Why double-paned windows get hazy or collect condensation inside them..

So you had your windows cleaned but they still look dirty. Then you realize what you see is actually between the panes and it isn't possible to even get to it. If multiple-pane windows appear misty or foggy, it means that the seal protecting the window assembly has failed.

Double-pane windows have a layer of gas (usually argon or air) trapped between two panes of glass that acts as insulation to reduce heat loss through the window. Other types of gas used in this space have various effects on heat gain or loss through the window. Some windows also have a thin film installed between panes that separates the space between the panes into two spaces, further reducing heat loss and heat gain through the window.

Although double-paned windows appear to be stable, they actually experience a daily cycle of expansion and contraction caused by thermal pumping. Sunlight heats the airspace between the panes and causes the gas there to heat up and expand, pressurizing the space between the panes. At night, the window cools and the space between the panes contracts. This motion acts like the bellows of a forge and is called thermal pumping. 
 
Over time, the constant pressure fluctuations caused by thermal pumping will stress the seal. Eventually, the seal will develop small fractures that will slowly grow in size, allowing increasing amounts of infiltration and exfiltration of air from the space between the panes.

According to industry experts, the glazing assembly can be replaced  approximately 75% of the time.  Occasionally, the sashes must be replaced, and only about 5% of those cases require that the entire window be replaced.

In summary, condensation in double-paned windows indicates that the glazing assembly has failed and needs repair or replacement. Visible condensation can damage glazing and is the main indication of sealant failure.

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