The glass combination is not the only thing involved in a double glazed window.
Rather than allowing normal air to exist at the gap in between the two glasses, they are filled with dehydrated air or gases like xenon and argon.
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Dehydrated air is air in which water vapour or moisture has been removed. When this is the case, the air is made an insulator. The presence of water vapour in air makes it a good conductor but when this has been removed, it becomes a perfect insulator.
This helps to ensure sufficient insulation effect between the outdoor and the indoor panes of the windows and hence reducing the loss of heat between the two.
Single glazed windows are also excellent options but they cannot be matched with double glazed ones in terms of the features obtainable from them. Single glazed windows are probably most suitable in the tropics where the temperature is almost warm or moderate all through the day as they will give room to the cooling of the air.
However, double glazed windows are desirable in conditions of extreme coldness or warmness. In a condition of extreme coldness, such windows will help to ensure that the room is warm all through the day owing to its insulation effect.
The dehydrated air trapped in between the glasses is responsible for the excellent insulation effect.