Primitive man’s needs for protection from the forces of nature were provided by Nature herself in the form of caves, trees, etc. Gradually he learnt to build with natural materials such as tree branches, animal skins, etc., which he used to build humble huts and other simple shelters. But with the dawn of civilisation some 5000 years ago man has shown a penchant for construction of a more permanent nature. Different civilisations across the world developed their own styles of architecture in response to the climatic conditions in their regions. They used the environmental conditions to their advantage by using the natural space conditioning effect of the Sun, wind and water to keep rooms comfortable. Selection of orientation, form, colour and materials for the building, windows, shape, style and location of external, shading was done meticulously for maximum benefit. Such an approach to building that takes advantage of natural conditions for keeping the living space comfortable is called solar passive architecture. But since the Industrial Revolution we seem to have abandoned such features relying instead, in our arrogance, on artificial space conditioning to achieve comfort levels in our buildings. These methods are not only energy intensive but are, in effect, damaging to the environment.
The design of buildings that respond to the environment involves the use of principles of solar design as also a detailed understanding of the complex interrelationship between architectural design, building materials, human behaviour and climatic factors. This kind of design, not restricted just to the use of solar energy but including the utilisation of all forms of natural energy to provide required comfort conditions within the built-up space may be defined as climatic design. Before elaborating on the ways and means to achieve human comfort in the built-space, it would be useful to define what exactly constitutes comfort.
Thermal comfort criteria
Comfort levels are influenced by three main factors:
Mean radiant temperature (MRT): Temperature is one of the main parameters on which comfort of the inhabitants depends. The standards adopted for the establishment of comfortable temperature are however very demanding on the HVAC system of the building. In summer the acceptable temperature is considered to be 24-25 degrees C while in winter it is 22-23 degrees C. maintaining a temperature of 24 degrees C within the structure when the outside temperature is 35-37 degrees C puts a huge strain on the HVAC system leading to huge energy costs. Therefore, it would be wise to revise our criteria for thermal comfort and accept a standard for thermal neutrality instead, i.e. the person feels neither too hot nor too cold, nor feels any local discomfort due to asymmetric radiation, drafts, cold floors and furniture, non-uniform clothing, etc. At same time there has to be a willingness to adapt to the local weather conditions so as not to make unrealistic demands from the air-conditioning system. We should realise that the days of wasteful spending are now over and a measure of austerity has to be there in our energy spending.
Humidity : humidity is also a major factor affecting comfort levels within a space. The moisture content present in the air is called ‘humidity’. The level of humidity greatly influences evaporative cooling. Greater the moisture content in the air lesser is the effect of evaporative cooling. Therefore efforts to reduce humidity levels within a space result in better conditions. In the design of HVAC systems humidity level of 40-50% is considered acceptable, but one should also remember that this standard is not a law and human adaptability can be stretched to farther limits.
Air Movement : Air movement or ventilation can be used to considerably cool the interiors of a building. Air movement over the skin results in Evaporative cooling- as the air moves over the skin, the perspiration on the skin surface evaporates leading to cooling of the surrounding area. Air movement also affects conductive-convective heat transfer between skin and air. The velocity of the air is also important as stagnant air creates a suffocating effect as the air turns stale due to respiration, foul odours, smoke, etc. Therefore removal of this air and its replacement with fresh air is very important which directly depends on adequate cross ventilation of the spaces, which results in proper air movement and velocity.
A combination of these three factors is responsible for the maintenance of proper living conditions within the space. It is therefore, possible to maximise the cooling effect of these factors by making use of proper design elements and the principles of solar architecture to reduce our dependency on external energy to maintain a comfortable living environment.
The objectives in the design of a structure that responds to the environment should be to maximise solar gain in winter and minimise heat gain in summer. Heat gain can take place in one of the following ways of natural thermal transmission-conduction, convection and radiation in addition to evaporation, which plays a major role in cooling of indoor environment. There is thus an inherent contradiction in the tasks that the building envelope has to perform in summer and winter.
In a climate such as ours, cooling is the main factor affecting building design. Humidity levels are also quite high along with high summer temperatures in the high 30’s. Therefore the control of solar heat gain is the most important factor to be considered.
Sources of heat gain
The proper use of shading devices can prevent direct solar radiation from reaching all or part of the roof, walls or windows of a building. Natural vegetation, neighbouring buildings or the surrounding landscape can provide shading - for example on the north-facing slope of a hill or valley. Shading devices on the building (fixed or movable, the latter being manually or automatically controlled) can prevent radiation from reaching critical parts such as windows, doors and even roofs. Indirect solar gain from the sky or reflected from the surrounding buildings or the ground and air heated by irradiated surfaces such as roads and pavements can also contribute significantly to cooling load.
A significant amount of heat is also produced by appliances, electric lighting and occupants, which during the overheating season can lead to uncomfortably high temperatures. The use of Natural Daylight to replace artificial light where appropriate and the use of high efficiency artificial lighting can reduce cooling costs drastically, especially in commercial buildings. Heat producing appliances should be placed such that the heat can be quickly removed from the building to reduce cooling load.
Control of heat gain
Solar control involves the prevention of unwanted solar heat gain taking into consideration the following factors in design: