Why are my windows still streaky after cleaning?
Windows are usually still streaky after cleaning because of mineral deposits left behind by tap water, a thin detergent film, or dirt being redeposited from a cloth rather than removed. Any one of these on its own is often enough to leave visible marks once the glass dries.
The streaking typically becomes visible only once the water has evaporated, which is why a window can look clean immediately after washing but show marks an hour later in sunlight.
This delayed appearance often leads homeowners to blame their technique, when the underlying cause is actually the water and cleaning product used rather than how carefully the glass was wiped.
Do mineral deposits from tap water cause streaks?
Yes, tap water contains dissolved minerals such as calcium, and these stay behind on the glass as fine spots once the water evaporates. This happens with or without detergent, since the mineral content is in the water itself rather than in whatever's added to it.
Hard-water areas see this most, where mineral levels in tap water are naturally higher. A squeegee clears the bulk of the water off the glass, but it can't remove minerals that are already dissolved within the remaining film.
These mineral spots tend to be small and scattered rather than in streaks, which is one way to tell mineral spotting apart from a detergent haze or a missed patch left by an uneven squeegee pass.
Does detergent leave a film even after rinsing?
Detergent can leave a faint surfactant film on glass even after rinsing, particularly if the rinse water itself contains minerals that combine with the remaining product. That film dries with a slight haze rather than a fully clear finish.
This film also changes how the glass behaves afterwards, since a surfactant residue attracts airborne dust more readily than bare glass. Windows cleaned with detergent can appear dirty again sooner than windows finished with plain, filtered water.
Rinsing with more tap water after detergent doesn't fully solve this either, since the rinse water introduces its own minerals even as it clears away some of the product. The two issues compound rather than cancel each other out.
How does drying speed and sunlight affect streaking?
Drying speed matters because water that evaporates quickly, such as in direct sunlight, leaves dissolved minerals behind before they can be wiped off evenly. Slower, even drying in shade generally leaves a clearer finish, since there's more time to remove excess water before it evaporates in place.
This is one reason professional cleans are often timed to avoid the hottest part of the day, particularly on south-facing glass that dries fastest.
Wind has a similar drying effect to direct sun, so an exposed window on a breezy day can streak for the same reason even without strong sunlight involved.
| Cause | Why it streaks |
|---|---|
| Tap water minerals | Dissolved calcium dries as visible spots |
| Detergent film | Surfactant residue dries with a faint haze |
| Fast drying in sunlight | Minerals settle before they can be wiped away |
| Dirty cloths | Redeposits grime onto otherwise clean glass |
Most streaking traces back to one or more of these four causes rather than a single fault, which is why switching just one habit — a different cloth, say — often only partly improves the result.
When to call a professional
A professional clean is worth calling in once streaking persists despite careful technique, since this usually points to the water itself rather than the method being used. De-mineralised water avoids mineral spotting, detergent film and uneven drying all at once, without needing a different wiping technique.
Persistent streaking despite a careful clean is usually the clearest sign that the water itself, not the wiping technique, is the actual cause.
Booking pure water window cleaning removes the guesswork around tap water and detergent entirely. Homes near Gorey looking for a lasting result often choose streak-free window cleaning in Wexford for exactly this reason.
Frequently asked questions
Why do windows streak more in direct sunlight?
Direct sunlight speeds up evaporation, so any water left on the glass dries before it can be wiped away fully. Fast drying gives dissolved minerals and detergent residue less time to be rinsed off evenly, which is why cleaning in shade or overcast conditions usually gives a cleaner finish.
Does the type of cloth used affect streaking?
Yes — a cloth that's already carrying dirt, dust or old detergent residue can redeposit that grime onto the glass as it's wiped, causing new streaks even on an otherwise well-cleaned window. Clean, lint-free cloths reduce this, though the underlying mineral and detergent issue remains unless the water itself is filtered.
Can hard water alone cause streaking without any detergent used?
Yes, hard tap water alone can leave streaks and spotting as it evaporates, since the dissolved calcium and other minerals stay behind on the glass regardless of whether detergent was used. This is common even with a careful squeegee technique in hard-water areas.
Is streaking worse on new double-glazed windows than older glass?
Streaking depends more on the water and technique used than the age or type of glazing, so new double-glazed units aren't inherently more prone to it. Any glass surface will show mineral spotting from hard tap water if it's not rinsed with filtered or de-mineralised water.