Double-glazed sealed units are only referred to as broken down (failed) if moisture or “misting” develops between the two double-glazed glass panes. More importantly, when misting arises, it is no longer a “sealed unit”. So, as a result, it will not be fulfilling the role of a sealed double-glazed unit. The broken-down unit can no longer keep the cold out or enable heat generated by your heating system to be kept in, allowing solar gain to be held inside the home.
The seal can rupture for several reasons, including the age and condition of the window frame. On occasions, this could be due to a problem with the production of the unit. It could also be down to how the window was initially installed. Chemical cleaning materials applied on windows can also cause the seal to degrade over time.