What causes exterior condensation?

Exterior condensation occurs when moist air comes into contact with cool surfaces, such as glass.  This type of condensation appears when the dew point in the air is higher than the temperature of the glass.  This occurs when a cool night follows a warmer day, most typically during the spring and fall seasons.

How does low-emissivity (Low-E) glass affect exterior condensation?

Low-E glass reduces heat conducted through the glass from the warm interior of the home to the outside glass surface.  Heat conduction can be reduced by as much as 50 percent with an efficient Low-E coated glass.  This reflected heat energy reduces the outside glass temperature and can result in condensation on the glass.  Exterior condensation is actually an indication that the insulating glass in the window is performing as it should.