Why are my windows cloudy between the panes?

Before a cause makes sense, the object itself has to be clear. Cardinal Glass defines insulating glass as "two or more panes of glass separated by a spacer and sealed around the edges to create an insulated airspace," usually filled with argon or plain air. That airspace is not incidental; Cardinal says it "improves thermal performance, reduces heat transfer, and minimizes noise infiltration."

Cardinal says the spacer contains desiccants, which help prevent moisture buildup between the panes. Cardinal writes that "desiccants are vital components within insulating glass (IG) units, specifically designed to maintain a moisture-free environment between the glass panes." The whole assembly is engineered to stay dry on its own, which is what makes cloudiness inside it worth explaining rather than just wiping.

Cardinal names the sealant that carries that load: "PIB acts as a primary barrier, preventing moisture ingress and retains the gas fill, ensuring the unit's longevity and performance." Its resistance to moisture is the whole point: "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." Everything about the anatomy above depends on that seal holding.

PGT, CGI and WinDoor's own care guide states the consequence plainly for their insulated windows: "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." Condensation itself is defined the same way regardless of where it happens: the National Weather Service calls it "the physical process by which a vapor becomes a liquid or solid; the opposite of evaporation," the same process that forms dew outside once a surface cools past the dew point.

Put the anatomy and the maker's line together and the mechanism is straightforward: a broken edge seal lets moisture into the airspace the sealant and desiccant were built to keep dry, and that moisture condenses inside the unit. What replaces a failed unit, and when, is the window maker's or a glazier's call, not a cleaning question.

Cardinal names a different mechanism entirely: chemical fogging, which Cardinal describes as occurring "when materials used in insulating glass, such as sealants, grilles, spacer systems, desiccant, and more, produce a fog within the unit." That fog is produced by the unit's own components inside the sealed cavity, not by an outside breach. Cardinal is direct about the result: it "can obstruct visibility and detract from the aesthetic appeal of the glass."

A seal failure and chemical fogging both put haze inside the same sealed airspace, but they start from opposite ends: one is moisture getting in from outside, the other is the unit's own materials acting up from inside. This page does not merge the two, and telling them apart on a given window is, again, the maker's or a glazier's job.

Cardinal states that its regular Low-E products "are applied to the #2 or #3 surface in a dual pane," inside the sealed cavity, while NGA warns that glass can carry Low-E or reflective coatings "some of which may be on the exposed surface of the glass." Where a coating sits is a separate question from cloudiness, taken up under how the industry finds out where a coating sits.

Whether a particular unit has actually failed, and what replaces it, is decided by the window maker, a glazier or a window contractor, not from a description.

• Whether fog specifically on the outside face of a pane is a cleaning problem at all is answered on can foggy windows be cleaned.
• Whether cleaning chemistry itself can hurt a window's edge seal is answered on can window cleaning hurt the window seals.
• What seal failure means as a term, on its own, is defined in the glossary.
• What glazing materials do and do not stop from reaching that same edge is covered on what the frame does not stop.
• Cloudiness inside a sealed unit is out of reach from either face of the glass; what an exterior cleaning does cover is taken up on whether exterior window cleaning is enough.