Why does Condensation form on windows? 

All air contains some water vapour, but warm air can hold more vapour than cold air. When warm, damp air is cooled it will reach a temperature at which is cannot hold all the vapour, and the water will condense out. This temperature is called the dew point. 
 
Warm damp air passing over a cold surface may be cooled locally below the dew point in which case condensation will take place. This effect causes the familiar condensation on the inside of cold windows. 
 
Modern Secondary Glazing will reduce condensation by up to 95% as you can see in the picture. 
 
We took this picture half way through a large installation of a property in Braintree Essex. We installed the right hand window and on returning the next morning to finish the bedroom we had the perfect opportunity to take this picture to show first hand how much Secondary Glazing reduces condensation. 
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