Can foggy windows be cleaned?

Start by working out which face of the fogged pane the fog is on, because the window makers' own condensation section turns the whole answer on that. The PGT, CGI and WinDoor Care and Maintenance Instructions open it with a plain rule: "Water or frost on windows is condensation. It is formed when warm moist air comes in contact with cooler dry air." They go on to say that "the inside or outside of your window can sweat or fog because of temperature differentials."

An insulated window adds a third place for moisture to be: the sealed space between its two panes. Fog that clears when a hand is drawn across the glass from outside the house is on the outside face. Fog that clears when it is wiped from inside the room is on the room side. Fog that neither wipe touches is between the panes.

• Outside face: condensation from the outdoor air. It lifts by itself as the glass warms, and the fog itself is not dirt.
• Room side: moisture carried by the air inside the house, which makes it a question about the rooms behind the glass.
• Between the panes: moisture inside a sealed insulated unit, which the window makers treat as a question about the seal.

The same three places exist on every pane of a house, from a sliding door to a transom window over a door.

Fog on the outside face of a window is the same thing as dew on a lawn. The National Weather Service defines dew as "Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature." A pane is one of those objects. When its outer face sits cooler than the dew point of the air around it, moisture from that air settles on the glass as fog.

The weather service's glossary describes dew point as a measure of atmospheric moisture, and adds that "A higher dew point indicates more moisture present in the air."

Condensation, in the same glossary, is the process by which a vapor becomes a liquid or solid, "the opposite of evaporation." That is why outside fog reverses on its own. As the day warms the glass back above the dew point, the water evaporates. The fog was water, not dirt, so a cleaning neither causes it nor cures it, and wiping it away removes only water the air would have taken back.

Whether the heavy air of the Gulf coast also explains why a house's exterior surfaces get dirty is a different argument, taken up at whether Naples humidity is why exteriors get dirty.

Fog on the room side follows the same physics, with the house supplying the moisture. The care instructions put the cause plainly: "The moisture in the air causes condensation." They also make a seasonal point about the building itself: "During the hot humid summer, your house absorbs moisture."

The same section heads off the suspicion most people start with. In the makers' words: "Faulty windows do not cause condensation. Glass is usually the first place you notice condensation because glass surfaces have the lowest temperature of any surface in a house." That is PGT, CGI and WinDoor speaking about their own windows and the rooms they sit in. Read plainly, a fogged room-side pane is telling you about the air indoors before it tells you anything about the window.

The room side of the glass, and the humidity of the rooms behind it, sit outside an exterior cleaner's work. Window cleaning at 239 Exteriors is exterior window cleaning, and the line where that kind of job stops is drawn in what an exterior cleaner is not.

Fog between the panes is the one case that involves the window itself, and it is the one case no cleaning of either face can reach. The care instructions are direct about it: "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 name for what that sentence describes is seal failure. Why a sealed unit clouds on the inside is its own subject, set out at why windows go cloudy between the panes. Whether a particular unit has failed, and what replaces it, is for the window maker or a glazier to say, not for anyone judging from the outside of the glass.

So the question in the title has three answers, one for each face. Outside fog needs no cleaning and clears with warmth. Room-side fog is about the moisture in the house. Fog between the panes is about the seal. What an exterior window cleaning removes is something else again: the dirt and buildup on the outside of the glass, which the National Glass Association says should be cleaned "as dirt and residue appear."