MUMBAI, March 15: Windows are possibly the most interesting elements in residential design. They provide light and fresh air, and offer views to connect the interior spaces with the outdoors. On the other hand, windows also cause unwanted heat gain in summer and significant heat loss in winter, resulting in significant expenditure in terms of energy costs. Fortunately, in recent years, windows have seen remarkable technological changes which are effective in reducing the entry of hot summer sun and preventing the loss of warmth in winter.A study carried out by the Indian Institute of Technology, Delhi, of the climatic zones and requirements across the country throws new light on the importance of windows in terms of saving costs and conserving energy. Says Dr N K Bansal, professor and head, Centre for Energy Studies, IIT Delhi, ``Our research has revealed that it is now possible to have expansive views and daylight without sacrificing comfort or energy efficiency.
Any house can be made considerably moreenergy efficient by using high performance windows. Also, technologically advanced windows perform so much better and differently than old-time windows that many of the assumptions of both traditional and more recent energy efficient design must be reexamined.''
The IIT study took climatic data of Leh, Shillong, Delhi, Jodpur, Chennai and Bangalore. The atmospheric dynamics and climatic profiles of these cities were thought to be representative of six types of climatic variations in the country. The research focussed on reducing the heat transfer from one side of the window to the other. Adds Bansal, ``In most cases, window buyers base most of their decision on appearance factors such as frame style and material. But a window is much more than just a piece of glass. In fact, if people have a very parochial view about windows, it will be a huge drain of resources and energy.''
The IIT study took into consideration parameters such as resistance to condensation, sound control, maintenance requirements,durability, glare control, thermal comfort and ventilation, keeping in view the data from the six cities.
What emerged from the research was a need for double glazed windows which have a very fine air gap. Explains Bansal, ``With the use of low-emittance coating, heat transfer across a gap is dominated by conduction and natural convection. Air being a good insulator prevents much of the heat from entering a room in summers and at the same time checks heat loss from a room in winters.''
At present, the double glazed windows being sold by Sintex Industries Ltd and Uniplast, following the IIT research, have air in the gaps. While air is a relatively good insulator, other gases such as argon, krypton and carbon dioxide with lower thermal conductivity are also suitable. Using one of these non-toxic gases in an insulating glass unit can reduce the heat transfer between the glazing layers. The energy efficient windows were tested by the National Physical Laboratory in Delhi.
Bansal stresses that these windowswill be particularly helpful in air conditioned houses and can check energy losses up to 30 per cent. Giving an example, he explains: ``For a one tonne air-conditioner in a 10'x10' room, the energy consumption is 35 kwh/day. This will cost about Rs 100 per day with an approximate monthly bill of Rs 3,000. If double glazed windows are used you can do with a 0.7 tonne air-conditioner in the same room. The energy consumption will be down to 20 kwh costing Rs 60 a day which translates to 1,800 per month. This is an overall saving of at least Rs 1,200 per month. Apart from being energy efficient, these windows also reduce unwanted noise by about 20-25 decibels.''
In an Indian household or office building, the glass area varies from 15 to 50 per cent. In fact, modern offices and houses are increasingly relying on the use of glass to enhance appearance and aesthetics. So the importance of energy saving windows will increase with time, says Bansal. The cost of these double glazed windows is Rs 800 per sq mt morethan the corresponding single glazed window.
The research showed that for Leh, Delhi, Jodhpur and Chennai type of climatic conditions, the use of double glazed windows will be extremely useful. For more temperate conditions like Shillong and Bangalore, the pay back period (return on investments) is about five years, considering an energy cost of Rs 2 per kwh. For the Assembly Building in Rajasthan, greater use of double glazing resulted in an annual saving of 314,000 units of electricity. The pay-back period in this case is 30 months. If air tight windows are used, the annual savings are 52,68,000 units of electricity with a pay-back period of only two months.
Says V M Mendirattha, branch manager, Sintex Industries Ltd, ``So far the use of glass windows in our buildings has been only from the point of view of lighting. The concept of reducing energy losses and saving money by using windows which have more resistance to flow of heat is beginning to get importance, particularly in air-conditioned buildings.The research has led to a better understanding and techno-economic viability of the double glazed windows.''
With each passing day, as the mercury soars, it will be prudent to take a new look at the windows in your air-conditioned house to cut down on energy costs.
Copyright © 1998 Indian Express Newspapers (Bombay) Ltd.
