Chapter 20 Thermal comfort and occupant behaviour Fergus Nicol and Michael Humphreys Abstract This short chapter introduces the causal relationship between the behaviour of building occupants, their comfort and the energy used by the buildings. It suggests that much of the behaviour is motivated by the desire of the occupants to make themselves comfortable and to optimise the environment. The provision of comfortable conditions in domestic buildings in hot-humid climates is highlighted. An annex also introduces ways in which the comfort-related behaviour can be understood and allowed for in predictive simulations of indoor temperature and energy use. Keywords Occupant behaviour, Adaptive comfort, Energy used, Hot-humid climates, Cooling strategies 20.1 Basic principle of adaptive behaviour The behaviour of building occupants has often been seen as a problem by environmental engineers and building simulators. Their actions can often appear to be random in both their motivation and their effects. But they can make a big difference to the success of any strategy for ensuring comfort in a building. In particular behaviour can radically change the amount of energy used by mechanical systems. Buildings which are physically identical can vary the energy they use because the building occupants differ in their use of the building. This can lead to an increase in the energy use, but properly targeted behaviour can also help to cut the use of energy and allow the building to remain comfortable without using any energy for much of the time. The adaptive principle “If a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort” is the basis of the adaptive approach to thermal comfort [1]. It introduces the idea that in a changing environment behavioural responses may be needed to ensure human comfort. These responses will be directed towards restoring comfort and can therefore be 1 F. Nicol () London Metropolitan University, Low Energy Architecture Research Unit, Sir John Cass Faculty of Art, Architecture and Design, Calcutta Hose,Old Castle Street,London E1 7NT, UK. e-mail:
[email protected] M. Humphreys () Oxford Brookes University, School of Architecture, Faculty of Technology, Design and Environment, Headington Campus, Gipsy Lane, Oxford OX3 0BP, UK. e-mail:
[email protected]
seen as arising from the thermal environment through the mediation of the human subject. In their paper Understanding the adaptive approach Humphreys and Nicol list over 30 common actions a person might take to make themselves comfortable [2]. Over time it is essential to balance the metabolic heat produced by the body with the heat lost to the environment in order to keep the body temperature constant. They identify five types of behavioural actions which change the physical/physiological heat balance in five basic ways: 1. Regulating the rate of internal heat generation (e.g., change activity) 2. Regulating the rate of body heat loss (e.g. change clothing) 3. Regulating the thermal environment (e.g. open a window, turn down the heating) 4. Selecting a different thermal environment (e.g. move to another room, go out) 5. Modifying the body’s physiological condition (e.g. vasoregulation, sweating, shivering and changes of posture) These changes mean that the actions people take either change their own response to achieve comfort in an existing environment or changing the environment to suit them (Figure 20.1). The means they use to enable them to keep the thermal balance (the windows, shades, blinds as well as any heating or cooling system) are often referred to as ‘adaptive opportunities’ [3] Some actions show aspects of more than one type of behaviour. Opening a window for instance can increase the air movement inside a room which will help cool occupants by convection and encourage evaporative cooling and increase the temperature needed for them to be comfortable. At the same time it will encourage mingling of the inside and the outside air which will cool or heat indoor air depending on the temperature difference between the two. At the same time it is important to remember that there are other reasons for opening a window – maybe to improve the view out, to reduce indoor pollution or to hear the birds singing in the garden. The window may be closed to reduce the glare or keep out the smell and noise of traffic.
Comfort is achieved by the occupants adapting to the building
Occupant
Building
Or by the occupants adapting the building to suit them
This has to be done within the existing climatic, social, economic, architectural and cultural context. Buildings should be designed to provide acceptable conditions
Figure 20.1 basic model of ways in which building occupants achieve comfort in a building (Nicol) The interaction between the environment and behaviour is illustrated in Figure 20.2 based on surveys in Pakistan. As the indoor temperature increases the mean values for clothing insulation (in Clo units) falls and the air speed (ms -1) to rise as windows are opened or fans turned up. The metabolic rate (wm-2) is not noticeably effected as these Pakistani office workers go about their business but mean skin moisture [on a scale suggested by Webb [4] 0 (none), 1(slight), 2 (moderate) and 3 (profuse)] rises as the physiological response of the body seeks to increase heat loss through evaporation.
Figure 20. 2 The responses of Pakistani office workers to indoor temperature (Nicol) 3
20.2 Comfort and climate: choosing the right behavioural control system An adaptive approach to design will as far as possible allow the building and its occupants to remain comfortable for as much of the time as possible and with the minimum use of energy. The accepted vision of the built environment is that the building and its services should provide comfort for the occupants. In contrast the adaptive approach suggests that the building (and its services) should provide the occupants with means to make themselves comfortable. Any particular building has to deal with the specific climate in which it is built. In addition to a heating system a building designed for a site in a cool climate might thus concentrate on providing ways the occupant can capture and store ‘natural’ heat in the warmer times of the day or the year. There are many methods used for the capture and storage of solar heat. These are ways that have been developed to allow building occupants to access solar or other sources of natural energy when it is needed. In the built environment the most common site for energy storage is in the thermal mass of the building. Methods such as Trombe walls which absorb solar heat and pass it to the building’s interior, heated cores which store heat in hot weather and pass it on as needed, and earth coupled heat transfer which can store heat in the ground during hot weather and use it to heat the building in colder seasons. In addition to this stored heat are the immediate sources of heating or cooling examples are window shades, opening windows which let in outside air and encourage air movement and so on. In climates where overheating is more likely to be the problem than getting too cold, then systems will concentrate on providing ‘coolth’ rather than warmth. These may include shading to keep out the sun using shading, air movement to encourage heat loss by convection and evaporation, night ventilation to cool down the thermal mass using low night-time air temperatures and so on. One assumption of the adaptive approach is that the building occupants will understand the role of the different adaptive opportunities provided and be sufficiently familiar with their purpose and the way they work to use them in the way the designer intended. If they are to use the opportunities to make themselves comfortable they must know what they do, how they do it and when they will be most effective. The designer also should ensure that the controls do not conflict with one another – or indeed with other aspects of the building such as security. 20.3 Seasonal changes Most areas of the world have a climate which varies seasonally from one part of the year to another. In the equatorial tropics the seasonal temperature change is relatively small as is the daily temperature range. This is the classic situation in what is referred to as a hot humid climate. Because of the high level of evaporative heat in the air any change in the temperature involves relatively large amounts of heat exchange. The climatic variation from one part of the year to another is kept small by the generally high level of humidity. In these areas the climate will change not from hot to cold but from a relatively humid or ‘rainy’ season to a relatively dry season. It is also possible to have a ‘hot humid’ season
where the summer and the winter seasons are quite different but the hotter months behave like the equatorial climate. The way in which the adaptive opportunities are used in the hot humid season will be similar to those in the equatorial climate but the designer has to keep in mind that there may be contrasting requirements in the cooler times of year. 20.4 Behaviour in a hot humid climate Understanding of the human thermal response to the hot-humid climate is complicated by the supposed effect of the high humidity on the thermal sensation. The high humidity will affect the ability of the body to keep cool by the use of the evaporation of sweat from the skin surface. A number of estimates of this have effect have been made and are surveyed by Nicol [5]. He found that the effect on comfort of humidity, though real, is actually smaller than is often imagined. A high humidity is equivalent to a rise in the temperature of about 1K, compared to a low humidity. The high humidity will also reduce the range of temperatures which are comfortable. The effect of high humidity may lead to other types of discomfort, such as increased sweating, but measured in terms of the ASHRAE scale the effect is small. An important cooling strategy in the hot-humid climate is the use of air movement. This can cool by convection if the air is cool and by evaporation. The evaporative effect will be most important in the hot humid climate because most people will have some sweat, but the rate of evaporation may be reduced by the humidity of the air. Movement of the air over the skin is therefore necessary to ensure heat is lost by evaporation. Heat loss by the use of night-cooling of the building can be important, though the temperature of the mass of the building may be too not be as different from the day time as could be expected in a dry climate. Nicol [5] estimated from a survey in Pakistan that the comfort temperature in a room with fans running is about 2K higher than when the fans are not running over an outdoor temperature range of 20°C to 32°C. Above 32°C very few offices did not have fans running. The aim of the designer in the hot humid climate must be to use low energy technologies to make occupant comfortable. The solar heat must be kept out so that roofs and ceilings should be well insulated and where possible extended by the use of shading especially over openings. Air movement by cross ventilation should be readily available and controllable by the occupants. Low-energy fans are essential. Buildings in equatorial sites should also where possible be arranged with their major axis is an east-west direction to reduce the solar heating of the major walls which can be most easily shaded on the south and north sides. There is a possible conflict if the hot humid weather is in one particular season and the building needs to encourage solar heating in another season. Careful and thoughtful design can overcome some of the problem by using well designed moveable shading or the clever use of deciduous vegetation. 5
20.5 Mechanical cooling and heating and adaptive comfort The control of indoor conditions using mechanical cooling or heating is always a powerful option if the system is provided by the building and may be essential in more extreme weather. Nicol [6] has found that the availability of mechanical control can result in a wide range of indoor temperatures in the domestic buildings and must be controlled by occupants. Indeed the indoor temperature range in heated and cooled buildings is found in most cases to be greater that than in free-running buildings. In the mechanically conditioned building the choice of indoor temperature is left to the occupants who are free to decide on the basis of their own preference (and their ability to afford to run the buildings at high or low temperature). In the free-running building it is the layout and materials of the building itself which decides indoor temperature according to the physics of the situation and the occupants will have adjusted themselves to this.
20.6 Annex: adaptive behaviour as a stochastic phenomenon The scientific method suggests that we should propose a model of the underlying process of, say, the opening of a window or windows in a building motivated by indoor temperature and then check this proposition against measured data from the field. In the Figure 20.3 (a) we are considering a single occupant with two possibilities window open or window closed. If we start from the bottom left of the diagram with a low temperature and the window closed, as we move right we will get to a point where the temperature is too hot with the window closed and the temperature reaches a ‘trigger temperature’ (c/o) at which the occupant will open the window to cool the room. At temperatures above this trigger temperature we can assume that the window will be open. If we start from the top right of the diagram and the room cools we will reach a second ‘trigger temperature’ (o/c) at which the occupant will close the window to prevent the temperature from falling any further. At temperatures below this trigger temperature we can assume the window will be closed. Between o/c and c/o we cannot be sure whether the window is open or closed as this would depend on the way the temperature has changed. It could for instance have increased to c/o. the window is opened and the indoor temperature falls, but the window will remain open unless it drops below o/c.
(a) Single occupant window opening
(b) The effect of numerous different occupants Figure 20. 3 Developing a model of window opening behaviour (from Rijal et al. 2012 [8]) In the Figure 20.3 (b) we suggest what the effect will be if there are a number of occupants. Each occupant will have a slightly different value for o/c and c/o which means that instead of a sharp trigger temperature there will a sigmoid one where the average value of the c/o and the o/c will be the temperature at which there is a 50% chance of the window being open or closed. Between the two sigmoid curves the window can be either open or closed.
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Figure 20. 4 Measured values of the proportion of windows open with indoor globe temperature. Each point is the mean proportion of windows open at the given temperature. Data from surveys in the UK reported in Rijal et al. 2007 [7]. Figure 20.4 shows how the likelihood of the window being open in occupied rooms actually changes with temperature. Each dot on the graph it the average number of windows open in 25 rooms at the temperature shown on the x-axis. As the temperature rises the number of windows that are open increases. The middle of the three lines on the graph is the 50 percent line and 83% of points lie between the two outer lines. Such a graph can be used to estimate the likelihood a window will be open. The method is given in Rijal et al. [7]. It can be used for other similar behavioural responses to environmental stimuli such as the running of fans [9] or use of lights and allows the simulator to estimate the indoor temperature from the usual input variables for a simulation 20.7 Conclusions This chapter has looked at the behaviour of building occupants in relation to their thermal comfort and buildings’ energy use. Adaptive comfort suggests that much occupant behaviour is directed towards avoiding discomfort as suggested by the adaptive principle. The palette of behaviours which will help ensure indoor comfort will depend on the climate. In the hot humid climate the key adaptive behaviours are to exclude solar radiation and to ensure a source of variable air movement. Minimising the heating effect of solar radiation is essential in an environment where even a small increase in temperature can give rise to overheating. The roof of the building is the most vulnerable surface especially in equatorial regions where the altitude of the sun in the sky normally high but it is also important to shade walls and openings.
Air movement will help the body to lose heat by evaporation but needs to be controllable. The possibility of controllable air movement is essential to fit the different comfort preferences of occupants. In many cases cross ventilation can be used and it can be augmented by fans. Night cooling of the building structure is possible but is less effective because the diurnal variation of outdoor air temperature is relatively small and in addition a light-weight structure is often preferred in a hot-humid context. Mechanical cooling and air conditioning is increasingly used and is relatively controllable but there is a considerable energy cost involved with both the cooling load and the need for air circulation. References 1. Nicol F, Humphreys M, Roaf S (2013) Adaptive Thermal Comfort, Principals and Practice, Routledge, London p.8. 2. Humphreys M, Nicol F (1998) Understanding the Adaptive Approach to Thermal Comfort, ASHRAE Transactions 104 (1): 991-1004. 3. Baker N, Standeven M (1995) A behavioural approach to thermal comfort assessment in naturally ventilated buildings, Proc CIBSE National Conference, Eastbourne 76-84. 4. Webb C (1959) An analysis of some observations of thermal comfort in an equatorial Climate, BJIM 16(3): 297-310. 5. Nicol F (2004) Adaptive thermal comfort standards in the Hot-Humid Tropics. Energy and Buildings 36(7): 628-637. 6. Nicol F (2017) Temperature and adaptive comfort in heated, cooled and freerunning dwellings Building Research and Information DOI 10.10180/09613218.2017.1283922. 7. Rijal HB, Tuohy P, Humphreys M, Nicol F, Samuel A, Clarke J (2007) Using results from field surveys to predict the effect of open windows on thermal comfort and energy use in buildings, Energy and Buildings 39 (7): 823-836. 8. Rijal HB, Tuohy P, Humphreys MA, Nicol JF, Samuel A (2012), Considering the impact of situation-specific motivations and constraints in the design of naturally ventilated and hybrid buildings, Architectural Science Review 55(1): 35–48. 9. Rijal HB, Tuohy P, Humphreys MA Nicol F, Samuel A, Raja IA, Clarke J (2008) Development of Adaptive Algorithms for the Operation of Windows, Fans and Doors to Predict Thermal Comfort and Energy Use in Pakistani Buildings, ASHRAE Transactions 114 (2): 555-573.
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