Windows, glass and mirrors

Cleaning glass is the household task most people find frustrating, and the frustration is nearly always about residue rather than dirt.

Why streaks happen

Three causes.

Too much product, which leaves a film that dries as streaks.

Residue from previous cleaning, particularly from products containing soap or additives, which builds up.

Drying too fast, which happens in direct sun or on warm glass and leaves marks before the surface can be wiped.

Which means the solutions are less product, an occasional deep clean to remove build-up, and not cleaning windows in direct sunlight.

The method

What professional window cleaners actually do.

A little detergent in water, which is all that is required for most glass.

A few drops of washing-up liquid in a bucket, not more.

Wash the glass with a cloth or an applicator, covering the whole surface.

Squeegee it off in overlapping strokes, wiping the blade between each.

The squeegee is the tool that produces a streak-free result, and it is what distinguishes professional from domestic window cleaning.

Dry the edges with a cloth.

For anyone unwilling to use a squeegee, the alternative is a microfibre cloth to wash and a second dry one to buff, which works reasonably well on small panes.

The materials

What works and what does not.

Microfibre cleans glass effectively with water alone in many cases, which is the simplest approach for interior glass and mirrors.

Newspaper, which is traditional and works because of the paper rather than the ink, and which modern printing has made less effective.

Vinegar solution, which cuts through residue and is useful for a deep clean, though it should not be used on glass adjacent to natural stone.

Proprietary glass cleaners, which work and which are frequently used in excessive quantity, producing the streaking they are meant to prevent.

Paper towel, which leaves lint.

The specific situations

Hard water spots on shower screens and exterior glass are limescale, which requires acid rather than detergent.

A vinegar or citric acid solution left in contact for a period, then washed off.

Severe long-standing deposits may have etched the glass permanently, which no product removes.

Greasy interior glass, particularly in kitchens, needs a degreasing wash before the final clean.

Exterior glass benefits from being rinsed first to remove loose dirt.

Mirrors, where spraying directly can allow liquid to run into the edges and damage the backing.

Applying to the cloth rather than the surface avoids this.

Screens — televisions, monitors, phones — which require a dry or barely damp microfibre cloth and nothing containing alcohol or ammonia, both of which damage coatings.

Frames and sills

The parts that determine how clean a window looks.

Clean these before the glass, since dirt runs down.

Drainage holes at the bottom of frames should be clear, since blocked ones cause water to accumulate and eventually to leak inside.

Seals and gaskets should be checked and cleaned, since deteriorated seals cause draughts and condensation between panes.

Hinges and locks benefit from occasional lubrication.

Condensation between panes

Worth distinguishing from ordinary condensation.

Misting between the panes of a double-glazed unit indicates that the seal has failed and the unit has lost its inert gas fill.

This cannot be cleaned, and the affected unit requires replacement — the whole frame does not.

It is also a sign of reduced insulation performance, which makes replacement worthwhile rather than cosmetic.

Working safely

The practical caution.

Falls from height in domestic settings are a substantial source of serious injury, and window cleaning is a common cause.

Extension poles with attachments reach most first-floor windows from the ground and are considerably safer than a ladder.

Where a ladder is used, it should be secured, at the correct angle, on firm level ground, with someone else present.

And anything beyond the first floor is worth paying someone for.

Condensation on the inside

A frequent complaint that cleaning does not solve.

Water condenses on glass because the glass is the coldest surface in the room and the air holds more moisture than that surface temperature can sustain.

Which makes it a humidity and ventilation problem rather than a window problem, and it is covered at more length elsewhere on this site.

The short version: reduce moisture production, ventilate to remove it, and improve the surface temperature of the glass where possible.

Wiping it up matters too, since water pooling on sills causes rot and mould within weeks.

Condensation between the panes of a sealed unit is a different matter entirely and means the unit has failed, which cleaning cannot address.