Hassan Fathy 7

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Hassan Fathy 7: El factor solar Artifex - © 2003

(http://www.unu.edu/unupress/unupbooks/80a01e/80A01E01.htm )

4. The sun factor

Orientation
Shading
Facades
Openings
The roof

Orientation

In hot climates, the sun is the major source of heat. To plan any site, the position of the sun must be determined for all hours of the day at all seasons as well as the direction of the prevailing winds, especially during the hot season. For the direct rays of the sun, it is sufficient to know the angles of declination and altitude for the summer and winter solstices (21 June and 21 December, respectively) and the autumnal and vernal equinoxes (21 September and 21 March, respectively), from which the position of the sun at any time of day on any intermediate date can be deduced. These dates, rather than averages, represent the extreme cases which the architect must consider. Appendix 4 gives this information for the city of Cairo, Egypt, which is located at latitude 30° N. Similar tables for any city can be obtained from the local meteorological office. In addition, for an ensemble of buildings forming a sector, there will be reflection from adjacent buildings and wind screening by clusters of buildings, which contribute to a specific microclimate for each location in the sector. Wind movement and humidity also are important and should be considered simultaneously with the direct and indirect effects of the sun.

The main objective is to establish the optimum orientation with regard to the sun and the prevailing wind. The problem is complex, and it is useful to begin by considering the simple case of a block consisting of a single row of buildings. On the basis of this, more complex cases can be understood.

Appendix 4 indicates that the optimum orientation of the building block with regard to the sun factor is east-west. In this case, the north facade is exposed to the sun's rays at the summer solstice from sunrise at 5:00 A.M. to about 9:00 A.M. These rays have an angle of altitude of 0° at 5:00 A.M., but at 9:00 A.M. the angle of altitude is 49°30', the angle of declination 88°13', and the rays hit the facade at an angle of only 1°03'. For the south facade, the angle of altitude is 83°36' at noon, which is 6°24' off the vertical. Solar radiation does not penetrate the south facade openings, and a slight overhang properly positioned could easily shade the openings and wall surface. The east and west facades constitute the end walls of the entire row and are not provided with openings. In winter, the angle of altitude at noon is 36°34', which allows the sunlight to penetrate into the interior for warmth.

Meteorological records show that the cool wind in Cairo blows from the northwest. Thus the optimal orientation with regard to wind is such that the long side of the row is aligned northeast to southeast so the wind can be as normal to the long surface as possible.

At first glance, the obvious solution to the requirements of these two factors would be to orient the row from northeast-east to southwestwest, bisecting the angle between the two optimal orientations as shown in figure 9. This solution would be correct if the windows were to serve as wind inlets and outlets to ensure air movement indoors. However, people in the hot arid and warm humid zones devised the malqaf or windcatch, whereby air high above a building can be captured and forced through the interior, as explained in the next chapter. With the wind problem solved with the malqaf, the row can be aligned east-west, which is optimal for the sun, as indicated in figure 10. This innovation permits flexibility in design with regard to the wind factor and makes it possible for the designer to concentrate on orienting his buildings with respect to the sun factor.

Shading

Although the optimal orientation for single buildings and blocks of row houses is with the long side aligned from east to west, for many reasons this cannot always be applied so simply over the entire plan of a city or sector. Some single buildings or row houses must face streets and squares that may be oriented at any angle from the north, with each case requiring an appropriate means of shading, depending on its orientation.

Generally, a building with a facade opening to the west is the worst case encountered, owing to the heat gain of the surrounding environment during the day and the angle of altitude, which allows the sun's rays to penetrate into the interior. However, for a sector with the long facade facing west and east, blocks of buildings can themselves shade one another. To ensure this, the height of the blocks must be designed according to the width of the street and the angle of altitude of the sun, which can be obtained from data like that contained in Appendix 4 for any geographical site. In this way, areas that will be exposed to the sun can be defined, either for the facades or for street surfaces, and the duration of exposure can also be calculated.

Facades

Northern Facade

This facade is least exposed to the sun. In fact, exposure occurs only in the early and late hours of summer days when the angle of altitude is low and the angle of declination is such that the sun's rays are almost tangential to the surface of the wall, as illustrated in figure 11. An advantage to rooms opening on this facade is that their illumination is always evenly distributed, making them ideal for hospital operating rooms and for school classrooms.

Southern Facade

With regard to the sun factor, an advantage of southern exposure in the Tropics and Subtropics is that the sun is high over the horizon in summer and can be shaded using a relatively small overhang. In winter it is low, allowing the sunshine to penetrate when it is most desirable. This situation is outlined for a particular case in figure 12. However, with regard to the wind factor, a disadvantage of the southern exposure is that it receives no wind, since the cool prevailing winds generally blow from a northerly direction in the Northern Hemisphere.

Although the sun's rays cannot be manipulated and directed at will, there are ways of directing airflow to rooms with a southern exposure, either by architectural design or by such devices as the malqaf, the windescape, and even the indoor mashrabiya as seen in some traditional houses in Jeddah, Saudi Arabia.

Eastern and Western Facades

The eastern facade is exposed to the sun's rays only from sunrise to noon. The walls cool down considerably by evening, making this exposure more suitable for bedrooms than the western exposure.

Shading of the facades of buildings can be achieved by covering the streets, as is often found in older cities and oasis villages of West Asia and North Africa, examples of which are illustrated in figures 13-15. For a single building, shade can be provided by architectural elements such as balconies, covered loggias or open galleries, and verandas to shield the facade, or by introducing special devices such as the venetian blind, the brise-soleil, and the mashrabiya to shield the openings. In Iraq, walls ventilated and cooled by surrounding the rooms with an outside corridor with arcades and colonnades, as shown in figure 16.

Openings

Window openings normally serve three functions: to let in direct and indirect sunlight, to let in air, and to provide a view. In the temperate zones these functions are conveniently combined together in the window, the size, form, and location of which are determined by local climatic conditions. However, since in hot arid climates it is rarely possible or desirable to combine these three functions in a single architectural solution, several solutions were developed which concentrate on each function separately.

The Venetian Blind

One device which can be added directly to the window is the venetian blind. This blind is made of small slats, about 4-5 cm (1.6-2 inches) wide, closely set in a wooden frame at an angle that will intercept the sun's rays. The slats are often movable so the angle can be changed. This feature of adjustability renders venetian blinds very useful in regulating solar radiation and wind flow into rooms. Using the venetian blind, the sun's rays can be blocked out without obstructing the breeze, which generally blows from the northwest in most hot arid areas, including Egypt, Iraq, and North Africa. When the blinds are drawn, they completely obstruct the view to the outside as well as considerably dim the light reaching the interior.

However, sometimes the venetians blind is not a satisfactory solution to the problem of adjusting radiation and airflow. In summer, the blind can be adjusted to deflect the wind downward onto the occupants, but this permits the sun to shine directly into the room, as shown in figure 17a. Alternatively, by changing the position of the blind to block the direct sunlight, the wind is redirected uselessly over the heads of the occupants, as figure 17b illustrates. Also, if the slats are made of metal, they then absorb some incoming radiation and reradiate it into the room as heat.

The Brise-soleil

The brise-soleil or sun-breaker is a new shading device that requires a special sophisticated support. It is generally used to shield entire facades of glass-wall and concrete or steel frame buildings. Originally, the glasswall concept was introduced to provide an outside view through the entire side of a room. Standard glass is transparent to ultraviolet radiation and opaque to infrared or heat radiation. Therefore, when a glass wall of a room measuring, say, 3 x 3 m (about 10 x 10 ft) is exposed to the sun's rays, it lets in 2000 kcal (nearly 8000 Btu) per hour throughout most of the day. This light strikes the solid material inside, including the walls, floor, and furniture, and is transformed into infrared radiation to which the glass is opaque. The glass wall thus traps the heat, creating a phenomenon known as the greenhouse effect, and two tons of refrigeration per hour are required. Thus additional energy, and therefore cost, is required to maintain a comfortable microclimate in the room.

A brise-soleil properly designed to intercept the sun's rays reduces this heat gain to at most one-third, which although an improvement is still inadequate. Furthermore, there is the additional disadvantage of using the brise-soleil with regard to the view to the outside, which was the original purpose for using the glass wall. The brise-soleil is in fact a transposition of the venetians blind, with the slat width increased from 4 to about 40 cm (1.6 to about 16 inches) to suit the scale of the entire facade instead of just the window opening in a solid wall. When the angles of altitude and declination for screening direct sunlight are calculated, the required space between the slats is much larger than for the venetians blind. The result is a view slashed by large dark stripes interspersed by offensive glare. This is why photos showing the brise-soleil in architectural magazines and books are always taken from the outside and never from the inside looking out, as in figure 18. Nevertheless, the brise-soleil concept need not be discarded. It may be used advantageously in some cases of modern architecture if comprehensively articulated in the facade with due regard for reduction of physical glare and for aesthetics.

The Mashrabiya

The name mashrabiya is derived from the Arabic word "drink" and originally meant "a drinking place." This was a cantilevered space with a lattice opening, where small water jars were placed to be cooled by the evaporation effect as air moved through the opening. Now the name is used for an opening with a wooden lattice screen composed of small wooden balusters that are circular in section and arranged at specific regular intervals, often in a decorative and intricate geometric pattern. Figure 19 shows such a mashrabiya that of the As-Suhaymi house in Cairo.

The mashrabiya has five functions. Different patterns have been developed to satisfy a variety of conditions that require emphasis on one or more these functions. These functions involve: (1) controlling the passage of light, (2) controlling the air flow, (3) reducing the temperature of the air current, (4) increasing the humidity of the air current, and (5) ensuring privacy. Each mashrabiya design is selected to fulfill several or all of these functions. In the design, it is the sizes of the interstices (spaces between adjacent balusters) and the diameter of the balusters that are adjusted. Different names identify certain of these patterns.

Daylight entering a room with an opening facing south has two components, the direct high-intensity sunlight that enters at very large angles normal to the plane of the opening, and the lower-intensity reflected glare, which can enter nearly normal to the opening. Since direct sunlight passing through the opening will heat surfaces in the room, it is best to block such radiation. The reflected glare, while less intense and not very effective in heating room surfaces, does produce uncomfortable visual effects.

The sizes of the interstices and the balusters of a mashrabiya placed in such an opening are adjusted to intercept direct solar radiation. This requires a lattice with small interstices. The balusters, round in section, graduate the light reaching their surfaces, thus softening the contrast between the darkness of the opaque balusters and the brightness of the glare entering through the interstices, as illustrated in figure 20. Therefore, with the mashrabiya the eye is not dazzled by the contrast as in the case of the brise-soleil. Figures 21 and 22 show the effect of a mashrabiya under conditions of severe glare. The characteristic shape of the lattice with its lines interrupted by the protruding sections of the balusters produces a silhouette which carries the eye from one baluster to the next across the interstices, vertically and horizontally. This design corrects the slashing effect caused by the flat slats of the brise-soleil and harmoniously distributes the outside view over the plane of the opening, superposing it on the decorative pattern of the mashrabiya so that it resembles a darkened glass made of lace. This effect is shown in Figure 19.

At eye level, the balusters of the mashrabiya are set close together with very small interstitial spacing both to intercept direct sunlight and to reduce the dazzle of contrasting elements in the pattern. But to compensate for the accompanying dimming effect, the interstices are much larger in the upper part of the mashrabiya, as in the example from the Jamãl Ad-Din Adh-Dhahabi house in Cairo, shown in figure 23. Figure 24 shows the striking effect that can be achieved for a room with a high ceiling. This arrangement permits reflected light to brighten the upper part of the room, while an overhang at the top of the opening, as seen in the outside view of a second story mashrabiya in figure 25, prevents direct sunlight from entering. Similarly, in openings on a northern facade, where direct sunlight is no problem, the interstices are quite large, to provide adequate room lighting.

To provide airflow into a room, a mashrabiya with large interstices will ensure as much open area in the lattice as possible, as shown in figure 26. Where sunlight considerations require small interstices and thus sufficient airflow is not provided, an open, large-interstice pattern can be used in the upper part of the mashrabiya near the overhang. For this reason, a typical mashrabiya is composed of two parts: a lower section with fine balusters in close mesh, and an upper section filled with a wide mesh grill of turned wood in a pattern called sahrigi, as shown in figures 23 and 25. If this solution still does not provide sufficient air movement due to the small interstices required to reduce the glare, the dimensions of the mashrabiya can be increased to cover any size opening, even to the point of filling up the entire facade of a room. Figures 27 and 28 show inside and outside views of a facadesized mashrabiya designed to solve this problem in the As-Suhaymi house in Cairo. The very large size of such a mashrabiya also helps to compensate for the dimming effect of the screen. In some places, the mashrabiya is used indoors between rooms for cross-ventilation, as in some houses in Jeddah, Saudi Arabia. The mashrabiya concept has been universally used in hot arid areas, particularly throughout the Middle East and North Africa, but even in India, where it is called the jãli.

Its cooling and humidifying functions are closely related. All organic fibers, such as the wood of a mashrabiya readily absorb, retain, and release considerable quantities of water. Plants can provide some regulation of their skin temperatures by the successive processes of transpiration and evaporation (called evapo-transpiration). Thus, the sap flows through the fibers to the plant surfaces, where it evaporates and cools the skin. Wood fibers retain this ability even after they are cut from the tree and used in buildings, as long as the pores are not covered by an impervious paint.

Wind passing through the interstices of the porous-wooden mashrabiya will give up some of its humidity to the wooden balusters if they are cool, as at night. When the mashrabiya is directly heated by sunlight, this humidity is released to any air that may be flowing through the interstices. This technique can be used to increase the humidity of dry air in the heat of the day, cooling and humidifying the air at a time when most needed. The balusters and interstices of the mashrabiya have optimal absolute and relative sizes that are based on the area of the surfaces exposed to the air and the rate at which the air passes through. Thus if the surface area is increased by increasing baluster size, the cooling and humidification are increased. Furthermore, a larger baluster has not only more surface area to absorb water vapor and serve as a surface for evaporation but also more volume, which means that it has more capacity and will therefore release the water for evaporation over a longer period of time.

In addition to these physical effects, the mashrabiya serves an important social function: it ensures privacy from the outside for the inhabitants while at the same time allowing them to view the outside through the screen. Therefore, a mashrabiya covering an opening that overlooks the street has small interstices except at the top far above eye level. A striking example of the feeling of security and external view a mashrabiya can provide is shown in figures 29 and 30. With the focus on the lattice, the mashrabiya appears as a lighted wall. When focusing beyond the lattice, the external view is quite clear and only slightly obstructed.

Figure 31 shows how mashrabiya can be used in the design of a modern villa. This design for Saudi Arabia includes mashrabiya high in the top of the dur-qã'a and others at a lower level in adjacent rooms, as well as a malqaf on the right.

The roof

If the outdoor air temperature is higher than the indoor temperature, the outer surface of the roof exposed to the sun is heated as it absorbs radiation, and, being in contact with the outside hot air, also is heated by conduction. The roof then transmits this heat to the inner surface, where it raises the temperature of the air in contact with it by conduction. At the same time, it radiates heat that is absorbed by people and objects indoors, thereby affecting thermal comfort.

Therefore, the reflectivity of the outer surface of the roof and the thermal resistivity of its materials are of primary importance. Shade can be achieved by using a double roof with a layer of air between or by covering the roof surface with hollow bricks. Insulating materials such as fiberglass, styrofoam, and lightweight blocks are often used. This solution, however, requires special commercial materials and increases the cost of the building beyond the means of most inhabitants in hot arid zones.

The idea of using a roof with a lightweight cover as a living space has been further developed in the modern example of the roof garden with a trellis. Soil is a good heat insulator, and plants can provide shade. Plants also transpire and cool the air in contact with the roof. Again, this idea requires special structures to ensure a strong and waterproof roof, and is also too costly for most inhabitants of these regions. Psychologically and aesthetically, people appear to prefer to live on the level of tree trunks, branches, leaves, and flowers, rather than to feel as if they were living under the roots.

A useful idea is to shade the roof more naturally by designing it to suit popular traditions. In hot arid countries, since the air temperature drops considerably during the night, the inhabitants have arranged the roof architecturally into loggias or open galleries and lightweight roof covers. These loggias and roof covers have the double function of shading the roof during the day and providing physiologically comfortable living and sleeping spaces at night. Examples from Iraq and Rosetta, Egypt, are shown in figures 32 and 33, respectively.

The shape of the roof is also of considerable importance in a sunny climate. A flat roof receives solar radiation continuously throughout the day, at a rate that increases in the early morning and decreases in the late afternoon due to changes in both solar intensity and angle of the sun.

Pitching or arching the roof has several advantages over a flat structure. First, the height of part of the interior is increased, thereby providing a space far above the heads of the inhabitants for warm air that rises or is transmitted through the roof. Second, the total surface area of the roof is increased with the result that the intensity of solar radiation is spread over a larger area and the average heat increase of the roof and heat transmission to the interior are reduced. Third, for most of the day, part of the roof is shaded from the sun, at which time it can act as a radiator, absorbing heat from the sunlit part of the roof and the internal air, and transmitting it to the cooler outside air in the roof's shade.

This latter effect is particularly effective for roofs vaulted in the form of a half-cylinder and those domed in the form of a hemisphere since at least part of the roof is always shaded except at noon when the sun is directly overhead. Domed and vaulted roofs also increase the speed of any air flowing over their curved surfaces due to the Bernoulli effect, discussed in the next chapter, rendering cooling winds more effective at reducing the temperature of such roofs.

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