Seal failure

Cardinal Glass Industries starts with the sealant: “As a sealant, PIB acts as a primary barrier, preventing moisture ingress and retains the gas fill, ensuring the unit's longevity and performance.” It then names what that barrier is for: “PIB is renowned for its exceptional resistance to moisture vapor transmission. This ensures that the space between the panes in an IGU remains dry, preventing fogging or condensation inside the unit.”

The unit around that sealant, in Cardinal's glossary, is “two or more panes of glass separated by a spacer and sealed around the edges to create an insulated airspace.” Cardinal adds that the spacer contains desiccants, which help to prevent moisture buildup between the panes and are “specifically designed to maintain a moisture-free environment between the glass panes.” Those are Cardinal's descriptions, written about insulating glass and its own units; how a particular window was built is its maker's to say.

PGT, CGI and WinDoor put the consequence in one sentence of their care instructions: “If you see condensation between the two layers of glass in an insulated window, the airtight seal has probably been broken, and the glass will need to be replaced.” The word probably is the maker's, and it matters, because fog inside a unit is not always a seal failure.

The reasoning follows from Cardinal's description. Once a seal lets moisture into an airspace the sealant and desiccant were built to keep dry, the fog is inside the unit, where no exterior cleaning reaches. Cardinal also names a separate cause, chemical fogging, produced within the unit by its own materials; how the two differ is taken up at why windows are cloudy between the panes.

Diagnosing a particular unit and deciding whether its glass is replaced belong to the window's maker or a glazier. Whether fog is a cleaning problem at all is answered at whether foggy windows can be cleaned. The National Glass Association's caution points the other way: glazing materials, it writes, are not a sufficient barrier to keep cleaning agents out of the glazing pocket and away from the insulating glass unit seal. That is a risk from improper cleaning materials, not something a cleaning does for a seal, and it is covered at what the frame does not stop.

The term does not change with where the unit sits, whether in a door, a window at eye level or a clerestory high on a wall. Fog on the outside face of a pane is a different event: the window makers say the inside or outside of a window can sweat or fog because of temperature differentials, and the moisture measure behind that is the dew point.