Do I need ventilation?
If there is not an ample level of background ventilation in the room replacement windows can have trickle vents incorporated into the frame. This lets in a small amount of controlled ventilation.
In some conditions condensation can also occur on the outside of the new double glazed window. This is because the glass reflects heat back into the home, which keeps the outside pane cool. In this scenario, condensation can build up in cold weather. This isn’t a problem.