Fix It Britain

To stop condensation on windows, you need to tackle two things at once: how much moisture is in the air and how cold the glass is. Wiping the panes treats the symptom, but left alone the water soaks into sealant, frames and sills until black mould takes hold. This guide explains why windows stream, how to tell condensation from other damp, and the steps that make the biggest difference in a typical UK home.

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At a glance

• Difficulty: Easy. Most of the fixes are habits rather than DIY jobs.
• Time needed: 5 minutes a day to wipe and ventilate, plus an hour or two to clean up any mould and set up a hygrometer or dehumidifier.
• Best time to do it: Early autumn, before the heating goes on full time and the first cold mornings arrive. It’s worth doing as soon as you notice streaming glass, whatever the month.
• Best for: Bedrooms, kitchens and bathrooms with wet windows in the morning, black spots on seals or frames, and homes where washing is dried indoors.

Why your windows get wet (and what kind of wet it is)

Condensation forms when warm, moist indoor air touches a surface cold enough to make the water vapour turn back into liquid. In most homes the window glass is the coldest surface in the room, so that’s where it shows up first. Everyday living produces a surprising amount of moisture: according to condensation advice published by both South Kesteven District Council and the City of Lincoln Council, the average UK household puts about 12 litres of moisture into the air every day, and homes that dry clothes indoors can produce over 20 litres a day.

Before you start, check where the water is, because it tells you different things:

• On the inside of the glass: this is the problem this guide is about. Too much moisture indoors, not enough ventilation, or a very cold pane (common with single glazing and older aluminium frames).
• On the outside of the glass: usually nothing to worry about. The Energy Saving Trust says external condensation is harmless and is a sign that your windows are keeping heat in well, so the outer pane stays cold.
• Between the panes of double glazing: the sealed unit has failed. The Energy Saving Trust notes this means the unit is letting air in, and you can often replace just the glass unit rather than the whole window. No amount of wiping or ventilating will clear it.

Rule out other damp too. Condensation is worst on winter mornings, appears on cold surfaces such as glass, wall corners and behind wardrobes, and often comes with black spotty mould. A tide mark rising from the skirting, or a patch that worsens after rain, points to rising or penetrating damp instead, often from a leaking gutter or failed render. Drying the air won’t fix those.

What you’ll need

• A window vacuum or a squeegee and absorbent microfibre cloths
• A digital hygrometer (a small humidity and temperature monitor)
• A dehumidifier for the worst room (optional, but often the single most effective fix)
• A fungicidal mould wash with an HSE approval number, rubber gloves and a disposable cloth
• Pan lids, a clothes airer and somewhere to dry washing that isn’t on a radiator
• A soft brush for cleaning trickle vents

Step 1: Wipe the windows dry every morning

Start by removing the water that’s already there. Left on the glass, it runs down into the bottom rubber gaskets and silicone, sits on the sill and feeds mould on the frame. A window vacuum is the quickest way to do it, because it sucks the water into a tank rather than spreading it around. A squeegee and a dry microfibre cloth work almost as well.

Work from the top of each pane down, then wipe the bottom of the frame and the sill. Wring cloths out in the sink, not over a radiator, or the moisture just goes back into the room. The amount you collect is a useful gauge: you should see it drop as the next steps start working.

Step 2: Measure your humidity with a hygrometer

A cheap digital hygrometer takes the guesswork out of condensation. Put it in the room with the worst windows, away from the window itself, a radiator or the kettle, and check it at different times of day. The Healthy Homes guide to damp and mould published by Westmorland and Furness Council recommends that indoor humidity averages between 40% and 65%, and notes that mould spores multiply when relative humidity reaches 70–80%.

Kitchens and bathrooms spike while you cook or shower, which is fine if the reading falls back quickly. A bedroom sitting above 65% overnight needs more ventilation or a dehumidifier.

Step 3: Open your trickle vents and ventilate little and often

Most modern uPVC windows have trickle vents: slim plastic slots along the top of the frame that let a steady trickle of air through when the window is shut. They’re designed to stay open, especially in bedrooms. Slide or flip each one open and clean out dust and dead insects with a soft brush so air can actually pass through.

On top of that, open windows properly for a short time each day. The Westmorland and Furness guide suggests around ten minutes a day, ideally at midday when the outdoor air is drier. A short blast swaps damp air for drier air without chilling the walls; a window left open a crack all day in winter cools surfaces and can make things worse.

If you’re having windows replaced, ventilation rules apply. LABC’s summary of Approved Document F (the ventilation part of the Building Regulations in England) explains that if your old windows had trickle vents, the new ones must have vents at least as large, and if they didn’t, ventilation must be no worse than before, usually by fitting trickle vents. A registered installer will handle this.

Step 4: Cut moisture at the source

The less water vapour you create, the less ends up on the glass. These habits appear in almost every UK council condensation leaflet:

• Cook with lids on and use the cooker hood on its extract setting, or open a window. Don’t leave a kettle boiling.
• Run the bathroom extractor fan during and after a shower, and keep the bathroom door shut so steam doesn’t drift into colder bedrooms. Never switch off or block an extractor fan because it’s noisy; get it repaired.
• Dry washing outside whenever you can. If you must dry it indoors, use one room with the door shut and a window or trickle vent open (or a dehumidifier running), and never drape it over radiators.
• Vent tumble dryers outside through a wall or window kit, or use a condenser or heat-pump dryer and empty the water container regularly.
• Avoid bottled gas and paraffin heaters. They release a lot of water vapour as they burn, and councils such as South Kesteven don’t allow them in their homes.
• Never cover airbricks or permanent wall vents. As the City of Lincoln Council points out, blocking them traps moisture and can stop gas appliances getting the air they need, which risks carbon monoxide.

Step 5: Keep the heating low and steady

A home heated in short, hot bursts and then left to go cold is a magnet for condensation: the air warms up and holds lots of moisture, but the walls and glass stay cold. Low, consistent heat keeps surfaces warmer.

The Westmorland and Furness guide suggests keeping homes above 16°C, with 18–21°C as the ideal range, while the City of Lincoln Council recommends leaving background heating on at around 15°C even when you’re out. Make sure radiators are working properly; a radiator that’s cold at the top needs bleeding (see our guide below). Leave a gap between furniture and external walls so warm air can circulate; the Healthy Homes guide suggests around a foot where you can manage it.

Step 6: Use a dehumidifier in the problem room

If you’ve done all of the above and a room still reads high on the hygrometer, a dehumidifier is the most direct way to pull water out of the air. There are two main types. Compressor (refrigerant) models are generally more efficient in rooms kept at normal living temperatures. Desiccant models use an absorbent material rather than a cold coil, so they keep working well in cooler spaces such as unheated bedrooms, conservatories and garages, though they typically use more electricity.

Run it in the worst room with the door and windows shut. If it has a humidity setting, aim for 40–65% rather than drying the room out completely, and empty the tank daily.

Step 7: Clean any mould off frames, seals and sills

Once the moisture is under control, deal with any black mould left behind, wearing rubber gloves with a window open. Use a fungicidal mould wash that carries an HSE (Health and Safety Executive) approval number and follow the instructions on the bottle, including how long to leave it on. The Westmorland and Furness guide advises removing mould while it’s damp and avoiding sweeping or vacuuming it, as dry brushing spreads spores. It also warns that bleach isn’t an effective way to remove mould.

Use a disposable cloth and bin it afterwards. Mould deep in silicone sealant usually can’t be cleaned out, so the sealant may need replacing once dry. If the surrounding wall is affected, wash it, let it dry and use a fungicidal paint; the Healthy Homes guide warns against covering this with normal emulsion, which stops it working. If mould covers more than about one square metre, the same guide says you should get expert advice and wear protective equipment.

Step 8: Make the glass warmer

The colder the glass, the more water it collects. Curtains and blinds pulled tight at night trap cold air against the pane, which is why bedroom windows stream behind them by morning. Open them during the day so the glass and frame can dry out.

With single glazing, secondary glazing makes the inner surface warmer. The Energy Saving Trust describes three types: DIY shrink film (cheapest but least effective and short-lived), removable magnetic panels, and permanent secondary glazing, which suits listed buildings.

Common mistakes to avoid

• Closing trickle vents to stop draughts. It feels warmer for a day, but the moisture has nowhere to go.
• Drying clothes on radiators. It’s one of the biggest single sources of indoor moisture.
• Heating in short bursts. Warm air and cold walls is exactly the combination that causes condensation.
• Painting over mould. Ordinary emulsion over untreated mould just hides it for a few weeks.
• Relying on moisture-trap crystals. They absorb a little moisture in a cupboard, but nowhere near the litres a household produces each day.
• Pushing wardrobes and beds against outside walls. The trapped, cold air behind them is a classic spot for hidden mould.

When to call a professional

Get help if condensation and mould keep coming back after you’ve followed every step for a few weeks, or if damp patches appear away from windows and cold corners, which suggests rising or penetrating damp. Fitting an extractor fan or a positive input ventilation (PIV) unit involves fixed electrical work, so use a qualified electrician. If you have gas appliances and suspect vents are blocked or inadequate, call a Gas Safe registered engineer. Misted double-glazed units need a glazier. If you rent, report persistent damp and mould to your landlord in writing: the NRLA notes that government guidance published in 2023 expects landlords to investigate the underlying causes rather than blame tenants’ lifestyles.

Frequently asked questions

Is some condensation on windows normal?

A light mist on a cold morning, or after cooking or a shower, is common and usually clears quickly once you ventilate. It becomes a problem when the glass is streaming every day, water pools on the sill, or black mould starts appearing on seals and frames.

Why is there condensation on the outside of my windows?

External condensation happens on clear, cold mornings when the outer pane drops below the temperature at which moisture in the outdoor air condenses. According to the Energy Saving Trust, it’s harmless and shows your glazing is insulating well, because less heat is escaping to warm the outer pane.

Does opening a window in winter really help?

Yes, as long as you do it briefly. Ten minutes or so with the window wide open replaces damp air with drier outdoor air without letting the walls and furniture cool down much.

Will new double glazing stop condensation?

It makes the inner pane warmer, so you’ll usually see less condensation on the glass. But new windows are more airtight than old ones, so if the moisture isn’t dealt with it can simply move to the next coldest surface, such as wall corners or behind furniture. That’s why Building Regulations guidance expects replacement windows to keep or add trickle vents.

What humidity should my home be?

The Westmorland and Furness Healthy Homes guide recommends an average of 40–65% relative humidity indoors, with kitchens and bathrooms allowed to run a little higher. Readings that sit at 70% or above for long periods make mould growth much more likely.

Related guides: How to Bleed a Radiator · How to Clear and Fix a Leaking Gutter · How to Paint a Room Without Streaks