Glazing seal failure
Glazing seal failure is the breakdown of the sealed edge of an insulated glass unit, letting moisture and air into the space between the panes. The most visible sign is fogging or condensation trapped between the glass layers.
Glazing seal failure happens when the perimeter seal of an insulated glass unit (IGU) stops keeping the air space between two or more panes closed off from outside air and moisture. Most skylight glazing is built this way: two panes held apart by a spacer, with the gap filled with air or argon and the edges sealed to hold that gas in. When the seal fails, humid air gets in and the inside faces of the glass fog up or collect droplets.