Windows installed within the last 20 years have double pane insulated glass units, also know as Thermopane windows, double pane windows, or IGU's (insulated glass units).
IGU window technology offered a huge improvement in insulating abilities compared to single pane windows, or storm windows. The chamber between the panes is typically filled with an inert gas called argon or krypton to further improve the window efficiency and prevent heat loss and heat gain.
Over the life of the IGU the seal deteriorates, and after many years (typically 15-25 years) the rubber breaks down to the point that it allows the argon gas to escape and ambient air to enter. After a dramatic change in temperature the window may begin to show signs of condensation between the panes. For example, after a warm day if there is a dramatic cooling the warm moist air in the window condenses and sticks to the inside surfaces of the glass. After repeated occurrences of condensation forming minerals begin to bond to the glass and leave shadows, even when no moisture is present.
The seal may fail well before condensation appears, but the air is dehumidified with desiccants engineered into the window when it was manufactured. These desiccants only work for a period of time before they lose effectiveness and the window is diagnosable as having a failed seal.
The three primary causes of a failed IGU:
The solution to correcting a foggy window is to replace the IGU. The window frame usually does not need replacement, just the IGU can be removed and replaced.