Simple anti-fog trick: washing-up liquid or glycerin to stop condensation on windows

A tiny household trick can make a real difference - and it’s one our grandparents already knew.

In January, many people wake up to a familiar sight: not the view outside, but a cloudy window with beads of water running down the inside. The heating may be on and the flat might feel warm, yet everything still seems damp and chilly. Rather than battling the glass every morning with a towel or turning the thermostat up yet again, there’s a surprisingly straightforward workaround that costs next to nothing and uses a kitchen staple most people already have.

Why condensation on windows is so persistent in winter

Water on the inside of your windows isn’t just an annoying cosmetic issue - it’s a completely normal physical process. Warm indoor air can hold far more moisture than cold air. When that warm, moisture-laden air hits a cold window pane, it cools rapidly at the surface. The excess moisture then settles on the glass as tiny droplets: the dew point has been reached.

What’s irritating isn’t only the fogged-up view.

The water that runs down the pane in the morning will, sooner or later, end up in frames, seals and on the windowsill - an ideal breeding ground for mould.

Over weeks and months, those small droplets can attack seals and frames, particularly on wooden windows and older uPVC. Black specks along silicone seals, a musty smell and crumbling frames are common consequences.

Many people respond in ways that don’t really help: drying the glass every day, adding a portable fan heater, or keeping the heating permanently set high. That costs money, wastes energy and rarely fixes the root cause - the cold glass surface remains.

The simple trick: washing-up liquid or glycerin as an invisible protective film

Instead of buying expensive anti-fog sprays from a DIY store, an old household method works with two basic options: washing-up liquid or glycerin. Both act as surfactants, meaning they alter the surface tension of the glass and change how water behaves on it.

Normally, countless small, rounded droplets form and create that familiar pale haze. If you apply a very thin film of washing-up liquid or glycerin, the effect changes: moisture is less able to gather into distinct droplets, so it spreads more evenly or runs off as a fine sheet. The glass stays clearer, even though moisture is still physically condensing on the surface.

A microscopically thin film of washing-up liquid or glycerin helps ensure moisture on the glass is no longer visible as dense mist.

Divers have long used this on masks, and motorcyclists on helmet visors. The same approach can work remarkably well on home windows, bathroom mirrors and shower screens too.

How to apply it step by step

Whether it works well depends heavily on how you apply it. Use too much and you’ll be looking through smears. Get the amount right and you won’t notice anything - until the next morning.

Prepare the window

• Clean the inside of the window thoroughly so there’s no grease, dust or fingerprints left on the glass.
• Dry it with a clean cloth - the pane really does need to be completely dry.

Apply the washing-up liquid or glycerin film

• Put a tiny amount - roughly a single drop - of washing-up liquid or glycerin onto a slightly dampened microfibre cloth.
• Rub it evenly over the inside of the pane using circular motions. Treat as much of the surface as possible.
• Then buff firmly with a second, dry, soft cloth until there are no visible streaks.

One crucial point: don’t wipe the glass again with water afterwards. That would rinse away the remaining effective film. If faint smears remain, it’s usually simply because too much product was used. In that case, more vigorous dry buffing with a lint-free cloth will normally sort it.

How long does it last - and where is it most worth doing?

In day-to-day use, the anti-fog effect typically remains noticeable for several days; in some homes it can last a week or even longer. Longevity depends mainly on two things: humidity levels and cleaning habits.

• High humidity from lots of houseplants, drying laundry indoors or frequent cooking shortens how long the film remains effective.
• Frequent cleaning with glass cleaner or multi-surface spray removes the film more quickly.

This trick is especially useful in rooms with a big temperature difference and lots of moisture:

• bedroom windows, particularly if you sleep with the window closed
• bathroom windows and mirrors that regularly fog up after showers
• kitchen windows near the hob
• windows in small, rarely aired spaces such as storage cupboards

The anti-fog trick doesn’t replace ventilation - but it helps a lot

As useful as the protective film can be, it doesn’t remove moisture from the air. That moisture still needs to leave the room - otherwise it will settle somewhere else. So if you treat your windows, it’s still important to keep an eye on your ventilation routine.

The thin film on the glass prevents visible droplets, but only airing the room properly reduces humidity in the long term.

The most effective approach is several short bursts of ventilation each day: open the windows wide, create a draught for five to ten minutes, then close them again. Leaving windows on the latch for hours tends to cool walls and window reveals without properly exchanging the air.

If you regularly see heavy mist on the panes in the morning, it’s also worth checking:

• Is someone sleeping with the window shut in a small room?
• Are towels or laundry often drying in the bedroom?
• Is there no ventilation break after showering or cooking?

Even small changes - such as moving laundry to another room, or keeping the bathroom door shut and opening the window wide after showering - can reduce indoor humidity significantly.

Which products work best - and where the limits are

For this method, a straightforward washing-up liquid is best - ideally without heavy balm additives or special “shine” polymers. The simpler the formula, the lower the risk of smearing. Glycerin is available from pharmacies and well-stocked chemists; it’s commonly used in cosmetics and for baking.

Both options go a long way. A single small drop is easily enough for one standard-sized window pane. If you’re treating large glass areas or a patio door, it’s best to work pane by pane so you can control the dosage.

This trick has its limits where the glass stays extremely cold for long periods - for example, very old single glazing in unheated stairwells. In those situations, condensation can be so heavy that even a good film loses effectiveness quickly. Then more structural solutions make a bigger difference: properly sealing windows, adding extra draught-proofing tape, or, in the longer term, replacing the windows.

More comfort, less mould risk - with minimal effort

Once you’ve tried it, you’ll often notice the change the very next morning: the pane stays largely clear, the room immediately feels more pleasant, and the daily wiping routine becomes unnecessary. At the same time, there’s less risk of damp frames and mould spots because less water collects and drips from one area.

This is particularly handy in rented homes where major upgrades or costly technical fixes such as electric dehumidifiers aren’t an option. A drop of washing-up liquid or a little glycerin, plus two cloths - that’s all it takes to noticeably reduce a common winter nuisance.

If you’re already getting your windows ready for winter, this anti-fog trick pairs well with other measures: checking seals, avoiding blocking radiators completely, and hanging curtains so warm air can still reach the glass. Together, these small, low-cost actions can make the indoor climate feel much more comfortable.

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