With improvements in the thermal performance of glass the phenomenon of condensation on the exterior surface of glazing has become more prevalent. External condensation occurs in particular climactic conditions with high humidity levels and/or particularly cold weather/ similar to pictures shown. It is not a defect in the glass or the windows but it does demonstrate that the item is doing the job it's supposed to - keeping heat in.
Low emissivity inner pane reflects heat back into the building preventing the outer pane from warming up. The outer pane presents a cool surface and, given the right temperature and humidity conditions, water vapour from the air will condense on it.
External condensation is more likely to occur in the spring and autumn, and it’s all down to something called the dew point.
This was not an issue with traditional double glazing when there was much more heat loss through the inner pane so that the outer pane was warmed up by this wasted energy. Today, because of energy prices, global warming, the need to comply with building regulations and reduce carbon emissions, it is no longer possible to install inefficient glazing. In Northern European countries which have much colder winters than we do, glazing with very low U-values has been used for some time and the phenomenon of external condensation is understood and accepted. it is considered much more important to conserve energy and have a warm comfortable indoor environment.
Customers are sometimes perplexed by the fact that condensation may occur on one house but not on another, on one window but not on another, indeed even on one pane but not on another. This happens because the surface temperature of the glass is affected by the degree of shading from a roof overhang, a projecting reveal or lintel, a tree, another house or by a very minor difference in orientation.
There is nothing that can be done to predict where external condensation will occur or to avoid it. It does however indicate that the glazing is very energy efficient and is saving money by conserving heat. In most cases the condensation will disappear as soon as the window is exposed to a little sunshine or breeze.
It is possible that this condition or phenomena will dissipate entirely on its own, over time, due to
normal atmospheric conditions, climatic conditions, and regular cleaning. Should the appearance of these marks be disconcerting in the interim, it may be possible to minimize and (potentially) remove the surface demarcation with a very mild abrasive cleanser like cerium oxide. If attempting this method be sure to clean the surface area of the glass first using regular methods. Then apply the cerium oxide and use a wet, clean cloth to assertively rub the surface. We also recommend that you test this method on a small area first.
Further information on this topic can be found on major glass manufacturers websites eg. or down Glass & Glazing Federation some advice & some causes.