Should windows be cleaned before listing photos?

Three marks show on exterior glass in a picture, and each one traces back to a step in how the glass was, or was not, cleaned.

• Residue. Dirt and film that were never taken off. The National Glass Association's routine maintenance rule is that glass “should be thoroughly cleaned as dirt and residue appear,” and its do-not list says not to “allow dirt and residue to remain on glass for an extended period of time.”
• Spots. What water leaves behind when it evaporates carrying dissolved solids. NGA's pure water paragraph starts from that fact: “Rinse water is generally allowed to evaporate from freshly cleaned surfaces.”
• Streaks. The same deposits drawn into lines, or cleaning solution that dried on the glass before it came off. NGA names spotting and streaking together as the marks its water specification exists to prevent.

A camera does not add those marks, and it does not hide them. It records what is on the pane when the shutter opens, which is why the useful question before listing photos is less whether to clean than how the cleaning finishes. The camera point rests on NGA's account of what drying water and lingering residue leave on glass, not on any photography source.

Water with nothing left to deposit

Where the rinse is left to evaporate, NGA sets a number for the water itself: “the pure water used in both the wash and rinse must have a total dissolved solids content (TDS) of 20 parts per million (PPM) or less to prevent spotting and streaking of cleansed surfaces. The use of tap water is not acceptable.”

The specification, and the pole the trade uses to deliver that water, are covered under pure water and the water-fed pole. Why pure water dries clear, in more depth, is its own answer: why pure water dries without leaving spots.

No tinted or coated glass in direct sun

NGA's do-not list includes “DO NOT clean tinted or coated glass in direct sunlight,” and the PGT, CGI and WinDoor care guide says to clean glass “during cooler temperatures without direct sunlight.” Those are conditions of the method rather than a clock, and a cleaning that ignores them is one of the ways marks end up in a listing picture. The traditional soak, clean and squeegee procedure NGA publishes is set out under the published window cleaning method.

The sun condition applies to tinted and coated glass, so the first question is which panes are which, and a pane does not always say. NGA publishes a pencil test for an exposed reflective coating, explained under how the industry finds out where a coating sits. Whether a particular window carries a tint or a coating is a question for the glass or window maker.

A window maker's care guide also limits the tools. PGT, CGI and WinDoor list “A power washer or garden hose” among improper tools that “may void your warranty and/or damage your products.” That is those makers' own line for their own products, and on their windows it puts a quick hose-down before a photographer arrives on the improper list; the hose question in general is taken up in cleaning windows with a garden hose. What a given window's warranty allows is in that maker's care guide.

How far ahead of a photographer to clean is a separate question with its own answer, in how soon before an event windows should be cleaned. The mechanism on this page says only that what is on the glass on the day is what the picture shows. NGA's reason for keeping up with glass is broader than any single picture: glass products “can be permanently damaged if infrequently or improperly cleaned.”

Some marks are past routine cleaning altogether, and a picture taken after a cleaning will still show them; those are taken up in spots regular cleaning does not remove. A house is also more than its glass, and whether the roof, the pavers and the windows together are worth cleaning ahead of a sale is weighed in whether exterior cleaning is worth it before selling, and why the roof reads differently in the same pictures is in why a clean roof photographs differently.

239 Exteriors' window service is exterior window cleaning, which removes dirt and buildup for a clearer view and a brighter property exterior.