|Climate Responsive Building - Appropriate Building Construction in Tropical and Subtropical Regions (SKAT, 1993, 324 p.)|
|3. Design rules|
The main points:
· Minimize heat gain during daytime and maximize heat loss at night in hot seasons, and reverse in cold seasons.
· Minimize internal heat gain in the hot season.
· Select the site according to microclimatic criteria
· Optimize the building structure (especially regarding thermal storage and time lag).
· Control solar radiation.
· Regulate air circulation.
3.1.1 Climate and design in general
In general, in tropical and subtropical regions the daytime temperature is uncomfortably high, particularly during the warmer seasons and in low altitude locations. However, the differences between regions are immense, depending mainly on the distance from the equator and on altitude.
Air humidity is also of great importance. This factor influences the precipitation pattern and the amount of solar radiation that reaches the earths surface. The influence of a cloud cover is most obvious, but invisible humidity in the atmosphere also alters the amount of radiation. Whereas with dry air conditions the radiation is strong and direct, humid air results in a less intense but diffuse radiation and also reduces the amount of re-radiation to the night sky.
These factors result in mean temperatures that differ highly
from place to place. Annual and diurnal fluctuations also vary sharply.
(also see Chapter 2.2)
Design objectives and response
The main objective of climatic design is to provide comfortable living conditions with a minimum and meaningful input of artificial energy. This also reduces investment and running costs as well as ecological damage.
The above-mentioned main points are the framework for design in tropical and subtropical climate conditions. They have to be adapted to each climatic zone because the dominant climatic factors differ highly between these zones. This leads to different solutions for various climate types.
Such solutions are described in the corresponding chapters.
3.1.2 Settlement Planning
Different factors have to be considered when planning settlements. Transportation means and ways, water access, water supply, available materials and technical means, infrastructure, social structure and defense considerations are but a few of them.
In view of the general goal of protection from the harsh climate as well as risks, the following main criteria have to be considered :
The main points
· Topography, to benefit from
· Orientation, to optimize sun and wind impact.
· Wind, to achieve the required ventilation.
· Pattern and form, to optimize the reciprocal impact between buildings.
· Hazards, for safety reasons
188.8.131.52 Topographical location of settlements
In selecting the location for a settlement, the microclimatic advantages caused by topographical features of different sites should be considered.
a) Locations on slopes, hills and in valleys
In general, elevated sites are preferable. Locations at higher altitude have lower temperatures due to the adiabatic phenomenon. The mean temperature decreases by 1°C with 100-m altitude difference.
Settlements are preferably placed on northern slopes to avoid excessive sun exposure, using natural shade. West slopes should be avoided. At higher altitude south exposure maight be adequate for reasons of passive heating.
Valley bottoms are additionally heated by reflection of sun radiation from the surrounding slopes .
c) Wind - orientation
Locations situated at the bottom of valleys are often handicapped. Air movement is usually much better at higher locations. Valleys tend to have lower wind velocity and hence the cooling effect by wind is reduced.
d) Air pollution
Further negative effects of a site located in a valley can be caused by air pollution, especially when polluting industries are combined with poor air movement.
Under certain circumstances the air movement in a valley can be reduced by inversion. It occurs when a relatively cooler layer of air accumulates at the bottom of a valley. If no dynamic winds prevail, this cooler air cannot be replaced because the phenomenon prevents air movement by thermic winds. An air trap may result, and with it, a dangerous increase in air pollution.
e) Location near water bodies and green areas
Where possible, settlements should be placed near large bodies of water such as lakes - preferably on the leeward side - and green areas. Water has a regulating effect on the climate because the water temperature is near to the annual mean temperature. Due to the large thermal capacity of water it can absorb surplus daytime heat and reduce the nighttime drop. The resulting temperature difference between the land area and the water surface furthermore produces thermal winds, which blow towards the land during the day and at night away from the land. Green areas have the advantage of cooling by shade and evaporation.
Floods and landslides
A threat to building in valleys may be the danger of floods and landslides. Although seldom, even in arid regions heavy rain can occur, causing torrent streams combined with masses of mud, rocks and boulders.
In almost all areas, heavy winds occur and a firm structure is required. Special care, however, has to be taken in areas that are threatened by hurricanes and sandstorms.
Despite the fact that earthquakes are not a topic of climatic design, the location of settlements has to be checked for possible earthquake risks and safe constructions have to be made. They may be in contradiction to traditional design or climatic construction requirements.
184.108.40.206 Urban forms and external space
Urban forms depend strongly on climate and are designed differently in each climatic zone. Basic concerns are the provision of shading and air movement by alternative means.
The urban form cannot change the regional climate, but can moderate the citys microclimate and improve the conditions for the buildings and their inhabitants.
The influence of the climate on the external space of traditional settlements can be well illustrated by the following examples:
Settlements for hot, dry climates are characterized by optimal protection against solar radiation by mutual shading, which leads to compact settlements, narrow streets and small squares which are shaded by tall vegetation.
Fig 3/9 Typical settlement for hot-dry regions
Settlements for warm humid areas are laid out to make maximum use of the prevailing breeze. Buildings are scattered, vegetation is arranged to provide maximum shade without hindering natural ventilation.
Fig 3/10 Typical settlement for warm-humid regions
Although modern requirements are often in contradiction to traditional patterns, their advantages should be adapted as far as possible.
The use of vegetation in landscaping
Designs using vegetation in the urban environment are of functional, aesthetic as well as climatic importance for its radiation absorbent surface and its evaporative and shade-giving properties. The vegetation in and around cities also has definite effects on air movement.
Vegetation is desirable both for providing shade, thus reducing the temperature in such shaded areas, and for reducing the effects of strong solar radiation on the walls of buildings and structures. Also, by forming a thick barrier of foliage, the velocity of strong wind is reduced. The foliage of different types of wooded land (e.g. hedges) acts as a filter and purifies the atmosphere by keeping down dust.
Advantages of vegetation
Landscaping using vegetation has many advantages:
· It improves the microclimate both outdoors and indoors.
· It checks hot and dusty winds in arid regions.
· Through the transpiration of leaves temperatures are lowered.
· Its shade lowers daytime temperatures and heat emission at night is also reduced, thus resulting in more balanced temperatures.
· It balances the humidity. During precipitation much of the free water is absorbed and during dry periods water is evaporated.
Plants offer longterm energy saving free of cost, both in financial and in ecological terms.
In hot-arid areas with limited water reserves, plants with high water requirements may not be possible, but plants adapted to local conditions are always advantageous.
Moreover, plants increase the value of indoor and outdoor living space. Outdoor space becomes a more useful area and can accommodate a variety of functions which are not possible in a barren area.
The cooling effect of vegetation can be illustrated by the following measurements which were taken in South Africa:
Slate roof in the sun
Concrete surface in the sun
Short grass in the sun
Leaf surface of tree in shade
Short grass in shade
[ 12 ] (also see Fig 3/94 in Chapter 220.127.116.11)
Selection of plant species
When selecting the plant material, it is strongly advisable to consult local plant nurseries about their stocks and their experiences. The suitability and performance of plants depends highly on the specific local conditions:
· the climatical factors,
temperature, air humidity etc.
· soil condition
· soil moisture (ground water level)
If there are doubts, plants should first be tested under local conditions, before they are used in a larger scale.
In large cities, where water in abundance can be made available, the excessive use of vegetation and water surfaces can also create a less comfortable microclimate because of too much evaporation that increases the humidity.
For the use of vegetation also see Chapter 18.104.22.168, 22.214.171.124, 126.96.36.199, 188.8.131.52, 184.108.40.206, 220.127.116.11 and Appendix 5.6.
Natural elements of landscape design include the meaningful use of trees, streetscaping with vegetation, surface water management and with it the utilization of the cooling effect of water.
Trees and shrubs are a very effective means of improving the climate on a larger scale. They are the simplest way of shading outdoor space and buildings.
It is important to select the appropriate type of tree
One simple solution for regulating shading by trees throughout the year is the use of deciduous trees, which provide shade during the hot season and allow solar radiation in winter.
Another factor that can help in the selection of the right tree is its cooling factor. When measuring the radiation intensity in the shade of a tree the efficiency of different species varies.
The cooling factor for the examples given here indicates the radiation intensity compared to unshaded conditions.
Fig 3/11 Selected trees and their cooling factor (drawing H. Haas)
b) Streetscaping using vegetation
The furnishing of space with trees and hedges greatly improves the microclimate and quality of life.
Fig 3/12 Green street space
c) Surface water management
An important aim of road planners is usually, to design drainage systems that ensure a rapid rainwater run-off. Such systems, together with a high percentage of paved surfaces - as is common in urban areas - have the disadvantage that shortly after rainfalls the surroundings are dry again and the cooling effect of the water is lost Furthermore, the functioning of the drainage systems depend to a great extent on their maintenance. Blocked drainage systems may cause dangerous flood situations. Floods can also occur further down near the river, because the water quantity is not balanced.
Fig 3/14 Problems of quick drainage systems: rainwater is moving fast from the sealed surfaces, forming forceful streams
d) Utilizing the cooling effect of water
An alternative approach would be to retain as much surface water as possible for a longer period. This can be achieved by keeping surfaces unpaved wherever feasible. Public open spaces, streets, squares and parks should only be covered by hard top when absolutely necessary.
In addition, drainage systems can be combined with ponds and artificial lakes e.g. in park areas. The advantages are obvious:
· The increased water content in the air and soil improves the microclimate. It also supports and promotes vegetation which is an additional factor for a favourable microclimate.
· Such a system also feeds the ground water, which is an important factor with regard to water supply.
· The drainage system can be designed for smaller peak flows.
· The danger of floods due to blocked drainage systems is reduced.
Fig 3/15 Advantages of slow drainage systems: intercepts the rainwater in the greenery, in the ground and ponds, the natural way to prevent floodings
Fig 3/16 Water cycle with unpaved surfaces: the soil absorbes water during rain, stores it and feeds it to the ground water and back to the air.
3.1.3 Building design
The main points
· Orientation and room
placement, for optimal response to sun and wind.
· Form, providing protection where required.
· Shade, as much as required.
· Ventilation, by excluding climatically adverse side-effects.
18.104.22.168 Orientation of buildings
To define the optimal orientation of a building, three factors have to be considered:
· Solar radiation
· Prevailing wind
To define the optimal orientation with regard to heat gain by solar radiation, it is useful to analyse the radiation intensity on differently oriented surfaces, its diurnal change and its change with seasons.
The diagram (Fig 3./17) shows an example of an analysis for 1o South (Nairobi). It indicates, depending on whether heat gain is desired or not:
· What is the optimal
· Where are large openings, small or no openings desirable ?
· What kind of structure and shading devices are appropriate for a given surface ?
Fig 3/17 Analysis of the solar radiation intensity in Nairobi [ 8 ]
Optimal sun-orientation reduces radiation to a minimum in the hot periods, while allowing adequate radiation during the cool months.
East and west facing walls receive the highest intensities of radiation, especially during the hot periods. These walls should thus normally be kept as small as possible and contain as few and small openings as possible.
Fig 3/18 In general, north and south facing is the preferred orientation
By plotting the directions of maximum radiant gain for both hot and cool months, it is possible to determine the optimum orientation for any given situation. Some compromise must be made in order to achieve the most satisfactory distribution of the total heat gained in all seasons. [ 10, 11 ]
Usually cooling by ventilation is desired. Buildings should therefore be oriented across the prevailing breeze. This direction often does not coincide with the best orientation according to the sun. Here a compromise should be found, paying more attention to the effects of solar radiation, because the direction of the wind can be influenced to a certain extent by structural elements
Fig 3/19 In general, facing the wind is the preferred orientation
The surface of the surroundings may store and reflect solar radiant heat towards the building, depending on the surfaces angle relative to the solar radiation and on the type of surface. Where this solar heat is not desired, the orientation of the building should be changed or the surface of the surroundings should be covered with greenery that improves the microclimate.
The topography may also alter the prevailing wind and provide shade at certain time of the day. Such elements should also be considered.
Fig 3/20 Topography reflecting solar radiation
22.214.171.124 Shape and volume
The functional as well as socio-cultural requirements and particularly the climatic conditions define the form of the buildings.
The heat exchange between the building and the environment depends greatly on the exposed surfaces. A compact building gains less heat during the daytime and loses less heat at night. Therefore, the ratio of surface to volume is an important factor.
A simple model calculation on differently arranged building units illustrates this.
12 building units of 7 x 7 m width and 3 m height are arranged as individual bungalows, as row houses or as a compact 3-story building. The volume : surface ratio changes drastically.
a) as individual bungalows
b) as row houses
c) as compact 3-story building
Fig 3/21 Volume to surface ratio by differently arranged building units
A similar phenomenon can be observed when comparing large buildings with small buildings of the same shape.
This can be demonstrated when comparing cubes of differing volumes:
a) cube 3 x 3 x 3 m
b) cube 7 x 7 x 7 m
c) cube 20 x 20 x 20 m
Fig 3/22 Volume to surface ratio by different sized cubes
In general, where little heat exchange between the interior and the environment is desired, the surface to volume factor should be small. The indoor temperature will be near to the average outdoor temperature.
Where heat exchange is desired, for instance to gain from cool nights in warm-humid areas, the surface to volume factor should be bigger. This also favours a higher ventilation rate.
126.96.36.199 Type and form of buildings
The suitable form of buildings differs very much between the main climatic zones. Traditional regional dwelling types illustrate this clearly.
a) The compact, inward oriented house of the hot-arid zone (see Chapter 3.2.3).
Massive wall and roof structures even out the indoor climate in conditions of hot days and cold nights. The surface is kept at a minimum compared to the volume so that the exchange of heat and cold is minimized. Ventilation should be controlled: minimized during the heat and increased during periods when the outdoor temperature is at comfort level.
Such types are generally appropriate in areas with large temperature differences between day and night.
Fig 3/23 Typical house of the hot-arid zone
b) The open, outward oriented, detached, built on stilts house
of the warm-humid zone
(see Chapter 3.3.3)
The surface is large compared to the volume and therefore the exchange of heat energy high. As a consequence the indoor temperature approaches the outdoor temperature. The walls are light and maximum ventilation can easily be achieved. Large overhanging roofs are the main important element.
This type is appropriate in zones with even day and nighttime temperatures.
Fig 3/24 Typical house of the warm-humid zone
c) A compromise between the two extremes is the house of the
(see Chapter 3.4.3)
It is composed of shading roofs as well as protective walls which are less massive than in a) above.
The windows are of medium size, providing good ventilation and moderate solar heat gain.
Fig 3/25 Typical house of the temperate zone
When designing the floor plan of a building, apart from the functional arrangements, room connections and privacy requirements, the following aspects should be considered:
· At what time of the day will
the room be used ?
· Is the room of prime importance or is it an auxiliary space ?
Important rooms should be located at places with climatic advantages. For instance, in hot climates a bedroom is preferably located on the east side where it is relatively cool in the evening, whereas the living room is placed on the northern side. Auxiliary spaces should be located on the disadvantaged sides, mainly west.
Rooms with high internal heat load, such as kitchens, should be detached from the main rooms.
Fig 3/26 Typical room arrangement
Minimize internal heat gain
Internal heat gains, in the form of heat output from human bodies, equipment, cooking and lighting (often referred to as wild heat), can present quite a problem and should be minimized in hot seasons. In cool seasons it can be welcome as a heating source.
It is not possible to avoid these heat sources, but one aspect for reducing the indoor temperature in buildings is to minimize their quantity as well as their impact on the main rooms. This involves technical measures and also has consequences with regard to the room arrangements.
a) Heat gains from human bodies
As far as is possible, the number of people living in a house should be reduced. To provide more space is, of course, very much an economical question.
To avoid overcrowded indoor areas the outdoor space should be designed in such a way that as much activity as possible can take place there.
Daylight provision should be adjusted to the necessary level only, not too excessive and diffuse rather than direct.
Where artificial lighting is needed, high efficiency light sources should be used which produce less heat.
Unnecessary lighting should be avoided and background lighting should be of low level.
In hot seasons heat producing equipment should be placed remotely, away from occupants.
When placing such equipment, the prevailing air movement should be considered. It should be placed on the lee-side of the main rooms, if possible in a separately ventilated, detached room.
A high ventilation around heat-producing equipment may be required.
Separate zones for day and night, summer and winter
Separate day and night zones may be provided in the house. The day zone would be a heavy structure retaining the coolness of the night and oriented towards west. The night zone would be a light structure which cools down quickly after sunset and is oriented towards east.
Fig 3/27 Use of heavy and light building parts as day and night space
Similarly, variation in living spaces used in summer time or in
winter time could be provided - a concept which is feasible mainly in temperate
(see Chapter 188.8.131.52)
Fig 3/28 Orientation of space used in summer or winter
184.108.40.206 Immediate external space
In tropical and subtropical regions the outdoor space is actively used. A major part of the social life and the daily routine work takes place there.
Depending on the climatic conditions, various forms of courtyards, protected niches and alcoves are common. Such elements should be carefully designed.
Trees and other plants are important elements of immediate outdoor spaces. They are inexpensive elements which regulate and improve the climate. At the same time they add to the attractiveness of this space.
When planting trees, some basic rules should be kept in mind:
a) Basically, the same considerations for designing shading devices are also applicable to trees:
· At what time of the day and at
what seasons of the year is shade desired ?
· What is the suns path?
b) A tree planted close to the building, even with the crown covering the roof, provides the best protection from the intense midday sun, but allows access to the sun in evening hours, when in certain situations this is welcome.
Deciduous trees allow enough heat gain for passive heating and daylight during the winter season.
Fig 3/29 Tree close to a building
c) A tree planted within a certain distance of a building provides shade only during evening or morning hours, but not at midday, the hottest time.
Fig 3/30 Tree far from a building
d) Planting a tree close to a building does not necessarily harm it. While growing, trees always adapt their shape according to the nearby building form. Certain constant observations and maintenance measures are however necessary. These include some trimming and removal of branches which are likely to break off.
Fig 3/31 Tree adapting to the form of a building
3.1.4 Building components
(Technical data see Appendix 5.1 )
The main points
· Heat storage and time lag, which provide a balanced indoor climate and take advantage of outdoor temperature fluctuations.
· Thermal insulation, which prevent undesired heat gain, but do not impede emission of surplus heat.
· Reflectivity, absorption and emissivity, which regulate the radiation from and to the sky and the surroundings.
All building components should work together as a balanced system to create a comfortable indoor climate.
The appropriate design of floors, walls, roofs and openings varies greatly with different climatic zones. Solutions cannot therefore be generalised and have to be worked out according to the individual situation as well as to basic physical principles.
In the following section, the main characteristics of heat storage, time lag, thermal insulation and reflectivity are discussed, their influence on the indoor climate explained.
The most commonly used building materials and details are listed and then their main properties and suitability described.
220.127.116.11 The principles of heat storage; time lag; thermal
insulation; reflectivity, absorption and emissivity; and condensation
(also see Chapter 2.4)
Heat storage and time lag (see table in Appendix .5.1 )
The capacity of building components to store heat and to release it later has an important regulating effect on the indoor climate. A high internal mass reduces the indoor temperature swing. During the daytime it is thus cooler and at night warmer than outdoors.
The main performance range is shown in Fig 3/32.
The indoor temperature of a light structure (1) is similar to the outdoor temperature (To) with a slight time lag. Without proper reflection of the solar radiation this temperature can also rise far above the outdoor temperature.
The indoor temperature of a heavy structure (2) remains near the average outdoor temperature, with a longer time lag.
The temperature can be considerably lowered during the day by combining a heavy structure with proper night ventilation (3).
The effect of heat storage and time lag in conditions of a wide diurnal temperature range can clearly be seen in Fig 3/32.
Fig 3/32 Wide diurnal temperature range
This effect can be ignored in conditions with a narrow diurnal temperature range as illustrated in Fig 3/33.
Hence, heat storage is only valuable in climates where the diurnal temperature range is wide and falls below comfort level at night. In this situation one likes to get rid of surplus heat - or part of it - during the day; on the other hand, this heat may be welcome in the evening or during the night.
Fig 3/33 Narrow diurnal temperature range
Active heat storage capacity
The amount of heat stored depends on the effective thermal storage capacity. The entire building mass cannot be activated to store heat.
· Outer walls and roof:
If thermal insulation is used, only the mass inside of the insulation is active in storage.
· Internal materials:
The amount that can be used depends on the extent of the active heat storage capacity. For areas exposed to direct solar radiation (primary mass) this is 15-25 cm and for areas not exposed to direct solar radiation (secondary mass) this is 8-10 cm.
The primary mass is much more effective than the secondary mass with regard to active heat storage capacity.
Fig 3/34 Primary and secondary masses
The time lag determines when maximum heat is emitted (see Chapter 2.4). According to the function of a building or room, the components can be designed to achieve the desired effect.
Storing heat over periods longer than a couple of days is only possible with special storage elements, e.g. large, well-insulated watertanks.
Comparison of heat storage requirement
A large thermal mass with high heat storage capacity is desired
in most cases in order to keep houses cool in daytime and to achieve a
comfortable night temperature, despite severe outdoor temperatures.
Because the narrow diurnal temperature range does not usually
fall below comfort level, heat storage capacity should be avoided, at least for
rooms also used at night.
A compromise between conflicting requirements is necessary. Too
little storage capacity results in overheating in summer, too great a storage
capacity makes the building unheatable in winter.
The time between peak temperature being reached on the outer surface and the same on the inner surface is called the time lag. This is important where internal heat gain is desired later in the evening.
For passive heating, the building shell has ideally a time lag covering the hours between the greatest heat gain outside and the desired heat gain inside
Estimating the required time lag:
Depending on the orientation of a surface, the hours of maximum heat gain (radiation) varies. In addition, the time at which heat emission to the interior is desired or does not cause any disturbance, varies as well. As a consequence, the ideal time can also vary. See diagram (Fig 3/35)
· An office space that does not require any heat gain, would best be designed as a structure with a time lag which takes effect after office hours only.
· A living or sleeping space should be designed with a time lag which takes effect when the outdoor temperature drops below comfort level.
Fig 3/35 The outer surface temperature and the desired time lag
In cases where cooling during the daytime is desired, the principle can be reversed. The desired time lag would be defined as the time between the period of maximum heat loss to the night sky and the period of desired internal cooling.
In areas where the outdoor temperature does not fall below the level of comfort or where the diurnal change is minimal, the time lag is not relevant. Here, reflective insulation, shading and ventilation are the main instruments for controlling the indoor climate.
(see table in Appendix 5.1 )
In the case of a temperature difference, heat energy always travels from hot to cold. Thermal insulation reduces such heat transfer. As a consequence, it reduces daytime surplus heat entering a building, but prevents the building from cooling down at night. In general, this dual function makes insulation unsuitable for naturally-climatized buildings.
In the theoretical case of a highly insulated structure with no heat storage capacity, the indoor temperature would always be exactly the same as the outdoor temperature, because the minimum ventilation which is always required would bring in the air which is at the outdoor temperature.
In some cases a partial thermal insulation is nevertheless appropriate; for example in roof structures where, due to solar radiation, extreme daytime heat occurs.
The thermal insulation capacity of a structure is indicated with the U-value (see Chapter 2.4).
Thermal insulation and storage mass
If thermal insulation is used in combination with heat storing materials, this storage mass must be on the inside, e.g. in a massive shell construction, or in the internal walls or floor slabs.
Fig 3/36 Storage mass on the inside of the insulation is effective
If insulation separates the storage mass from the interior, its effect is lost.
Fig 3/37 Storage mass outside of the insulation it is not effective.
A building with thermal insulation and sufficient internal heat storage mass can be suitable, provided that a very reliable and efficient ventilation at night removes the daytime surplus heat.
Thermal insulation and active cooling or heating
In cases of active cooling or heating thermal insulation has clear advantages and is often indispensable. It reduces the heat load considerably.
To avoid damp condensation, care has to be taken in placing insulation in relation to the damp-proof material (plastic, metal, aluminium foil etc.). In this case the damp-proof material has always to be on the warm side of the insulation.
Reflectivity, absorptivity and emissivity
(see data in Appendix 5.1 )
Much of the heat received by a building is through radiation, mainly solar radiation. The treatment of the outer surface is therefore important.
The quantity of radiant heat a surface receives depends not only on the sun angle, but to a large extent on the properties of reflectivity and absorbance.
Heat emission at night is also important. It takes place only towards cooler surfaces, that is, mainly, towards the clear night sky. There is no radiant heat emission towards other buildings and surfaces that have the same surface temperature.
(see Chapter 2.4 )
Therefore, the main properties to be considered for constructions and materials are:
· Reflection of radiant
· Absorption of radiant heat
· Re-emission of stored heat
(see data in Appendix 5.1 )
Reflection of radiant heat
Where heat gain is not desired, a reflective surface, e.g. white or bright metallic, is appropriate. Lightweight constructions should always possess such surfaces. Dull surfaces such as older galvanized iron sheeting are poor in this respect.
Absorption of radiant heat
Where heat gain for nighttime is desired, absorbent surfaces, which are generally darker and non-shiny are preferred. Such surfaces should only be used for buildings with a high thermal capacity. Buildings with a low thermal capacity would immediately overheat.
Where radiant heat loss is possible, for example to the sky, a white surface allows less net gain. Where opposing surfaces are warm, there is no radiant loss, and aluminium is preferred.
Re-emission of stored heat
Where a re-emission of stored heat to the environment and the sky at nighttime is desired, surfaces should preferably be of a porous nature. Plaster and brick surfaces are more efficient than metallic surfaces. The degree of brightness (color) is not of relevance.
Fig 3/38 Reflection, absorption, emissivity of white metal surfaces (a) and bright aluminium (b) [ 8-]
With regard to reflectivity, the property of the roof surface is of the greatest importance because it receives a far greater amount of radiant heat than any vertical surface, and can also re-emit more than other surfaces. Hence, it has to be carefully selected. If an absorbent surface is used, the time lag should usually be at least 8 hours. Lightweight roofs should have reflective surfaces combined with thermal insulation or a ventilated ceiling.
When selecting the building materials, their thermal properties should be analysed so that materials suitable to the local climatic conditions can be chosen. When considering exposure to solar radiation, the solar heatgain factor (SHF) is an important criterion to be taken into account, especially in the case of the roof. It is more important than the U-value.
Appendix 5.1 contains tables with the most important thermal properties of typical wall and roof constructions.
When the inner surface of the building shell cools down far below the indoor air temperature at night, then condensation may occur. This is often the case with single metal sheeting and can be countered by a properly ventilated double shell construction.
A secondary problem with condensation may arise when the inside surfaces of a building remain cool and warm and relatively humid air enters. This may cause condensation and mould growth, which must be countered by additional ventilation.
Further information see [ 2, 4, 8, 10, 11, 162 ]
18.104.22.168 Foundations, basements and floors
(also see Chapter 22.214.171.124, 126.96.36.199, 188.8.131.52)
Basements and floors generally have a large thermal storage capacity and can therefore act as a climate regulating element. It depends on the specific climatic conditions, whether these properties are an advantage or whether the rooms have to be insulated against it.
Common building materials, properties and suitability
Solid floor, concrete, stone burnt clay bricks and tiles, earth
Good materials for heat storage; help to balance indoor
temperature. Suitable for hot zones with large diurnal temperature
Multilayer floor with insulation materials
Suitable for upland climates.
Single planking timber floor, ground detached
Suitable for warm-humid climate, for comfort at nighttime.
(also see Chapter 184.108.40.206, 220.127.116.11, 18.104.22.168)
Walls (exterior and interior) can have several functions:
Beside being a structural element, they provide protection from heat, precipitation, wind, dust and light and serve as a means of space definition and partition. The properties should therefore be selected according to the main functions of a wall.
COMMON BUILDING MATERIALS FOR WALLS, PROPERTIES AND SUITABILITY
SOLID WALLS EARTH, STONE, BRICK
Good materials in hot-arid zones, combined with few openings and light colored outer surface. Takes best advantages of time lag, with heat emission at night. In warm-humid zones only useful for daytime rooms.
BURNT CLAY BRICKS
Good thermal resistance, depending on the porosity. Medium to high heat storage capacity, good humidity regulating property.
UNBURNT CLAY BRICKS
Better thermal resistance and humidity regulating property than burnt bricks. Less resistant to mechanical stress. Needs protection from driving rain and rising moisture. Improved products with low cement content are somewhat less vulnerable.
SOLID CONCRETE BLOCKS
Poor thermal resistance and high heat storage capacity.
HOLLOW CONCRETE BLOCKS
Less heat storage capacity than solid blocks but improved insulation, thus better suited for temperate climate.
Has similar properties to concrete, but less thermal storage capacity due to the reduced thickness; suitable for warm-humid zones.
Good thermal resistance, high heat storage capacity, good regulation of humidity.
MATTING OF BAMBOO, GRASS, LEAVES
Good material in warm-humid zones, with no thermal storage capacity, not airtight and thus allowing proper ventilation.
Various natural and artificial materials are available and have to be selected carefully. They prevent not only heat gain, but also heat loss. The danger of overheating at night has to be considered as well.
Simple and low cost, yet effective methodfor making a surface highly reflective. The emission at night remains high.
Has many advantages, especially in hot-arid zones. Reflective surface in the cavity (e.g. aluminium foil) reduces radiant heat transfer. Ventilation of the cavity takes the heat away and reduces conductive heat transmission to the interior.
Light weight walls, traditional matting, frame construction with
thin infill panels
Indoor and outdoor temperatures remain much the same, provided the walls are shaded.
If unshaded, indoor temperature rises quickly above outdoor temperature.
Suitable for warm-humid climate, taking full advantage of cooler night temperature.
Suitable in hot-arid regions for rooms used at night only, where the outdoor temperature does not fall considerably below comfort level.
Heat insulated light weight wall
Mainly used for air conditioned rooms, especially if exposed to direct solar radiation.
The application of multilayered construction is in many cases an economic question. Where the resources are available, it can be used; however, a careful assessment of its thermal performance is needed.
Placing a lightweight insulating material on the outside of a massive wall or roof will give a time lag and decrement factor greater than that of the massive wall alone. On the other hand it prevents heat dissipation to the outside at night, thus making internal ventilation imperative.
Fig 3/40 Insulation outside: night ventilation is important
Placing insulation on the inside will result in an indoor climate performance similar to the one in a lightweight structure with a highly reflective outer skin, because the balancing effect of the thermal mass of the outer wall is cut off.
Fig 3/41 Insulation inside: high indoor temperature during the daytime if not mechanically cooled
The time lag is thus minimal and the indoor temperature is always close to the outside temperature.
Such inside insulation can be appropriate in actively cooled or heated buildings.
A ventilated and reflective outer skin is an efficient, although expensive solution, to reduce radiant daytime heat. Heat dissipation at night is more efficient than with a structure using outside insulation.
Fig 3/42 Ventilated and reflective outer skin with heavy inner structure
One way of reducing the radiant heat transfer between the two skins is the use of a low emission surface on the inside of the outer skin (e.g. aluminium painted white on the outside but left bright on the inside) and a highly reflective surface on top of the ceiling. Bright aluminium foil can be used to advantage in both situations.
22.214.171.124 Openings and windows
Design(also see Chapter 126.96.36.199, 188.8.131.52, 184.108.40.206, 220.127.116.11)
The design of the openings is greatly influenced by the prevailing climate. In general it can be said that
· in hot-arid zones, openings should be of minimal size or adjustable in size by shutters, and the view not directed towards the ground (glare) as far as considerations of natural lighting permits. The seasonal difference of the sun angle should be taken into account. Airtight closing should be possible.
· in warm-humid zones, openings should be as large as possible, and the view directed to surrounding grass or trees, with the sky blocked by roof overhangs or sun breakers. Air circulation should not be blocked by vegetation. An airtight construction is not needed.
Outlet openings should be located at high levels, where hot air accumulates.
Bedroom windows are best placed at the height of the bed or pivoted to direct the airflow towards the sleeping body. Louvres are a suitable accessory to assist the channeling of airflow. (also see Chapter 18.104.22.168)
Common building material for windows, properties and suitability
A wide range of special heat-absorbing and heat-reflecting glass types is on the market, but they are generally only suitable for air-conditioned buildings. Most of them are limited in their effectiveness because either their own temperature is raised, which increases the heat convected and re-radiated into the internal space, or they tend to reduce light rather than heat. In addition, availability and costs have to be considered.
Sealed double-glazed window panes can only be used for air-conditioned buildings. They are expensive and difficult to replace. In naturally cooled buildings they have little advantages.
(also see Chapter 22.214.171.124, 126.96.36.199, 188.8.131.52)
The most important element is the roof because the strongest thermal impacts of heat loss and heat gain occur here. The roof is the part of the building receiving most of the solar radiation, and its shading is difficult. Therefore, this building part should be planned and constructed with special care. Naturally, this applies to single story buildings and to for the top floor of buildings only.
The thermal performance depends to a great extent on the shape of the roof and the construction of its skin, whereas the carrying structure has little influence.
The shape of the roof should be in accordance with precipitation, solar impact and utilisation pattern (pitched, flat, vaulted, etc.)
Fig 3/43 Basic roof types
COMMON ROOFING MATERIALS, PROPERTIES AND SUITABILITY
Good thermal insulation and emissivity, suitable in dry climates.
BURNT CLAY TILES
A traditional material still very suitable today, with rather good thermal properties. Relatively heavy, requiring a strong support structure; medium heat storage capacity. Are permeable to air through the gaps between the tiles.
Similar properties as clay tiles but somewhat reduced heat resistance.
FIBRE CONCRETE (FCR) AND MICRO CONCRETE (MCR) TILES
Similar properties but lighter than concrete tiles, hence less heat storage capacity.
Fairly good thermal performance, medium reflectivity. Disadvantages: low mechanical strength, asbestos fibre is harmful to health (carcinogenic).
MONOLITHIC CONCRETE SLAB
Poor thermal resistance and high storagecapacity. Due to the big mass relatively cool during the morning, but re-radiating the daytime heat to the interior in the evening and at night.
NATURAL STONE (FLAG STONE, SLATE)
Thermal performance similar to concrete tiles depending on the thickness and the surface (brightness).
ORGANIC, VEGETAL ROOFING MATERIALS BAMBOO, LEAVES, THATCH, WOODEN SHINGLES
Climatically suitable, but of relatively low durability. Applicable for semi-permanent and self-built houses.
Problematic in the tropics, quick deterioration due to the intense solar radiation.
INSULATION MATERIALS see Chapter 184.108.40.206
SINGLE SKIN CORRUGATED GALVANIZED IRON SHEETING (CGI)
One of the most widely used, simple constructions, of low weight allowing an economical support structure.Has no significant thermal resistance, aged sheeting has no significant reflectivity, reradiates the received solar radiation into the building creating intolerably high indoor temperatures during the daytime. Rapid cooling at night with the problem of condensation in humid climates. Low life-span, noisy during rain.
A fairly expensive material but with good thermal reflectivity and long life span, preferable to galvanized iron sheeting. Reduces the heat load due to the low heat storage capacity and high reflectivity.
THIN SINGLE SKIN ROOF
Solar heat transmittance and heat conductance is high.
INSULATED ROOFS IN GENERAL
Prevent heat entering through the roof but also prevent heat escaping at night, thus their use has to be carefully considered.
INSULATION ABOVE A MASSIVE ROOF
The time lag is four times longer than with insulation placed inside, but also prevents cooling at night.
INSULATION BELOW A MASSIVE ROOF
Allows excessive heat storage, for which the insulation can hardly compensate. The slab exposed to the sun receives very high temperature differences that may be harmful to the structure.
CONCRETE SLAB WITH SCREED AND FIBRE BOARD CEILING
Resistance to heat flow is insufficient. Only useful for rooms used in daytime, not in the evening and at night.
DOUBLE SKIN ROOF WITH TWO LIGHT LAYERS
The outer skin shades the inner layer and reflects as much solar radiation as possible. The accumulated heat between the two skins must be removed by ventilation. Suitable in warm-humid climate, reduces the heat load in daytime and allows quick cooling at night.
DOUBLE SKIN ROOF WITH A LIGHT OUTER SKIN AND A HEAVY INNER LAYER WITH REFLECTIVE SURFACE
Suitable for hot-arid zones, keeping the indoor night temperature at a higher level than the outdoor temperature. A reflective surface in the cavity (e.g. aluminium foil) reduces the radiant heat transfer. Ventilation between the two layers must take the heat away. (see also Chapter 220.127.116.11). A separate roof and ceiling is the obvious solution for warm-humid climates. If for some reason it is used in hot-dry regions, the roof should be light and the ceiling massive. (also see Chapter 18.104.22.168 and 22.214.171.124)
Air which has passed through a double roof space and can reach the living zone (e.g. discharged towards a verandah) should be avoided, as this air will be much hotter than the normal outdoor air.
3.1.5 Special topics
(Passive cooling and heating)
Principles for the design and construction of special devices for passive cooling and heating, such as shading, natural ventilation, evaporative cooling, energy storage and temperature exchange between day and night, are described in this section and under the separate chapters on climate.
126.96.36.199 Shading devices
A major part of the heat a building gains is through solar radiation. This radiation is experienced in the form of increased air temperature, radiant heat and glare. Adequate shading reduces these effects drastically.
In certain climates a limited radiant solar heat gain may be welcome. It is possible to allow for this by a differentiated shading concept.
The following considerations provide the basis for the shading concept:
· At what time of the year and day is solar heat gain desired; when is it not ?
· What is the geometry of the suns path in relation to the building and its facades, and what is its change with the seasons ? (see Chapter 2.2 and Appendix 5.3 )
· What is the quality of solar radiation: is it strong or weak, direct or diffuse ?
Depending on the type of climate shading should cover openings either fully or partly. But under extreme conditions it should cover wall surfaces as well. This is possible with fins covering the entire wall or with double shell construction.
Fig 3/47 Shading of the entire wall surface
Shading can be provided by means of building shape, double shell construction, shading devices as attached accessories, facade greenery and roof gardens.
Shade can be provided by the shape of the building itself; for instance, by cantilevered upper floors or arcades.
In hot arid climates, shading can also be provided by placing buildings closely together, where other factors (traffic, hygiene, daylight) allow it.
Fig 3/48 Shading by building shape
Double shell construction
A double shell construction should have reflective properties protecting the building from direct and diffuse radiation. The outer skin should be placed fairly close to the facade and be properly ventilated. Such methods are suitable mainly for warm-humid climates.
Shading devices as attached accessories
A common means of shading is the use of shading devices placed outside the facades. The suns path is the main criterion for its design. Therefore, each facade has to be planned separately. (also see Chapter 188.8.131.52)
When designing a shading device, various factors beside the suns path have to be considered. The shading effect depends not only on the geometrical shape and orientation of the fixtures, but also on the material used and on the surface treatment and color.
The ratio of influence can be estimated as follows :
· geometry, shape, orientation
· material properties
· surface treatment, color
The efficiency of different measures can be roughly estimated and compared with the following chart, indicating the transmitted radiation impact:
· regular glass
· internal venetian blind, white
· internal venetian blind, dark
· external venetian blind, white
· continuous overhang on south side
· external movable louvres
Geometry and form
In general, shading elements on east and west facades should be vertical, because the sun is low.
On south and north facades the shading elements should be horizontal. Here, shading can often be provided simply by roof overhangs.
The shape of the elements should prevent radiation being reflected directly through the openings.
Types of shading devices
The variety of shading methods is large and the designer has the choice of many options.
When selecting the type of shading device, apart from shading, other factors should also be considered :
· The airflow through the
openings should be reduced the least possible, never stopped
· The view should not be obstructed.
· Daylight should not be reduced too much.
Elements attached to the building are:
a) Horizontal screening .
This is very efficient against high midday sun, especially on north and south facades. It can take the form of a roof overhang, a slab projection and verandahs, or with fixed or adjustable louvres.
Fig 3/51 Horizontal screening
b) Vertical screening
Such elements are best against low sun, thus on east and west facades. Optimal efficiency can be obtained with movable elements. A simple form of vertical screening can also be achieved with window shutters and doors.
Fig 3/52 Vertical screening
c) Egg-crate types
A combination of vertical and horizontal elements may be used where only horizontal or vertical protection alone would not provide shade. It may be required on east to southeast and on west to southwest oriented surfaces. It could be made of precast concrete or brick elements, timber or other similar material.
Fig 3/53 Egg-crate types
d) Screening, curtains
Traditional wooden trellis-work (mashrabiyas) or similar elements, e.g. bamboo screens, provide protection against sun as well as glare.
Curtains of any flexible material can easily be fixed in any door or window opening.
e) Pergolas, balconies, loggias, porches, arcades
A pergola can be made of bamboo or wooden components. The horizontal screening can be overgrown with creeping vegetation for better shading. Balconies and loggias as architectural elements can be helpful in providing shade.
When covering large horizontal areas, such elements are also a very efficient protection for roof surfaces.
Materials for shading devices
Generally the use of materials with a low thermal capacity is recommended for shading devices near openings, thus ensuring that they cool quickly after sunset.
Materials that do not overheat should be used.
Guidelines for detail design
· Screening should generally be placed on the outside of a building. If inside the glass, it provides only protection against glare.
· Horizontal shading elements should be detached from the facade, so that rising warm air is not prevented from escaping. A gap of 10 to 20 cm should be maintained between the horizontal screen and the facade.
· Thermal bridges between the building structure and shading elements should be kept to a minimum. Shading elements, when exposed to intense solar radiation, heat up. Through massive connections to the building the heat can flow to the inside and cause a considerable heat gain in the interior. Therefore, the fixing points should be kept to the minimum required for structural reasons.
Fig 3/55 Elimination of thermal bridges
· Adjustable shading devices can balance seasonal differences.
Solar control glass
Solar control glass can reduce direct radiation but cannot offer complete protection. If the windows cannot be opened, air conditioning is unavoidable. Furthermore, such glass is expensive, its life span uncertain and it is difficult to replace.
Facade greenery (for shading with trees see Chapter 3.1.2)
A green cover on the facade shades the wall surface and thus reduces solar radiant heat gain. It also protects the walls from heavy winds and driving rain.
Facade greenery can be planted on the ground adjoining walls or, in higher buildings, in plant boxes on terraces or hung onto the facades.
To give protection from certain insects that may be attracted to the greenery, it is recommended that mosquito-screens are used in the openings.
It will often be necessary to water the plants, which may be a problem in areas with limited water supply.
Plants with aggressive roots should be used with care, as they may harm the structure.
Fig 3/56 Facade greenery: two possible approaches
Plantation on roofs which are flat or have a slight slope, has a strong regulating effect on the indoor temperature due to the heavy earth coverage and the shading effect:
· Solar radiant heat gain is
· The ceiling temperature is fairly even throughout day and night.
· The temperature of the roof slab also remains stable, and the thermal stress on the structure is reduced.
· Further advantages are the aesthetic values, the reduction of dust and the improvement of the microclimate.
The disadvantages of roof gardens, however, also have to be considered:
· A heavy load is added on the
· It is not easy to achieve a reliable waterproofing of the roof.
· Heat emission at night is reduced.
· Clogging of drainage channels and outlets may occur.
· In dry regions the high water consumption may cause difficulties.
For roof gardens, the following plants are recommended:
For 10 cm thick soil cover:
Wedelia trilobata, Syngonium spinosa, Setcresea putzpurea, Cythyla, Hemigraphis spec., Pandanus spinosa, Rhoeo spec., Rhoeo tricolor.
For 20 - 30 cm thick soil cover:
Ipomoea Batatas, Ivoras, Sanchezia Nobilis, Stromanthia sanguinea, Strobilanthius dyerianus, Excocaris.
For 40 - 50 cm thick soil cover
Polyscia filicifolia, Hymenocallis spesiosa, Dieffenbachia marinne, Dieffenbachia tropic sun., Heliconia latispathia, Heliconia rostrata, Heliconia speciosa, Alpina purpurea, Alpina speciosa variegata, Alpina sanderae, Costus speciosus, Phaeomeria magnifica, Pandanus, Akalysrha wilkesoniana, Cordiline speciosa, Wrigthia religiosa, Ravenala madagaskariensis.
Fig 3/57 Types of roof garden
184.108.40.206 Natural ventilation
Air movement is a major factor influencing indoor climate and should be considered when planning and constructing buildings. Similar to the suns radiation, existing winds should also be incorporated in the design concept.
For planning purposes, it is important to distinguish between regular wind patterns and winds that occur only occasionally.
Occasional winds, such as in storms, have to be considered when designing the structure in order to guarantee sufficient strength. For the purpose of climatic design, only regular winds are relevant.
Wind for cooling
Regular winds can be utilized for cooling. If the temperature of the circulating air is below the indoor temperature, then the cooling effect is obvious. But a breeze with a slightly higher temperature can also be felt as cool because it increases the perspiration of the skin. As soon as the temperature of the wind exceeds the temperature of the human body, such an effect is no longer possible.
To avoid discomfort caused by indoor ventilation, the speed of the air should not exceed a certain velocity. (see Chapter 2.3)
In composite climates, wind can also cause undesired cooling when the outdoor air temperature is below the desired room temperature. In this case, the building should be built fairly airtight to minimize infiltration. Designing the surroundings with wind protection is also an effective measure to reduce such cooling.
Sand and dust driven by the wind can cause great problems, mainly in arid regions. Such winds can also cause erosion on facades and other exposed elements, requiring specially resistant building materials.
To prevent sand entering buildings and courtyards, suitable construction details and room arrangements are required.
· Hot air entering a building
heats it up, cold air cools it down.
· Air circulation striking the human body provides evaporative cooling which at certain times and in certain circumstances is most welcome, at other times not.
As a consequence, the ventilation system of a building should be planned in order to optimize the indoor climate.
There are, however, limiting factors:
· Ventilation can only reduce temperatures higher than the outdoor temperature.
· The air circulation should not exceed a certain speed (ca. 1,5 m/s under warm-humid conditions) because this would create discomfort. (see Chapter 2.3.2)
· On the other hand, complete blocking of air ventilation is also not possible because a minimal air change is needed for reasons of hygiene and oxygen requirement.
· The removal of internal humidity too, demands a certain degree of ventilation because mould growth has to be avoided.
· In assembly areas (e.g. schools, meeting halls, etc) it is almost impossible to keep the internal air cooler than the external, other than for short periods. When the bodily heat output exceeds the rate of heat absorption by the building fabric, the air temperature increases. When it reaches the outside air temperature, further rises can be avoided by ample ventilation.
Ample ventilation at night
When the stored heat is to be dissipated at night, ample ventilation is necessary. The indoor air stream at night should be directed so that it passes the hottest inside surfaces, which are likely to be the ceiling or the underside of the roof. The placement of openings, louvres etc. should be designed accordingly.
Types of air circulation
Basically, two types of air circulation can be distinguished:
a) External winds
Air circulation can be induced by external winds. They produce wind pressure on the building, positive on the windward side, negative on the leeward side.
Fig 3/58 Wind pressure distribution
b) Thermic circulation
Air circulation can also be induced by thermic movement. Any material, including air, expands when heated. Warm air is lighter than cool air and rises. This, so-called stack effect can be used to increase ventilation where the breeze is not sufficient.
Fig 3/59 Principle of thermic effect
When designing for optimal ventilation the following information is required:
· What is the pattern of
existing winds (speed, direction, temperature) ?
· How do these wind characteristics change during the course of the day and with the seasons ?
· When is increased air circulation desired for cooling or heating, when is it not ?
· When air circulation is desired, in which room; and in which zone and at what level in the room ?
For instance, in bedrooms, particularly in warm-humid zones, the main airflow should be in that part of the bedroom where the beds are located and at a height a little above bed level.
Fig 3/60 In warm humid zones air movement at body level is desired
Means of controlling ventilation
To either benefit or to protect from cooling winds, the pattern of the airflow in a building can be influenced by
· measures outside the building
and building shape
· measures relating to the building shell, openings, louvres, shutters, etc.,
· measures relating to the interior and special ventilation devices,
· devices that create a stack effect ventilation.
There are many possibilities for directing and deflecting winds. Deflection of up to 90o is possible.
Fig 3/61 Deflection by hedges
Fig 3/62 Deflection by parapet walls
Fig 3/63 Relation of trees to parapet wall
Fig 3/64 Protection from wind by vegetation and topography
In the hot seasons, before entering a building, wind should not pass over hot surfaces.
Influence of building shape on wind
Every building creates wind-protected areas and may deflect the wind direction. This may be important for neighbouring buildings. Some general examples illustrate this aerodynamic phenomenon:
The wider a building, the larger is the windshade behind it. [ 153 ]
Fig 3/66 Influence of building depth
The higher a building, the deeper is the windshade area behind it.
Fig 3/67 Influence of building height
When grouping buildings in a row parallel to the main wind direction, a large distance between buildings is needed to guarantee proper ventilation.
Fig 3/68 Buildings grouped in a row
When grouping buildings in a staggered pattern, the distance between buildings can be reduced.
Fig 3/69 Buildings grouped in a staggered pattern
The grouping of buildings also affects the airflow pattern. Typical examples are:
· The jet-effect, where a funnel
situation causes accelerated wind speed through a narrow passage.
· The gap-effect, creating a dispersion of the airflow after a gate-like situation.
· The diversion-effect created by staggered buildings.
Fig 3/70 Typical effects on airflow pattern
Orientation of the roof
To keep roofs cool, they should be sloped towards the prevailing breeze and any obstructions which would prevent the airflow along the roof surfaces should be avoided. High solid continuous parapet walls around the roof would, for example, create a stagnant pool of hot air, and should, therefore, be avoided. [ 8 ]
d) Building shell design, openings and louvres
The size of the openings and their location influence the velocity of air circulation and its main route in the interior.
The larger the windows, the higher the indoor air speed; but this is true only when the inlet and outlet openings are increased simultaneously. When a room has unequal openings and the outlet is larger, then much higher maximum velocities and slightly higher average speeds are obtained.
In Fig 3/71 the air speed outside is taken as 100, the inside values are expressed as a percentage of this.
Fig 3/71 Influence of size of openings
A loggia opening leewards, with only small openings windwards, will have a steady airflow through the building because the airflow over and around it creates a low pressure within it, thus pulling in air in a steady stream through the small openings. Therefore, the greater the ratio of outlet area to inlet area, the greater the airflow through the building. [ 122 ]
Placement of openings
The location of openings may create a deflection of the indoor air circulation. When the opening is placed asymmetrically in a facade, unequal pressure on both sides of the opening influence the airflow.
Fig 3/72 Pressure distribution by openings
This effect can be observed in the horizontal direction when a window is not centred in the plan.
Fig 3/73 Deflection in the horizontal direction
The same is also true in the vertical direction. This is best illustrated when adding another floor on an existing building and thus changing the proportions of the facade.
Fig 3/74 Deflection in the vertical direction
Fins and projecting slabs also influence the pressure distribution on the facade and with it, too, the direction of the airflow inside the building. In this case, the airflow is influenced both in the horizontal as well as in the vertical direction.
A fin on one side of a window diverts the airflow
A canopy over a window directs the airflow upwards
A gap between it and the wall ensures a downward flow
This is further improved in the case of a louvred sunshade
Fig 3/75 Deflection by fins
Effect of louvres and their position.
(also see Chapter 220.127.116.11)
Although the indoor airflow pattern is mainly influenced by the size and position of the openings, it can also be influenced and controlled by adjustable louvres. In this way, incoming air can be diverted to the desired level within the room.
Fig 3/76 Effect of louvres
Double roof ventilation
If a double roof, or a separate roof and ceiling are used, the heat transfer from the outer building skin to the ceiling has to be considered. This will be partly radiant (approximately 80%) and partly conductive. As the roof is warmer than the ceiling, and hot air rises to the roof, there will be no convection currents. If the roof space is closed, the enclosed air may reach a very high temperature, thus increasing the conduction of heat.
This can be avoided by ample ventilation of the roof space. Ventilation will also reduce radiant heat transfer by lowering the temperature of the inside surface of the outer skin and thus reducing the temperature of the ceiling.
Attention must be paid to the design of the openings from this space and their orientation in relation to the prevailing breeze. Even if this breeze itself is warmer than is comfortable, (it will, therefore, be excluded from the room itself), the roof temperature both on the outside and on the inside of the outer skin is likely to be much higher: the opening will thus still help in removing some of the heat.
To achieve a reliable air circulation, buildings must be designed for cross-ventilation.
Care must be taken not to impede such cross-ventilation with incorrectly designed interior partitions. When a room is divided by means of a partition - or when there are several rooms together with inlets and outlets separated by doors or halls - the air changes direction and speed as it passes through the room. This, in general, reduces air movement. By creating a turbulent, circulating movement of air within the room, however, an effective ventilation of more of the area may result.
Partitions arranged parallel to the airflow may divide this stream, but do not reduce the velocity.
Fig 3/77 Arrangement of partition walls affects air flow pattern
Electric fans(see Chapter 18.104.22.168)
Mounted electric ceiling or other types of fans may be used where there is little or no breeze, but these will normally only provide air movement and not induce the exchange of air.
Device utilizing external wind
To benefit more efficiently from existing winds, various devices mounted on the roof can be used.
Fig 3/78 Examples of devices using external winds
Devices utilizing the stack effect
Often regular winds do not exist but there may be solar radiation and diurnal temperature fluctuations. These phenomena can create a stack effect that can be utilized to increase ventilation. (Also see [ 8 ] )
The stack effect can also be induced by placing openings near the floor and near the ceiling. It can be regulated by window shutters to obtain the desired heating or cooling effect.
Fig 3/79 Use of stack effect
Solar chimneys and induction vents
Solar chimneys make use of solar heat to reinforce natural air convection. A black coated metal pipe chimney is heated by the suns radiation and so is the air inside. The latter then rises taking the interior air up and out. This system is self-regulating, the hotter the day, the faster the air motion
Fig 3/80 Black coated pipe as solar chimney
A variation is the glazed solar chimney. Such chimneys, when facing west, are favourable for ventilation during the hot afternoon. If a thermal storage mass is added behind the glazing, the system will store heat and keep on expelling air after sunset.
Fig 3/81 Glazed solar chimney
Induction vents use solar air ramps, windows with radiant barrier curtains, or solar mass walls. Sunlight is trapped behind south or west facing glazing and the heated air rises and is allowed to escape to the outside. This causes the internal air to be pulled into the heated space and expelled.
Air taken from the shaded north side may be used to replace the expelled air inside the building.
Fig 3/82 A variation of solar chimney with solar air ramp [ 2 ]
22.214.171.124 Passive cooling means
(also see Chapter 126.96.36.199)
a) Roof ponds
A water body covering the roof functions similarly to a soil cover, minimizing the diurnal temperature range. It is thus appropriate in climates with a diurnal average temperature within the comfort zone. It has the advantage that it can easily be removed during periods when this effect is not desired. Open roof ponds are difficult to maintain and require an absolutely watertight and costly roof construction. Shortage of water in arid zones is another disadvantage.
Fig 3/83a Roof pond cooling in summer
Fig 3/83b Roof pond heating in winter
A special system works with a layer of bags (15-20 cm) containing water that are placed on the roof and are covered with movable insulating panels (5-10 cm), which appear to regulate the internal temperature at comfort level. In summer, these panels are closed during the day to insulate the bags from solar radiation and to allow heat to be drawn from inside, while at night the insulation is removed to allow the water to radiate heat to the night sky. In winter the process is reversed.
The system is good for cooling, since it faces the night sky, but does not have an ideal angle for collection of heat. However, it is a complicated and expensive solution which also requires the daily attention of the users. [e.g. 7, 10, 12, 136, 138 ]
b) Trombe walls and water walls
These systems are mainly suited for heating and thus dealt with in Chapter 188.8.131.52. Under certain circumstances they can also be used to induce cooling by ventilation (see Chapter 184.108.40.206)
220.127.116.11 Active cooling devices
(also see Chapter 18.104.22.168)
a) Electric fans
A simple active device for the improvement of the indoor climate may be the use of electric fans. In most cases this widespread method can provide a sufficient means of evaporating perspiration and cool the skin at a fraction of the cost of air conditioning.
Fans can be used in various ways:
· Placed too closely to the body
may be a health hazard, especially for the elderly.
· Remote or slow revolving overhead fans are recommended.
· Indirect and remote placing gives a steady mild flow and is safe for health.
· Pivoting fans produce a strong but intermittent flow, which may not suit everybody. [ 147 ]
Fig 3/84 Various ways of using an electric fan
b) Forced ventilation
Air circulation and air changing by electric ventilators is another possibility of cooling. Ventilators may be placed directly in the outer wall or may be combined with an air duct system.
c) Evaporative cooling
(also see Chapter 22.214.171.124)
Cooling can be achieved by humidification. The evaporation of water is a physical process which requires heat energy. This energy is taken from the air, and its temperature drops accordingly. Thus this phenomenon can be used for cooling. The possibilities of evaporative cooling depend on the potential of the air to absorb humidity. The drier the air, the greater is the cooling potential, because a greater amount of water can be evaporated. The method is thus best suited to hot-arid climate zones.
Fig. 3/85 Direct evaporative cooler
In some maritime, coastal areas and in warm-humid climates, this potential is small because of the high relative humidity. Here only indirect cooling using a heat exchanger is possible, and the efficiency is less.
Fig. 3/86 Indirect evaporative cooler
d) Air conditioning
Under extreme conditions, active devices in the form of air conditioners are often unavoidable because sufficient passive cooling is very difficult to achieve.
Air conditioning requires a fundamentally different concept of construction. Aspects of thermal insulation, vapour diffusion, double glazing etc. need to be considered; of less importance are heat storage and time lag. Thus, the decision has to be made right at the beginning of planning and designing a building. However, many passive means such as orientation, shading, limited window surface, etc. are also beneficial for air conditioned buildings by drastically reducing energy consumption and running costs. [ 136 ]