Fogging windows are caused by seal failures, which allow air and moisture to seep in between the panes of glass, where the moisture condenses. Fogging doesn’t affect the structural integrity or the function of the window, but When I do an inspection, I will note if windows are fogged.
Generally, fixing fogged windows means replacing the entire sash, since the windows have factory seals on them. If the windows are new, this should be covered by the warranty. However, on older windows, the replacement can be expensive.
As part of my Continuing Education as an ASHI member, I attended a seminar on a new service to repair fogged windows, which would actually fix the existing window, rather than replacing the sash.
The process involves drilling a small hole in the interior glass and cleaning out the air space between the panes. Tiny vents are then installed which allow moisture to escape the gap without affecting the thermal qualities of the window.
Because the process involves drilling a hole, this method cannot be used on tempered (safety) glass, such as sliding doors, because this would shatter the glass.
If you are planning on selling your home, fogged windows can be seen as a defect by a potential buyer, as well as being unattractive. Clients have asked me how they can fix fogged windows, and until now I have only been able to recommend replacement.
For more information, check out their website at http://www.theglassguru.com.
(Note: I am not affiliated with The Glass Guru in any way, and this post does not imply endorsement of their services.)