Condensation on a double glazed window means one of three quite different things depending on which surface it is on, and only one of them is a fault with the window. Working out which you have takes a few seconds and saves buying the wrong remedy, because two of the three are not solved by new glass at all.
Condensation between window panes: the unit has failed
If you cannot wipe the moisture off from either side, it is inside the sealed unit and the edge seal has failed. Moist air is now getting in, condensing, and often leaving a permanent mineral haze on the inner faces. This is the one genuine window fault of the three. It does not repair: drilling and venting units is sold occasionally and it does not restore the gas fill, the coating or the appearance. The fix is a new sealed unit, which usually goes into the existing frame.
Double glazing condensation inside the room: the house, not the window
Moisture wiped off the room side is normal physics. Warm damp air meets the coldest surface in the room and condenses there, and the window is almost always that surface. It means there is more moisture in the air than the ventilation is removing, and it often gets worse after new windows are fitted because the house has been made more airtight. Trickle vents, extraction in the kitchen and bathroom, and drying washing outside do more than replacing anything.
On the outside: the window is working
Condensation on the external face on a clear cold morning means the outer pane is staying cold because very little heat is escaping through the window. It is a sign of good thermal performance, it is most common on new energy efficient units, and it clears as the sun comes round. Nothing needs doing.
What to do about cloudy windows and a failed unit
Measure the affected units, check whether the frames and hardware are sound, and get prices for replacement glass rather than replacement windows. Where the windows were recently installed the unit may be under the manufacturer's or the installer's guarantee, so find the paperwork and the installation certificate before paying for anything. Where several units across the house have gone at once, they were probably all fitted at the same time and will keep going.
Questions people ask about condensation in double glazed windows
Is foggy window repair possible on misted double glazing?
Not properly. Drilling and venting is offered by some firms and it does not restore the gas fill or the coating, and the haze usually returns. Replacing the sealed unit in the existing frame is the real fix.
Do I have to replace the whole window?
Usually not. If the frame, hinges and locks are sound, only the glass unit needs replacing, which is far cheaper.
Why did condensation get worse after new windows?
New windows are more airtight than what they replaced, so moisture that used to leak out now stays in. That is a ventilation question rather than a glazing fault: trickle vents and extraction are what fix it.