Thermal Comfort ZoneThermal comfort

Thermal Comfort Zone
Thermal comfort is complex and partly
subjective. It depends on many factors, of which
air temperature, humidity, air movement, thermal
radiation, the metabolic rate and the level of
clothing are fundamental. The impacts of these
factors on the thermal balance of the human body
irrespective of adaptation to the local climate form
the basis on which theoretical comfort models/
standards, such as Fanger’s PMV [9], its derivative
ISO 7730 and most versions of ASHRAE Standard
55, were developed. However, adaptive models,
such as those developed by Auliciems [10],
Humphreys [11] and Szokolay [12], also consider
acclimatisation an important factor in comfort
sensation. This difference leads to the adaptive
models predicting comfort zones which vary
according to the prevalent local climates, while
the theoretical models predict comfort zones which
are independent of local thermal conditions.
Results from a large number of field
studies have indicated that theoretical models
which neglect the impact of acclimatisation can
significantly underestimate the thermal and
humidity tolerance of the occupants of freerunning
buildings in hot humid climates [13-25].
For example, while ASHRAE Standard 55-1992
Addenda 1995 [26] suggests that the summertime
comfort zone ranges from about 23.5๐C at 25%
relative humidity (RH) to about 26๐C at 60% RH,
comfort is experienced at a temperature as high
as 32๐C at over 85% RH in Bangladesh [17], and
within higher ranges of temperatures and relative
humidities of 25-31.5๐C and 62.2-90% RH in
Thailand [15]. Adaptive models, however, usually
predict comfort zones which are closer to the field
study results.
This tolerance to relatively high temperatures
and humidities is likely to be a result of
adaptive activities, such as opening windows and
removing clothes, which form part of the daily life
in hot humid climates [27], as well as the homoeothermic
mechanisms of the body. Indeed, the
degree of adaptive opportunity can influence
thermal comfort expectation: people tend to
accept warmer environments more readily in their
homes than in offices, as they have more control
over their environments and activities in the
former situation [27]. Such impact of acclimatisation
in extending the comfort zone suggests
that passive design probably has greater potential
to provide thermal comfort in hot humid climates
than is generally believed. Also, theoretical comfort
standards which neglect acclimatisation, such as
ISO 7730: 2005 [28], are likely to be inappropriate
for free-running buildings in hot humid climates.
The impact of acclimatisation observed
in the above field studies has also led to the
modification of existing comfort standards/models.
A key example is the work by de Dear and Brager
[25] that contributes to the introduction of an
adaptive comfort model for naturally conditioned
spaces for the first time in ASHRAE Standard 55,
in its 2004 version [29]. Other examples include
the work by Srivajana [18] that adjusts the
Standard Effective Temperature (SET) comfort
scale originally defined by Gagge et al. [30] to
accommodate prediction of thermal sensation
under higher air velocities and lower clo values
commonly found in hot humid climates. Jitkhajornwanich
[15] modifies Olgyay’s bioclimatic chart [31]
to take into account the acceptance of higher
temperatures and humidities in hot humid climates
(Figure 1). And Khedari et al. [16] put forward a
ventilation comfort chart for a higher range of
indoor air velocities often encountered in the
climates.
As attempts to identify the appropriate
comfort zones for different local conditions
continue, the impact of humans’ acclimatisation
to global warming on their thermal tolerance and
preference should perhaps also be taken into
consideration. To observe and understand this
impact, research which involves long-term
monitoring is probably required. Moreover, as the
economies of certain parts in hot humid regions
grow peoples’ tolerance to higher temperatures
and humidities may diminish due to increased
expectations [22]. The impact of economic and
social factors on thermal sensation is another area
which offers opportunity for research.