2.1 Climate zones

The climates prevailing around the globe vary greatly, ranging from the polar extreme to tropical climates. These are primarily influenced by the sun’s energy heating up the land and water masses. At the regional level, the climate is influenced by altitude, topography, patterns of wind and ocean currents, the relation of land to water masses, the geomorphology, and by the vegetation pattern.

Accordingly, the tropical and subtropical regions can be divided into many different climatic zones, but for practical reasons, in this publication three main climate zones are considered:

· the hot-arid zone, including the desert or semi desert climate and the hot-dry maritime climate
· the warm-humid zone, including the equatorial climate and the warm-humid island climate
· the temperate zone, including the monsoon climate and the tropical upland zone

The main climatic factors relevant to construction are those affecting human comfort:

· air temperature, its extremes and the difference between day and night, and between summer and winter temperatures.

· humidity and precipitation

· incoming and outgoing radiation and the influence of the sky condition

· air movements and winds

Fig 2/1 World climatic zones

2.1.1 The hot-arid zone

This zone is situated in two belts at latitudes between approximately 15° and 30° North and South of the equator. Its main characteristics are the very hot summer season and a cooler winter season, and the great temperature difference between day and night.

Temperature in summer

In the hot season the air temperature rises quickly after sun rise up to a mean maximum well above 40°C, with a recorded maximum of 58°C. At nighttime the temperature falls by about 20°C.

Temperature in winter

In the cool season the mean maximum lies at about 30°C and falls at night by about 10 to 20 °C or more, according to altitude. In addition ground frost is possible at night.

Coastal areas

In the maritime region the temperatures are somewhat less extreme but in the hot season the mean maximum temperature also reaches about 40°C and drops at night by 10 to 15°C . In the cool season the mean maximum lies at about 25°C with a similar drop at night.

Humidity and precipitation

The relative humidity is very low in the continental areas and varies between 10% and 55%. In the coastal areas, however, it can reach up to 90% which, together with the high temperature, makes the climate very uncomfortable. Precipitation is scarce, irregular and unreliable.

Radiation

The sky is mostly clear, with some haze in the coastal regions, allowing a very strong solar radiation during the daytime. A considerable release of the heat stored during daytime takes place in the form of radiation toward the cold night sky.

Wind

The winds which vary greatly are usually caused by thermals created by humidity and temperature differences. During the daytime they are often strong and violent with a tendency to evolve to sand or dust storms. In the coastal regions a regular wind pattern exists, blowing landward from the sea during the daytime and seaward at night.

2.1.2 The warm-humid zone

This zone covers an area around the equator extending from about 15° N to 15°S. There is very little seasonal variation throughout the year.

Temperature

The air temperature varies very little throughout the year or between day and night. It reaches a mean daytime maximum between 20°C and 32°C and a nighttime minimum between 21°C and 27°C.

Humidity and precipitation

The relative humidity varies between 55% and 100%, but generally lies around 75%.
Precipitation is high throughout the year and often occurs in the form of torrential rains with heavy winds and storms.

Radiation

The sky is fairly cloudy throughout the year; in coastal regions, however, it is often clear. Accordingly, the solar radiation is to a great extent diffused and partly reflected by the high vapour content. Thus at night the accumulated heat is not readily dissipated.

Wind

The wind velocity is generally low except during rain squalls, when usually one or two dominant wind directions prevail. In coastal regions, however, regular thermic winds provide relief from heat and humidity. Storms are common in this region.

2.1.3 The temperate, monsoon and upland zones

These climatic regions are generally located around the Tropic of Cancer and the Tropic of Capricorn. The climate is neither consistently hot and dry, nor warm and humid. Their characteristics change from season to season, alternating between hot, dry periods and periods of concentrated rainfall and high humidity.

Three seasons

Three main seasons can thus be distinguished:

· the hot and arid pre-monsoon season,
· the warm-humid monsoon period,
· the moderate or even cool winter period.

Temperatures in lowland areas

The lowland monsoon area is characterized by air temperatures which are highest in the pre-monsoon season, i.e. around 35 to 45°C in the daytime and a drop at nighttime of about 10 to 15°C. With the start of the monsoon rains the temperature drops considerably. In winter the lowlands have moderate temperatures.

Temperatures in upland areas

In the upland areas the temperature naturally depends on altitude. In winter night frost is possible. This can also happen in continental areas.

Humidity and precipitation

The relative humidity varies in the dry season between 20% and 55%, and in the wet period between 50% and 100%, depending on precipitation.

Radiation

The sky condition varies with the seasons. In the dry and cool season it is clear with intense direct solar radiation. In the hottest period the sky is rather hazy and radiation is more diffused. During the monsoon period, heavy and low clouds often cover the sky, alternating with periods of clear sky and intense solar radiation.

Wind

Winds are variable and influenced by topographical conditions. During the dry period winds are dusty and hot in lower areas. In mountainous regions, strong and regular valley winds of thermic origin occur in the afternoon.

Fig 2/2 Diagrams showing the typical mean temperature curves for the three zones over 24 hours during the hot and cool seasons.

2.1.4 Microclimate

The above described division into three climatic zones is very generalized, since many areas exist with differing climates or a combination of types. Local conditions, however, may also differ substantially from the prevailing climate of a region, depending on the topography, the altitude and the surroundings which may be either natural or built by humans.

Cold air pool

A phenomenon often observed is that of the cold air pool. Because cold air flows downwards, similar to water, it causes cold air “lakes” in depressions and in the bottom of valleys if there are insufficient outlets. This occurs especially at night, but can also prevail over a longer period and can prevent air circulation. In urban areas which are located in such depressions this phenomenon favours the development of smog conditions.

Local wind

Local wind conditions strongly influence the climate. They are determined mainly by the topography. When a wind blows over an obstacle such as a hill or tree, its velocity on the windward side is greater than on the wind-protected leeward side, and is greatest on the crest.

Water bodies

Large water bodies such as lakes and seas generally have a balancing effect on the temperature in the adjacent areas due to the great thermal storage capacity of the water. Water is also a source of local winds because it accelerates thermic air movements.

Urbanization

Heavy urbanization of an area (townships) generally increases the temperature compared to the rural surroundings. Differences of up to 10°C are possible. Wind velocity and its ventilation effects are generally decreased, but the channeling effect of narrow streets can also cause the opposite to occur.

Altitude

Altitude is a major factor influencing air temperature. As a rule of thumb, the temperature is reduced by 2°C for every 300 m increase in altitude.

Ground surface

The properties of the ground surface cover also influence the climate. Bare or denuded surfaces store little or no humidity, but absorb solar heat radiation and heat up. Surfaces covered with vegetation heat up much less, and thus have a regulating effect on the temperature and increase humidity. The more intense the vegetation, the greater is its balancing effect.

Response to microclimate

While considering the general climatic characteristics may be sufficient in working out the rough concept of a building, the individual site conditions, as observed according to the above criteria, need to be considered in designing the details. If possible, these factors should already be considered when selecting the construction site.

2.2 Climatic factors.

The main natural elements that define the climate, are

· solar radiation,
· wind, and
· humidity, in the form of vapour and precipitation.

Their characteristics and relevance for construction depend largely on the geographical location, but also on the topography, altitude and properties of the earth’s surface and its coverage.

2.2.1 Sun

The earth receives almost all its thermal energy from the sun in the form of radiation. Thus the sun is the dominant factor that influences climate.

2.2.1.1 Solar radiation

The spectrum of solar radiation extends from ultra violet through visible light, to infrared radiation. The latter is the main medium of energy, in the form of heat.

Fig 2/3 Spectrum of the solar radiation

The solar energy from the sun is always constant. How much heat is received at a given point on earth depends on

· the angle of incidence
· atmospheric conditions
· the length of the day

Fig 2/4 With the changing angle of incidence the radiation intensity changes

At an angle of 30°, a given area (a) only receives half the amount of solar rays it would at an angle of 90°.

The distance (d) that solar rays have to pass through the atmosphere at an angle of 30° is double that if the angle were 90°.

This increased distance reduces the energy received on the earth surface considerably, especially if the atmosphere is humid or dusty.

The angle of incidence changes not only in the course of the day, but also with the seasons. This is due to the earth’s path around the sun.

Fig 2/5 The angle of incidence changing with the seasons

The amount of energy received on a given surface varies during the course of the day, depending on the angle of incidence of the sun. The graph below illustrates a typical amount of energy received by a south facing and inclined solar collector surface. The total energy received during this day amounts to about 5 kWh/m².

Fig 2/6 Sun intensity on a south-facing, inclined surface in January during a clear day, latitude 27° North

Major factors influencing the amount of solar energy received are the weather and the pollution content of the atmosphere.

Fig 2/7 Solar energy received on a surface vertical to radiation (angle of incidence 90°). Source: [ 137 ]

Similar to the energy gain during daytime, nighttime heat loss by radiation to the sky is also greatly dependent on atmospheric conditions. A clear sky allows maximum, a thick cloud cover minimal heat loss.

(see Chapter 2.4.1)

Length of the day. According to the geographic latitude, the length of the day and hence the duration of sunshine varies during the year.

Fig 2/8 Length of the shortest and longest day at different latitudes in the northern hemisphere.

2.2.1.2 The sun’s path

While designing buildings anywhere in the world, the sun’s path must be considered as an important factor.

The position of the sun depends upon

· the geographic location (latitude)
· the time of year (season)
· the time of day (hour)

It can be determined most easily, and for our purposes sufficiently exactly with the help of the diagrams given in Appendix 5.3

How to read the diagram:

· Select the diagram for the latitude of the building site.
· Find the point at which the time of the day and date you are interested in cross each other.
· Read the solar altitude and the azimuth.

Fig 2/9 How to use the sun path diagram

2.2.1.3 The geometry of shadows

Knowing the sun’s position, the geometry of shadows on buildings, facades and shading devices can be derived.

Detailed methods, with the help of the sun-path diagram and a shadow angle protractor, are found in the literature.
[i.e. 2, 8, 11, 13 ]

If these tools are not at hand, the following simple geometrical method can be used:

Fig 2/10 Geometry of shadows

Method:

1) Draw a plan of the building part, e.g. a window with overhang, and enter the direction of the solar radiation with the help of the azimuth.

2) Draw section A-A parallel to the direction of the solar radiation and enter the solar altitude angle.

3) From the plan and section derive the elevation with the shadow picture.

4) Drawing the normal section B-B provides the shadow angle. This is always bigger than the solar altitude angle except in the case where the direction of the sun is at right-angles to the building elevation. In this case the two angles are identical.

This simple method, analogously applied, provides information about the shading performance of any shape of shading devices or building components, and also shading by surrounding buildings. It provides a basis for planning the orientation and grouping of buildings, and for the design of shading devices and openings.

(Relevance to planning and construction see Chapter 3.)

2.2.2 Wind

The phenomenon

The reasons for the development of winds are manifold and vastly complex. The main reason, however, is the uneven distribution of solar radiation over the globe. It results in differing surface heating and temperatures. This causes differences in air pressure and, as a consequence, the development of winds.

Typical main winds

The prevailing air pressure pattern on earth is fairly regular. Together with the rotation of the earth, the main winds can be determined.

Fig 2/11 Main global winds

Local winds

These main winds are overlaid with secondary winds, mainly of thermic origin.

The daily variations in heating and cooling of land and water surfaces (seas, lakes), of mountainous and flat land areas, and of bare and land covered with vegetation, cause regular wind patterns in certain areas, such as sea winds or valley winds in the daytime, and land winds or mountain winds at nighttime.

Valley and mountain winds; Sea and land winds

Fig 2/12 Thermic wind pattern varying between day and night

Thermic bubbles

Strong solar radiation also causes irregular local thermic winds. This is due to air that is heated near the ground and rises from time to time in the form of bubbles.

Influence of topography

Topography influences wind characteristics. Valley bottoms are generally wind protected areas whereas elevated locations receive more and stronger winds.

Fig 2/13 Topographical influence on wind speed

Monsoon wind

The monsoon wind is a result of seasonal differences in the heating up of land and sea areas and is of great importance to a large area in the tropics.

Characteristics of winds

Depending on the origin of the wind, its quality differs. It can be dry or humid, clean, dusty or sandy, hot or cool compared to the prevailing temperature, constant or irregular. Its speed too can vary.

Accordingly, wind can either be utilized for improvements to the indoor climate of buildings or measures must be taken to protect against it.
(Relevance for planning and construction, see Chapter 3.)

Storms

Wind can have a disastrous effect in the form of storms. Due to climatic constraints, certain zones on the globe are prone to storms. In these areas buildings require special structural protection. In most other zones, however, storms occasionally occur as well, but probable with less intensity. As a consequence, adequate protection is also required there. (see Chapter 3.1.4)

Fig 2/14 Regular storm zones. Source: [ 11 ]

2.2.3 Humidity and precipitation

A major factor in climatic characteristics is water. It occurs as rain, hail, snow, clouds and vapour.

Relative humidity

Vapour is water in the form of gas, absorbed by air. Depending on the temperature, the absorption capacity of the air varies.

Fig 2/15 Saturation point

The curve shows the maximum absorption capacity in relation to the air temperature. This represents 100% relative humidity.

Relative humidity is defined as

humidity at saturation point (g/m³) . 100 / effective humidity (g/m³) = ... %

Fog, clouds and precipitation

Air temperature fluctuates considerably during the day and night, and with it the saturation point. Because the absolute humidity remains constant, the relative humidity changes. If, however, the absolute humidity exceeds the saturation point, the surplus water condenses and occurs in the form of fog, clouds, dew or precipitation. The same can be observed when air rises and thus cools down. Strong thermic upwinds result in cumulus clouds; winds crossing mountains create clouds and precipitation.

Fig 2/16

Due to topography, distribution of water bodies and winds, the types and quantity of precipitation varies strongly.

Fig 2/17 Annual precipitation values, based on [ 11 ]

Not only the quantity, but also the types and seasonal distribution of precipitation are manifold. For example, in monsoon areas rainfall is concentrated over a certain period of the year and can be extremely intense and long-lasting. In warm-humid regions it can occur over the whole year with short downpours almost every day.

These differences in precipitation patterns are reflected in construction details and building types, at least traditionally. This can be illustrated by typical building types for different regions (Relevance for planning and constructions see Chapter 3)

Thermal capacity of water

Water has an extremely high thermal capacity, and can thus store and emit large quantities of thermal energy. For instance, the temperature of 1m³ earth increases five times more than that of 1m³ water, when putting in the same quantity of heat energy.

This explains the temperature-regulating effect of large water bodies such as seas or lakes, resulting in the typical maritime climate on the one hand and the continental climate on the other.

Fig 2/18 Difference between continental and maritime climate

A similar balancing effect is caused by a thick vegetation cover such as a forest, partly because it contains large quantities of water.

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).

Fig 2/19 Ways of thermal exchange by the human body

Conduction

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.

Convection

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”).

Fig 2/20 Insulation values of different kind of clothing (1 clo = 0.155 m²K/W). Source: [ 121 ]

Radiation

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).

Fig 2/21 Metabolic rate of different activities (1 met = 58 W/m²) [ 121 ]

2.3.2 Thermal comfort zone

Definition

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
· temperature of the surrounding surfaces (radiant heat)
· relative humidity
· air velocity

Fig 2/22 Physical factors of climatic comfort

The relation of these four factors is well illustrated in the bioclimatic chart.

Fig 2/23 Bioclimatic chart according to [ 13 ]

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.

Fig 2/24 Comfort zone in differing air and surface temperatures

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.

Fig 2/25 Percentage of dissatisfied persons in relation to uneven surface temperatures [-121 ]

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.

Humidity

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.

Wind speed

As the figures below shows, air circulation influences the temperature felt. The cooling effect of wind increases with lower temperatures and higher wind speed.

Source: [ 136 ]

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.

Source: [ 164 ]

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.

Fig 2/26 Optimum room temperature in relation to activity and clothing

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.

Example:

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.

Other factors

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.

Conclusions

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 ?
· When do these activities take place during the course of the day ?
· Where and in which room do these activities take place ?
· What are the anticipated seasonal changes for these functions ?

Working space

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.

Residential space

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.

Seasonal differences

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.

2.3.4 Limitations

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.

2.4 Physics

Obviously, indoor climate depends largely on outdoor climate, especially in the case of passive buildings that are neither heated nor cooled. To a certain extent, however, the indoor climate can be influenced with the help of appropriate designs and materials. This influence depends on the physical processes that occur.

General principles

In order to gain a general understanding of the most important processes, the main physical principles are explained. Together with the physical data given in Appendix 5.1 a rough assessment of the characteristics of the most common materials and composite constructions is possible.

The main physical processes that govern the indoor climate are:

· Thermal radiation
· Heat transmission
· Convection
· Heat storage and time lag
· Internal heat sources

Practical recommendations

This chapter explains only the basic physical phenomena. Evaluation and recommendations for particular materials and for a specific situation are given in Chapter 3.

Detailed information

To verify the exact thermal performance of building components is a rather complex task. Detailed information and calculation methods necessary for the study of specific problems can be obtained from various technical books [-8, 11, 127-]

2.4.1 Thermal radiation
(also see Chapter 3.1.4)

Definition

Radiation is the heat transfer from a warmer surface to a cooler surface which are facing each other. This happens in the form of waves and a transmitting media (e.g. air) is thus not required.

Emittance

The warmer surface emits thermal energy in the form of radiant heat always towards a cooler surface. The quantity of emitted energy depends on the temperature difference between the surfaces, and also on the material property (emissivity) of the warmer surface.

Fig 2/27 Emittance e

Absorption and reflectance

Depending on these surface properties the radiation received by the cooler surface can be partly absorbed and partly reflected. These properties are called absorbance-(a) and reflectance-(r).

(a)-+-(r) always equals 1.

Light-colored, smooth and shiny surfaces tend to have a higher reflectance. For the perfect theoretical white surface the reflectance is 1 and the absorbance is 0; for the perfect “black body” absorber the reflectance is 0 and the absorbance is 1.

Fig 2/28 Absorbance a and reflectance r

Geometrical location

The quantity of radiant heat that a body receives depends also on the geometrical location with regard to the heat source.

Surfaces which directly face each other exchange the greatest thermal radiation, whereas surfaces that are turned away from each other exchange less.

Fig 2/29

Balancing effect

As a consequence of this radiation, the warmer surface cools down and the cooler surface heats up.

(Values of emittance and reflectance of the main building materials see Appendix 5.1 )

2.4.2 Heat transmission
(also see Chapter 3.1.4)

Heat always flows from a higher temperature to a lower temperature. The quantity of heat transmitted through a material depends on

· its conductivity;
· the temperature difference between outside and inside;
· the thickness of the material; and
· the surface conductance.

The conductivity k (W/mK)

In conduction, the spread of molecular movement constitutes the flow of heat. The rate of heat flow varies with different materials and depends on its thermal conductivity (k). It is defined as the rate of heat flow through a unit area of unit thickness of the material, by a unit temperature difference between the two sides. The dimension is W/m°C. This value is used to compare the thermal insulation effectiveness of materials that are homogeneous in composition. Its value ranges from 0.03 W/m°C for thermal insulation materials up to 400 W/m°C for metals. The lower the conductivity, the better an insulator is the material.

(k-values of different materials see Appendix 5.1 )

(The k-value corresponds with l in the German system)

Fig 2/30 Conductivity k

Air is a most efficient insulator

Air has an extremely low k-value. The higher the percentage of air enclosed in the material, the better is its insulation value, as long as convection does not occur. To avoid convection, the air enclosures must be fine. The finer the air inclusions, the less convection takes place.

Low weight materials tend to contain more air, thus their conductivity is less. This relationship is generally true for materials of the same kind but of varying densities, and of the same materials with varying moisture content.

Humid materials are poor insulators

Water has a conductivity of 580 W/m°C versus 0.026 W/m°C for still air. Therefore, if the air enclosed in the is replaced by water, the material’s conductivity is rapidly increased. For example, an asbestos insulating board in dry conditions has a conductivity four times lower than that of the same board soaked with water.

Resistance R (m²K/W)

The resistance depends on the conductivity and the thickness of a material.

It is defined as thickness / k = R

How much heat is prevented from passing through a non-homogenuous section?

The total resistance of a composite construction is the sum of the resistance of its components, thus R1 + R2 + R3....= R total

Fig 2/31 Resistance R

Heat transfer at the surface or surface conductance f (W/m²K)

A thin layer of air film separates the material surface from the surrounding ambient air, and this air film has a specific conductance (f) in relation to the transfer between material and the surrounding air. Surface conductance includes the convection and radiant components of the heat exchange at the surfaces. The resistance of these films is expressed as 1/f.

For internal surfaces this resistance (fi) is around 0.15-m²°C/W, and for external surfaces (fo) it varies between 0.1 and 0.01 m²°C/W depending on wind exposure.

Transmittance U (W/m²K)
(see Appendix 5.1 )
Adding the surface resistance 1/f to R total, the total heat transmission can be calculated:

The reciprocal value is the thermal transmittance U.

(The U-value corresponds with the k-value in the German system)

Quantity of transmitted heat

The U-value represents the total heat transmitted through a composite construction by a temperature difference of 1°C. Multiplying it with the effective temperature difference gives the total heat energy transmitted:

Total heat transmission = U·(ti - to)·(W/m²)

This value, however, is only valid for the theoretical case of stable temperature conditions over a longer period. In reality, the outdoor temperature fluctuates during the course of the day. This is of special relevance in the case of warm climates, where the houses are neither heated nor cooled and the heat flow is thus not unidirectional. Here the time lag, the decrement factor and the thermal capacity play important roles.

2.4.3 Heat storage
(also see Chapter 3.1.4)

Specific heat (Wh/kgK)

This is defined as the amount of energy required for a unit temperature increase in a unit mass of material. The higher the specific heat of a material, the more heat it will absorb for a given increase in temperature. Of all common materials, water has the highest specific heat.

Heat capacity Q (Wh/m² K)

This is defined as the amount of heat energy required for a unit temperature increase in a unit of area.
Thickness x specific mass x specific heat = heat capacity (Q)

Time lag O (h) and decrement factor

The time lag is defined as the time difference between the peak outer surface temperature and the peak inner surface temperature; it is actually the time required for the heat to pass through a material. It is of importance, for instance, in the case where one wants to take advantage in the evening of day time surplus heat energy.

Fig 2/32. Source [ 8 ]

Decrement factor

The decrement factor is the ratio between the temperature fluctuation on the outer and the inner surface. It is the measure of the damping effect. Generally, the higher the thermal capacity or the higher the thermal resistance of a material, the stronger is the damping effect.

The time lag can be controlled by the selection of materials and their thickness. It depends on the thermal capacity Q and the resistance R.

For heavy materials the time lag can be roughly calculated using the formula

time lag O = 1.38 + (Q x R)^1/2

For composite constructions, an additional estimated lag should be added to the individual sum of the time lags. It is customary for two layers and light construction walls to add an additional 0.5 hour; for three or more layers, or for very heavy constructions, one additional hour lag is assumed.

(Time lag values of common materials and composite constructions see Appendix 5.1 )

Active heat storage capacity

The heat storage capacity and the time lag of a building structure can be utilized for balancing the indoor temperature. In such a case, however, the so-called active mass only, and not the entire building mass, is taken into account ( see Chapter 3.1.4 ).

2.4.4 Solar heat gain factor

When selecting construction materials in areas with intense solar radiation an important criterion is the solar heat gain factor (SHF). This is defined as the rate of heat flow through the construction due to solar radiation expressed as a percentage of the incident solar radiation. [ 8 ].

SHF (%) = 100 x transmitted solar energy / incident solar energy

As this value can be related to the increase in the inner surface temperature, a performance standard can be established on the basis of experience. Its value should not exceed 4% in warm-humid climates or 3% in hot-dry climates.

A graphic method exists for calculating the SHF. [ 120 ].

For instant practical use a table with the values for common constructions can be found in Appendix 5.1

2.4.5 Vapour diffusion

Water in the form of vapour diffuses through the outer building shell when the outside and inside vapour pressures differ. Vapour usually diffuses from the warmer towards the cooler side of the shell.

This phenomenon requires attention in the case where there is likely to be an area of condensation inside the shell (e.g. “vapour barrier” on the cooler side). This happens when the saturation point is reached, particularly in heated or constantly cooled buildings. In air-conditioned buildings, especially, this aspect requires consideration. However, in naturally climatized buildings such conditions usually do not occur. Hence vapour diffusion is not dealt with in this publication.