We get a number of calls each week at this time of year regarding condensation on the outside panes of glass on our 'A' rated windows. This is not a fault with the window - quite the reverse! Because these windows transmit barely any heat from the inside of your house to the outside, the outer pane mists up when weather conditions are right.
This will quickly clear as the day warms up. A normal double glazed window looses a proportion of the house internal heat through the inner pane and airgap. This in turn will heat the outside pane and prevent condensation build up. Effectively you are heating up the outer glass with your heating. Since the rated windows contain argon gas and special spacer bars as well as planitherm total and diamonte glass, this does not happen. The insulating properties of these windows are superb.