There are a few ways to defog your windows but before you go ahead there are some considerations that you should bear in mind. The fog that appears in a double glazed window is caused by a build-up of moisture between the panes of glass. As this moisture goes through daily heating and cooling cycles it accumulates on the internal surface. If left untreated over a period of time the moisture can cause permanent glass damage. To prolong the life of your window it is possible to have a professional company defog the glass for you. However, defogging is not a miracle solution.
Having your window defogged is a short term, lower-cost solution to replacing the whole thing. The reason that moisture is building up inside the window is that the spacer bar in between the panes of glass is saturated with moisture. By drilling a small hole in the glass and inserting a valve it is possible to allow future moisture to be expelled and prevent fogging. Eventually, however, the window will reach the end of its useful life and have to be replaced. The defogging process simply delays that for a little while.
As mentioned above, having your windows defogged is cheaper than replacements. However, it's still not cheap and depends on the size of your windows. Estimates place the defogging cost at between a half and a third of the cost of replacing them. If you have a number of large windows this will still be quite expensive to do. Check any alternative solutions such as replacing only the glazing before committing to the defogging process.
When you hire a professional company, check to see what length of warranty they will give for the work and also if they will return to fix any streaks that appear in the weeks after the window valve has been installed. It is common for the window to fog up periodically during that time as the spacer bar dries out.
Not a Miracle
If a window has had a build-up of moisture for quite some time then it is possible that the glass has been permanently damaged and the defogging process will not be able to recover it. When getting a quote be honest about the length of time that the problem has existed so that you don't waste your money on a pane of glass that simply needs to be replaced.
Argon and Krypton Gas
In new windows, argon and krypton gases are used to increase the insulation properties of the window. The defogging process will not replace these chemicals and even if it did they would be expelled through the valve over time. The restored window will therefore not be as efficient at insulating as a brand new glazing unit. However, if your window was fogging up then any original gas is no longer present and not providing any insulation anyway.
Due to the properties of tempered glass, it is not possible to drill the holes in the glass that are necessary for the defogging process. Windows that may have tempered glass can be found in conservatories/solariums or where a window sill is at a low level. Normally there is a white mark in the corner of the glass to make them easily identifiable.