Solar Access Shading And Building Form Geometrical Study Of The Traditional Housing Cluster In Sanaa

WREC 1996

SOLAR ACCESS~SHADING AND BUILDING FORM: GEOMETRICAL STUDY OF TtlE TI~XD!TIONAL HOUSING CLUSTER IN SANA'A KHALED A. AL-SALLAL, PH.D. Faculty of Engineering, Departmer,t of Architecture Sana'a, Republic of Yemen

ABSTRACT The study employs three-dimensional computer modeling for visualizing the solar access and shading in the traditional housing duster of Sana'a historical city. It investigates the geometrical relationships among the building form and the sun location, and motion, in the critical times of the year It also investigates the methods for controlling the different components of the solar energy (solar radiation, sunlight, daylight) and its potential impact on the thermal performance of the houses. It analyzes the results and establishes design rules of thumb that are graphically and textually described. The rules of thumb are valuable design tools in the initial stages of design and for evaluating design alternatives or existing buildings.

KEYWORDS Solar access; solar shading; solar heating; three-dimensional modeling; Sana'a architecture

INTRODUCTION To maintain comfort with minimal cost, the people in Sana'a have depended on guidelines and rules of thumb for orienting and shaping their city, buildings, and b u l l i n g elements for hundreds of years. There has not been any kind of studies that investigate the solar access and sb.ading for buildings in Sana'a. As a contribution for exploring the values of the vernacular arc.Ntectnrc in Yemen, this study explores the issues of solar access and shading in the housing cluster of the historical city of Sana'a. The objectives of this study are threefold: to plot and e~amine the solar access and shading in a selected case study of housing cluster, to analyze its performance with respect to solar heating, shading, and daylighting, and to develop and present the rules of thumb in formats that can aid the architectural design. The most challenging demand for solar access in Sana'a is solar space heating in winter. Solar access for domestic hot water can be easier to achieve because much of the solar collection occurs during the higher sun angles of spring and summer. Access for daylighting is also less demanding because of the year-round use of the sun and because of both diffuse sky radiation and reflected sunlight are useful. Thus, daylighting can be achieved with considerably less solar access than for space heating. It is generally agreed that solar access should be maintained, if possible, from 9 AM to 3 PM during the winter months. During those six hours over 80% of a winter's day total solar radiation will fall on a building (I_e,chner, 1991.) Tile traditional housing duster contains several tower houses that surround the urban garden and attached 331

WREC 1996 That is because solar access for the first two floors in the vernacular Sana'a house is not necessary because these floors are used mainly for hand mill, grain and timber storage, and stable for animals. One should notice that these shadow patterns are based on a 12 m high pole, which was selected to represent fourfloors building high, the most dominating height in Sana'a, and these patterns should change for higher buildings. However, with the very high houses (8-10 floors), one could allow the shadow to reach the third floor, since third floors in these houses are used occasionally as guest rooms. The same method was applied to check the solar access for buildings oriented to southeast. As shown in Fig. l(b), one should allow larger distance, 9 m, between buildings so that the shadow cast on the southeast or southwest walls does not exceed the height of two floors. The shadow pattern of the case study was determined using 3D computer modeling, and plotted for the critical times of the year, as seen in Fig. 2., Fig. 3, and Fig. 4.

In the winter solstice, at 9:00 AM, the sun strikes south and southeast facades except for few buildings that do not have enough external space toward the south direction such as the ones located at the far right edge of the garden. Also, the minaret of the mosque blocks partially solar access to one of the houses located on the west side of the social square. At 12:00 Noon, the sun strikes almost all south and southwest facades. At 3:00 PM, the sun strikes south and southwest facades except for few buildings that do not have enough external space toward the southwest direction such as the ones located at the northern boundary. Because of the low angle latitude of Sana'a city (15° N)--due to its nearness to the equator-the sun moves to the northern hemisphere of the earth in the summer and thus does not strike south, southeast, or southwest facades. This phenomenon provides a design asset that helps to prevent the occurrence of summer overheating. This could be achieved by orienting the main activity rooms toward south and service spaces toward north with minimal openings. In fact, the people in Sana'a have been following this rule of thumb for hundreds of years. In the vemal or autumnal equinox, at 9:00 AM, the sun strikes southeast facades except for few buildings that do not have enough external space toward the southe.ast suc~ as the, one~ located at the far right edge of the garden. At 12:00 Noon, the sun hardly reaches to souLh facaxles with a very high altitude angle (74.18°)and does not strike may other fa,cade~,~. At 3:00 PM, the sun stTike~ southwest fa~des except for few buildings that do not have enough external space toward the southwest such as the ones located at the northern boundary.

ANALYSIS The mass arrangement of this housing cluster case study serves solar access for the following reasons. The housing section is located along the north-northeast boundary which provides appropriate orientation toward south and southwest directions. The placement of the marketplace buildings 333

Fig. 3

Shadow pattern on March, 21.

WREC 1996

Fig. 1

Shadow pattern of a pole, cast on south wall (a) and cast on southeast wall (b).

to the social square and the road. Some of these houses reach up to nine floors; yet, the majority are three to five floors high. They were built of squared black volcanic stone on the lower levels, and of baked brick above that (Lewcock, 1986; I_~me, 1~,~8.) Sana'a house looks outwards in order to provide its occupants with fascinating views and access to the comfortable climate. Being a tower building and looking outwards gives the windows of the old Sana'a house a significant role. They are responsible to provide the occupants' needs of daylighting, winter heating, summer cooling, outdoor view, and aesthetics (AI-Sallal and Cook, 1992.) The selection of a housing cluster for the study depended primarily on the availability of data. The one selected, however, was judged to be sufficiently representative for the typical housing duster in Sana'a and appropriate for the purpose of the study. It contains a garden in the center of the cluster surrounded by tower houses at the north and northeast sides, a mosque at the northwest side, and the market and caravanserais at the south and east sides. The garden is linked to the road that includes another row of houses from the north side. The road is linked to the social square which exists in the northwest side and includes al-Shahidayn mosque and other houses.

SHADOW PATTERN A shadow pattern is a composite of all shadows cast during the winter hours when access to the sun is most valuable. The shadow pattern of a pole (area = lm x lm, height = 12 m) at 9:00 AM, 12:00 Noon, and 3:00 PM for the winter solstice day, December 21, was determined using computer modeling, as seen in lower part of Fig. l(a). The upper part of Fig. l(a) shows the shadow for these hours cast on a vertical wall. The clear distance between this wall and the pole was determined, as 6 m, so that the shadow cast on the wall did not exceed the height of two floors (the average height of two floors in a vernacular Sana'a house is 6.5 m.) 332

Fig. 2

Shadow pattern on December, 21

WREC 1996 along the south-southwest boundary of the garden does not block the south sun from the housing section because they are single story buildings. The placement of the mosque on the south side of the social square does not block the south sun from the housing section that exists around the social square. That is because of the relatively low height of the mosque that balances with the north-south depth of the social square. The road directions of the case study maximize solar access to buildings because the roads run mostly east-west which allow the sun to reach south facades. In fact, placing the roads along the east-west axis was dominant in most parts of the Mstofical city of Sana'a. The size and proportion of the garden and the social square permit appropriate solar access to the housing sections located along their boundaries. That is because the dimension of any side of the garden or the social square boundaries exceeds the minimum clear distance, 6m, explained in section Shadow Pattern. Also, the proportion of east-west to northsouth dimensions (1.66:1) of the garden m~'dmizgs south solar access to large number of buildings. The narrow roads in some parts of the case study hinder the solar access for some buildings.

CONCLUSION AND RECOM~IviENDATiON o The mare activity spaces in the Sana'a house should be oriented toward south and the service spaces should be oriented toward north with minimal openings. • Buildings that are most demanding for solar access should be placed along the north-northeast boundary of the urban garden or the social square. • Buildings that are one to two floors high, such as the marketplace, should be placed along the south-southwest boundary of the urban garden or the social square. • Roads should run east-west in order to allow south solar access for maximum number of buildings. • For four-floors buildings dominated areas, the roads that run east-west should not have less than 6m width and the roads that run northeast-southwest, or northwest-southeast, should not have less than 9m width. The same method can be applied to determine the appropriate road widths for high building dominated areas.

REFERENCES A1-Sallal, K. A. and J. Cook (1999.). Sana'a historical windows: integration between comfort and aesthetics. In Proceedings of 17th National Passive Conference of American Solar Energy Society (S. Burley and M. E. Arden, ed.), Vol. 17, pp. 197-202, ASES, Boulder. Lechner, N. (1991). Heating, Cooling, Lighting: Design Methods forArchitects. Wiley Inc., New York. Lewcock, R. (1986). The Old Walled City of Sana'a. United Nations Educational, Scientific, and Cultural Organization, Paris. Lane M. B. (1988). Sana'a: Pilot Restoration Projects. UN-DPiUNESCO YEM,'88/O06 334

Fig. 4

Shadow pattern on June, 21.