How to Stop Condensation on Windows UK 2026: What Works, What It Costs

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To stop condensation on windows you need to change one of three things: how much water vapour is in the air, how cold the glass is, or how fast the moist air is being replaced. Every genuine fix works through one of those three levers. Everything else - wiping, squeegeeing, washing-up liquid - manages the puddle rather than the cause.

This guide takes each lever in turn, says what it costs to run at the October 2026 price cap rate of 26.32p per kWh, and is honest about which of the popular hacks actually do anything.

What causes condensation on windows

Air holds water vapour, and warm air holds more of it than cold air. When that air touches a surface colder than its dew point, it cannot hold the water any more and drops it as liquid. Your window is almost always the coldest surface in the room, so it goes first.

The dew point is not a fixed number. It depends on how warm and how humid your air is.

Now put that against how cold the inside face of your glass actually gets. On a 0°C night with the room at 20°C, the inner pane sits at roughly:

Read the two tables together and the whole problem falls out. A room at 60% humidity has a dew point of 12°C. Single glazing at 5.4°C is far below that, so it streams. Pre-2002 double glazing at 12.2°C is right on the line, so it fogs some nights and not others. Modern double glazing at 16.4°C stays dry - until the humidity climbs past 70%, at which point even good glass gives up.

That is why two houses in the same street behave completely differently, and why the right answer for your home depends on which of the two numbers is out of place.

Where the water comes from

A normal household puts a surprising amount of water into the air every day. The commonly quoted UK figures:

Drying washing indoors is the biggest controllable source on that list, and it is why condensation problems spike in exactly the months people stop hanging things on the line. Our dehumidifier running costs guide works through where that 2-3 litres per load ends up, and our heated airer running costs guide covers the cheapest way to get it out of the room.

The bottled gas heater line is worth flagging separately. Unflued bottled gas and paraffin heaters produce roughly a litre of water for every litre of fuel burned, straight into the room. If you are using one and fighting condensation, those two facts are the same fact.

How to stop condensation on windows overnight

Bedroom windows are the classic case: dry all day, streaming by morning. Two sleeping adults release the better part of a litre of water, the heating is off so the glass drops several degrees, and the door is shut so nothing escapes. Three things stack up at once.

The overnight fix, in order of effect:

Leave the trickle vents open. Those slots in the top of the frame exist precisely for this. They cost nothing, lose a negligible amount of heat, and keep the air changing slowly all night. Most people with condensation have them shut.

Leave the bedroom door ajar, or the window on night vent. Moist air needs somewhere to go. A closed door turns the bedroom into a sealed box with a cold window in it.

Do not dry washing in the bedroom. One airer in a closed bedroom overnight will beat every other measure on this page.

Purge-ventilate for ten minutes in the morning. Open the windows wide, let the wet air out, close them again. This is cheaper than it sounds, for a reason worth spelling out.

What opening a window actually costs you

This is where most condensation advice goes quiet, because "just open a window" sounds like it must be expensive in a British winter. The sums say otherwise, provided you do it properly.

Replacing all the air in a 90 m³ house and reheating it from 0°C to 20°C takes about 0.6 kWh. On gas central heating at 8.86p per kWh of delivered heat, that is roughly 5p. Doing it every morning through a six-month heating season costs about £9.

The reason it is so cheap is that air has almost no heat capacity compared with the bricks, plaster and furniture around it. In ten minutes the air is fully changed but the fabric of the room has barely cooled, so the room comes back up to temperature quickly.

What is genuinely expensive is the opposite approach: leaving a window cracked open all day, every day. That is a continuous heat leak rather than a short flush, and it can add tens of pounds a month. Short and wide beats small and permanent. If you are draught-proofing the rest of the house - and you should, see our draught-proofing cost guide - this is the one place to deliberately keep an air path.

The fixes, ranked by what they cost to run

A few of those deserve explaining.

A humidistat extractor fan is the best value item on the list. A 5W unit running continuously on trickle and boosting when humidity spikes uses 0.12 kWh a day, which is 3.2p a day or about £12 a year. It removes moisture at source, in the bathroom and kitchen where four litres a day are being generated, before it ever reaches the bedroom window. A humidistat extractor fan is a fitted item rather than a plug-in one, so budget for an electrician.

Moisture traps are the worst value item on the list, by a wide margin. A crystal tub absorbs perhaps 0.3 to 0.5 litres before the refill is spent, at £1.50 to £2 a refill. That is roughly £4 per litre of water removed. A compressor dehumidifier does the same job for about 7.9p per litre - fifty times cheaper. Traps are fine for a wardrobe, a caravan or a boat. For a room with a real condensation problem they are an expensive way to feel busy.

A dehumidifier is the right answer when you cannot ventilate more. Flats with no through-draught, homes with a genuine need to dry washing indoors, north-facing bedrooms in terraces. At £31 to £46 for an entire October-to-March damp season it is far cheaper than most people fear, and the full arithmetic is in our dehumidifier running costs guide.

Warming the glass is the lever people forget. Condensation is as much about cold surfaces as wet air. Steady low heat holds the glass above the dew point; heating in short hot bursts lets it fall below overnight. If your windows are the original single-glazed units, thermal window insulation film shrink-fitted to the frame creates a still-air gap and lifts the inner surface temperature by several degrees for a few pounds a window. It is not pretty, but it works, and it is the cheapest intervention on the temperature side.

Before spending on any of this, spend a few pounds on a hygrometer. You want relative humidity between 40% and 60%. Below 40% the air is uncomfortably dry; above 60% you are in mould territory. Knowing the actual number in each room tells you whether you have a moisture problem, a temperature problem, or both - and stops you buying a dehumidifier for a room that was already at 48%.

Does the washing-up liquid trick work?

It is one of the most-searched condensation remedies in Britain, so it deserves a straight answer: no, not in the way people think.

Wiping a dilute washing-up liquid solution on the glass leaves a surfactant film. Surfactants lower the surface tension of water, so the condensation that forms spreads into an even, near-invisible sheet instead of beading into droplets. It is the same chemistry as anti-fog spray on a diving mask.

What that means in practice:

• The same amount of water still condenses on your window. Nothing about the humidity or the glass temperature has changed.
• It looks clearer, and it is less likely to run down and pool on the sill, which is a genuine if modest benefit.
• It wears off, so it needs redoing every week or two.
• Too strong a mix leaves streaks and a film that attracts dust.

Treat it as a cosmetic measure for a window you cannot fix this month, not as a solution. The water is still in your house, and it will find the next-coldest surface - usually the corner of an external wall behind a wardrobe, where you will not see it until the mould appears.

A similar verdict applies to anti-condensation paint. It is an insulating, mould-resistant emulsion that raises the surface temperature of a wall by a fraction of a degree and resists mould growth on the finish. On a cold window reveal or a chronically damp wall patch it is a reasonable product. It cannot be applied to glass, and it will not fix a room sitting at 75% humidity.

Condensation on the outside of your windows is good news

If the misting is on the outside - you can wipe the inside and it stays clear, and it burns off by mid-morning - nothing is wrong. That is external condensation, and it means your glazing is working.

It happens on clear, still nights when the outer pane radiates heat to the sky and drops below the outdoor dew point. For it to get that cold, almost no heat can be escaping from inside. It is most common on new A-rated double and triple glazing, and installers field complaints about it every autumn from people who have just spent thousands on the best windows they have ever owned.

There is nothing to fix. It clears as the sun comes up.

Condensation between the panes is the one to worry about. If the misting is sealed inside the unit and cannot be wiped off either side, the perimeter seal has failed and the unit is no longer insulating properly. That is a replacement rather than a repair - although it is usually just the glazing unit, not the whole window and frame.

Double glazing and aluminium frames

Two specific cases come up constantly.

Condensation on double glazing in winter usually means one of two things. Either the units are old - pre-2002 double glazing runs an inner pane around 12°C, which is below the dew point of a normal 60%-humidity room - or the units are modern and your humidity has climbed past 70%, which no glazing will save you from. Check with a hygrometer before blaming the windows.

Condensation on aluminium windows is nearly always the frame rather than the glass, and it is a thermal bridging problem. Older aluminium frames have no thermal break, so the metal conducts cold straight through and the frame sits close to outdoor temperature even when the glass is fine. You will see a wet frame and a dry pane. Short of replacing the frames with thermally broken ones, the realistic measures are to keep humidity down and to avoid heavy curtains that trap cold air against the frame.

When it is not condensation

Condensation is a ventilation and temperature problem. Rising damp and penetrating damp are building faults, and no amount of airing will touch them. Some quick tests:

• Condensation appears on cold surfaces, is worst in the morning and in winter, and follows cooking, showering and drying washing.
• Penetrating damp appears after rain, often as a defined patch on an external wall, and may trace back to a blocked gutter or failed pointing.
• Rising damp starts at floor level, rarely climbs above a metre, and often leaves a tide mark.

If the damp is at skirting height, or appears after rain rather than after a shower, stop buying dehumidifiers and get a survey. A professional damp survey costs £150 to £300 and is money well spent before any remedial work.

If the underlying problem is that your walls are simply cold, the fix is insulation rather than ventilation. Our home insulation guide covers what raising wall and loft surface temperatures does to condensation risk, and our loft insulation cost guide has the numbers for the cheapest of those jobs.

Frequently asked questions

How do I stop condensation on my windows overnight? Open the trickle vents, leave the bedroom door ajar so moist air can disperse, keep laundry out of the bedroom, and purge-ventilate for ten minutes in the morning. Two sleeping adults release close to a litre of water in eight hours, and in a sealed bedroom it has nowhere to go but the glass.

Does washing-up liquid stop condensation on windows? No. It leaves a surfactant film that makes the condensation spread into a clear sheet rather than beading into droplets, so the window looks clearer and is less likely to drip onto the sill. The same amount of water still condenses, and it wears off within a week or two.

Why do I get condensation on the outside of my windows? Because your glazing is working well. The outer pane radiates heat to a clear night sky and falls below the outdoor dew point, which only happens when very little heat is escaping from inside. It is most common on new A-rated double and triple glazing, and it clears by mid-morning.

Is a dehumidifier or a window vacuum better for condensation? A dehumidifier treats the cause and a window vacuum treats the symptom. The vacuum is quick, cheap to buy and stops water pooling on sills and rotting the frames, but the moisture is still in the house. If condensation is appearing daily on more than one window, a dehumidifier at £31 to £46 a season is the better spend.

Does opening windows in winter waste a lot of heat? Not if you do it in short bursts. Fully changing the air in a 90 m³ house and reheating it costs about 0.6 kWh, or roughly 5p on gas. Leaving a window permanently cracked open is a different matter and can cost tens of pounds a month.

What humidity should my house be to avoid condensation? Between 40% and 60% relative humidity. At 60% in a 20°C room the dew point is 12°C, which modern double glazing stays comfortably above. Past 70% the dew point reaches 14.4°C and even good windows will start to fog.

Will turning the heating up stop condensation? It helps, because warmer glass condenses less, but steady low heat does far more than short hot bursts. Heating the house to 22°C for two hours and letting it fall overnight gives you a cold window at exactly the point moisture peaks. Warm air also holds more vapour, so heating without ventilating can make the problem worse rather than better.

Does anti-condensation paint work? On walls, partially. It is an insulating, mould-resistant emulsion that lifts the surface temperature slightly and resists mould growing on the finish. It cannot be used on glass and it will not rescue a room sitting at 75% humidity. Treat it as a finishing product after the ventilation is sorted, not instead of it.

The bottom line

Condensation on windows is arithmetic, not bad luck. Air at 20°C and 60% humidity condenses on anything below 12°C, and your glass is the coldest thing in the room. You either take water out of the air, get it out of the house faster, or make the glass warmer.

The cheapest effective package for most UK homes costs almost nothing: open the trickle vents, run the extractor when cooking and showering, keep laundry out of bedrooms, and purge-ventilate for ten minutes each morning for about £9 a year in lost heat. If that is not enough - and in a flat with no through-draught it often is not - a humidistat extractor fan at £12 a year to run, or a dehumidifier at £31 to £46 a season, will finish the job.

What will not fix it: wiping the glass, moisture traps at £4 a litre, and washing-up liquid. Those manage how the water looks. The water is still in your house, and if it is not on the window it is behind the wardrobe.