My double glazed sealed units have steamed up why is this?

We are often called out to house owners who complain that their double glazed windows have started to steam up on the inside of the units. Upon inspection it is often the case that the units have been incorrectly fitted in the first place. The sealed unit is supposed to be centralised in the outer frame and be supported on all sides with spacers of varying thickness`. In particular the base of the sealed unit must not rest on the bottom of the window frame. There are specific bridge packers designed to lift the unit off the frame allowing any water ingress to escape through the outer drainage channels. If this is not done the sealed unit will block the drainage in the outer frame and after time the unit will break down. Properly designed packers come in boxes of 1000 and invariably if the windows have been fitted by someone other than a bona fida company, the person who originally completed the installation will have done it incorrectly.