Condensation is a common problem on windows, even highly energy efficient windows. This occurs more often in the winter but really any time the temperature of the glass reaches or goes below the temperature of the dew point of the air inside the house.
The dew point is the temperature at which the air will no longer hold the moisture that is in it. Generally the more dry the air the lower the temperature of the dew point which is why condensation on windows actually tends to be more prevalent in newer houses or in houses with new highly energy efficient replacement windows.
Older houses tend to be more drafty and as a result, the mixing of the dry air that seeps in from outside with the more humid inside air keeps the overall inside air dry enough to prevent condensation.
Over the years houses have become better sealed to keep the warm air in and the cold air out in the winter. While this more efficient sealing of houses lowers the cost of heating it will also tend to keep the moisture in the house when the house is not ventilated properly or contains a lot of moisture producers.
The condensation then occurs because even on a highly energy efficient window the temperature of the inside pane of glass will be somewhat cooler than the air inside the house being heated by the furnace.
When the wet air inside the house creates a high dew point temperature and comes in contact with the cooler glass on the window the moisture in the air is drawn out and forms condensation on the glass.
The solution to this problem is to reduce the moisture contained within the inside air so that the dew point is lower than the temperature of the inside pane of glass.
The moisture in the house can be reduced by using exhaust fans in your bathrooms and kitchens and venting clothes dryers or gas burners to the outside which all expel a substantial amount of water vapor.
It is also a good idea to make sure that the outside sensors on your furnace humidifier are set up properly so that the humidifier is not putting too much water in the air.
