4.6 Buildings in Kathmandu, Nepal
The main points:
• In buildings with adequate storage mass, insulation and control of solar radiation, the temperature is acceptable in summer and winter, except for cold nights where an additional heating source is required. This is in sharp contrast to poorly-designed buildings.
• A well-designed building is up to 7ºC cooler in summer than the poorly-designed “concrete box”.
• A floor heating system with passive solar collectors - the collectors measuring 1/3rd of the floor area - increases the temperature in winter by 10°C.
Source: Paul Gut
4.6.1 Geographical location and climatic characteristics
Kathmandu lies at an altitude of 1350 m and a latitude of 28o-North. It is situated in a wide valley of about 20 km circumference, surrounded by hills reaching up to 3000 m height.
The climate is characterized by three main seasons:
• In winter-time temperatures are relatively low, ranging between 0°C at night and 15 to 20°C in the daytime. Sometimes light frost appears over clear nights. The cold air lake phenomenon, which is typical for a valley location like Kathmandu, keeps temperatures between December and February uncomfortably low. However, the frequent and strong solar radiation, which is common during this season, improves the situation and provides an excellent opportunity for passive solar room conditioning.
• The pre-monsoon season is hot and dusty, mainly in May and the first half of June. Temperatures rise up to 35°C in daytime and drop to around 20°C at night. The solar radiation is often intense, and protection is required. During this season dusty storms are frequent.
• During the monsoon season temperatures hardly reach 30°C and the diurnal differences are less. Periods of pouring rain and heavy clouds alternate with periods of clear sky and glaring sunshine. The humidity is high and proper ventilation is required.
Considering the climatic conditions which change drastically with the seasons, the design concept of a building should respond to these differences.
Cold season requirements
Passive solar heat gain is welcome during the cold season. The main rooms and the large windows should be south oriented. To provide an acceptable indoor climate in winter, buildings usually require active heating as well, unless they are very well designed and equipped with special passive solar heating devices. The heat storage capacity should be moderate, not too excessive; otherwise the space becomes non-heatable. Airtight construction is another important aspect; it is more important than the thermal insulation properties. Inner surfaces should not be highly conductive which would result in low surface temperature and uncomfortably high conductive heat losses from the human body when in contact.
Hot season requirements
During the hot season protection against solar radiation is necessary. Windows should be shaded and a proper cross-ventilation should allow accumulated heat to escape in the evening. Here again, a moderate heat storage capacity is appropriate, keeping the daytime indoor temperature at tolerable levels.
Special care is required in the design of the roof. Its inner surface should not heat up too much and it should not store much heat. The worst solution is the plain concrete roof slab, which is a common solution these days. It heats up to extreme temperatures and makes living conditions during the evening unbearable.
During the monsoon period the most important factor is cross-ventilation
4.6.2 The monitored buildings
To illustrate the effect that different design features have on the indoor climate, two different buildings have been evaluated under winter conditions (building A and B).
In the summer season the performance of two additional buildings has also been recorded. (buildings A, B, C and D).
A modern residential house, located on a southern slope with mainly south-oriented rooms. The main windows also face south and are partly protected by overhangs from the summer sun. The walls consist of 35 cm thick solid brick masonry; fair-faced outside and inside, with lime white wash on the inside. The floors are made of timber beams supporting brick vaults, covered with clay flooring tiles. The roof is pitched, covered with clay tiles and with timber panelling inside.
The windows are made of specially well seasoned timber (timber from a dismantled old building) and are built with double grooves for air tightness, equipped with imported fittings.
An interesting feature is the solar floor heating system in the living room which works entirely passively as a thermo-syphon, even without a regulatory mechanism. The system is described in more detail at the end of this chapter.
(Mana Niwas, Arch: P. Gut, built in 1980)
This is an office building, hence only the thermal performance in daytime is of relevance. Of special importance is a increased temperature during the winter mornings, when most buildings are freezing cold and non-heatable, thus it becomes extremely difficult to work in.
As a consequence, all offices are located on the main front which is oriented south-south-east. This elevation is designed in such a way that all windows receive winter sun, from sunrise to sunset. During summer an overhanging curved slab shades the windows entirely. Deciduous trees in front of the building help to control the effects of the sun.
The walls consist of 35 cm solid brickwork, the floor slabs are of concrete and the flat roof is additionally covered with a 5 to 10 cm thick screeding and clay tiles.
(National Parks Department Headquarter Building, Arch: P. Gut, built in 1981)
This old palace with small windows and massive, 70 cm thick brick walls. Floors are of timber structure with a thick layer of mud covered with clay tiles. The room monitored is south facing.
This building is a modern bungalow of “international concrete box” type. The walls are of 35 cm brick masonry, the windows are rather large with only minimal protection from the summer sun. The roof slab is of 12 cm plain concrete without any cover. Climatic considerations were not applied.
This type of building is the most common solution for modern development in Kathmandu, as well as in many other places in the developing world.
4.6.3 Climatic performance and conclusions
The outside temperature varies between 19 and 34°C, hence the average temperature more-or-less lies within the comfort zone.
The buildings B and C show the most even temperature swing, slightly above the average temperature. This is due to the consistent shading of the windows in the case of building B and due to the excessive heat storage capacity of building C.
Building A shows somewhat higher daytime temperatures because of windows facing east and west which are less protected against solar radiation. These windows, on the other hand, allow for a better ventilation at nighttime, resulting in more comfortable conditions during the night.
Clearly worse is the performance of building D. Due to poor protection of the windows against solar radiation, and mainly due to the non-insulated flat concrete roof, temperatures constantly lie above the outside temperature and above comfort level. Characteristically, the temperature remains high during the evening, the greatest difference, compared to building A, reaches 7°C at midnight. In addition to the high air temperature the high surface temperature of the concrete ceiling has to be considered, which is not expressed in the diagram. The inner surface of such concrete roofs reaches temperatures up to 50 or 60°C, resulting in unbearable indoor climatic conditions due to radiant heat.
Whereas the outdoor temperature varies between -2°C and 16°C, the temperature in an unheated, west facing room in house A is rather even, at the uncomfortable low level of 6 to 10°C.
During the day the temperature of house B lies at about 5°C above this level due to the consequent utilization of direct solar heat gain through the windows. The temperature during working hours for an office building is still low but bearable, due to the direct solar radiation in the rooms.
The living room of house A, which is equipped with a solar floor heating system, performs well with temperatures about 10°C higher, thus lying within the comfort zone. The surface temperature of the floor, which varies between 20 and 28°C, is a further contribution to the comfortable climate.
4.6.4 The solar space heating system
House A is equipped with a passive floor heating system. It heats the main living room during the unpleasantly cool winter months. The system consists of a flat water heating solar collector situated in front of the room at a lower level. It works entirely passively on a thermosyphonic basis, without a pump and even without regulating instruments. As experiences over 10 years have shown, the system works extremely reliably and gives no problems with regard to maintenance.
The total collector surface measures 9 m², that is 28% of the floor area of the heated room. In January the total solar energy received by the collector amounts to about 5 kWh/m² per day with a peak of 900 W/m².
The collector is divided into 8 elements, each working independently with a separate steel pipe loop laid in the floor of the room. These 8 loops, although covering the entire floor area, are short and thus guarantee a reliable circulation of heated water. The only mechanical parts of the system are the three-way valves which are necessary to switch from winter to summer operation. Each of the 8 collector elements can be individually controlled; thus a fine regulation of the system is possible.
During the warm seasons the collected heat is diverted by these valves to a boiler which is equipped with a heat exchanger, providing pre-heated water to the electric drinking water boiler.
Except for the three-way valves, all parts of the system are produced in a local workshop. This suggests a low technological level resulting in a probably somewhat lower efficiency, compensated by the dimensioning of the collector surface. On the other hand, this has kept costs down to a reasonable level.
The adjoining structural elements are also of local manufacture, without imported insulation materials. The floor structure consists of a layer of boulders covered by a 40 cm thick layer of brick waste collected from the construction site. On top of this the heating pipes were laid in sand and carefully levelled to avoid backslope. Clay flooring tiles laid in a concrete screed form the floor finish.
This structure provides a moderate heat insulation and a large thermal mass resulting in a very inert thermal performance. Overheating of the room and also of the floor surface is avoided. The floor surface temperature never rises above 30°C.
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