Why Windows Streak After Cleaning, and the Order That Stops It

You washed every pane on Saturday and by Sunday morning the front windows look worse than they did before you started: long smears in the low sun, a cloudy band along the bottom, a haze that only shows from one angle in the living room. The reason why windows streak is almost never dirt that got missed. It is that something was left on the glass and allowed to dry there — soap, minerals, cloth fibres, or grit-laden water that ran down off the frame after the glass was already finished.

Why do windows streak after you clean them?

A streak is residue that dried in place. Glass is smooth enough that ordinary dust has nothing to hold onto, so what shows up in raking light is a thin film of whatever was in the cleaning water — detergent, dissolved minerals, dust lifted off the frame — spread out and left to dry on its own. This is also why a second pass with the same cloth and the same bottle usually makes it worse rather than better: it lays down a fresh coat of the same residue on top of the one already there.

Two conditions decide whether that residue ends up on the pane or in your bucket. The first is how fast the water dries: anything that evaporates before it is physically removed leaves its dissolved contents behind on the glass. The second is whether the last object to touch the pane was itself clean and dry. Almost every streak on a house window traces back to one of those two, and both are controlled by working order rather than by product choice.

The five things that actually leave a streak

Streaks are readable. The shape and position of the mark usually name its cause before you touch anything, which saves the common mistake of rewashing a pane for the third time using the method that failed twice.

The third row is the one that catches most people, because it happens after the work looks finished. The top edge of a sash and the underside of the head casing both hold water you cannot see from where you are standing. Ten minutes later that water runs down over a clean pane, carrying whatever was sitting on the frame with it, and the streak appears on glass you would swear you did properly. Washing the frame before the glass is the whole fix, and it costs nothing.

The last row is not a technique problem at all. A chalky film that shrugs off soap is dissolved mineral left behind by evaporating tap water, and it needs a mild acid and dwell time rather than more scrubbing — that is covered in full in hard-water spots from sprinklers. Rule it out before blaming your squeegee, because no amount of technique removes it.

The order that stops streaks, from the frame to the final check

Order matters more than product here. The sequence below is arranged so that nothing dirty ever gets moved onto a surface that has already been cleaned, and so that no water is left anywhere it can run down later.

1. 1Pick the side of the house that is in shadeGlass that is warm to the touch dries faster than you can pull water off it, and everything dissolved in that water stays behind. Work the west side in the morning and the east side in the afternoon, or take the whole job on an overcast day. Wind dries glass quickly as well, so a bright breezy day is poor conditions for this rather than good ones.
2. 2Do the dry work before any water appearsBrush or vacuum the sill, the track and the corners of the frame while everything is still dry. Dry grit sweeps away in seconds; the same grit once wet becomes mud that spreads across the pane and scratches it on the way. This single step removes most of the dirty-water band along the bottom edge.
3. 3Take the screens out and wash them somewhere elseScreens hold fine dust that transfers straight onto wet glass the moment they are disturbed. Wash them flat with a soft brush and clean water, then stand them somewhere to drain completely. A screen put back while still damp drips down the pane you just finished.
4. 4Wash the frame, sash edges and head casing nextIncluding the top edge of the sash and the underside of the casing above it, which is where hidden water sits. Use a different cloth from the one that will touch the glass, and wipe those edges dry before moving on.
5. 5Wet the whole pane, edge to edgeCover the corners and the perimeter properly rather than only the middle. A pane wetted in the centre and squeegeed outward drags dirt from the dry edges back into the clean area, which is exactly the pattern that shows up as marks around the border.
6. 6Pull the water off in overlapping passes, drying the blade each timeOne continuous stroke per pass, each one overlapping the last by a couple of centimetres so no dry strip is left between them, and wipe the squeegee blade dry on a clean cloth between every pass. A wet blade redeposits what it just removed. If the rubber has a nick in it, it will draw a line on every pane in the house until it is replaced.
7. 7Detail the perimeter with a dry, lint-free clothA thin bead of water always survives along the frame line. Run a dry microfibre cloth around the edge, keeping it off the middle of the pane so you do not reintroduce anything. This is the cloth that must never have touched the frame or the sill.
8. 8Check from an angle, then from insideLook along the pane rather than straight at it, from at least two positions, and then from the other side of the glass. Marks that are invisible head-on show up immediately in raking light, and checking now costs a minute against redoing the whole window next weekend.

How much soap, and why the wrong soap is worse than none

Soap in window work is there to make the glass slippery enough for a squeegee to glide without chattering. It is not doing the cleaning — the water and the mechanical action do that. A few drops in a bucket is the working amount; a squirt that produces visible suds is far past it, and every bit of surfactant above what is needed for slip has to be removed again or it dries as haze.

Which soap matters as much as how much. Read what the bottle sells itself on: a dish liquid marketed on hand softness, added shine or rinse-aid performance is telling you it deposits something that stays behind on a surface after the water leaves. That is a selling point on dishes and the wrong behaviour on glass. A plain, additive-free detergent in a small quantity gives you the slip without the film.

Commercial glass sprays are a different tool for a different job. They work well on interior fingerprints and small panes where there is not enough water to squeegee, and they run out of usefulness fast on a full-size exterior window, where the volume of dirt simply overwhelms a cloth. Ammonia is a common ingredient in window cleaners, glass cleaners, bathroom cleaners and furniture polish, which is worth knowing before anything else comes out from under the sink.

How to tell which side of the glass the streak is on

Change your stroke direction by side and the glass will tell you. Pull the squeegee vertically on the inside and horizontally on the outside, and any streak you find afterwards names its own surface: a vertical mark is interior, a horizontal one is exterior. Without that, people commonly rewash an interior pane three times to chase a mark sitting on the outside face.

The distinction is worth the effort because the two faces collect different things at different rates. The interior face collects what the household puts into the room air, builds up slowly and evenly, and comes off with an ordinary wash. The exterior face collects what the outdoors delivers — road dust, pollen, runoff from above — and it returns on a schedule set by the weather and by the house, not by how well the last cleaning went.

That last part is worth checking before you blame the cleaning. If an eavestrough is spilling over its front edge, every heavy rain sheets grit-laden roof water down the wall and across the top of the windows below it, and the glass is marked again within days of being cleaned. The signs of a gutter problem visible from the ground include exactly this kind of staining under the overflow point, and no window routine can outrun it while the cause is still there.

When the marks are not dirt at all

Some of the things people book a window cleaning for cannot be cleaned off, because they are not on either face of the glass. Knowing which is which before you spend a Saturday, or someone else's visit, on it is the most useful thing on this page.

The first row is the one that gets misdiagnosed most often, because it looks like a cleaning failure from across the room. Natural Resources Canada is direct about it: where condensation forms inside a sealed double-glazed unit because the seal has broken, the problem is best corrected by replacing the glazing unit, and the window should be checked for remaining warranty. Some specialty companies will refurbish a failed sealed unit, but NRCan describes that as a temporary fix that does not restore the original energy performance.

The second row is a genuinely different situation with a cheap fix. On non-sealed glazing or a storm window, NRCan attributes the fogging to moist house air leaking past the inner pane and condensing on the outer one, and notes it is common on second storeys, where stack effect pushes more air out through the upper part of the house. The remedy is weatherstripping the inner sash and confirming the storm window's weep holes still drain to the outside — those holes are easy to close off by accident when a frame gets painted, or when someone caulks around it to stop a draft.

The third row surprises people who have just replaced their windows. NRCan notes that homeowners sometimes buy new windows only to find interior condensation gets worse, because the old leaky units were quietly venting humidity out of the house; the tighter new ones stop doing that and indoor humidity rises. Wiping the glass every morning treats the symptom. Reducing the moisture load and improving ventilation treats the cause.

When it is worth calling someone, and when it is not

Ground-floor windows, on an overcast day, with a bucket and a decent squeegee, are a straightforward afternoon for most people. The order in this article is the whole method, the tools are ordinary hardware-store items you keep, and hiring anyone for four panes you can reach from the lawn is not the best use of the money.

The calculation changes with height rather than with difficulty. Second-storey glass means an extension ladder set correctly on stable ground, or a water-fed pole, and the risk sits in the setup rather than in the cleaning — that is covered separately in reaching a two-storey window safely. It also changes after construction: renovation dust on the glass normally arrives with a whole house carrying the same dust on every other surface, which makes it a full interior clean rather than a window job.

And there is a category where nobody should be hired at all. A failed sealed unit, etched glass, or condensation driven by indoor humidity will look exactly the same after a professional window cleaning as it did before, because none of those sit on a surface anyone can reach with a cloth. They are glazing work, ventilation work and glass restoration respectively, and none of them belongs on a window-cleaning quote.