Condensation or fog trapped between two panes usually means the seal on an insulated glass unit has failed. Unlike room-side condensation, it does not clear permanently with ventilation because moisture has entered the sealed space between the panes.
Confirm which surface the moisture is on
Wipe the room-side and exterior glass. If the fog remains visible and cannot be touched from either side, it is inside the glass unit rather than ordinary indoor condensation.
A failed seal can reduce insulating performance
Moisture entry shows that the cavity is no longer sealed as designed. The window may still open and close normally, but loss of insulating gas and persistent fog can reduce performance and visibility.
Cleaning the glass surface will not repair the unit
Anti-fog cleaners or dehumidifiers can help with surface condensation, but they do not restore a failed seal between panes. Drilling or venting the cavity may remove fog temporarily without restoring the original insulating performance.
Consider glass-unit replacement if the frame is sound
Many windows allow replacement of the insulated glass unit without replacing the entire frame. A glazier or window professional can confirm dimensions, glass type, and whether the frame also has damage or leakage.