Condensation Between the Panes Means One Thing
Misting on the inside of the room is normal. Misting inside the sealed unit is not, and no amount of ventilation will fix it.
The Seal Has Gone
A double-glazed unit is two panes bonded to a spacer bar, with a desiccant inside the spacer to absorb the small amount of moisture sealed in at the factory. When the perimeter seal fails, outside air gets in, the desiccant saturates, and from then on every cold morning you get fog inside the glass where you cannot reach it.
Once that has happened the unit is finished as an insulating component. The gas fill, if it ever had one, is long gone, and the U-value is somewhere near a single pane.
You Replace the Unit, Not the Window
This is the part that saves people money. The sealed unit is a separate item from the frame. If the frame is sound, a glazier removes the beads, takes out the failed unit and fits a new one, usually inside an hour. You do not need a new window and you should be suspicious of anyone who says you do.
Get the sizes measured rather than guessed. Units are made to order and a unit that is 3mm out is scrap.
Why Units Fail Early
Standing water is the usual culprit. If the drainage slots in the bottom of the frame are blocked with debris, the unit sits in a puddle and the seal breaks down from the outside in. Clearing those slots once a year costs nothing and is the single most useful thing you can do for a glazed unit.
South-facing units also work harder, because the daily expansion and contraction cycle is more severe. That is unavoidable, but it does explain why one elevation fails and another does not.
