Energy-efficient facades for tropical and subtropical climates

As economic growth is shifting globally, many of the current large-scale construction projects can be found in the emerging markets. Most of these emerging economies are in regions with tropical or subtropical climate. Reason enough to look more closely at the design of energy-efficient facades in hot climates.

Many emerging economies such as India, Brazil and large parts of China are located in the tropics and subtropics. The economic and population growth in these regions involve growing energy needs, but also the risk of further shortage of energy resources and gradual environmental degradation.

Buildings in general are among the main polluters. In Europe, they are responsible for about 40% of the total energy consumption. Energy-efficient building concepts can therefore contribute significantly to the reduction of adverse environmental impacts. The facade, being the climate boundary between inside and outside, plays a key role in these building concepts.

But what should a climate-friendly and thus energy-saving facade design for hot climates be like? All too often, many modern buildings in emerging markets reveal a mindless application of western standards without adaptation to the local climate.

Traditional buildings as a blueprint

A promising approach is the study of traditional buildings in tropical and subtropical climates (1). People of earlier centuries obviously didn't have access to today's building technology and energy resources. They had to resort to highly energy-saving concepts in order to create a pleasant indoor climate.

However, it would be too simple to lump together all regions with a hot climate. Instead, a division into the following three zones seems appropriate: hot-humid, hot-arid and subtropical climates.

Hot-humid climates are located near the equator and characterised by mostly overcast skies, high humidity and frequent rainfall. Air temperature is always high with minimal daily and seasonal fluctuations. Traditional buildings in this climate have many structural similarities. The facades are usually divided into several layers and made of materials with low thermal storage capacity such as wood, bamboo or grass. They often consist of half-open, air-permeable wattle which provide optimal natural cross-ventilation. Roof overhangs and elevated building structures offer shaded, ventilated outdoor areas. In addition, the steep roofs often have ventilation holes and consist of two structural layers with cavities that act as climatic buffers.

Mashrabiya in Cairo: small facade opening, bright, reflective surfaces and translucent shading devices reduce the solar energy input

Hot-arid climates, on the other hand, have mostly clear skies with high irradiation, low humidity and little precipitation. Wide differences between day and night temperatures are used to cool the building structure by means of nocturnal ventilation. The building structures are therefore mostly made of solid materials with high heat storage capacity such as clay and natural stone. Small openings in the facade, bright reflective facade surfaces and translucent shading devices like the traditional Arab Mashrabiya reduce the solar energy input.

The typical facade concepts of hot-humid and hot-arid climates can also be found - often to a lesser extent - in subtropical climates.

Climatic zoning

The climatic zoning of the building is a strategy that can be found in all hot climates. This zoning can be applied in a concentric or linear manner. The concentric zoning is characterised by central main rooms which are protected from sunlight and heating by surrounding secondary rooms ('onion skin principle'). Buildings with a linear zoning consist of rooms aligned from east to west. The large main facades face towards north and south. The facades of the east and west sides are closed and small in order to reduce the energy input from the low morning and evening sun.

Energy-efficient facade design: Palace of Justice in Chandigarh by Le Corbusier

The Palace of Justice in Chandigarh (1961) by the architect Le Corbusier shows the successful application of traditional facade concepts by means of linear zoning and a layered building envelope. The main facades of the building face towards north and south. Sun-shading structures - so-called 'Brise-soleil' - are placed in front of these facades and protect the glazed areas from the sunlight. The east and west facades are narrow and closed. Thus, the solar heat gain is minimised. The roof gets the main solar energy input but is efficiently ventilated by means of a double layer construction.

Requirements for modern buildings

However, traditional approaches can not always provide an adequate response to the challenges of modern societies. Increasing urbanisation, new life styles and working environments require new types of buildings. In addition, new building envelope materials like large glass facades and technical building equipments like air conditioning have become integral parts of modern buildings.

Air conditioners are responsible for a large part of the energy consumption in modern buildings in the tropics and subtropics. Due to increasing comfort demands, their use will continue to increase. This in turn may cause the inhabitants of hot climate areas to lose their adaptability to the natural (outdoor) climate. Studies have shown that the psychological adjustment is affected by the use of air conditioning: the comfort temperature range in naturally ventilated rooms is significantly higher than in air conditioned rooms. While the comfort zone in HVAC buildings doesn't exceed about 24 ° C/ 75 F, it reaches about 30 ° C/ 86 F in NV buildings (90 % acceptability; outdoor temperature about 33 ° C/ 91 F) (2).

The energy consumption of air conditioners to cool and dehumidify buildings varies greatly between different climate areas. It is highest in hot-humid areas mainly due to the high air humidity and relatively warm nighttime temperatures. Thermal simulations have shown that the annual energy demand of an air-conditioned office space in Singapore (hot-humid climate) is - depending on the window size - two to three times higher than in Cairo (hot-arid). Compared to Tokyo (subtropical climate) the energy demand is even about five times higher (3).

Hot-humid areas: the challenge of the future

Modern buildings in hot-humid climates present a challenge not only because of their much higher energy consumption. In addition, many energy-efficient concepts such as night ventilation for cooling purposes can not be applied effectively here. The use of photovoltaic systems for energy production (eg. for solar air conditioning) is limited because of mostly overcast skies. Therefore, the following three solution approaches seem be the most promising: a return to the climate adaptability of man, naturally ventilated buildings and the design of building zones with different thermal comfort levels according to the 'onion skin principle'. If however air conditioning is mandatory, exterior facades should be insulated and as airtight as possible. Interior thermal insulation for HVAC buildings in hot climates appears to be a promising, albeit largely unexplored approach.

Conclusion

Strategies of traditional architecture - from deep multi-layered facades with effective shading systems to climatic zoning concepts to natural ventilation - are perfectly suited to save energy even in modern buildings. In any case, the decisions made at the beginning of the design process are crucial. Client, architect and facade engineer must work hand in hand to achieve a successful energy concept.

References

(1) cf. Lauber, Wolfgang. Tropical Architecture, Munich et al. 2005
(2) Brager, Gail S .; de Dear, Richard. Climate, Comfort and Natural ventilation, Berkeley 2001, p. 5 et sqq.
(3) Junghans, Lars. Klimagerechte Buerohausfassaden, Zurich 2005, p. 70 et sqq.

Note: This article was published in the journal: Fassade - Technik und Architektur , Issue 1/2013 (original article in pdf-format can be downloaded here)