A double glazed window is one which has two layers of glass and a layer of inert gas is sealed between these two layers of glass.
This is the reason why double glazed windows gives almost double the insulation as compared to the single glazed windows.
The double glazed windows have many advantages. It helps you in making a lot of cost savings in energy. The double glazed windows provide thermal insulation because of which the flow of incoming and outgoing heat is reduced. Hence, less energy is required to heat or cool down the place.
There are other advantages, like it provides sound insulation, safety, reduced damage to the furniture, etc.
However, there are also a few disadvantages associated with the double glazed windows. A major disadvantage is that they cannot be repaired. The space between the two layers of glass traps air and this forms a layer of insulation. If the seal of the window is not air tight (because of some crack or a pinhole) then condensation will appear between the panes of the window.
This happens because the moisture gets into the window through the crack and is getting condensed in your double glazed window. Once they are sealed the panes cannot be pulled apart and repaired. This is the time when the window needs to be replaced. The heat trapping characteristic of the double glazed window is very beneficial during winters, but during summers this leads to a stuffy and uncomfortable room. A solution to this problem is tinting the window panes to block the heat but this will add to the cost of installation of double glazed windows.
When you face such a problem with misted or fogged double glazed windows you must contact a good local glazier who can suggest what exactly your requirement is. You must understand that there is no possibility of repairing your misted window and you will have to get it replaced. You can also replace the unit yourself if you know how to handle it. Just make sure that the unit that you are fixing in should be of the exact size including the thickness of the unit.
You can also use this as an opportunity to upgrade your double glazed window. If the room gets a lot of sun light, then you can use the solar control units which will help you in keeping the room cooler.
All about double glazed windows & doors here: www.doubleglazing-pro.co.uk