3.2 Design for hot-arid zones
The main points:
• Provide maximum shading of direct and reflected sun radiation in the hot season.
3.2.1 Climate and design in general
The climate of hot-dry zones is in general characterized by high temperatures (40 - 50°C in summer), with sharp variations in both diurnal (day / night) and seasonal (summer-/ winter) temperatures; and precipitation (rainfall, snow) which is scarce, irregular and unreliable, but may nevertheless cause severe floods. Cold winds and dust/sandstorms prevail in winter. The solar radiation intensity is high and enhanced by the radiation reflected from the ground. The air humidity is low and this climate is generally healthier than those of warm-humid lands. Different climatic zones can be distinguished within desert regions according to their specific geographical characteristics. Particular conditions in maritime desert regions mean that the high humidity causes definite discomfort in summer. On the other hand, the humidity tends to reduce diurnal variations and moderate temperatures. (also see Chapter 2.2)
Design objectives and response
The main goal of climatic design, on a macro (settlement) and micro (building) level, is hence to reduce uncomfortable conditions created by extremes of heat and dryness. Buildings must be adapted to extreme summer / winter and day / night conditions to achieve a well balanced indoor climate. Not only cooling is needed; passive heating may also be needed in winter and during cold nights. Protection is required from the intense radiation from the sun, ground and surrounding buildings, from dust, sandstorms and insects (flies). Glare has to be reduced and dust penetration prevented. Settlements and buildings, therefore, have to be compact, providing shade and controllable ventilation.
In maritime desert regions, the high humidity requires more air circulation (ventilation) in summer. It is difficult to design buildings for this climate.
Hot-arid zones or desert regions with scarce vegetation and saline soils are distributed throughout the world. 15 percent of the world’s population lives in arid zones; 1/3 of the world’s land mass and 22% of all potential arable land lies in the arid zone. Most of the world’s energy reserves (oil) are within or adjacent to these zones. [ 112 ]
In the last half century, technological changes have had a major impact on urban forms and housing throughout the world. The introduction of the car into the settlements has also drastically altered the traditional urban pattern of hot-arid regions. The new wide streets reduce the potential for shading. In addition, the great amount of heat-discharging air conditioners and large paved surfaces have contributed to changes in the microclimate of urban situations. Moreover, a change in lifestyles and means of livelihood has occurred. Mud or adobe buildings, dark interior spaces (very few and small windows) and sleeping on the roof are probably no longer acceptable to society in general, but still reality for low income groups. In addition, the proper handling of climatization devices properly, and the limitations of passive means are problems which should not be neglected. (also see Chapter 3.1.1)
3.2.2 Settlement Planning
The main points:
• Topography, to enhance the efficiency of passive means
22.214.171.124 Topographical location of settlements
The positioning of settlements can help to take advantage of local features to improve the micro-climate with regard to comfort. Attention has to be paid to the topographical altitude, the geomorphology and the most suitable orientation regarding sun exposure and prevailing winds. Differentiation must be made between locations on top of hills, on slopes, in valleys, on flatlands and near water. (see Fig 3/1 to Fig 3/8)
Compact settlements should be located on shaded slopes (north-sloping) and at higher levels. The general preference for the orientation of slopes referring to sun exposure (on the northern hemisphere) is: 1st: north; 2nd: east; 3rd: south; 4th: west. This can vary in relation to the local conditions, topography, vegetation, sun angle and exposure time. e.g. sites on north / southeastern slopes are also acceptable. Near the equator, south slopes are preferred over east and west slope.
Depending on how much passive heating during night and cold seasons is required, south slopes can be advantageous. (also see Chapter 3.4)
ocations are preferred where the effect of cool airflow can be utilized and controlled. High altitudes and locations with evaporative possibilities are advantageous. Settlements have to be properly oriented regarding prevailing winds. Winds are more frequent and relatively cooler at higher elevations. Blowing over a water-body can result in a drop of a few degrees in the temperature of a wind. Wind can also be caused by specific direction and conditions in a valley.
Location in flat regions
Compact settlements in flat areas have, in general, less natural features, such as hill sides, slopes, and rock formations which have to be integrated to improve the micro-climate. Such settlements should include vegetation because the air is cooled while crossing green shaded areas. A draft is created through cooling the hot air in the shade and by the humidity of plants or water ponds, a phenomenon well known from traditional oasis settlements. (also see Chapter 126.96.36.199, 188.8.131.52)
Sand and dust storms, sand dunes
The reduction of the effects of sand storms can be achieved through the location of settlements at higher elevations and landscaping cities with plants and water, which lead to less sand in the air. The open space pattern (network of streets and squares) has to be planned accordingly, e.g. an irregular pattern to break strong winds (see Chapter 184.108.40.206). Particular attention has to be paid to the moving direction of sand dunes, which can slowly bury houses and entire settlements.
In desert areas, the so-called wadis (dry valleys and rivers) can be very dangerous places because of their bad drainage and sudden inundation in case of heavy precipitations.
Heavy precipitations can cause landslides both at the bottom of valleys and on slopes.
Safe constructions can be in contradiction to traditional design or climatic construction requirements, particularly in the case of simple mud (adobe) or brick buildings.
220.127.116.11 Urban forms and external spaces
The following are the main design objectives:
• Provide maximum shade in summer and adequate heat gain in winter.
• Minimize reflection (indirect solar radiation) in streets and open spaces.
• Moderate the effects of undesired winds.
• Plan narrow winding alleys and streets, which are shaded and relatively cool and break stormy winds, but allow through-ventilation and adequate natural lighting.
• Design suitable building forms.
• Plan close proximity of urban services and daily functions within walking distance; wide roads can thus be omitted or at least reduced.
• Avoid large open spaces within the city where hot air can collect during the day and which are conducive to duststorms.
• Provide ample shaded public spaces.
• Select light colors for every open space.
• Include green areas of plants around and within the settlement to provide shade and cool air and to stabilize the soil.
• Plant and cultivate xerophytes that require little or no water.
• Integrate water bodies, which evaporate and therefore reduce temperature.
Minimal sun-exposure in summer and therefore compactness and shade are the main principals for building in hot-arid zones. Hence, compact planning for groups of buildings is required in order to give shade to each other and to provide a shaded network of narrow streets and small spaces in between as patio-like areas. Arcades, colonnades, cantilevered buildings or building components, membranes and small enclosed courtyards are traditional responses to the climate; even larger public open spaces should be enclosed, inward looking and shaded for most of the day. Of equal importance is natural lighting and ventilation. Air circulation can be improved through wind channelling in shaded narrow streets in the direction of the main wind. The grouping of buildings and alleys or lanes should allow for proper ventilation or even increase the airflow. The location near a water source and the incorporation of vegetation is most important. [ 9 ]
Settlement patterns and street-networks (also see Chapter 18.104.22.168)
Urban forms are not only a result of physical and functional, but also of social and cultural factors and traditions in a region. There are different ways of properly designing an urban form in an arid region taking into account solar radiation and wind.
Some basic possibilities:
a) Grid diagonal to east-west axis
The grid pattern maximizes radiation throughout its straight streets, but by orienting the grid pattern diagonally to the east-west axis, the sun exposure and shade is better distributed on the streets; such a grid still supports the dynamic movement of air. More important, however, is the form of alleys and buildings. [ 5, 143 ]
b) Narrow, zigzagging alleys
Winding or zigzagging narrow alleys receive minimum radiation, reduce the effect of stormy winds, establish shaded spaces throughout the day which provide a cool and comfortable microclimate and also stay relatively warm during cold nights and in winter.
c) Blocked streets and alleys
Street orientation and housing patterns are significant and must be planned carefully. Straight and parallel streets open the city to wind ventilation. Storm effects can be reduced by blocking streets. Two-story buildings with closed patios open to the sky will maximize shade, minimize radiation, yet still retain ventilation and reduce the effects of stormy winds. Buildings should be attached (cluster) to reduce exposed surfaces.
External space design
External space design(also see Chapter 3.1.2)
The town structure and the public spaces should thus counteract heat with a shaded and dense layout. There should be a close connection between public spaces and residential areas. Dwelling units or groups should create patio-like areas. Paved open spaces within a flat cityscape should be avoided or kept to a minimum size.
Most important is the design of the whole urban configuration, because the ratio of shaded space to space open to solar radiation affects air temperature significantly. The temperature in and around buildings can either be tempered or aggravated by the nature of the surrounding surface. The temperatures shown in Fig 3/94 were recorded in a hot-dry climate when the air temperature was 42°C [ 106 ]
Particular attention has to be paid to the needs of the pedestrians, walkways and the scale of the environment. Half and full shade protection by arcades, membranes etc., and vegetation (trees) is desirable; exposed paved surfaces should be avoided; pools of water are beneficial.
b) Landscaping with vegetation
Trees, hedges and plants in an urban context can have a dramatic effect on the microclimate and help to tie down sand and dust [ 1 ]. As vegetation is generally sparse, an oasis-like concentration of plant and grass-covered areas is desirable. Nevertheless, landscaping should not always imply the inclusion of very high water consuming lawns and grassed areas. Local desert plants as well as rock and stone garden as well as gravel coverage should also be considered as adequate design elements.
c) Pattern of green areas
The vegetation in and around the city promotes and controls air movement. Apart from water areas, evaporation and cooling takes place only in green areas. Green areas located near and in a city will therefore improve the urban climate. The difference in temperature between green areas and built-up land causes minute air cycles and a horizontal exchange takes place. An arrangement of small parks and lanes could facilitate the ventilation of the town. The wind from the countryside is encouraged to penetrate as far as possible into the built-up area. [ 124. 134 ]
3.2.3. Building Design
The main points:
• Orientation and placement, to minimize sun exposure in summer.
22.214.171.124 Orientation of buildings
Proper orientation and location of buildings allow for sun and wind protection and controlled wind channelling (airflow).
The orientation of a building is influenced by the amount of solar radiation falling on different sides at different times. Buildings are best arranged in clusters for heat absorption, shading opportunities and protection from east and west exposures. Protection from solar radiation is particularly important during times of excessive heat when there can be a difference of as much as 3°C in air temperature in a building between the best and least favourable orientation. The larger building dimension should face north and south (generally, west orientation is the worst: high air temperature combined with strong solar radiation) [ 9 ]. The optimum orientation for any given location has to be determined in order to achieve the most satisfactory distribution of total heat gain and loss in all seasons. At high altitude enough heat gain for passive heating should be possible.
In general, the best orientation is: north-south with 25o south easterly direction [ 13, 161 ]. Attention should be paid to solar radiant heat reflected from the surroundings (topography, slopes, rocks) to the building.
Main walls and windows should face the prevailing (cool) wind direction in order to allow maximum cross-ventilation of the rooms.
126.96.36.199 Shape and volume
The shape and volume of buildings should be compact, yet somewhat elongated along the east-west axis; (e.g. the optimum shape is 1:1.3), because large, compact building volumes gain less heat. In general, the optimum shape is that which has a minimum heat gain in summer and the maximum heat gain in winter. Under winter conditions an elongated form is ideal; under summer conditions a square shape is better [ 9 ]. A compact “patio” house type is therefore preferable. Adjoining houses, row houses, and group arrangements (all continuous along the east-west axis), which tend to create a volumetric effect, are advantageous, as are high massive buildings [ 13 ]. Lithospheric arrangements (subterranean) are also applicable.
188.8.131.52 Type and form of buildings
Dense settlement patterns require a particular type of building consisting of compact structures and forms. Subterranean spaces are also adjusted to climatic stress. In hot-arid zones, external and internal living spaces have to be protected against solar radiation, glare, and hot, dusty winds. Compactness can be achieved by “carpet-planning” layouts with courtyard houses or cluster settlements of high buildings to create suitable patterns. Particular solutions may utilize underground (subterranean) buildings or caves. Some heat gain and storage in the winter season is desirable.
The main objectives are:
• Compact and massive design, mainly inward-facing buildings.
• Minimize surface areas and openings exposed to the east and west sun and orient the building accordingly.
• Allow heat gain and storage in winter.
• Group buildings closely to each other. Especially east and west walls should be placed closely together for mutual shading.
• Create thermal barriers (non-habitable rooms, such as stores, toilets etc.) on the east and especially on the west side of the building.
• Promote ventilation and access to cooling winds.
• Provide sufficient natural lighting (no excessively deep rooms).
• Plan short internal circulation distances and avoid unnecessary stairs.
• Shade roofs, walls, openings and windows and outdoor spaces.
• Include small enclosed courtyards with arcades, colonnades for light and air and outside day-to-day activities. Courtyards provide shade, cool air pools, and protection from hot and dusty winds.
• Treat the external space as carefully as the building itself to reduce glare and reflected heat radiation.
It is difficult to meet all the different functional and climatic requirements. Regarding the volume, the “patio-house” is the most suitable form and can benefit in summer from the microclimatic effects of cool air pools that occur in courtyards. Although winter conditions in hot-arid regions would permit an elongated house design, the heat in summer is so severe that a compromise is required. The very old, traditional solution - particularly for flat land - is a compact, inward-looking building with an interior courtyard. This minimizes the solar radiation impact on the outside walls and provides a cool area within the building. It also meets other requirements such as safety, defense, privacy, lifestyle etc.
In the typical oriental courtyard house, the covered terraces, which are usually on two or three sides of the courtyard, and the identical covered gallery on the first floor help to reduce the heat gained during the day and provide shaded areas. The correct ratio between the height and width of the courtyard should always allow for adequate shading, even when the summer sun is almost directly overhead. When the courtyard is provided with water and plants, it acts as a cooling source and modifies the microclimate accordingly.
In areas with cold nights or winters the court yard has to allow for adequate south exposure for passive heat gain and should be equipped with movable shading devices for the hot period.
However, the one or two storied courtyard building type cannot always fulfill today’s functional and urban planning requirements, where high population density, economic land use, adequate car traffic, accessibility and suitable public transportation, etc. are required.
In certain regions, such as mountainous and coastal areas, (North Africa, Arabian Peninsula, etc.) high, compact buildings are the traditional solution, having also had an important defense purpose in the past. Cooler air from the lower floors is channeled through the building. High walls with integrated ventilation shafts are built at the back on the shady side. In maritime regions, large openings or bay windows for cross-ventilation are protected with wooden screens such as “Rowshans” or “Mashrabiyas”. (see Chapter 184.108.40.206)
Underground dwellings have been known for thousands of years. At a depth of about 2.5 m, the temperature of the earth is practically constant and remains close to the average yearly temperature. The indoor climate of structures built underground or covered with a thick layer of soil benefits from the huge thermal mass of the adjacent ground and is thus not affected by hot days and chilly nights. Structures can be carved into suitable rock formations or may consist of a structural shell (even several floors underground), which is mainly concrete and covered by soil. (The provision of natural lighting might cause difficulties.)
• Where the diurnal temperature range is wide, but the daily average is within the comfort zone, a soil cover is appropriate.
• Where the annual average temperature is within the comfort zone, structures built 2-3 m underground are suitable.
• High rooms (ceilings) are not necessary.
• Natural lighting must also be considered.
• Protection against surface water (flooding) may be required. Structures within the groundwater table should be avoided.
[ 5, 7, 9, 136 ]
Buildings in maritime, coastal regions
In certain regions, the high summer humidity in maritime areas makes designing buildings here extremely difficult. More ventilation is required at times and high thermal capacity structures are less effective. Tall buildings and building components with lightweight structures which utilize the breeze for rooms used in the daytime are good traditional solutions to reduce discomfort. The use of high thermal capacity structures, although still useful, will not be as effective as in other hot-dry regions. The coastal wind blowing off the sea during the day may be utilized to ameliorate thermal conditions. On the other hand, the nighttime wind carries hot inland desert air, possibly dust, towards the sea, which can be very unpleasant. Protection from this wind should be provided.
Perhaps the only solution is to provide alternative spaces: one with high thermal capacity walls and roof, for use at night, especially during the cooler part of the year; and one of lightweight construction, the roof providing shade only and the facades facing and opposite to the sea being left almost completely open. This is the best solution for daytime use, especially during the hottest part of the year.
It is in this climate that wind catchers, scoops and wind towers have their greatest benefit. [ 8, 9 ] (also see Chapter 220.127.116.11, 18.104.22.168, 22.214.171.124 and 126.96.36.199)
Table: The concept of alternative day and night space
Room arrangements (also see Chapter 188.8.131.52. and 184.108.40.206)
The room layout depends on the building type. A courtyard design has certain advantages. Heat-producing areas should be separated from other areas of the house. Non-inhabitable spaces should be placed on the west side to check the sun’s impact. Internal heat gain can be avoided by a functional layout.
Bedrooms should be on the east side, and outdoor or roof sleeping possibilities should be considered. Living rooms should be on the north or south side. The depth of interior spaces should allow for proper natural lighting. Nevertheless, modern floor plan requirements, multi-family housing (high density) and different values, such as access to a view, might be in contradiction to climatic design principles.
220.127.116.11 Immediate external space
The walls of houses and courtyards, cantilevered building parts and plants should provide shade to outdoor living areas. Half and full shade protection by arcades or loggias, membranes and trees is desirable; exposed paved surfaces should be avoided; pools of water are beneficial for cooling.
3.2.4 Building components
The main points:
• Control of heat transfer through thermal storage and time lag by proper construction and materials
The comfort of people inside the buildings depends largely on the thermal properties of the outer and inner walls and the roof. Depending on the function of the building components specific insulating and/or thermal storage qualities are required. (Basic explanations see Chapter 3.1.4)
Buildings in hot-arid zones are traditionally constructed with thick walls and roofs and with very small openings. An internal thermal storage capacity is very important to decrease the temperature variations and to make it possible to profit from an increased night ventilation by “storing the cool of the night until the day” during summer. The best materials are those that do not conduct heat.
Sun-dried earth brick is one of the poorest conductors of heat, partly because of its very low natural conductivity and partly because mud is structurally weak and necessitates thick walls. Yet thick mud bricks are not a perfect means of keeping cool; they retain heat for a long time. Therefore, it is important to calculate and plan the proper time lag. A big thermal mass can keep cool during the daytime and not be too cold at night. (see example Chapter 4.4) High heat capacity walls are essential. The traditional principle is to shelter behind very thick mud walls by day, and to sleep on the roof under a tent at night. [ 122 ]
Construction concepts and details
The different building components require adequate design and material properties to act as a balanced system.
• Thermal insulation is important to suppress surface temperature variations, but is only applicable in connection with adequate inner ventilation and cooling means or in combination with light structures (Insulation can also reduce necessary heat loss at night). Roof insulation is especially important in decreasing summer temperatures. The outside application of insulation is preferable because the structure and the construction materials are less exposed to thermal stress, and the storage capacity of a heavy structure material helps to balance the inner temperature. The additional needed skin for the building or roof must protect the insulation against damage by physical, mechanical forces, and should be of a hard material. The required insulation value depends on the sun exposure. (see Chapter 3.1.4)
• Time lag properties of building parts and its materials should be used for energy storage and temperature exchange between day and night. Necessary time lags for internal heat balance are: Walls, east: 0 hours; south: 10 hours.; west: 10-hours.; north: 10 hours or no lag; roof: 12 hours [ 13 ] (also see Chapter 3.1.4)
• Shading devices, such as a heavily ventilated double roof, and radiation reflection by a white surface are necessary to decrease heat gain from solar radiation - mainly through the roof - during the hot period.
• External colors are required as a combination of high reflectivity of solar radiation and high emissivity of infrared radiation to the cool sky at night: white, non-shiny surfaces, avoid all dark colored surfaces. White paint has a high reflection ratio on sun exposed surfaces. Dark absorptive colors are usable where reflection towards the interior should be avoided (such as under eaves). Deep-set surfaces can be dark-colored for winter radiation absorption. Bright color contrasts should be in agreement with the general character of the region. [ 13 ]
• Internal colors, such as “cool” and bright colors can be used psychologically as a cooling contrast to intense outdoor heat and to distribute natural light for deep room arrangements.
18.104.22.168 Foundations, basements and floors
The ground is a valuable means of heat absorption; therefore the building should have maximum contact with the ground. Ground floors should be solid and built directly on to the ground or into the ground with heat absorbing materials (stone, adobe, earth, high density burnt clay or cement products) Ground floors should not be suspended and on no account be built on stilts. Flooring materials should be of high thermal conductance. The ground near the building should be shaded during the day, but fully exposed to the night sky, so that the emission of radiant heat is not obstructed. [ 8 ]
During the hot season, walls of daytime living areas should be made of heat-storing materials; walls of rooms for nighttime use should have a light heat capacity. East and west walls should preferably be shaded. High reflective qualities are desirable for both thermal and solar radiation. [-13 ]
In regions with a less extreme diurnal temperature range and where the night temperature does not fall below comfort zone, the internal walls and intermediate floors should have large thermal masses, whilst the outer walls and roof need a high resistive insulation and reflectivity [-8 ]. Double walls with insulation in between are a suitable solution. (multylayer construction, see Chapter 3.1.3)
In regions with large diurnal temperature ranges and night temperatures below comfort level, inner and outer walls and - especially in the absence of a ceiling - roofs should possess a large thermal capacity with an appropriate time lag to balance temperature variations. To achieve this they must be constructed of heavy materials. The use of exterior or interior insulation has to be considered carefully and its suitability depends on the particular requirements and technical possibilities.
22.214.171.124 Openings and windows
Openings and windows are necessary for natural lighting and ventilation, but heat gain in summer should be minimal. During the daytime, the absence of openings would be desirable, especially on the west side; or the openings should be as small as possible and be shielded from direct radiation and located high on the walls to protect from ground radiation [ 13 ]. At night, the openings should be large enough to provide adequate ventilation for the dissipation of heat emitted by the walls and the roof. Hence larger openings should be closed during the day with insulated shutters and opened at night. Such systems are not always reliable because they require the attendance and readiness of the inhabitants. Other considerations such as desired privacy and safety may prevent the correct use of a system with shutters.
Appropriate natural lighting is important. The depth of rooms and the size of windows have to be coordinated. Glare of direct natural lighting can also be avoided by the use of internally reflected light.
Orientation and size of openings
Main openings should face north and south, but the latter should be shaded either by shading devices, roof overhangs or by deciduous trees. The size of the windows on the west and east sides should be minimized in order to reduce heat gains into the house in the early morning and late afternoon, or also be protected by particular shading devices. A moderate, south-facing glass area catches the solar radiation during the cold season, but should not be affected by direct radiation during the summer.
Window glass (also see Chapter 126.96.36.199)
Generally, single glazing is sufficient. Insulating and special heat-absorbing and heat-reflecting glass is basically only suitable for air-conditioned buildings. Generally, single glazing is sufficient. Tight closing joints and window profiles are important to prevent the penetration of hot air, sand, dust and insects.
Placement of openings
Windows and other openings must be placed in suitable positions relation to the prevailing (cool) breeze to allow a natural airflow through the building, to achieve air movement across the body for evaporative cooling and air changes for driving out excess heat. An internal draft (cross-ventilation) can be channeled by louvres set in an upward position towards the ceiling or in a horizontal position towards the human body. Outlet openings should be located at a high level where hot air accumulates.
In buildings in coastal areas, openings for cross-ventilation should be equipped with movable shutters. Because of the hot land wind which occurs at night, openings facing the inland direction should be closable. [ 1 ] (also see Chapter 188.8.131.52)
For comfort, ventilation openings should be at the level of the occupants. High openings vent the hot air collecting near the ceiling and are most useful for convective cooling.
Different forms of roofs are possible or can be traditionally applied, the latter mainly determined by local materials and technical means. In hot-arid regions the vault, the dome and the flat roof are the traditional roof shapes. The common construction method of today, a 10 to 15 cm exposed concrete is the worst possible solution, because the inner surface temperature can reach up to 60°C, which remains till late in the evening.
As the roof is the most critical part, high solar reflectivity and emissivity for long-wave radiation are essential, as well as thermal insulation and/or adequate time lag. Outside application of insulation is preferable for reasons mentioned earlier, but needs an additional, robust skin which protects the insulation from damage.
The rounded form of a hemispherical vault (dome) has a larger surface area than its base. Solar radiation is thus diluted and re-radiation during the evenings is also greatly facilitated. [ 9 ]
The flat roof is practical in areas where it seldom rains. It is also a good reflector and re-radiates heat efficiently, especially if it consists of a solid, white painted material. (see Chapter 3.1.5, 184.108.40.206, 220.127.116.11) [ 13 ] High solid parapet walls along the edge of the roof can on the one hand provide daytime shade and privacy, but can have the disadvantage of creating an undesired stagnant pool of hot air. The construction and exact placement of parapet walls should therefore be carefully examined. [ 8 ]
A separate roof and ceiling are still today less common in hot-arid regions, whereas they are the obvious solution in warm-humid climates. This efficient, but expensive solution (pitched or flat ventilated double roof) contrasts with the traditional form of most desert buildings. However, the sloping roof with wall shading overhangs and a well-ventilated space between roof and ceiling appears to be an appropriate, contemporary solution. [ 147 ]
If it is used, the material of the roof should be light and the ceiling material should be massive. The air enclosed in a double roof, or between the roof and ceiling, may reach a very high temperature. This can be avoided by ample ventilation of the roof space by openings facing the prevailing breeze. In addition, roofs (slopes) should be orientated towards the prevailing breeze and any obstructions which would prevent the airflow next to the roof surfaces should be avoided.
A somewhat less effective but also less expensive construction would be a simple ceiling with a ventilated roof space (also only common in warm-humid climate zones). A shaded, ventilated roof is applicable primarily over rooms used at night.
Sloped roofs could also provide cold airflow towards a courtyard. A membrane covering the courtyard in the daytime allows retention of cool air and provides shade, but needs attendance by the inhabitants.
The efficiency of the central courtyard is increased by stretching a curtain across the courtyard early in the morning during the summer months to trap the cool air. In the evening, this is removed to maximize the night radiation potential. [ 106 ]
3.2.5 Special Topics
18.104.22.168 Shading devices
In hot-arid zones, shading of the direct sun’s radiation and its reflection by surroundings is essential; diffuse radiation is less of a problem. Shading can be provided by different means, such as placing buildings closely together, the shape of the building itself (overhangs etc.), vegetation such as deciduous trees, or attached, special shading devices.
In hot maritime regions, the traditional “mashrabiyas” or “rowshans” are common. These projecting, screened (bay) windows or non-projecting screened windows consist mainly of wooden, shading screens over large openings and allow cross-ventilation as well as the passage of daylight while preserving family privacy. Some contain evaporative cooling means such as an earthenware water pot.
Similar devices can be designed by contemporary means (see Chapter 22.214.171.124).
126.96.36.199 Natural ventilation
Basic principles and concepts
Ventilation is essential and must be regulated to achieve the highest efficiency in keeping hot (and dusty) air out during the daytime, and cooling the thermal mass at night by air movement; if possible together with outside vegetation. Ventilation can only reduce temperatures higher than the outside air temperature. However, if the air is very dry, any breeze also helps to evaporate sweat and thus to cool the body. High rooms promote air circulation and increase the distance to a radiating ceiling. A low ventilation rate during winter decreases the temperature variation and thus raises night temperatures. A high night ventilation rate in combination with an internal thermal storage capacity is preferable during summer.
During the daytime, openings should be closed and shaded and ventilation kept to the absolute minimum necessary for hygienic reasons. Openings should be placed according to the prevailing winds and allow cross-ventilation. Air intake openings should be located so that the coolest and most dust-free air is taken and, if necessary, the air can be conveyed to the points in the building where it is needed. Thus the cool conditions existing at dawn can be maintained inside the building for the longest possible period. Internal heat sources should, if possible, be isolated and separately ventilated.
Electric fans (ceiling mounted etc.) may be used where little or no air movement occurs. (see Chapter 188.8.131.52)
Windcatchers (also see Chapter 184.108.40.206)
Windcatchers are a significant feature in the traditional structures to ventilate and cool buildings in hot desert and hot, coastal regions. Wind pressure forces air down the wind catcher. Air circulation inside the building is achieved if there are openings on the opposite side allowing suction of inner air by lower pressure.
Depending to the region, they have a variety of forms, details and ways of functioning, and are known in the Middle East as “malqaf” and/or “badgir” [ 122, 149, 155 ]
a) Roof windcatcher
One kind of windcatcher (also called wind “chimney”) is built onto the roof. In some places the catchers are unidirectional and orientated to catch favourable winds or are facing away from it to draw cool air from the court yard through rooms, and expel stale air and smoke. By change of wind they are anticipated to reverse their function.
In other places pivoted scoops and multidirectional wind towers utilize winds from any direction. Generally, windtowers are square in plan and have four internal shafts.
The principle involved is to catch an unobstructed breeze at a high level and channel it to areas in the bottom parts of the building. The increased air-velocity supports perspiration and is thus cooling. The ducts are preferably built in a massive way to absorb the heat of the incoming air and not exposed to solar radiation (e.g. northern wall), to enhance efficiency. In addition they should be equipped with evaporative cooling means, such as porous water jugs, moist matting, wet charcoal etc., to achieve efficient cooling (also see Chapter 220.127.116.11).
The inlet of the catcher should have a shutter to regulate the air movement and to protect against too cold or too hot air and against sand. [ 9 ]
In the Middle East, wind catchers can provide sufficient ventilation and cooling during approximately six months of the year for comfortable inner climatic conditions of today’s comfort requirements, without additional devices or the use of mechanical cooling or heating systems. [ 155-]
b) Mid-wall and parapet windcatchers
Structurally integrated wind catchers or scoops and air ducts are a special kind of vents and selective ventilators. A recessed, horizontal niche on the external wall, e.g. on the floor level and in the roof parapet, creates a slot between two vertical, structural posts. These mid-wall or parapet wind intakes or series of them may allow for enough cross-ventilation through the internal spaces in humid weather, while preserving visual privacy. Shutters are necessary to control the air movement. Vertical air shafts integrated into the wall provide air circulation within the building. [-155, 166-]
Solar chimneys and induction vents(see chapter 18.104.22.168)
Electric ventilators or fans represent simple active devices. They may be placed directly in the outer wall or combined with an air duct system.
22.214.171.124 Passive cooling means
Cooling means should be integrated into the general ventilation concept of a building. Cooling can be achieved by the evaporation of water. The dryer the air, the greater is the cooling potential.
A courtyard house with a dry, hot yard and a cool yard with vegetation and a pool represents a good example of such a ventilation concept. A draft which passes through an evaporative cooler before entering the main rooms is created by the two yards.
External cooling External cooling through humidification can be achieved by keeping the surfaces of roofs and / or walls moist. (e.g. lawn sprinkler) The surface temperature can be reduced by up to 30°C. However, the water consumption is excessive.
Air cooling and humidification or simple air conditioning devices are important means of internal cooling. Warm and dry air passing over water is cooled by evaporating the water. Evaporative coolers have a limited effect and should only be used in relatively dry climates.
a) Moist matting
An open weave matting of vegetable fibre (straw) is stretched on a wooden frame and is kept moist. The matting should be as fine as possible, placed in front of windows and in the path of the natural airflow. The natural airflow should not be reduced and can also be supported by a fan. The damp matting humidifies and cools the air as well as filters out the dust. [ 136 ]
b) Earthenware pots
Another simple system entails the use of large, porous earthenware pots filled with water which seeps through the walls of the pot moistening the outside and, as it evaporates, cools the passing air. [ 9 ]
c) Wet charcoal and water pools
In wind catchers, beds of wet charcoal over which the air passes before entering the room, are sometimes used. The same principle can be applied by channelling breezes over pools or water sprays before they enter buildings. A spray pond is more effective than a still pool of the same size and has the additional advantage that the air is not only cooled, but also cleaned by binding the dust particles. Availability of water and maintenance aspects should not be neglected. [ 122 ]
Roof pond (also see Chapter 126.96.36.199)
A water body covering the roof functions similarly to a soil cover: it minimizes the diurnal temperature range. It is a technically demanding and expensive solution. It also requires the daily attention of the users and is not very suitable for hot-arid regions of the Third World.
Thermal walls and solar collectors (also see Chapter 188.8.131.52)
A wall exposed to the sun can be built in the form of a solar collector and used to create a draft. The air warmed up by the solar collector creates a buoyancy which moves the air in the room. The air entering from there is cooled down by an absorber and perhaps additionally by an evaporative cooler.
184.108.40.206 Active devices
Active devices, such as air conditioners, are often unavoidable and require a different building construction. Many passive means of climatization, however, are also beneficial in that they drastically reduce running costs. With the increased possibilities for using solar energy, active devices may become the means of the future.
In certain regions, particularly on higher altitudes, heating might be necessary in winter. (see Chapter 220.127.116.11)
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