RESEARCH NOTE - THERMAL COMFORT

Health,
Wellbeing and Productivity in Offices
Research Note: Thermal Comfort
RESEARCH NOTE - THERMAL COMFORT
We are particularly grateful to Paul Appleby, Guy Newsham, Derek Clements-Croome and Ashak
Nathwani for their efforts in leading the production of this research note.
Introduction
Human beings spend a significant amount of their lives in “artificial environments” - at work, at leisure / pleasure venues or at home. Thermal Comfort, within these built environments, plays a significant role in occupant health and wellbeing, which in turn has an impact on performance and productivity.
Climate change and the urgency of minimising the carbon footprint of the built environment are driving technological innovation in the way thermal comfort is delivered to building occupants. In recent
building developments, international attention has been increasingly focused on the way buildings
are designed and operated. There is no doubt that high-quality environmental design can be an investment, as occupants should be healthier, staff-retention rates should be higher, productivity
should increase and sustainability ideals are more likely to be met. With the ever increasing focus on
climate change mitigation and energy costs major efforts are being directed at improving thermal
performance of building envelopes and enhancing the energy efficiency of building services within
them. In most instances, however, such initiatives carry implications for the indoor environmental
conditions of buildings, particularly on building occupants. To meet ambitious energy targets, and
subsequent reductions in greenhouse gases, many sustainable building design and operation strategies are exploring new ways to provide comfort. It is not only occupant comfort that is being examined; occupant productivity is also coming under scrutiny.
Benefits for Occupants
Thermal comfort is defined in the International Standard ISO 7730 as: ‘that condition of mind which
expresses satisfaction with the thermal environment.’ Thus ‘thermal comfort’ describes a person’s
psychological state of mind and is often referred to in terms of how one feels - generally in relation
to air temperature. However, thermal comfort is more complex and needs to take into account a
range of environmental and personal factors. These factors make up what may be termed as the
‘human thermal environment’. Hence thermal comfort, as a parameter, can be assessed by subjective evaluation (ANSI/ASHRAE Standard 55).
Thermal neutrality is maintained when the heat generated by human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. The main factors that influence
thermal comfort are those that determine heat gain
and loss, namely metabolic rate, clothing insulation,
air temperature, mean radiant temperature, air speed
and relative humidity. However, it is also argued that
neutrality is not necessarily the preferred condition that people often prefer to feel warmer than neutral in
winter, and cooler than neutral in summer.
Asymmetry can also impact on thermal comfort: for
example different surface temperatures within a space
(radiant temperature asymmetry), temperature gradients between head and feet and fluctuating air velocities (turbulence intensity). Sitting next to a cold surface such as a single glazed Thermal Comfort
window in winter can cause discomfort due to radiant temperature asymmetry as Parameters
well as from a cold draught from both air being cooled and dropping down the
Source: HSE - UK
window surface and infiltration air leaking through gaps around the window.
Psychological factors, including individual expectations, also affect thermal comfort. According to
the UK Health and Safety Executive (HSE), the best that one can realistically hope to achieve is a
thermal environment that satisfies the majority of people (around 80 - 85% as an average) in the
workplace, or put more simply, ‘reasonable comfort’. By deduction up to 20% of occupants in an office could complain of “thermal discomfort”.
According to a 2006 survey by recruitment consultants Office Angels and the UK trade union USDAW:
15% of workers have arguments over how hot or how cold the temperature should be; 81% of workers
find it difficult to concentrate if the office temperature is higher than the norm; and: 62% of workers
state that, when they are too hot, they take up to 25% longer than usual to complete a task.
The Predicted Mean Vote (PMV) model stands among the most recognized thermal comfort models. It
was developed using principles of heat balance and experimental data collected in a controlled climate chamber under steady state conditions. The PMV scale stretches from plus 3 to minus 3 with
zero as the neutral point. A relationship has also been developed with Predicted Percentage (of people) Dissatisfied (PPD). At a PMV of + or - 3, 100% of any group is dissatisfied. At zero, at least 5% of a
group is still dissatisfied - which takes into account the variability in response by a population to the
same thermal environment (Fanger 1970).
In contrast to Fanger’s laboratory-based comfort model the Adaptive Model was developed based on
a great many field studies with the idea that occupants dynamically interact with their environment.
The adaptive comfort theory contrasts with simple static comfort metrics as a function of temperature and humidity. Adaptive comfort considers the behaviour of occupants, who adjust themselves
and their surroundings to retain comfort in changing environments (Nicol and Humphreys, 1973). It
also recognizes that people may prefer fluctuations in the thermal environment, rather than constant, neutral, conditions. The adaptive approach was brought into mainstream thinking in comfort
research and practice by ASHRAE when it commissioned de Dear and Brager in the mid-1990s to develop a rigorous adaptive comfort model from quality-assured field data, collected across the major
climate zones of the world (de Dear and Brager, 1998). Occupants control their thermal environment
by means of donning or removing clothing, unconscious changes in posture, choice of heating, moving
to cooler locations away from heat sources and other techniques such as operable windows, fans,
personal heaters and sun shades. The problems arise when such choices (e.g., to remove jacket, or
move away from heat source) are not available, and people are no longer able to adapt. In many instances the environment within which people work is a product of the processes of the job they are
doing, so they are unable to adapt to their environment (HSE).
It should also be noted that Fanger developed PMV and PPD to account for both activity (met) and
clothing (clo) and therefore adaptation through adjusting either or both of these (usually clo). However adaptation also relies on occupants responding on the basis of their thermal experience – their
more recent experience being the more important. Hence they will tolerate higher indoor temperatures when outdoor temperatures are higher.
A study by Kosonen and Tan published in 2004 estimates the relationship between productivity loss
and thermal conditions using PMV. This indicates for example a 15% loss in productivity when 'thinking and writing' when room temperature reaches 27C at 50%RH or 26C at 80%RH, or PMV reaches 0.5.
Cui et al (2011 & 2012) has also presented an argument that lower PMVs could result in higher
productivity but the outcomes are not conclusive.
However according to de Dear (2013), “despite the large volume of research effort directed at it over
the last couple of decades, our understanding of the thermal comfort effects on productivity is far
from clear. This is largely due to diverse definitions of the productivity metric and their varying degrees of validity. Performance has variously been quantified by educational achievement tests, psy-
chological tests, neurobehavioral tests, simulated office tests, commercially accepted work-place
task performance indices, and subjective self-assessments of productivity, to mention just a few.
There is also a plethora of approaches to the quantification of thermal comfort in this productivity
literature, ranging from simple air temperature, through operative temperature, up to rational comfort indices (ET*, SET*, PMV), and finally self-rated subjective thermal assessments on the familiar 7point scales. In view of this inconsistency in definitions of independent and dependent variables, it is
not surprising the results are confusing (CIBSE, 1999; Fisk and Rosenfeld, 1997). For example, even
within one paper (Pepler and Warner, 1968) contradictory associations were found with classroom
temperature; school children performed mental tasks faster at 20°C, but made fewer mistakes at
27°C. Some researchers claim to have shown that thermal conditions providing thermal comfort do
not correspond with maximum efficiency (e.g., Wyon and Wargocki, 2006) but these are in a minority. Different task types, exposure times, or workers’ psychological factors such as motivation or
arousal level are all potential confounders to the relationship between thermal comfort and productivity”.
de Dear goes on to outline “The industry standard building use studies (BUS) methodology for POE
(Leaman and Bordass, 1999, 2001) contains a scale of perceived productivity. The question does not
relate specifically to temperature within the building, and nor are there concurrent physical measurements of temperature accompanying the POE questionnaire, so Leaman and Bordass (1999) have
relied on statistical correlations with other items on their POE questionnaire to make inferences
about the impact of thermal comfort (or rather, discomfort) on productivity. Their Probe studies
(2001) showed a pronounced difference in perceived productivity between occupants who reported
that their building was comfortable and those describing their building as uncomfortable. Uncomfortable staff reported productivity impacts attributable to their indoor environment of 8.8% below
‘normal’, whereas comfortable staff reported productivity gains of 4.0% above their normal expectation. Of the 39 articles on thermal comfort and productivity retrieved in literature search (carried
out by de Dear), the Leaman and Bordass productivity publications ranked very highly: third and fifth
in terms of citations in the 1996–2010 census period, suggesting that their pragmatic solution to the
difficulties of productivity metrics resonated with researcher and end-user communities alike.”
Control and energy/resource use
The thermal environment is strongly influenced by the environmental services strategy: methods of
temperature and humidity control, air distribution and their associated controls. Clearly there will be
significant differences between the thermal environments in naturally ventilated and fully air conditioned buildings, whilst mixed-mode buildings will perform like naturally ventilated buildings when
external conditions allow. The area and location of heated or cooled surfaces and the air movement
induced by the supply air terminal devices (or windows in the case of naturally ventilated windows)
have a major impact. However the method of control provides the interface with occupants and has
an impact not only on occupant comfort and empowerment, but also on energy consumed.
Given the large percentage of energy consumed by existing buildings (for HVAC and water heating as
well as for lighting and appliances), the existing building stock is becoming one of the key targets for
public policy, and research has shown that interventions in existing building stocks can substantially
reduce carbon dioxide (CO2) emissions (e.g. Urge-Vorsatz et al. 2007). The method by which air is
distributed through a space has a significant impact on energy consumption, thermal environment
and indoor air quality (see also IAQ & Ventilation). For example the distribution of air from a ceiling
requires momentum to encourage heat exchange with the air below and the supply air temperature
must be cooled to between 10 and 14̊C to deal with maximum heat gain. Delivering air with low momentum close to the floor on the other hand requires significantly lower fan energy and air must be
supplied at between 19 and 20̊C to avoid draughts. This is known as displacement ventilation or
buoyancy-driven air supply because the room air is displaced by the supply air, moves through the
room by natural convection induced by the heat sources therein, and is extracted at high level. This
encourages stratification in the room and a temperature gradient which is the limiting factor for design, since this must not exceed 3K between foot and head to avoid discomfort (CIBSE, 2001, Guide
B2, para. 4.2.5).
Displacement ventilation was developed originally in Scandinavia for industrial applications using
large area perforated or fibre faced outlets with large surface areas resulting in face velocities of
0.25 m/s or lower. The systems have generally been designed to provide 100% outdoor air, supplying
significantly more air than is required to meet occupancy needs, but with efficient recuperators to
recover heat from the exhaust air and exh aust temperatures of 30̊C not unusual. Floor to ceiling
heights need to be slightly greater than normal, preferably no less than 3m, to allow for stratification above head level. A system using a similar principle was developed in Germany employing outlets recessed into a suspended floor (sometimes referred to as under floor air distribution (UFAD) or
low level supply). Air is supplied at a higher velocity with a slight twist, which encourages some mixing close to the floor and therefore allows slightly lower supply air temperatures. The advantage of
this system is that it allows air distribution across a deep plan floor and with lower floor to ceiling
heights. However, the latter may be offset by the need for a raised floor of sufficient depth to accommodate the air supply distribution (plenum or ducted).
Also the method by which the room temperature is controlled impacts on both thermal comfort and
energy consumed. For example systems that incorporate large surface areas that are warmer or cooler than room air temperature such as radiators or chilled ceilings and beams rely in part on radiant
heat exchange between the surface and occupants’ bodies to provide heating or cooling. Radiators
can be used to offset the radiant cooling from bodies to cold window surfaces, for example, whilst
chilled ceilings/beams can offset the temperature gradient associated with displacement ventilation.
In applications where it has been appropriate, mixed-mode systems are being adopted. “Mixedmode” refers to a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled), and mechanical systems
that include air distribution equipment and refrigeration / heating equipment for cooling / heating.
A well-designed mixed-mode building begins with intelligent facade design to minimize cooling and
heating loads. It then integrates the use of air-conditioning when and where it is necessary, with the
use of natural ventilation whenever it is feasible or desirable, to maximize comfort while avoiding
the significant energy use and operating costs of year-round air conditioning. This type of application requires a high level of user engagement. A critical impact on the thermal environment is that
mixed mode allows ‘free running’ when the external temperature is at a level that creates comfort
conditions internally and is therefore compatible with the adaptation model referred to above.
Studies that have investigated the impact of openable windows on health, comfort and productivity
have not generally separated their impact on thermal comfort from air quality and inherent empowerment benefits of personal control. However the meta-analysis carried out by Carnegie Mellon University in 2004 makes it clear that overall buildings with natural or mixed-mode ventilation are more
popular with occupants than those which are mechanically ventilated or fully air conditioned (Carnegie Mellon, 2004).
Where electrical supply is not reliable openable windows become a necessity in the absence of reliable mechanical ventilation. Mixed-mode and naturally ventilated buildings have the potential to offer
occupants higher degrees of personal control over their local thermal and ventilation conditions, as
well as a greater connection to the outdoors, which leads to increased occupant satisfaction and reduced potential for IEQ problems. Numerous studies have found that building occupants prefer some
measure of personal control along with a wider range of indoor thermal conditions (including wider
temperature range) (Carnegie Mellon, 2004; Development Securities, 2010). de Dear and Hager concluded in their landmark 1998 paper ‘A variable indoor temperature standard, based on the adaptive
model of thermal comfort, would have particular relevance to naturally ventilated buildings and other situations in which building occupants have some degree of indoor climatic control.’
Recognition of geographical / cultural variation
In different areas of the world, thermal comfort needs vary based on climate and cultural aspects.
This clearly relates to the adaptive model referred to above: the thermal experience of office workers in a Norwegian winter is very different from those experiencing a Dubai summer, for example.
However the ability to adjust clothing may depend on the formality of the work environment and the
culture of the employer organization rather than climate.
In this context Japan has initiated the ‘Cool Biz’ campaign where, among other things, they encourage companies to increase their thermostat settings in summer to save cooling energy. Alongside this
the campaign suggests changes to business attire, replacing ties and jackets with short sleeved shirts,
for example. This campaign became especially important after the closure of nuclear power plants
following Fukishima (BBC, 2011).
Hot and humid climates present the greatest challenge in creating a comfortable thermal environment indoors whilst retaining a link with the outdoors via natural ventilation, for example. However
allowing adaptation of clothing and air movement can enable higher air temperatures and some degree of natural ventilation when external conditions allow.
However an increase in the incidence of ‘urban heat islands’ may mitigate against this. Heat islands
can occur over any urban or built up area with the correct conditions. Urban heat islands develop
where there are few trees and vegetation to block solar radiation or carry out evapotranspiration,
many structures with a large proportion of roofs and sidewalks with low reflectivity that absorb heat,
high amounts of ground-level carbon dioxide pollution that retains heat released by surfaces, great
amounts of heat generated by air conditioning systems of densely packed buildings and high traffic
densities.
Innovations
We have already seen that individual environmental control increases user performance (Leaman,
2005 and Kroner, 2006) and satisfaction (Bordass, 1993 and Leaman, 1996) and ‘adaptive opportunity’ forms an important constituent of this (Baker, 1995).In this context de Dear (2013) has concluded
that there is much empirical evidence that has accumulated over the last 20 years supporting the
comfort potential of increased air movement (Aynsley, 1999, 2008; Chow and Fung, 1994; Chow et
al., 2010; Gong et al., 2006; Ho et al., 2009; Scheatzle et al., 1989; Schiavon and Melikov, 2008; Toftum et al., 2003; Zhang et al., 2007b, 2009, 2010e; Zhou et al., 2006). Recognition is finally being
made in the standards. In Brazil, there is currently a proposal to include minimum air speeds in design guidelines for natural ventilation (Candido et al., 2011), and the most recent revisions in ASHRAE
Standard 55-2010 (ASHRAE, 2010) indicate how much warmer the comfort zone can be stretched by
increasing air speeds up to 0.8 m/s, without requiring individual occupant control and then beyond
0.8 up to 1.2 m/s when the occupants are granted control (Arens et al., 2009). These new provisions
represent a significant step forward in enhancing indoor environmental quality with elevated air
speeds, as well acknowledging the important role played by individual environmental control (and
perceived control).
Although air distribution systems that individually condition the immediate environments of office
workers within their workstations (personalized air supply devices – see IAQ & Ventilation) have been
around for many years it could be argued that they have not been exploited to their full potential,
and remain at the innovative end of the spectrum of air terminal devices. As with task lighting systems, the controls for these 'task conditioning' systems may be partially or entirely decentralized and
under the control of the occupants. Among the primary types of task conditioning systems (floor-,
desktop- and partition-based), floor-based designs are the most common, having been widely developed and used in South Africa and Europe, and are now gaining acceptance in the United States and
other Western Countries (Beauman et al 1995).
Building Management and Control Systems (BMCS) or Building Energy Management Systems (BEMS),
usually incorporating Direct Digital Controls (DDC), have been widely used for many years. Sophisticated software enhancements enable monitoring and trend logging of comfort vectors such as temperature and humidity. However the primary interface with the occupant is still typically a wall
mounted temperature sensor, with or without occupant adjustment. These require periodic recalibration, which is frequently overlooked.
In recent years a number of intranet-based tools have been developed that provide an occupant
feedback capability. In their simplest form they incorporate a screen view on computers, handsets or
smart devices, allowing occupants to report dissatisfaction with temperature, humidity, air quality
and lighting, for example. At the other end of the spectrum the US-based Centre for the Built Environment has developed an integrated facilities management tool that links reports from occupants to
a central database that provides recommendations to maintenance operatives whilst automatically
learning from their actions to improve the service (Federspiel and Villafana, 2003).
Shimmin and Koo (2002) developed a system for automatically analyzing ‘complaints’ using a fuzzy
logic engine for compiling ‘the occupant submissions for each interval and attempt to find the best
compromise to satisfy the complaints of discomfort in each zone by a form of democratic voting and
averaging.’
At the time of writing workers at the University of Sydney are developing a ‘comfortsat’ which not
only senses air temperature, mean radiant temperature, relative humidity and air velocity but also
enables input of average clothing and activity values whilst logging ‘Actual Mean Vote’ (AMV) for
comparison with PMV based on ISO 7730. The devices, located in various zones, will also be able to
record IEQ factors such as air quality, CO2 and VOC plus noise and lighting levels over real time periods - all interfaced to a dashboard that would provide valuable data for management of the facilities.
For more information
BBC (2011) Japan promotes 'Super Cool Biz' energy saving campaign. Available:
http://www.bbc.co.uk/news/business-13620900 Last accessed 4 September 2014
Carnegie Mellon (2004) Guidelines for High Performance Buildings - Ventilation and Productivity.
Available: http://cbpd.arc.cmu.edu/ebids/images/group/cases/mixed.pdf Last accessed 5 August
2014
CIBSE, 2001, Guide B2, para. 4.2.5).
Clements‐Croome, D. (2003), “Environmental quality and the productive workplace”, paper presented at CIBSE/ASHRAE Conference. Building Sustainability, Value and Profit. Edinburgh, Scotland, 24‐26
September.
de Dear RJ. (2013) Progress in Thermal Comfort Research in the Last 20 Years. Indoor Air 23, pp 442461
Kroner WM. (2006) Employee Productivity and the Intelligent Workplace in Clements-Croome,
D. (Ed.) Creating the Productive Workplace, Taylor Francis, London,
Leaman A. and Bordass B. (1994) Building Design, Complexity and Manageability. Available from
http://www.usablebuildings.co.uk/Pages/Unprotected/DesCmplxMan.pdf Last accessed 4 September
2014
Leaman, A. and Bordass, W. (2005), “Productivity in buildings: the ‘killer’ variables” in
Clements-Croome, D. (Ed.) Creating the Productive Workplace, Taylor Francis, London, pp 153‐80

Download Report