Explanation of the phenomenon of fogging up of external panes in glazing units

Fogging up of the outer surfaces of insulating glass units is a physical phenomenon and is not subject to complaint, unlike the fogging up in the space between the panes caused by unsealing of the glass.

This phenomenon consists in the condensation of water vapour on the surface of the glass when the temperature of the glass surface is lower than or equal to the dew point temperature for a given relative air humidity. It may occur more frequently in the case of low-emission ("warm") glass due to the fact that less thermal energy is radiated through this type of glass (this is due to the low-emission coating on the inner glass), and thus the surface temperature of the glass exterior is usually lower than in insulating glass without a low-E coating. Additionally, this phenomenon may be influenced by the immediate surroundings of the place where the window is installed. For example, a tree growing in front of the window makes it difficult to dry the glass from water vapour by the wind. A wide window sill covering the window from the bottom can also cause fogging up, as well as non-standard flashings around the window. In old-style wooden windows, which were often leaky, the warm air escaping from the rooms dried and heated the glass surfaces, preventing this phenomenon from occurring. The same applies to modern panes with "warm" panes when they are tilted – otherwise they cannot be dried other than by external forces (e.g. wind).

Unfortunately, the phenomenon itself is inevitable and occurs especially frequently in autumn and spring, when significant temperature differences between day and night and increased air humidity occur, and as such is not subject to complaints.