Condensation is not caused by double glazing (although new windows can sometimes make a condensation problem worse, by cutting out draughts).
Condensation on the inside of double glazed windows.
Condensation happens when water vapour in the air changes to liquid water. There can be many causes but put simply it generally occurs in the atmosphere when warm air rises, as it cools down it loses its capacity to hold water vapour. Water vapour then condenses on cool surfaces to form liquid water which is why often you will see windows with condensation on them.
Treatment for condensation is ventilation (to vent moisture-bearing air to the outside) and heating (to raise surfaces above dew point temperature).
Condensation on the outside of double glazed windows.
External condensation (dew) can occasionally occur on highly insulating glass units in temperate climates. Such occurrences will only happen on cloud-free nights when there is little or no wind and usually when a warm front follows a dry spell.
With modern Low E Glass products more of the heat is kept inside and the outer pane is not heated as much. Moisture condenses out of the air onto a cold surface that is said to be below the dew point.
In short the “problem” is caused by the fact that the Low E Glass is doing its job so well and reflecting heat back into the room that there is nothing to worry about if you see external condensation on your new double glazed windows.
Condensation between both glazed panels (inside the sealed unit).
If you find condensation/moisture inside your glass, then the sealed unit itself has broken down and has started pulling in moisture.
This may be because the unit is old (over time it has broken down) and will need to be replaced or if the sealed unit is new, then the seal around the unit was just not sealed off properly in the factory and has started pulling in moisture.