Can windows be cleaned in the summer sun?

Look for a number and you will not find one. The National Glass Association's cleaning paper, Glass Technical Paper FB01-00, and the PGT, CGI and WinDoor Care and Maintenance Instructions both address heat and sun, and neither sets a number of degrees at which exterior glass becomes off limits.

Each attaches its advice to the sun instead. NGA's line is about the sun on certain kinds of glass. The makers' line is about the conditions that give the best results. Those two lines are what the two documents publish on it, and they point to a different question from the one people usually ask. The question is not whether it is too hot to clean windows. It is which glass is in direct sun at the moment, and what kind of glass it is.

NGA's condition is specific to the glass: "Cleaning tinted and reflective glass surfaces in direct sunlight should be avoided, as the surface temperature may be too hot for optimum cleaning." The makers' condition is general to their products and framed as a tip: "For the best results, clean your glass during cooler temperatures without direct sunlight."

The paper does not explain further what happens on a surface that is too hot. What its method makes plain is that nothing is meant to dry on the pane. "Immediately following the application of the cleaning solution, a window cleaning squeegee should be used to remove all of the cleaning solution from the glass surface," and its quick-reference guide says not to allow water or cleaning residue to remain on the glass. NGA does not connect the two, so this is reasoning rather than its statement: a hot pane dries faster, and the method is written around removing everything before it can dry into a film or streaking.

The steps of that method, from the soak to the squeegee, are set out, with its line about direct sun, in the soak-and-squeegee method NGA publishes.

NGA's direct-sun sentence is written about tinted and reflective surfaces, which makes identifying the glass the first decision. The same paper notes that glass "can be clear or tinted and have pyrolytic or sputtered Low-E or reflective coatings, some of which may be on the exposed surface of the glass." A pane that looks ordinary may not be.

Whether a pane carries a coating on its outside face is something the industry has a way of checking, set out under finding out whether a pane carries a coating. How tinted glass in particular is handled is its own question, answered at whether tinted windows can be cleaned.

For clear, uncoated glass, the makers' line is the one that applies: cooler and out of direct sun gives the best results. That is advice about outcome, not a prohibition.

The heat is real, and it has an official shape. NOAA's 1991 to 2020 climate normals for Naples Municipal Airport put the normal daily high at 89.8 F in June, 90.8 F in July, 90.9 F in August and 89.6 F in September. At the other end of the year, the same normals put January's normal high at 74.8 F, the coolest month in the table.

Every one of those figures is an average over thirty years at one weather station. None of them forecasts a particular day, and none of them is a temperature that any glass document treats as a limit. The full year sits in the twelve-month normals table for Naples.

Put the two conditions against a real house and summer turns into geography. The sun moves across a building through the day, so a wall in direct light at one hour is in shade at another. Glass on the side the sun has left, or has not reached, is out of direct sunlight, which is half of the makers' condition; the cooler temperatures half depends on the day.

• Tinted or reflective glass in direct sun: NGA says to avoid cleaning it until the sun is off that surface.
• Any glass out of direct sun: the no-direct-sunlight half of the makers' best-results condition is met, and the cooler half depends on the day.
• Glass whose type is unknown: identify it first, because NGA's sun sentence depends on what the glass is.

So summer changes when and where on a house glass is cleaned, not whether. The heat asks similar questions of other surfaces, and whether a roof can be cleaned in the summer heat answers them for the roof. Summer does not change NGA's rule that splashed materials should not be left to dry on glass, which is taken up for one familiar deposit at whether bird droppings damage glass.