4.4 Experiments in Cairo, Egypt
The main points:
• A concrete structure of extremely poor design is compared to a well-designed mud structure.
• In contrast to the mud structure, the performance of the concrete structure is drastically different, being very hot in daytime and cold at night.
• The difference is obvious both for the air temperature and for the surface temperature.
Source: Research group under the leadership of John Norton, Development Workshop, Lauzerte, France
4.4.1 The project
Cairo lies in a maritime desert region. It is only a few metres above sea level and at a latitude of 30° North.
In the 1970s, several experimental rooms were constructed at the Building Research Center in Cairo to test prototype building solutions for poor rural areas in developing countries. Among the rooms built was one of mud brick vault and dome construction and another of prefabricated concrete slab construction, illustrating the contradictory proposals for rural development. One proposed a reinstatement of traditional modes of construction and the other an importation of ideas and materials from outside.
The mud brick structure was built by Professor Hassan Fahty, who used similar design features in many of his projects.
The researchers undertook comparative measurements of the thermal performance of these buildings. The tests were done at the end of March, thus the extreme temperatures of the climate in Cairo were not recorded. However, the results illustrate clearly the different performances of the two test rooms. Since each test room was built in an open space by itself, and each is oriented in the same direction, they can be considered to be affected by the same climatic factors and thus directly comparable.
4.4.2 The experimental buildings and test configuration
The prefabricated concrete test room consists of light weight concrete block walls 15 cm thick and a roof made of reinforced concrete with a prefabricated roof panel system.
The mud brick room consists of a lime stone foundation, walls of sun dried mud bricks 50 cm thick and a roof with a dome and vault shape made of the same material.
The first step in the recorded experiment was to close the doors and windows of the two test rooms. The house stayed closed for at least 24 hours to allow the air temperature in the rooms to stabilize and not be constantly renewed by air movement and wind. In this way the influence of heat transfer through the shell could be studied in an undisturbed manner.
4.4.3 Climatic performance
The tests show an impressive difference in the performance of the concrete structure and the mud structure. Whereas the temperature in the mud structure fluctuates by a few degrees only, in the case of the concrete structure the temperature varies between extremely high and very low. In practice, the difference may be somewhat less because air changes would increase the fluctuation of the temperature in the mud room; however, a clear difference would still remain.
As well as the air temperature the surface temperatures of the walls and the ceiling have to be noted. The temperatures on the outer surface (not shown in the graphs) are similar in both cases, but the temperatures on the inner surface are as drastically different as the air temperatures. This is an important factor because radiant heat from inner surfaces adds considerably to the comfort level. Furthermore it demonstrates the effect of the high thermal resistance (U-value) of the mud wall.
This example illustrates that a heavy structure with thermal properties similar to the mud room tends to have indoor temperatures close to the average outdoor temperature. As a consequence, such a room would need daytime ventilation during the cold period and nighttime ventilation during the hot period. In this way, an all year round acceptable indoor climate could be provided.
The lightweight structure, on the other hand, can hardly be regulated by ventilation because its storage capacity is not sufficient to store the heat surplus of hot days until nighttime in winter, and to keep the cool of the night until daytime in the summer. Such a structure would, however, be suitable as a temporary room, to be used alternatively at nighttime during the hot period and in daytime during the cold period.
The concrete structure monitored here is, of course, of extremely poor design. It cannot be concluded that modern structures in general would perform the same way. As seen in Example 4.7, a modern structure can also perform similarly to a mud structure, if it is well designed and sufficiently insulated.
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