperature may be a better outdoor index compared to the monthly mean temperature. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Thermal comfort Field study Adaptive thermal comfort Adaptive opportunities Climatic zones

1. Introduction With the introduction of sections concerning naturally conditioned spaces into standards like ASHRAE 55-2004, ISO 7730, and EN15251, the idea of adaptive comfort has graduated into the main stream of comfort design. Though certain conflicts persist on the definition of a naturally conditioned space as given by these standards, designers now have clear and recognized guidelines regarding the acceptability and minimal comfort requirements in these buildings. At their heart, adaptive models rely on the ability of occupants to adapt to changes in their thermal environment in such a manner as to restore their comfort [1]. These adaptive abilities act at three different levels: physiological, behavioural, and psychological. Since shaping of disposition of an individual at all these three levels is heavily influenced by the weather s/he is exposed to, it is a logical conjecture that adaptive models from similar climatic zones would have more similarities than differences. This idea has been used by multiple authors, with reasonable success, to validate their field study results against comfort surveys done in similar climates

[2e13]. Over the past couple of decades there has been a remarkable increase in the number of thermal comfort field studies. These studies have added many fold to the body of work existent at the turn of the century. The current work tries to group a number of such field studies on basis of the climatic zones they were conducted in and then examines their results for any underlying trends. 1.1. KöppeneGeiger climate classification The KöppeneGeiger system is one of the most used climate classification systems with widespread application in diverse fields ranging from climate research to biology and education. Kottek et al. [14] have published an updated version of the Köppen classification map and we have used their work as basis for classifying the comfort surveys into different climatic zones. Apart form climate of a place, what affects the adaptive patterns of a population is the culture. But as Humphreys and Nicol observe, climate and culture are interlinked [1]. 1.2. The climate types

* Corresponding author. E-mail addresses: [email protected], [email protected] (A.K. Mishra), [email protected] (M. Ramgopal). 0360-1323/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.buildenv.2013.02.015

KöppeneGeiger system divides climates into five major types (each type having several subtypes):

A.K. Mishra, M. Ramgopal / Building and Environment 64 (2013) 94e106

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Tropical Moist Climates (A) Dry Climates (B) Moist Subtropical Mid-Latitude Climates (C) Moist Continental Mid-latitude Climates (D) Polar Climates (E)

To the best of our knowledge, there do not exist any thermal comfort surveys from Type E climates. The closest a survey came to be done in the “Polar Climate” was the one done in a district of Nepal, along with four other districts having milder climates [15]. This is not a major hindrance considering the harsh and thinly populated nature of E zones. Several surveys have been conducted across most of the subtypes of the other four major climate types. Holmes and Hacker did an analysis of building comfort and performance of HVAC set ups for the near future using different scenarios of climate change [16]. Rubel and Kottek used similar scenarios for global climatic changes to predict shifts in the Köppen climatic regions [17]. As climate patterns do change, adaptive comfort designs for particular regions can be suitably modified for the future using Rubel and Kottek’s predictions as a basis. 2. Thermal comfort surveys from the different climate zones 2.1. Tropical moist climates Prominent surveys in this type of climatic zones come from Australia [18,19], Bangladesh [20], Brazil [21,22], Cuba [23,24], India [25,26] (in addition to the results from the study done in Hyderabad [27e32]), Indonesia [33,34], Nigeria [35], Malaysia [36,37], Singapore [4,10,38,39], Thailand [40e43], and the USA [44]. Surveys have been done across residences, offices, railway stations, and classrooms. Amongst these studies are those which deal exclusively with mechanically conditioned (MC) buildings [42] as well as those which have taken mixed samples of MC and free running (FR) buildings [33,38,41,44]. Apart form the surveys in Havana [23,24] and Chennai Central Railway Station [26], the other surveys are of reasonably long duration and across different seasons. Both these surveys were done over two weeks during summer. But we agree with Tablada et al. [23] that their results are remarkably similar to those from others done in Type A climatic zone and hence, quite useful. The indoor air temperatures measured in the Chennai survey vary from 31 to 40
C (87.8e104
F). The transient nature of subjects’ stay and the elevated range of temperatures experienced by them result in an atypically high neutral temperature of 31.93
C (89.5
F). This observation of an unusually high neutral temperature in unusually hot surroundings is actually in line with adaptive comfort hypothesis. In the same study, the authors also obtain a very narrow comfort zone (1.61
C/2.9
F for the range of 1 on the ASHRAE seven point scale). The authors have suggested that this could be due to the dearth of adaptive opportunities in waiting lounges. In addition, we suspect, the narrow nature of the comfort zone could be a testament to the highly humid nature of Chennai’s climate. In terms of operative temperature, the range of neutral temperatures observed for FR buildings in Type A zones are between 26 and 29.5
C (78.8e85.1
F). The comfort zones for FR buildings from different surveys lie between 22 and 32.5
C (71.6e90.5
F). Preferred temperature of these studies lies between 24 and 26
C (75.2e78.8
F) and is always lower than the neutral temperature. Studies that have considered MC buildings in these zones give neutral temperatures between 24 and 27
C (75.2e80.6
F) and comfort zones within 22e28
C (71.6e82.4
F). The MC buildings also show preferred temperatures lower than their neutral temperatures [19,44]. The neutral temperature reported for MC classrooms by Kwok is aberrant in being higher than the neutral

95

temperature of FR buildings in the same survey [44]. But as the author states, this value of 28
C (82.4
F) is determined at a low confidence level due to clustered nature of the data. 2.2. Dry climates There do not exist a large number of comfort field surveys in Type B climatic zones. The ones we consider here are from Australia [45], Egypt [46], Kuwait [47], Libya [48e50], and Tunisia [51]. In addition, some of the cities considered by Nicol et al. [52], during their studies in Pakistan, also have Type B climates. These surveys have been done mostly in work places and some surveys sample both homes and offices. Free running buildings in these types of climates can have comfort zones spread from rather low to rather high temperatures. While the data from Pakistan gives a comfort zone between 21 and 31
C (69.8e87.8
F) (globe temperature) for minimal reports of discomfort, the surveys from Libya and Tunisia [50,51] concur on a comfort zone between 16 and 26.5
C (60.8 e79.7
F). The last result is not surprising since Libya and Tunisia are neighbouring countries and share not just similar climates but also a similarity of culture. The study done by Cena and de Dear in Kalgoorlie-Boulder, Australia [45], was done in MC offices. They obtained a comfort zone between 22 and 25
C (71.6e77
F) ET* (effective temperature) with a neutral temperature (in terms of operative temperature) of 20.4
C (68.7
F) during winter and 23.3
C (73.9
F) during summer. Majority of subjects in this study were used to being in MC environments not just in their offices, but also at home and inside their vehicles. This acclimatization to an artificial climate shows in their choice of comfort votes. What is interesting however is that the neutral temperature from the year long study done in five different cities of Tunisia [51], in FR buildings, came up in between the summer and winter neutral temperatures from the Australian study. The neutral temperature for the study done in Cairo [46] was slightly higher at 24.5
C (76.1
F) but the 80% comfort zone in this study (between 19.7 and 29.3
C/67.5 and 84.7
F) was similar to that of the Pakistan study. The study done in Kuwait was exclusively in MC buildings but gave the highest neutral temperature amongst this group of studies at 25.2
C (77.4
F). The comfort zone found in Kuwait was similar to the Australian study. The study done in Ghadames, Libya [48], shows some interesting results. The sample buildings in this study included both MC and FR buildings. As part of government initiative towards building the new town of Ghadames, inhabitants of the MC buildings had been moved to such buildings a few years before the survey was conducted. The survey was conducted during the summer season alone. With conditioning, in the new buildings, residents were neutral between 25 and 31
C (77 and 87.8
F) globe temperature and without conditioning, thermal dissatisfaction was expressed for globe temperatures exceeding 32
C (89.6
F). In FR buildings the comfort range was between 30.8 and 32.5
C (87.4e90.5
F) and the neutral temperature was 31.6
C (88.9
F). These high values of comfort temperatures that were recorded in this particular study can be explained by the fact that the survey was during summer season and the residents of MC buildings were not living in centrally conditioned spaces. Rather, they were using window or split units sparingly to avoid too high utility bills. These inhabitants had still retained a high degree of acclimatization to their external environment. 2.3. Moist subtropical mid-latitude climates Numerous surveys have been done in these climates with a bulk of them coming from the humid subtropical region of China [2,7,8,11,13,53e69]. Studies have also been done across Nepal [15],

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A.K. Mishra, M. Ramgopal / Building and Environment 64 (2013) 94e106

Taiwan [5,6,70e72], and Portugal [73,74], and in France [75,76], Germany [77], South Korea [78], India [79], Italy [80e83], Iran [84], Japan [85e87] Pakistan [88], the UK [89e91], and the USA [92]. Both MC and FR buildings of different types d residences, work places, classrooms, dormitories, old age homes, library reading rooms d have been sampled in these surveys. Mid-latitude climates include a wide variety of subtypes within the broad classification. The humid subtropical subtype can have both cold winters and hot summers while the Mediterranean and oceanic subtypes are comparatively milder. Experiencing such a wide range of climates gives the inhabitants of Type C zones an equivalently broad range of adaptability. Neutral temperatures obtained in FR buildings from these climates can vary from 11.5
C (52.7
F) [55], for studies done exclusively in the winter, to 28.6
C (83.5
F) [57], for studies done exclusively during summer. Both of these studies were done in humid subtropical locations of China. For the mid-seasons, a study done across different building types of Portugal [73] and a study done in classrooms at Changsha [13] give close values of neutral temperatures at 21.9
C (71.4
F) and 21.5
C (70.7
F) respectively. Studies done in conditioned offices in Hong Kong give neutral temperature of 23.7
C (74.7
F) for summer and 21.2
C (70.2
F) for winter [62]. Studies done in conditioned spaces in northern Italy [83] and San Francisco [92] report values of neutral temperature in between the Hong Kong values: 22.9
C (73.2
F) for northern Italy, 22
C (71.6
F) for winter in San Francisco and 22.6
C (72.7
F) for summer in San Francisco. Comfort temperature ranges for FR buildings can be between 14 and 32
C (57.2e89.6
F) for the different subtypes and over summer and winter. Conditioned buildings have narrower ranges for comfort that lie within 19.5e 25
C (67.1e77
F). Preferred temperatures from the surveys have a much smaller spread than the neutral temperatures: 21.4e27.9
C (70.5e82.2
F). An observation unique to this climate type is that the preferred temperature was more than the neutral temperature in two studies done in classrooms and dormitories in Chongqing [54,56]. This is most likely a reflection on the rather extreme range of temperatures that Chongqing experiences d monthly averages spanning from 0 to 30
C (32e86
F) d and a high level of acclimatization of the residents to hot weather [56]. It would not be out of place here to note that Chongqing is often referred to as one of the ‘three furnaces of China’. 2.4. Moist continental mid-latitude climates Like the dry climatic zones, only few surveys have been done in Type D climates. The ones being considered here are from Canada [93], China [9,94e96], and South Korea [12]. The Seoul and Harbin studies were done in buildings that were FR type during the summer but had heating for winter months. The other two studies were in buildings with operational heating or cooling systems d as per the season. These surveys give summer neutral temperatures between 23.5 and 28
C (74.3e82.4
F) and summer comfort zones between 21.5 and 31
C (70.7e87.8
F). Winter neutral temperatures lie between 20 and 25
C (68e77
F) and winter comfort zones between 16.5 and 26.5
C (61.7e79.7
F). The broader ranges for winter are because the different winter time studies were done both with and without heating enabled. Preferred temperatures, where they have been calculated, lie between 22 and 23
C (71.6e 73.4
F). When the survey during winter time, with heating active, was repeated in Harbin after a gap of almost 10 years, a 1.1
C (2
F) lower neutral temperature was registered. The authors ascribe this to acclimatization of the occupants [96].

2.5. Change of neutral temperature over time What could provide valuable insight into the adaptive behaviour and acclimatization of people is how neutral temperature of certain populations change over the years. There are at least three places for which we have studies done about a decade apart. Table 1 summarizes the results of interest from these three cases. With just three instances it would not be ideal to generalize, but the indication is that neutral temperatures established by field studies do not go through big changes over reasonably long periods of time. 2.6. Adaptive comfort equations Apart from the adaptive comfort equations (ACEs) given in standards like ASHRAE Standard 55-2004 or EN15251, several field surveys have come up with their own adaptive equations too. Such equations are available from surveys done in Types B, C, and D climates. In Appendix A (Table A.1), we provide some of these equations, along with the equations from the standards and certain meta studies of global data, to act as a quick reference for the reader. 2.7. A summary of the field survey buildings In Table 2 we summarize the field surveys referred for this overview. We classify the studies done in different climatic zones on the basis of building type and the type of indoor conditioning used in those buildings. Surveys that involved both MC and FR buildings are put under a separate category. All offices, waiting areas, lounges, and other institutional buildings are grouped under Offices, residences and dormitories are grouped under Residences, while classes, reading rooms, libraries, and other educational rooms are grouped under Classrooms. Any studies that involved more than one building type have been cited under multiple categories. However, we must note here that, not all the studies that involved multiple building types have separated their analysis for the different building types. 3. Discussions At the outset of this section, we mention the surveys done with young children as subjects [89,97]. Children have different levels of thermal sensation, different metabolic rates, different clothing restrictions, and different sensitivities to temperature changes. Levels of response between children also has a lot of variance and classroom activities are more diverse than adult activities over a typical day [89]. We have not come across many surveys done with young children as subjects and much further work is needed in the field. So, we will avoid further discussions of surveys done with young subjects.

Table 1 Changes in neutral temperature with time. Location

Year of survey

Building type

Ventilation type

Neutral temperature

Bangkok

1988 [40] 2002e03 [43]

FR FR

28.7
C (83.7
F) ET* 28.0
C (82.4
F)

Singapore

1986e87 2000e01 2000e01 2009e10

Offices Offices and residences Residences Residences Residences Residences

FR FR MC MC

28.5 28.6 21.5 20.4

Harbin

[38] [4] [95] [96]




C C
C
C


(83.3 (83.5 (70.7 (68.7




F) F)
F)
F)


A.K. Mishra, M. Ramgopal / Building and Environment 64 (2013) 94e106 Table 2 Summary of buildings from different climatic zones. Climatic zone

Type of building

Type of conditioning

References

Type A

Offices

MC FR FR and MC FR

[19,22,38,42] [26,36] [33,37,40,41,43] [4,10,20,23e25, 27e32,34,35,38] [18,43] [21,22,35,39] [36,43,44] [45] [50,51] [52] [47] [50,51] [48,49,52] [46] [62,63,66,69,71,83,85,86] [7,53,65,75e77,79,84,88] [2,73,74,80,90e92] [68] [11,15,54,55,58,59,64,65,67,84] [2,8,57,69e71,73,74] [68,81] [6,13,56,59,65,72,78,82,89] [5,73,74,80,87] [60,61]

Residences

Classrooms Type B

Offices

Residences

Type C

Classrooms Offices

Residences

Classrooms

Type D

Climate chamber Offices Residences

Classrooms

FR and FR FR and MC FR FR and MC FR FR and FR and MC FR FR and MC FR FR and MC FR FR and MC

MC MC

MC

MC MC

MC

MC

MC

MC MC FR FR and MC MC

[12,93,94] [9] [95] [96] [94]

3.1. Comfort in free running buildings There are multiple examples of people feeling comfortable under free running conditions while environmental parameters were far removed from traditional comfort zones. We cite a few such examples from different climatic zones: Type A [4,20,21,35e37,44], Type B [48,50,52], Type C [6,13,15,55,57,58,70,78,84], and Type D [9,96]. These examples may also be classified according to the building types where the surveys were done as: residential buildings [4,9,15,20,35,48,50,52,55,57,58,70,84,96], work places and institutional buildings [36,37,50,52,84], and educational buildings [6,13,21,35,36,44,78]. However, amongst the numerous surveys done, a few do turn up where people have difficulty accepting the naturally ventilated environment, particularly during the hot season [22,23,31,34, 46,65,82]. Of these cases, there were also studies where more than 20% of occupants have voted in zone of discomfort [22,34, 46,82] despite the thermal environment being within the ASHRAE 55-2004 recommended 80% comfort limits for naturally conditioned buildings [98]. There was even a case where a great number of occupants voted to be uncomfortable even though PMV calculations suggested otherwise [82]. In most of these cases, the recorded air velocities were low. Also, in at least two of these surveys, the occupants had reduced flexibility with clothing adjustments [22,46]. 3.1.1. Neutral vs preferred temperatures Subjects often do not interpret the neutral point on a scale in the same manner as the surveyors might and if at all neutral is what a person wants to be is also debated [99,100]. de Dear and Brager observed a difference between a building’s neutral and preferred temperature in their analysis of field survey data from all over the world [101]. However, they found this difference to be significant

97

only for conditioned spaces. In many of the field surveys we discuss, we find this mismatch for surveys done in solely FR buildings [4,15,21,23,31,39,54,56], solely MC buildings [66,93], and in FR and MC buildings together [2,5,44,57,71,73,92]. Only in two of the instances was the annual average neutral temperature smaller than the preferred temperature [54,56]. In one case, the preferred temperature found was never actually recorded over the survey period [23]. This might have been simply due to the short duration of that particular survey. In FR buildings, occupants often take several adaptive measures to improve their comfort. The preference for such a person might be an idealized situation where s/he does not have to undertake any adaptive action and yet feels comfortable. So, as viewed by people, neutral and preferred conditions may not match. We further observe that subjects preferred to be cooler during warm periods/in warm-humid climates [4e6,23,31,34,41,44,54,56, 64,70,75,78,91e93], and to be warmer during cooler periods/in cold climates [54,56,64,81,82,97]. Even when we examine the preference of the same population during different seasons, a similar pattern turns up [65,73,74,76,80]. This trend has been best expressed by Humphreys and Hancock as: “People prefer sensations on the warm side of neutral if it is cool outdoors and warm indoors, while they prefer sensations cooler than neutral if it is warm outdoors and cool indoors” [100]. 3.2. Changes to the traditional adaptive model 3.2.1. Running mean temperature Traditionally, adaptive comfort models have related neutral temperature of occupants with a monthly mean outdoor temperature. The term monthly mean leaves a lot open to interpretation: mean of which month, the current or the preceding; is it a historic mean or a mean of the days during which the survey was taken; if a historic mean, how many years of data has been taken in its calculation? This situation is quite commonly faced in trying to understand end results from many of the surveys. Use of running mean temperature (RMT) puts an end to such predicament and provides a more fluid and sensitive indicator of occupant’s thermal history [1]. Successful use of running means in estimation of clothing changes with temperature [102], lead to an exponentially weighted running daily mean expression being used for RMT [1,103]. The form of RMT gives more recent daily mean temperatures a dominant role. This has been compared with a radioactive decay series and the RMT expression used with EN15251 has a “half life” of about half-week [104]. Similarly, ASHRAE Standard 55-2010 now requires the use of prevailing mean outdoor air temperature (PMOAT) in place of the mean monthly outdoor air temperature [105]. The calculation for obtaining PMOAT is essentially similar to calculating RMT, though ASHRAE gives more flexibility in the number of days of historical data used and the decay rate used. The ASHRAE standard also allows for the use of monthly mean temperatures if the required weather data is unavailable. We feel that use of RMT is able to remedy certain lacunae of monthly mean temperature quite ably. A month is a patently human construct and seasonal variations need not follow precise monthly intervals. Secondly, using monthly means with adaptive equations creates a set of rather discontinuous set point temperatures that change abruptly at the end of a month [106]. Lastly, the historic monthly mean temperature of a location does not reflect the thermal history of a person who has recently immigrated. For buildings where control algorithms have access to recent weather data, RMT does a much better job of assessing thermal history of occupants. That aside, the concept of using continuous weather data in RMT calculation is more in line with the idea of adaptability

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A.K. Mishra, M. Ramgopal / Building and Environment 64 (2013) 94e106

of occupants. Lately, authors are beginning to prefer use of RMT in their results and expressions [12,50,51,73,80,82]. 3.2.2. Extended PMV The PMV model, by accounting for changes of clothing and metabolic rates, accounts for certain behavioural adaptations and thus can be considered at least partially adaptive [101]. Fanger and Toftum introduced an extended model of PMV (ePMV) to take into account expectations of people based on local climate and popularity of mechanical conditioning [107]. In some cases, applying the ePMV model gives quite good match [7,59] and in others, the model manages to do only just better than PMV [25,39]. The determination and rationality of the expectancy factor used with ePMV models has been surrounded by controversy since it was proposed. So the ePMV model has failed to find any widespread use. We include it here primarily for its historical significance as it marks the turning point when rational models started trying to emulate adaptive comfort philosophies. 3.3. Age, gender, and seasonal differences 3.3.1. Age differences From a physiological point of view, all three major cold defences d shivering, vasoconstriction, and thermal perception of cold d get compromised with ageing [108,109]. This would imply that older subjects (>60 years of age) would either like to keep warmer or will rate warmer environments more favourably. Some of the field studies have the elderly feeling cooler/preferring a higher ambient temperature [7,29,69]. Two field studies also report differences in neutral temperature found for different age groups though their results were not conclusive [33,43]. The elderly do face greater rates of mortality from cold exposure [110] and this has also been noted in one of the surveys [15] though the authors could not give exact statistics due to lack of official death records. We must note here that the differences in thermoregulation in the elderly might be more due to the morphological differences, differences in fitness levels, and debilitating effects that come along with ageing than just ageing per say [110,111]. 3.3.2. Gender differences Studies that have analysed data for the two genders separately, invariably come up with different voting patterns, percentage of dissatisfaction, and comfort zone width for them [5,19,25,29,33,42, 60,61,67,73,80,83,85,93,95]. Clothing ensembles of men and women often have different clo values, with women showing more inter and intra seasonal variations. This is born out from studies in laboratory conditions [112,113] as well as in the field [5,27,45,80, 83,92,95]. In their meta analysis of field survey responses, Schiavon and Lee have also noted a statistically significant difference between the clothing insulation value of the two genders though they consider the difference to be negligible from “an engineering point of view” [114]. Women have several morphological differences compared to men, including higher surface area to volume ratios for body segments, a smaller average body size, lesser muscle mass, and a higher surface area to mass ratio [110,115]. Gender related differences in body morphology affect the heat balance and also cause differences in thermoregulation and thermal perception. Differences between the two genders manifested themselves in several forms across the works we have reviewed. Most common manifestations were differences in neutral temperature [25,29,33,45, 60,61,80,85,92,95] (with most cases showing that women preferred to be warmer), more dissatisfaction amongst female subjects [5,19,45,93], differences in clothing insulation and patterns of variation of clothing [45,93,92,95], and greater sensitivity of

women to temperature variations [5,60,67,95]. There were also examples of female subjects having a narrower comfort zone [5,80] and women adapting more effectively or more frequently to their environment [29,73]. 3.3.3. Seasonal differences Surveys that are conducted across the whole year or over part of summer and winter seasons, give different thermal sensations for the two seasons [8,12,15,19,25,34,43,45,52,62,65,72,76,86,88,93]. In some of the cases, the difference is small enough to be explained by seasonal clothing variations while in quite a number of cases, clothing differences alone are not enough to explain the change [8,12,15,43,52,72,88]. These findings suggest that people from different climatic zones have not inconsiderable capability for adapting to seasonal changes of weather. An analogous trend is seen for neutral temperature variation over a day. Two studies from two different climates report a higher neutral temperature for occupants during the later half of the day, when the outdoors is also warmer [33,77]. Circadian rhythm of human core temperature peaks during the afternoon [116] and part of the reason for a higher neutral temperature could be this variation in body set point temperature [117]. What needs further investigation is if the entire difference in neutral temperature found by these authors is ascribable to the circadian rhythm or if part of the variation is also due to our thermal history of expecting higher temperatures during the later half of a day. The observations made in this section could benefit from further investigations. A first step could be to establish conclusively that these phenomena exist. Following that, a better understanding of why they occur could help design more occupant responsive conditioning systems. 3.4. Effects of air movement and humidity In his work on adaptive comfort in tropical regions, Nicol describes that while humidity has an effect on the comfort temperature, this effect is small and difficult to consider [118]. For air movement though, he proposes that in hot climates, there can be an allowance on the comfort temperature depending on the velocity of air movement that can be provided to the occupants. This kind of allowance would not work for cold climates due to risk of an uncomfortable draft. The allowance proposed by Nicol is to raise the comfort temperature by:

DTð
C Þ ¼ 7 

50 4 þ 10Va0:5

(1)

over and above the value predicted by ACEs. Here Va is the air velocity and equation (1) is applicable when Va keeps consistently above 0.1 m/s (19.7 fpm). As a simplification, Nicol also proposed that if fans are available, one may add 2
C (3.6
F) to the neutral temperature values obtained from ACEs [118]. This correction factor given by equation (1) is also included in EN15251. Similar to EN15251, now the ASHRAE Standard 55 also allows for elevation of upper comfort temperature limits in naturally conditioned spaces when air speeds are in excess of 0.3 m/s (59 fpm) and operative temperature in the occupied zone is more than 25
C (77
F) [105]. The elevations allowed in comfort temperature limits are 1.2
C (2.2
F), 1.8
C (3.2
F), and 2.2
C (4.0
F) for air speeds of 0.6 m/s (118 fpm), 0.9 m/s (177 fpm), and 1.2 m/s (236 fpm) respectively. These values are based on equal SET (Standard Effective Temperature) values. Studies done in different climates show subjects expressing a desire for greater air movement and width of comfort zones increasing with aid of air movement [2,10,19e21,26,27,45,54,56,60, 61,65,75,76]. Zhang et al. have also observed from their analysis of

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the responses in Centre for the Built Environment’s (CBE) Indoor Environmental Quality (IEQ) survey that office workers complain more often about “too little” of air movement than about “too much” of air movement [119]. What they also point out is the demand for more air movement, amongst occupants voting warm thermal sensations, is more widespread than the demand for less air movement, amongst occupants voting a cool sensation. At least one study done in university classroom/drawing rooms shows that student activities are not disturbed at wind speed levels of 1 m/s (197 fpm) [21]. Additionally, Zhang et al.’s study suggests that more people view air movement as a positive influence in their work environment than as a disturbance [119]. Some studies done in MC offices have the respondents wanting more air movement even if their voted thermal sensation is in the slightly cool region [45,54]. On the other hand, the study done in Harbin during summer time [9] has the occupants’ peak preference air velocity at 0.4 m/s (78.7 fpm). As suggested by the authors, this is most likely due to low use of fans and air-conditioners amongst Harbin’s population. These occupants are satisfied with what little air velocity they can get from natural ventilation and have probably adapted to such velocities. In a dry environment, enhanced air movement is not required for sweat evaporation. So, in a hot and dry environment, high air velocities can be counter productive by increasing heat gain of the body [31]. In such situations, occupants would adaptively close windows or turn off ceiling fans. In high humidity locations, greater air movement not only aids convective heat transfer from skin, it also increases evaporation of sweat. This could be the reason why in certain situations and locations (especially humid places like Karachi and Chennai), adding just the air velocity to correlations improves predictability by a reasonable amount [25,52,60]. So, if the surroundings are warm and humid, higher air velocities can be very effective, and one of the sole ways for enhancing heat loss from human body. But, as Chow et al. [60] reason, more air movement can cause an inherent pleasant sensation of freshness in addition to thermal comfort. Occupants appreciate air movement even when it is not necessary for cooling action [119]. This could be crucial in conditioned spaces that do not have adequate ventilation measures in place. ASHRAE Standard 55-2010 has also raised the upper limit of air speed to 0.8 m/s (160 fpm) for operative temperatures above 25.5
C (77.9
F) in all types of buildings [21,105]. Considering this enhanced air speed is not conditional on availability of local control to occupants, it would seem that greater air velocities are here to stay. Givoni et al. have done a meta analysis of data taken from hot and humid regions and concluded that humidity has minimal effect on thermal sensation of ‘sedentary and near-sedentary occupants’ [120]. Adding humidity in correlations between thermal sensation and indoor temperature changes their predictive power very little [5,25,31,42,52,60,61]. As per EN15251, in the European context, humidity has a negligible effect on both thermal sensation and air quality perception for moderately active occupants [104]. People in locations that experience highly humid conditions on a regular basis are better acclimatized to such humidity levels [9,13,20,29,42,54,56,58,67,72]. The study done in Dhaka found occupants feeling comfortable at a mind boggling 95% relative humidity while the mercury soared close to 30
C (86
F) [20]. One of the explanations as to why humidity does not affect the thermal sensation of people could be acclimatization [120]. But, it has also been observed that in conditions of high humidity, comfort temperatures decrease and comfort zones become narrower [20,31,35,67]. This would imply that in humid climates, people get uncomfortable with smaller variations in temperature [118]. An ideal adjustment would be to offset the effect of humidity by designing indoors with slightly cooler temperatures even for acclimatized populations.

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At the other end, in a few surveys it has been seen that low humidity and warm environments together can cause symptoms like nose bleeds, eye/throat irritations, sore throat, and dry mouth [31,85,96]. Such situations lead subjects to give their thermal environments low ratings even though the root cause of their discomfort is not a thermal sensation. This also shows that subjects can be temperamental and are not always objective in estimating their thermal sensation. 3.5. Nature of the surveys For recommendations on survey methodologies, one may consult ASHRAE Standard 55 [98,121] and the Performance Measurement Protocols for Commercial Buildings (PMPCB) also published by ASHRAE [122]. Parts of the PMPCB that deal with “IEQ Thermal Comfort” discuss in great details about how to take the necessary environmental measurements, necessary instrumentation and their precision, and contents of subjective questionnaire. Though the PMPCB is intended for use in surveys of occupied commercial buildings, there is no reason as to why its advisories and guidelines may not be followed for surveys of other building types too. Over the past decades, surveys done around the world have tried to come closer to the ASHRAE standards. The ASHRAE sponsored field studies in Townsville [19], San Francisco [92], southern Quebec [93], and Kalgoorlie-Boulder [45] followed these protocols to the dot. The study done in San Francisco [92] was the trend setter which established the methodology followed not only by all subsequent ASHRAE sponsored surveys but also by many other surveys around the world. Not all surveyors are able to use such standard set ups and measure all the environmental parameters at three different heights. But there has been a continuous shift towards trying to get the measurements needed for PMV calculations and preferably in the immediate vicinity of the subject. Most surveys have preferred, for better mobility, the use of something along the lines of a cart loaded with all the sensors and data loggers. Measurements are taken at one height which is generally the sitting height of subjects. Sometimes authors have also measured other variables that impact comfort: sound level, illumination level, ventilation rate, concentration of pollutants etc. Outdoor climate conditions are generally taken from a nearby meteorological station or in case none is nearby, those values are also measured. There is no well defined convention when it comes to choosing how many responses to collect or over what duration to take the survey. Different authors have worked with a hundred to a few thousand responses, used longitudinal or transverse surveys and sometimes the same study has involved both longitudinal and transverse responses. Nicol et al. [52] have shown that one does not always need a costly instrumental set up to be able to conduct a field survey yielding consistent and valuable results. Certain suggested guidelines on choosing survey subjects could be: - Facing budget constraints, it is better to survey a large number of subjects with a simple set of instruments [1]. - If the survey is taken over a few days only, the thermal environments encountered may not be representative of the general conditions. There might also be temporary malfunction of HVAC equipments [123]. - When doing a longitudinal survey, if possible, replace subjects after about three months so as to avoid both the risk of bias and ‘survey fatigue’ [103]. 3.5.1. Survey questionnaire Over time, questionnaires have also evolved. Early surveys used to have just the question on thermal sensation. Most field studies now include the ASHRAE seven point scale for sensation, McIntyre

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scale for preference, some equivalent of the Bedford scale for comfort rating, a direct acceptability question, along with check lists for clothing and activity in the past half hour or one hour. Additionally, there may be questions regarding the different adaptive opportunities available and how frequently subjects use such opportunities, the subject’s rating on his/her productivity level, subjective rating of sound and illumination levels, air quality, odours. Questionnaires may have a section to collect background information on the subject d like height, weight, ethnicity, gender, use of air conditioning at home or in transport. Mostly, questionnaires are collected in form of multiple choice questions on paper. There have been instances of computer based [19,92] and web server based [86] questionnaires too. The PMPCB also gives advisory on how to form and what to include in comfort survey questionnaires [122]. The manual recommends that subjective responses be recorded on a continuous scale, through web based or PC terminal based surveying, for “more powerful statistical analysis”. Surveys that use the ASHRAE scale to determine thermal sensation and the Bedford scale to determine thermal comfort levels give a mismatch between resultant votes for the two scales [4,10,23,26,34]. One obvious explanation is that thermal sensation is not the sole determining factor for thermal comfort. What also plays a part in this inter scale discrepancy is that people need not interpret a neutral sensation as comfortable or for that matter, a cold/warm sensation as uncomfortable. As de Dear observed in the post occupancy surveys done for two green buildings in Melbourne and Sydney respectively, occupants may rate the buildings as warmer/colder in summer/winter than mechanically conditioned buildings but they are also more forgiving and accepting of the buildings’ environment [124]. 3.5.2. Adjusting to local traits Survey methodologies have themselves “adapted” to local situations when they needed to. Researchers have used person to person interviews either directly or through interpreters [15,31,69, 88], to suit local sensibilities, surveys in residences have been taken solely by female experimenters [47], and survey questionnaires have been translated to a multitude of world languages. The SCATs project was conducted across multiple countries in Europe and questionnaires were developed in all the local languages [104]. We cite here the works where authors have explicitly mentioned about use of questionnaires in a different language and the language used: Arabic [46,47,49], Chinese [60,68], Japanese [87], Kurdish [84], Nepali [15], Telugu [31], Thai [41], and Urdu [52]. 3.5.3. Determining preferred temperature Currently used survey practices do not follow a fixed process for determining preferred temperature. Most authors regard the point where equal number of subjects “want cooler” and “want warmer” surroundings as the preferred temperature [4,5,13,19,21,25,34,39, 57,70,80,93,95]. This method was also used by de Dear and Brager in their meta analysis [101]. Other methods include finding the point where maximum subjects vote for “no change” [9,92,96] or point where curves fitted with percentage of “warm dissatisfaction” and “cold dissatisfaction” intersect [56]. 3.5.4. Percentage dissatisfied Several methods are in use for estimating acceptability or lack of it from voting pattern and these different methods do not have a lot in common with each other. One of the more common ways is applying the PMV-PPD relation to field study votes and assuming that an average thermal vote of 0.5 corresponds to 90% acceptability while an average thermal vote of 0.85 corresponds to 80% acceptability [35,46,47,91,93]. This method makes the least amount of sense considering the

fundamentally different natures of a field study and Fanger’s rational approach. Another approach followed is classifying all votes outside the central three categories of the ASHRAE seven point thermal sensation scale as unacceptable [25,51,56,68,87,92]. The problem with doing so is that direct acceptability questions often reveal people outside the central three categories also voting their environment as acceptable or opting for ‘no change’ on the McIntyre scale [99]. The approach that we consider to be most straight forward and least likely to cause controversies is inclusion of an additional question in the survey regarding acceptability or lack of it [44,55,62]. Brager et al. observed that use of different methods to gauge dissatisfaction produces widely different results [99]. The same conclusion can be drawn from the results of a number of field surveys too [5,39,44,70]. Acceptability of occupants forms a major part of a building’s evaluation. A unified and universally accepted method for determining percentage of acceptability as well as preferred temperature would help comparison of results from different surveys. 3.5.5. Choice of survey parameters When calculating PMV values, there are two parameters which have an inordinate amount of variation between surveys: chair insulation and metabolic rate. Though all studies have considered near sedentary subjects, the range of metabolic rates they assume varies from 1 to 1.6 mets. It is to be noted that these are values approximated for standard activities from databases depending on the occupants answer to survey questions regarding their activity rate. These values are not based on any actual field measurement using calorimeters. One example of this difference is seen from the studies in offices in San Francisco, Townsville, and southern Quebec. All three studies were ASHRAE sponsored and had subjects performing similar office activities. Yet the three studies took metabolic values of 1.1, 1.3, and 1.2 mets respectively [19,92,93]. Similarly, a large range of values have been assumed for chair insulation starting from 0.01 clo to 0.15 clo. While 0.15 clo might make sense in certain office environments, an executive chair d for which the insulation would be 0.15 clo [98] d is quite unlikely to be at homes. So certain surveys done in residences using 0.15 clo for chair insulation [4,9,34,96] does not seem meaningful. We would like to stress here that values of metabolic activity or clothing insulation do not have a direct impact on development of adaptive models. Their use is confined to comparing the actual thermal sensation votes with PMV values. However, uniformity of such assumptions would help comparison between studies and interpretation of results by other experimenters. 3.5.6. What index to use for the indoors? In a field survey, the subjective comfort level reported needs to be related to some representative measure of the indoor environment. The surveys we have covered in this work have used quite a broad range of such indices: indoor air temperature, operative temperature, globe temperature, ET*, and SET*. One may go for complicated indices that include more of the environmental parameters and look more “complete”. But, as Humphreys observes, globe temperature can be an adequate representation of indoor thermal environment [102]. Later, de Dear and Brager found in their seminal work on adaptive comfort that the best correlations were obtained using operative temperature. They assert that behavioural adaptation makes more complicated indices not fare as well [101]. Nicol and Humphreys concluded that more complicated indices have less strong correlation with subjective expression of comfort [104]. They also put forth that operative temperature performs well enough as a representative of the indoor conditions and for sedentary subjects, operative temperature can be approximated by using a globe thermometer with a 40 mm diameter globe.

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3.6. Adaptive opportunities: their availability and use Choice of adaptive adjustments by occupants is governed primarily by three criteria: ease of use, effectiveness of use, and economy of use d the three Es [71]. When faced with a choice, occupants will try to alter their environment (open doors or windows, switch on a fan), then use methods affecting their person (change clothing, take a bath, take more cool or hot drinks), and finally try moving to more comfortable regions in the building [4,10,23,34,52,71]. The order of enacting these adjustments follows a decreasing sense of ease from the occupant’s viewpoint. An exception is seen in Nepal where the traditional architecture is such that houses have cooler and warmer regions (semi enclosed spaces or rooms with fire places) [15]. At the same time, we must remember that not all adaptive actions need be helpful in saving energy. The guiding force behind any adaptive action is to restore comfort. Once in a while, people will resort to such actions which are wasteful from energy consumption viewpoint but nonetheless enhance the feeling of comfort. One example could be occupants opening up windows to let fresh air in while air-conditioners are operational [66]. In what follows, we briefly cover the different adaptive opportunities available to occupants and their usage as recorded in different surveys. 3.6.1. Use of air-conditioners In a majority of the surveys we have considered, survey locations are in countries where penetration of centralized climate control is minimal. Though use of air-conditioners has increased many fold over the last few decades in Europe [74] as well as in countries like China, Brazil, India, Pakistan, and Singapore [2,7,10,32,68,125,126], this use is mostly confined to window or split units. Such buildings may be considered to be a type of mixed-mode building in their operation. The cooling systems are under direct occupant control instead of any automated controls. As noted by Brager and Baker, mixed-mode buildings have been observed to do much better than buildings with conventional air-conditioning when it comes to occupant satisfaction with thermal comfort and air quality [127]. They have further observed that even amongst mixed-mode buildings, those buildings had more occupant satisfaction which provided greater degrees of direct control to the user. Since the control of window or split units remains in the hands of the immediate user and there are no predetermined set point temperatures, several studies have considered such buildings in the same vein as FR buildings [10,27,33,42,51,52,71]. The idea is that since the occupant is still exerting considerable influence, its more useful to take the actual sensation vote approach than the predicted mean vote approach. For occupants in these buildings, switching the air-conditioner on/off is taken as just one more adaptive behaviour. And it does seem to have similar usage pattern as any other adaptive behaviour. What acts in favour of using air-conditioning is its ease and effectiveness and what acts against it are the costs involved. Unlike other adaptive measures, a person would just switch on the gadget and maintain a particular temperature. In those instances when economy of operation ceases to be an occupant concern, people get used to air-conditioning and start maintaining cooler temperatures, use adaptive opportunities earlier, are more sensitive to temperature changes, and give up other adaptive actions in favour of using the air-conditioner [27,68,70]. This has been described as ‘thermal indulgence’ by Indraganti [30]. Concerns for energy expense mean that an air-conditioner is either used less, or maintained at a high temperature or used only when temperatures are too high for other adaptive measures [2,18,28,53,71]. The study done across Taiwan by Hwang et al. finds that while the air-conditioners in offices were always on (as the utility bill would be borne by the employer), people in their homes used air conditioning sparingly and their first preference was

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opening windows [71]. This would be an example where economy trumps ease of use. At the same time, a few surveys done in MC buildings resulted in occupants rating their thermal sensation as cold/slightly cool [33,37,38,42,44,57,61]. This may imply that conditioned buildings, designed in keeping with rational standards like PMV, are often cooled beyond neutral/overcooled. But, such inferences may often get hidden by the narrow range of temperatures maintained in such buildings and the ability of PMV model to account for clothing and metabolic adjustments [101,118]. What is also true is that studies done in mechanically cooled or heated buildings have often come up with occupant complaints of stuffiness, lack of fresh air, and unpleasant odour. There are examples where the occupants have kept windows open, even while airconditioning is operational, to satisfy their craving for “fresh air” [66,69]. A few studies show that occupants have a perception of airconditioning as something unhealthy, of low ventilation quality, and something that cuts them off from the outside [9,18]. The study by Nakano et al. in air-conditioned offices d apart from showing incidences of Sick Building Syndrome (SBS) related symptoms d gives a rather high concentration level of CO2 inside the building (1200 ppm) [85]. For buildings using mechanical heating, incidences of non-thermal discomfort like dry mouths, throat itching etc. increases in the period of heating use [96]. This of course has more to do with the levels of humidity maintained during conditioning. 3.6.2. Adaptive opportunities of buildings Apart from a building’s design, the purpose of a building can also help or hinder the cause of thermal comfort. For example, in a classroom situation, not all the students have control over windows or fans and during lessons, there are further restrictions on how they use windows or shades [6,39,44,82,87,97]. Also, authors have noted that occupants have less adaptive choices in offices than at homes which leads them to be more stringent at judging office conditions [71,84]. This finding is in agreement with Oseland’s and Karjalainen’s works. Oseland concluded that even for similar thermal environment, clothing resistance, and activity level, people rate their homes better than their offices and their offices better than climate chambers [128]. Karjalainen’s survey showed that even the least satisfied group in home environment is more comfortable than the most satisfied group in offices [129]. It would appear hence that people vote closer to comfort if they are given more adaptive opportunities and they are more comfortable in places familiar to them. The concept of familiar surroundings getting better comfort ratings is also noticed by Indraganti in her field study of residences in Hyderabad [29]. Women, who spent more time at home, voted more often within the comfort band than men. The author goes on to suggest that the women’s familiarity with home surroundings may also be helping them to use the available adaptive features more effectively. The building type that has come up with a very bad record in terms of occupant satisfaction is the open plan office [75,83,85]. This is most likely due to the varied nature of occupants in such places and the very minimal adaptive measures that are available in such buildings. Certain adaptive opportunities are deeply ingrained in the building structure and are thus available to the inhabitants without need of extra effort. Buildings that have thicker walls or/and less glazed surface provide more comfortable indoors [20,33,39,88,90]. Living in the ground floor of a structure means occupants do not have to deal with direct solar radiation on their roofs and that also can keep them cooler [20,25,28,77]. For occupants in the top floors, this can translate to greater use of gadgets like air coolers and air conditioners [28]. Being exposed to higher temperatures, as a matter of course, changes the thermal history of the occupants of top floor. While the higher temperatures experienced lead them to vote less favourably and with lower acceptance, under situations where the available adaptive opportunities are enough, these

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occupants also report a higher neutral temperature due to their thermal history [32]. What may seem counter intuitive at first glance are claims by some occupants to be more comfortable in the higher (even top) floors [9,10]. We believe this could be in part due to greater air movement at higher floors and in part due to lower solar radiation in high latitude locations or locations with rainforest type climates. 3.6.3. Endemic adaptations Certain adaptations are specific to certain locations. In many countries of the Indian subcontinent, it is customary for people not to use footwear inside their homes. For these populations, this is an effective and economic adaptation, ingrained in their culture. This adaptation gets them in direct contact with the cooler floors and keeps them more comfortable during hot weather [15,20]. This can however be a disadvantage in certain locations across these countries that have cold winters and people have taken to wearing socks, or socks and shoes indoors, during cold weather [15]. Similarly, the study in Ilam [84] shows that while people at homes sit on the cooler floors during daytime, in workplace people have to sit on chairs thus adding to their overall clothing insulation. The study finally yielded a lower neutral temperature for the office population. This ‘sitting on the floor’ type of adaptation was also used by subjects in India and Libya [30,49]. The study done in Hyderabad [27] found people taking a post midday-meal siesta. This decreased their activity level and helped them to get past the hottest period of the day. We have not found other authors explicitly mention about such an adaptive measure though its likely to be common in most hot and humid regions. 3.6.4. Common adaptive measures Opening windows is one of the most favoured adaptive measure across countries [4,9,10,27,34,52,58,66e68,71,90]. Window opening is attractive in terms of its ease, effectiveness, and economy of use. Most of the above surveys also had high frequencies of door opening instances amongst occupants. Opening windows decrease the feeling of “stuffiness” and increases the wind speed indoors. Due to this, residents do not mind opening up windows even during cold seasons to let in some fresh air [67]. As Zhang et al. observe from the responses of office occupants, the three most common reasons for opening windows both during summer and winter were: “to feel cool”, “to feel more air movement”, and to “let in fresh air”. On the other hand, the primary reason for closing windows during summer was “to reduce outdoor noises” while in winter, windows were closed “to feel warmer” and “to reduce outdoor noises” [119]. Even if the outdoor temperature is higher, open windows do not allow solar heat gain to be trapped in and enhance wind velocities, thus aiding sweat evaporation [130]. Mostly however, people in hotter climates close their windows when outdoor temperatures go beyond a certain value [28]. Use of electric fans is quite widespread in tropical and humid climates [4,10,18,20,36,68,71,72] due to their effectiveness in obtaining larger indoor air velocities and economy vis-à-vis airconditioners and air-coolers. People also use smaller, personal fans to locally increase air velocity in their immediate surroundings [86]. The other extreme situation is found in places where a significant percentage of the population does not have fans or is not familiar to fan usage [9,34,58,90]. Under moderate conditions, people with access to fans have been observed to disregard other adaptive actions like opening windows, changing clothing etc. in favour of the ease of using a fan [30]. This observation of course sits well with the three E point of view. As temperatures rise, ceiling fans in rooms with exposed roofs end up aiding circulation of the warmer air from near the ceiling. This also leads occupants to switch off these fans and depend more on coolers, conditioners, and pedestal fans [28].

In cold climates, adaptive measures include use of electric or hand held heaters [52,53,55,88,96] or use of wood burning stoves over countrysides [15,55]. For winter, people may also choose thicker blankets to enhance comfort [15]. A rather obvious adaptive measure, as winters come up, is the closing of doors and windows [67]. 3.6.5. Clothing adaptations When thinking of use of clothings for thermal comfort, one is reminded of an old Norwegian proverb: “There is no such thing as bad weather, just bad clothing”. For long have clothing changes been an economical and effective method of achieving comfort in varied climatic conditions [112]. Even rational models of thermal comfort and corresponding comfort standards give considerable importance to clothing variations [98,131,132]. And in situations when occupants are allowed flexibility in their dressing pattern, varying clothing is seen as an easy, economic, and effective manner of adapting to the environment. So we discuss the nature of clothing adaptations in a section of its own. Clothing adaptations are often seen reaching the point that we would like to term as “adaptive saturation”. The phenomenon is mostly observed in hot seasons and less commonly during cold weather. Schiavon and Lee found that variation of clo values had greatest dependence on the outdoor air temperature measured at 6 a.m. and the indoor operative temperature [114]. As the climate of a location progressively gets warmer, clothing pattern gets lighter till it reaches a minimal socially acceptable limit. So from a certain point on, while the temperature might keep increasing, clothing remains the same. Thus, summer correlations found between clo values and indoor or outdoor temperatures are not very strong [2,6,52,53,56,64,70, 76,84,88,94]. Due to this reason, clothing patterns are less variable during summer and scope for adjustments in clothing patterns are lesser during warm periods [114]. A similar phenomenon is sometimes observed with winter clothing [52,56,88]. Rohles and McCullough have also observed that as people put on more layers to counter decreasing temperatures, a point does come up where the ensemble starts to get too bulky, unfashionable, and interferes with regular lifestyle [112]. However, occurrence of clothing adaptive saturation is much less common for winter clothing. In fact, when regression equations are developed between indoor temperature and comfort votes for warm and cold seasons separately, it is found that the slope of the line is smaller for winter equations [6,8,45,80]. This lower sensitivity to temperature changes and increased adaptability is likely because of the greater clothing variation. Several authors have found that traditional clothing of a location gives people much more flexibility and adaptability, allowing them to withstand the broad ranges of temperature they face in FR buildings [15,31,41,50,52,84,88]. A form of clothing adaptation that has been particularly observed amongst students is putting on and off extra clothing like jackets or sweatshirts, while moving between classes, reading rooms, and outdoors [5,44,87]. This behaviour is unlikely to be noticed in offices due to the formal surroundings. At residences, people know what kind of thermal environment to expect. They dress accordingly and require minimal changes over time. Students on the other hand are able to vary clothing to suit the transients and get maximum thermal satisfaction. Authors of two studies done in Libya and Tunisia [50,51] have developed a quadratic fit between the clothing and outdoor temperature data. Using these equations with high values of outdoor temperature, that one might likely encounter in a desert climate (40e50
C/104e122
F), the clothing resistance value increases. Normally one expects clothing to get lighter with higher temperature but the above phenomenon shown by the regression equations is nonetheless practical. People in deserts do try to cover up greater portions of their body to avoid exposure to direct sunlight.

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More clothing also encourages perspiration which ultimately contributes to keeping the person cooler in a dry climate [50,51]. 3.6.6. Impediments to adaptive measures A well accepted precept of adaptive theory is that “a person is not a passive receiver of thermal sensation but an active participant in a dynamic equilibrium with the thermal environment” [1,133]. As Humphreys and Nicol put it, if adaptation of the occupants is inadequate, it is likely due to the circumstances of those occupants rather than the thermal environment [1]. Common obstacles preventing full exercise of adaptive opportunities are : privacy, security concerns, pollution, noise, and bugs. Such obstacles often alter the perception regarding ease of use or effectiveness of an adaptive measure. Concerns for privacy, security, outdoor noises, or bugs entering living quarters often lead to occupants closing their windows [10,25,32,34]. In certain regions, power cuts hinder the active use of electrical appliances like fans, air-coolers, air-conditioners, or electric heaters [30,88]. Restrictions on dressing due to societal acceptance or workplace rules also lead to incomplete adaptation. Earlier, we have already discussed about adaptive saturation of clothing in summer and winter. Dressing options for women is often dictated by societal acceptance [27] or requirements of fashion [30] rather than thermal comfort. Use of uniforms minimizes variation of clothing resistance values across different seasons and limits choice of people to vary their clothing according to changing weather [22,84,86,87]. An interesting limitation to clothing choices has come up with globalization. Professionals around the world have given up their traditional clothing in favour of western business outfits, irrespective of the local climate. When people are not able to exercise their adaptive options, they try to use more power intensive methods like turning on the air-conditioner. But if such a course of action is not available, occupants are dissatisfied and rate their surroundings very poorly. 3.6.7. Some interesting contrasts The study done by Han et al. in Changsha and its neighbourhood [55] shows that rural residents have greater tolerance to similar climatic conditions and vote more acceptably than their urban counterparts. The reason could be a combination of better acclimatization and lower expectations of the rural population. Along similar lines is the finding of Indraganti from her study in Hyderabad [29]. Subjects from higher economic groups were found to be more dependent on gadgets like air-coolers and air-conditioners for comfort while neglecting adaptive measures. People not so well off were better adept at using adaptive control strategies. They showed more tolerance of their environment, higher satisfaction, and higher neutral temperature. The author found that even the comfort bands were different for the different economic groups. Yamtraipat et al. analysed their data after grouping the subjects on basis of their education levels and found that subjects with higher education voted at higher values of thermal sensation/warm discomfort [42]. The authors found that this could be a side effect of the fact that people with higher education often have such job profiles that require them to wear clothes with greater insulation d like multi-piece suits. Studies that have analysed their subjects by grouping them into those who have air-conditioning at home and those who do not, have also found different neutral temperatures and different thermal sensitivity for the two groups [25,42]. This result is essentially similar to the one obtained by Wang et al. in Harbin [96], where the occupants have a slightly lower sensitivity to temperature changes before heating started than when heating is in use. The study done in Hyderabad found that comfort votes were more favourable in residences where owners lived than in residences housing tenants [27]. The reason given by the author is that owners were more active in adding certain fitments to their places

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which made use of adaptive opportunities more possible without loss of privacy or the sense of security. Liu et al.’s field study of offices in the Chongqing university revealed that as levels of sunlight depend on a room’s orientation, so the orientation affects frequency and nature of occupant adaptive behaviours [66]. Cao et al. found that subjects who were not used to indoor heating during winter rated their surroundings warmer than those who had gotten used to the heating [94]. They also found that this kind of adaptation, to not using heating, took about an year to be lost. 4. Conclusions With changing climate, concerns regarding CO2 emissions, and fossil fuel depletion, there is an increased drive towards energy economy. Under these conditions, it is not a far fetched idea that in the near future, natural ventilation and low energy cooling systems would be the norm instead of the fringe. The imperative would be to have the right standards and design methods for the new systems so that we can effect energy economy without sacrificing comfort. There exists a large body of literature on thermal comfort field surveys and their applications to adaptive comfort models. Ideally, we could have a field study in every habitation and determine an ACE suitable for the particular location and culture. Since economic and time constraints mean that such an idealization is unlikely to happen, current standards provide generalizations of global data to aid and inform the building designers. Wherever possible, these generalizations can be improved with local field surveys. The current work tries to divide findings from field surveys on basis of climatic zones and further tries to find out important trends in thermal comfort of occupants. We expect these findings can be of aid in regions where no previous field study on thermal comfort has been undertaken. Several concepts in the field of adaptive comfort have been receiving increased attention and are being continuously better understood. Examples would be use of RMT as a preferred index of outdoor conditions, use and acceptability of greater air circulation in occupied zones, and acceptance that humidity levels (apart from rather extreme values) do not have a very large effect on thermal comfort sensation. There has been an attempt to make aspects of comfort surveys d like the clo and met values being used in analysis, questionnaires, instruments and measurement locations etc. d more uniform so that comparison of results and sharing of data can be aided. There is also the growing realization that both the non-availability of adaptive means and hindrances to their exercise can make occupants feel uncomfortable and rate their environments poorly. We summarize some of our more relevant observations here: - Excepting type A climates, other climate types have rather broad ranges of neutral temperature and comfort zones. This is expected since type A climates have minimal seasonal variations - Buildings, by their design and purpose of use, can help or hurt the cause of adaptive behaviours. - Available evidence suggests that neutral temperatures determined through field surveys do not change over reasonable intervals of time. - Adaptive opportunities are brought into play based on their effectiveness, ease of application, and economy. Occupants will always try to maximize all three Es and may employ multiple adaptive actions, in parallel, to achieve this. - When money is not a concern, people will prefer to ‘indulge’ in using the ease and effectiveness of gadgets like coolers and conditioners. In long term, such use makes them less dependent on other adaptive actions. - Certain adaptive actions, like opening windows, are popular in all climate types while certain actions, like sitting on cool floors, are limited by climatic and cultural traits.

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Appendix A. Adaptive comfort models. Table A.1 Adaptive comfort equations. Climatic zone

Location/source

Equation

Remarks

Meta studies

Humphreys [1] Humphreys [1] Humphreys [1] Auliciems [1] ASHRAE Standard 55-2004 [98] EN15251 [103]

FR buildings MC buildings MC and FR MC and FR FR buildings FR buildings

Type B

Pakistan [52] Tunisia [51] Shanghai [11] Hong Kong [62] Bari, Italy [80] Harbin [9]

Tn ¼ 11.9 þ 0.534To pffiffiffi 2 Tn ¼ 23:9 þ 0:295ðTo  22Þexpð½ðTo  22Þ=ð24pffiffiffi 2Þ Þ Tn ¼ 24:2 þ 0:430ðTo  22Þexpð½ðTo  22Þ=ð20 2Þ2 Þ Tn ¼ 9.22 þ 0.48Ta þ 0.14To Tn ¼ 17.8 þ 0.31To Tn ¼ 19.39 þ 0.302TRMT; TRMT > 10
C Tn ¼ 22.88
C; TRMT 
C Tc ¼ 18.5 þ 0.36Toh Tn ¼ 11.56 þ 0.532TRMT Tn ¼ 15.12 þ 0.42To Tn ¼ 18.303 þ 0.158To Tn ¼ 17.80 þ 0.315TRMT Tn ¼ 11.802 þ 0.486To

Type C

Type D

FR buildings, historic outdoor mean temperature FR buildings FR buildings MC buildings MC and FR FR buildings, in summer

Here, To is outdoor monthly mean temperature; Toh is the historical outdoor monthly mean temperature (averaged over 30 years in the given case); Ta is the indoor air temperature; Tn is the neutral or comfort temperature; TRMT is the running mean temperature. Both the local ACEs given here that use TRMT as an index of outdoor temperature, use similar formulations for TRMT as EN15251. All temperature units are in degrees Celsius.

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