When the warm air of the house meets a cold surface - water vapour in the warm air condenses out onto the cold surface - you windows are not sweating they're showing condensation - there may also be condensation on some walls that you can't see - the moisture actually being absorbed into the wall.
You have condensation - the solution is to keep the steam from kitchens and bathrooms under control (open windows) and to ventilate the house when you can (open windows).
Double Glazing and Central Heating often combine with poor ventilation to produce condensation. If allowed to continue unchecked black mould will begin to form - firstly on the top and corners of exterior walls and slowly spread. Also behind wardrobes in bedrooms etc.
Use the search box on this page and look for condensation.
Good luck...
Peccavi
April 2010
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