Is exterior window cleaning enough, or does the inside need cleaning too?

Start with what the service touches, because the question turns on it.

239 Exteriors describes its window service in its own words: exterior window cleaning removes dirt and buildup for a clearer view and brighter property exterior. The page on what each surface covers lists it as exterior glass. NGA's routine-maintenance rule does not single out one face: "For routine maintenance, interior and exterior glass surfaces should be thoroughly cleaned as dirt and residue appear." A pane has two faces that collect dirt, and exterior cleaning is the name for the outside one.

A hazy pane can mean more than one thing, and the causes sit in different places.

Dirt and residue on the room side of the glass are on a face an outside cleaning does not touch, and NGA's rule names that interior surface alongside the exterior one. An applied film is another possibility. NGA notes "There may also be exposed films applied to the interior glass surface," naming security and spall films as examples, so some glass carries a layer on its room side that no exterior pass reaches. Which cleaners that film tolerates is a question NGA sends to its supplier.

Some haze is not dirt at all.

PGT, CGI and WinDoor's care guide says condensation "is formed when warm moist air comes in contact with cooler dry air," and, comparing it to a fogged mirror, that "the inside or outside of your window can sweat or fog because of temperature differentials." That is the dew point at work on glass, and whether a fogged pane is something a cleaning addresses is the subject of whether foggy windows can be cleaned.

One more location is possible, and no cleaning pass on either face reaches it.

Cardinal Glass defines an insulating glass unit as "two or more panes of glass separated by a spacer and sealed around the edges to create an insulated airspace." For fog inside that airspace, PGT, CGI and WinDoor's guide says of an insulated window that "the airtight seal has probably been broken, and the glass will need to be replaced." That is the window maker's line about its own units, not a diagnosis of any particular window. Why a unit turns cloudy between the panes is answered on its own page, seal failure is defined in the glossary, and whether cleaning has anything to do with it is taken up on whether window cleaning can hurt the seals.

Put together, the question sorts by location.

• On the outside face: dirt and buildup, the surface exterior window cleaning is named for.
• On the inside face: dirt and residue that NGA's routine-maintenance rule covers just as it covers the outside, on a face an exterior cleaning does not reach.
• In a film on the room side: a layer NGA says may be applied to the interior surface, whose cleaners are its supplier's to name.
• As condensation on either face: moisture from temperature differences, which the window maker's guide says faulty windows do not cause.
• Between the panes of a sealed unit: on neither face, and in the window maker's words a sign the airtight seal has probably been broken.