Since we have been installing the most energy efficient low e double glazed units, customers have been ringing us with a concern, that in the morning , the replacement windows are showing signs of condensation - on the outside pane! Customers are most upset, thinking they have bought inefficient PVCu Windows that are not fit for purpose. On the contrary, this external condensation is actually a symptom of the Windows actually doing their job of keeping the heat in. This is a really positive sign and confirms the double glazing is working efficiently - so nothing to worry about ! As temperatures drop, especially during the night in Spring & Autumn, this natural phenomenon is caused by the outer pane being much colder than in the older PVCu systems and with more heat remaining inside, the result is external condensation, which, if it does occur, will only last a couple of hours in the morning, before disappearing. So if you are considering replacement windows , which these days are around 60% more energy efficient than your old plastic Windows, please be aware of this phenomenon and if you do get any condensation on your outside panes, it's all good news knowing the Windows are working to maximum efficiency!