There are many reasons for an older glazed unit to mist up and “break down”, the main ones being time and environment. Over time the sun, wind and everyday conditions force the sealed units to constantly expand and contract slowing sucking in and then releasing moisture laden air from within the glass panes. Eventually the desiccant that is inside the spacer bar becomes saturated, to a point that it can’t draw any more moisture from the air inside the unit, resulting in condensation between the panes.
Other reasons are that the sealed unit may have been badly manufactured, older types of materials used or perhaps just the way the sealed unit was fitted when the windows were originally installed. Eventually (usually over many years) this is a problem that will face all property owners who have installed double glazing.
If this has happened to your double glazed units, we will come and measure and fit new ones - even leaded and Georgian bar units!