The last decades of thermal comfort research have produced an irreconcilable debate between the two philosophies of thermal comfort: the “PMV” and “adaptive” models have contrasting assumptions about the way people respond to their environment. Ongoing research investigates if the application of a certain thermal comfort approach should depend on the type of building, e.g. naturally ventilated building, low-energy building, and air-conditioned building. The adaptive model of thermal comfort considers three categories of adjustment that people in buildings undertake to achieve thermal comfort: behavioral, physiological and psychological adjustment.
Everyday condition: Humphreys and Nicol (2002) argue that using a steady-state equation to predict responses of people in dynamic equilibrium is only an approximation. PMV can be seriously misleading when used to predict the mean comfort votes of groups of people in everyday conditions, particularly in warm environments. It is the common agreement among several researchers that the relationship based on laboratory experiments should be tested in the field before they are included in comfort standards.
Non air-conditioned buildings: de Dear and Brager (2001, 2002) found PMV to overestimate the subjective warmth sensations of people in naturally ventilated buildings. The bias in PMV is strongly related to the prevailing mean outdoor air temperature, in a manner that is non-linear. de Dear and Brager (1998) and Linden et al. (2006) show that occupants evaluate the indoor climate differently in buildings where they can individually influence the thermal indoor climate, e.g. operating windows, doors and blinds. In such buildings, higher indoor temperatures are more accepted than Fanger’s model predicts, especially in periods with higher outdoor temperature. Therefore, Linden et al. (2006) state that the static comfort model is only truly appropriate for sealed air-conditioned buildings, because the model can only take effects of behavioral adaptation into account (adjustment of clothing, level of activity, increase of air velocity). The generalization of the PMV-model for non air-conditioned buildings is considered as inappropriate.
Occupant’s control: Occupants with more opportunities to adapt themselves to the environment or the environment to their own requirement will be less likely to suffer discomfort [Nicol et al. 2002a]. Discomfort is increased if control is not provided, or if the controls are ineffective, inappropriate or unusable. Bordass et al. (2001a, 2001b) and Gossauer (2008) confirm these findings by comprehensive occupant surveys in the UK and Germany, respectively. Raja et al. (2001) and Rijal et al. (2007) state that windows are extensively used by occupants. At indoor temperatures in excess of 20 °C the number of subjects reporting the opening of windows rises steeply with indoor temperature and approaches 100 % at temperatures above 27 °C. The importance of individual occupant control is already acknowledged by the German Organization ‘Deutsche Gesellschaft für Nachhaltiges Bauen’ [DGNB 2009], where occupants’ influence on the surrounding condition is one factor for building certification. In addition, the U.S. ‘Green Building Council’s Leadership in Energy and Environmental Design’ accreditation system has three points associated with thermal comfort, of which one specifically applies to the control of operable windows [LEED 2006].
Occupant’s expectation: Thermal sensation is influenced by an individual’s experience and expectation of the building’s climate based on outdoor temperature of that particular day and of the preceding days. However, results of a study in Germany by Gossauer (2008) reveal that occupants might appreciate the opportunity to condition the office or influence the temperature at the workplace, respectively, at higher ambient air temperatures.
Effectiveness of intervention: Post occupancy evaluation studies demonstrate that occupant satisfaction with comfort correlates with opportunities to make interventions and with the effectiveness of these interventions [Gossauer 2008], [Leaman and Bordass2001]. Simplicity and convenience of intervention are paramount.
Overall comfort: Intensive research based on field studies has determined that satisfaction with workplace conditions is not exclusively driven by thermal comfort, but is also affected by very diverse factors such as visual and acoustic comfort, interior design, the occupant’s expectations and control of the surroundings, the user’s behavior as well as social, cultural and psychological parameters [Wagner et al. 2007], [Leaman and Bordass 2001,2007], [Bischof et al. 2003], [Humphreys and Nicol 2000a], [Humphreys and Hancock 2007], [Hellwig 2005]. It is often stated that physiological acclimatization to heat or cold does not affect the preferred bodily condition to thermal comfort, but it is generally agreed that it does affect people’s tolerance to body states that differ from that preferred one [Humphreys and Nicol 2002].
Health: Roulet (2001) states that, obviously, large energy consumption of the HVAC system does not result in better health. On the average, the higher the energy consumption, the larger are the number of sick-building syndromes (generally found in fully air-conditioned buildings, [Bischof et al. 2003]).
Forgiveness: In passively conditioned buildings (openable windows, sun shading systems), typically more adaptive mechanisms are available to the occupant for comfort and consequently a greater individual awareness of the available adaptive opportunities. Buildings, where people have easy access to a variety of building controls that enable direct effects on comfort, are found to have an attitudinal shift of occupant “forgiveness”. Leaman and Bordass (2007) coined this term as a description of how people extend their comfort zone by overlooking and allowing for inadequacies of the thermal environment [Kwok 2009]. Forgiveness factors are defined for different building types sorted according to the ventilation system employed. Buildings with natural ventilation are found to have the highest forgiveness scores, i.e., people may more likely tolerate otherwise uncomfortable conditions in buildings with natural ventilation.
Occupant’s tolerance: Investigations by Humphreys and Nicol (2000b) show that it seems to be increasingly common for users to tolerate conditions that are hotter than they were a generation ago. People were on average comfortable at about 20 °C in 1978 (from data put together in 1978 but collected earlier) and at about 23 °C in 1998 (from data put together by G. Brager and R. de Dear in 1998 [Leaman and Bordass 2007]. Low-energy buildings tend to be warmer in summer than conventional air-conditioned buildings, but thermal comfort has been rated as more comfortable and satisfying in low-energy buildings [Leaman and Bordass 2007]. However, it needs to be verified, if the findings of 1998 are applicable to today’s comfort requirements, since people spend a lot of time at conditioned spaces such as shopping malls, cars, trains, banks, etc. Therefore, higher indoor temperatures might be perceived as more unpleasant and uncomfortable.
Dress code: Changes in clothing and activity will change the condition which people find comfortable. If no dress code is required, the occupant can adapt better to the surrounding conditions by changing clothing [Nicol et al. 2002a] and, therefore might accept higher indoor air temperatures. Occupants are more satisfied with room temperatures if they do not have to adhere to a dress code [Gossauer 2008]. Haldi and Robinson (2008) found by means of questionnaires that occupants behave adaptively in terms of clothing if they do not have to adhere to a formal dress code. They found a clear linear relationship between the running mean ambient air temperature and the level of clothing insulation. Outdoor temperature was found to be twice as effective in explaining the variation in clothing level as indoor temperature.
Productivity: The great challenge in correlating interior comfort and occupant productivity is that there is no common agreement on the definition and the methodology to measure productivity concerning office work. Some researchers argue that studies concerning the relation between warmer environments and productivity are not very conclusive [van Linden et al. 2006]. Some studies demonstrate a decrease in productivity at higher ambient temperatures (e.g. studies by Wyon qt. in [Fitzner 2004]). Other studies show a positive effect on productivity when people have adaptive opportunities, like openable windows, fans and blinds to alter their subjective warmth. In addition, field studies reveal that perceived productivity does not vary with indoor air temperature and that productivity is positively correlated with the perception of general comfort and health. Therefore, some researchers believe that adaptive comfort models in moderate climates will have no adverse affect on productivity, required that adaptive opportunities are available. Occupants, who perceive that they are comfortable, also tend to say they are healthy and productive at work. Thus, health, comfort, and productivity are often surrogates for each other [Bordass et al. 2001a]. Fitzner (2003) presents a literature review on productivity.
Energy Savings: On the one hand, the provision of occupant comfort has a major bearing on energy consumption. On the other hand, an energy-efficient building that cannot provide comfortable and high-quality working conditions will either affect the well-being – and therefore the productivity – of the occupants or drive them to take actions that may compromise the energy economy of the building (e.g. subsequent installation of portable cooling devices) [Nicol 2007]. In brief, an energy-efficient concept without occupant comfort compromises the sustainability and profitability of the property. As a result, an optimum is needed between energy efficiency, interior comfort, and expenditure, both for new constructions and refurbishments. The EPBD (Energy Performance of Buildings Directive) Article 7 requires the inclusion of information on the interior climate of a building for the certification of energy use. Besides, post occupancy evaluation and field studies show that high-energy use for heating and cooling buildings does not correlate with high occupant satisfaction. Sustainability of the building and technical plant performance needs to be considered in the framing of standards [Roulet et al. 2006a, b], [Humphreys and Nicol 2002], [Nicol and Humphreys 2009].
