Do you have insul pane windows that fog up between the glass panes when the sun comes up in the morning?
Insulated glass that fogs up is an indication of glass failure. Over the years energy efficient glass has been perfected to compensate for nature's elements such as hot and cold temperatures and direct sun and wind. Insul glass is made up of two pieces of glass sealed together with 0% relative humidity in between giving clear vision with no fog during the cold weather. However, with aging some insulated glass will fail at the seal because of severe sun exposure. For example on a south elevation, continuous expansion and contraction takes place fracturing the seal and allowing moist air to get between the glass panes. During direct morning sunshine this moisture shows on the outside glass panel leaving a foggy film which is impossible to clean, therefore, replacement glass is the only answer when this happens.
Article courtesy of: Shell Busey