2.3 Human requirements regarding indoor climate
One of the main functions of buildings is to protect the inhabitants from outdoor climatic conditions which are often harsh and hostile. The building must provide an environment that does not harm the health of the inhabitants. Moreover, it should provide living and working conditions which are comfortable.
To achieve this, the physiological functions of the human body are to be considered. It is also necessary to know under which thermal conditions human beings feel comfortable.
2.3.1 Human physiology
Physiological factors are of primary importance with regard to comfort. The internal temperature of the human body must always be kept within narrow limits at around 37°C. Any fluctuation from this value is a sign of illness, and a rise of 5°C or a drop of 2°C from this value can lead to death.
The body has the ability to balance its temperature by various means.
This thermal balance is determined, on the one hand, by the “internal heat load” and on the other, by the energy flow (thermal exchange) between the body and the environment.
The thermal exchange between the body and the environment takes place in four different ways: conduction, convection, radiation and evaporation (perspiration and respiration).
The contribution that conduction makes to the heat exchange process depends on the thermal conductivity of the materials in immediate contact with the skin. Conduction usually accounts for only a small part of the whole heat exchange. It is limited to local cooling of particular parts of the body when they come in contact with materials which are good conductors. This is of practical importance in the choice of flooring materials, especially where people usually sit on the floor.
Heat exchange by convection depends primarily on the temperature difference between the skin and the air and on air movement. It can, to a certain extent, be controlled by adequate clothing.
The insulation effect of clothing can be expressed by a clothing-value (“clo-value”).
Radiation takes place between the human body and the surrounding surfaces such as walls and windows; and, in the open air, the sky and sun. In this process temperature, humidity and air movement have practically no influence on the amount of heat transmitted. This amount of heat depends mainly on the difference in temperature between the person’s skin and the surrounding or enclosing surface.
The body may gain or lose heat by above described processes depending on whether the environment is colder or warmer than the body surface. When the surrounding temperature (air and surfaces) is above 25°C, the clothed human body cannot get rid of enough heat by conduction, convection or radiation.
Evaporation (perspiration and respiration)
In this case the sole compensatory mechanism is evaporation by the loss of perspiration, together with, to a certain extent, respiration. During evaporation water absorbs heat, and as humans normally lose about one litre of water a day in perspiration, a fair amount of heat is taken from the body to evaporate it. The lower the vapour pressure (dry air) and the greater the air movement, the greater is the evaporation potential.
This explains why extreme temperatures in humid climates are less bearable compared to the same temperatures in dry climates.
Internal heat load
The “internal heat load” of a body depends on its metabolic activity and varies greatly (see table below).
2.3.2 Thermal comfort zone
The optimum thermal condition can be defined as the situation in which the least extra effort is required to maintain the human body’s thermal balance. The greater the effort that is required, the less comfortable the climate is felt to be.
The maximum comfort condition can usually not be achieved. However, it is the aim of the designer to build houses that provide an indoor climate close to an optimum, within a certain range in which thermal comfort is still experienced.
This range is called the comfort zone. It differs somewhat with individuals. It depends also on the clothing worn, the physical activity, age and health condition. Although ethnic differences are not of importance, the geographical location plays a role because of habit and of the acclimatization capacity of individuals.
Four main factors, beside of many other psychological and physiological factors, determine the comfort zone:
• air temperature
The relation of these four factors is well illustrated in the bioclimatic chart.
The chart indicates the zone where comfort is felt in moderate climate zones, wearing indoor clothing and doing light work. It also assumes that not only the air temperature, but also the temperature of surrounding surfaces lie within this range.
The sol-air temperature
Radiation and temperature act together to produce the heat experienced by a body or surface. (see Chapter 2.4)
This is expressed as the sol-air temperature and is composed of three temperatures:
a) outdoor air temperature
b) solar radiation absorbed by the body or surface
c) long-wave radiant heat exchange with the environment
Air- and surface temperatures often differ. This is especially the case where there are great differences between day and night temperatures and also where building components receive strong solar radiation. To a certain extent, high air temperatures can be compensated by low surface temperatures or vice versa, as is shown in the graph below.
The temperature difference between air and surfaces, however, should not exceed 10 - 15°C if comfort is still to be maintained. As research has shown, this fact is less valid for walls, but especially important for ceilings.
The graph shows how people react to different surfaces which have a temperature differing from the temperature of the other surfaces.
The design of the roof is therefore of the utmost importance.
The fact that the roof receives the greatest amount of solar radiation and re-radiates most at night is a further reason for the importance of roof design. A typical example of the effect of the roof design on inside temperatures is the plain concrete roof slab under a tropical sun which can result in an unbearable indoor climate in the evening, with inside surface temperatures of up to 50 or 60°C.
The humidity level affects the amount that a person perspires. It also influences, therefore, how temperatures are felt. High humidity reduces the comfortable maximum temperature; low humidity allows a tolerance for higher temperature. At the lower limit of the comfort level humidity has little influence.
Range of comfort in relation to humidity, with light summer clothes or 1 blanket at night
Humidity alone does not have a very significant influence on the comfortable temperature range, but in combination with air circulation it gains much importance.
As the figures below shows, air circulation influences the temperature felt. The cooling effect of wind increases with lower temperatures and higher wind speed.
This increased cooling effect of enhanced wind speed has another important consequence: the higher the air temperature, the higher the wind speed which is still felt to be comfortable .
Acclimatization and seasonal changes
To a certain extent human beings have the ability to become acclimatized. Therefore the resident population feels less stressed by a harsh climate than a passing traveler coming from another type of climate would. Analogously this can also be said for seasonal climatic changes, to which people can become adjusted. A certain temperature may be felt to be too cool in summer but too hot in winter.
The table below shows an example of the seasonal changes in the comfort zone as observed in Dhahran.
Changes between indoor and outdoor climate
Drastic changes which can occur, especially in air-conditioned buildings, may give discomfort (stress situation) and may also be negative for health.
Clo-value and met-value, tolerance
As mentioned above, clothing and metabolic activity have a great effect on the comfort zone. Moreover, they also influence the acceptable temperature range (tolerance). A physically highly active person can bear quite wide temperature differences, whereas a sleeping person is more sensitive to differences.
The figure below illustrates this relationship. The temperatures are valid for middle-European conditions.
Source: ISO 7730 (1984): Moderate environment, Determination of the PMV and PPD indices and specifications for thermal comfort, and element 29, Zurich, 1990
The white and shaded areas indicate an incidence of less than 10% of persons dissatisfied (PPD). This illustrates that the higher the clo value or the activity level of a person, the greater his tolerance for differences in temperature will be.
For a seated person wearing a suit (clo = 1.0; met = 1.2) the ideal room temperature is 21.5°C with a tolerance of +-2°C.
Factors other than climatic ones influence also the well being of the inhabitants, for example, psycho-social condition, age and health condition, air quality and acoustical and optical influences. Although these factors cannot be improved by climatically adapted construction, they should not be forgotten, because they may considerably reduce the tolerance. For example, ill people lying in a hospital or people under extreme noise stress are much more sensitive to climate than people enjoying a garden restaurant.
Due to the many factors described above which determine the comfort zone, it is not possible to describe it accurately in a single figure or chart. Summarizing, the bioclimatic diagram (Fig 2/23) may be applied considering the following parameters:
• Air and surface temperature may not differ more than 10 - 15°C.
• The temperature of the ceiling should not be much higher than the room temperature.
• At the upper limit of comfort, the temperature should be lower with increasing humidity.
• With increased air temperature, air circulation should be enhanced.
• The temperature that is felt to be comfortable changes with the seasons.
• The temperature that is felt to be comfortable also depends on the degree of acclimatization.
• The temperature that is felt to be comfortable is affected by the clothing worn and the physical activity level.
• With additional clothing and increased activity, the tolerable temperature range extends.
• Drastic temperature changes, as may be the case in air-conditioned buildings, should be avoided.
• Factors other than climatic ones (e.g. psycho-sozial factors) may decrease the tolerable temperature range.
2.3.3 Requirements for buildings according to their functions
Comfort conditions as described are not usually found outdoors and clothing alone is often not sufficient to compensate. An important function of buildings is to provide the necessary protection against the outdoor climate. However, not all types of buildings and not all rooms in a building have to fulfill the same requirements.
While designing a building and working out the thermal concept, the following functional parameters should be analyzed and considered:
• What type of activities and functions will be carried out in the building ?
Such areas are usually used in daytime only. As a consequence the design should be optimized such as to provide favourable conditions in daytime. The performance at night is of little importance. In areas where hard physical labour is carried out, the temperature should be generally lower than in areas, where sitting activities are predominate.
Structures for residential purposes are generally occupied throughout day and night. They should therefore be designed for an optimization over the whole period. Special attention should be paid to sleeping areas and their nighttime conditions, as the body is more sensitive to discomfort when at rest.
Similarly, requirements for buildings and rooms may differ throughout the seasons. A house which is used mainly in summer would certainly differ from a house used mainly in winter.
The daily routine of the inhabitants may also vary with the seasons. For example, in the hot season, people may start work early, thus benefiting from favourable temperatures. During the hottest hours a break may be taken. At this time the indoor temperature should still be at a comfortable level to allow relaxation. The late afternoon and evening hours may be spent outdoors when the temperature is past its peak. In the cold season the customs may be different: activities are started later in the morning, a great part of the day is spent outdoors and the evening is spent inside.
No ideal solution
No ideal solution From the technical and economical point of view it is usually impossible to provide buildings that fulfill the climatic requirements of all the inhabitants and under all prevailing climatic conditions throughout the year. As a general rule, buildings may be designed to satisfy about 80-% of the inhabitants during approximately 90% of the time during the course of the year. On exceptionally hot or cold days a greater degree of discomfort may be acceptable.
The hottest and coldest 10% of days do generally not have to be considered.
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