What Causes Condensation on Windows, and How to Stop It

Welcome to our guide to the most common winter complaint in British homes. You draw the curtains in the morning, and the bottom third of the glass is running with water. Sometimes there is a small pool on the sill. Sometimes there is a rim of black spotting where the frame meets the wall.

It is worth understanding properly, because condensation is both completely normal and a genuine warning sign, depending on how much of it you are getting and where. Here is what is actually happening, and what to do about it.

The Physics, in One Paragraph

Air holds water vapour in proportion to how warm it is. Air at 20°C can carry around 17 grams of water per cubic metre before it saturates. Cool the same air to 10°C and its capacity drops to roughly 9 grams. When warm, moist indoor air touches a surface cold enough to push it past saturation, the surplus water leaves the air and appears on that surface. The temperature at which this happens is the dew point.

In almost every room, the coldest surface is the window. That is why glass is where you see it first, even though the moisture came from somewhere else entirely, usually a kettle, a shower or a clothes airer.

At 20°C and 60 per cent relative humidity, the dew point sits at about 12°C. Raise the humidity to 70 per cent and the dew point climbs to roughly 14°C. Every degree of extra humidity puts more of your house below the line.

Is It Normal for House Windows to Have Condensation on the Inside?

A little, in winter, in the morning, yes. Overnight the heating drops, the glass gets cold, and several hours of breathing has to go somewhere. A thin film that clears within an hour of the room warming up is not a problem.

What is not normal is condensation that is still there at midday, condensation that pools deep enough to run onto the sill and soak the frame, or condensation accompanied by black spotting on the reveal. That combination means the room is holding more moisture than it can shed, and mould will follow. Mould gets going once surface humidity stays above roughly 80 per cent for sustained periods, which is exactly the microclimate a permanently wet window reveal creates.

Does Condensation Mean My House Is Too Cold?

Not quite. It means your house is too cold relative to how much moisture is in the air. Those are two different problems with two different fixes.

You can have a chilly house with almost no condensation if the air is dry and well ventilated. You can also have a warm house streaming with water if you dry laundry indoors and never open a window. Turning the thermostat up treats one half of the equation and often makes people feel the problem is unfixable when it does not work.

The pairing that actually works is steady background warmth plus deliberate ventilation. Warmth keeps surfaces above the dew point. Ventilation removes the moisture rather than relocating it.

What Causes Condensation on the Outside of Windows?

This one worries people unnecessarily, so it is worth saying plainly: condensation on the outside of your glass is a sign your windows are working well.

It happens on clear, still nights when the outer pane radiates heat to the sky and drops below the dew point of the outdoor air, exactly like dew on a car roof. The reason it appears on good windows and not bad ones is that a well insulated unit lets so little heat through that the outer pane stays cold. On a draughty single glazed window, escaping warmth keeps the outer surface too warm for dew to form.

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

Will Double Glazing Stop Condensation?

It will reduce it substantially, but it will not eliminate it, and this is where a lot of money gets spent on the wrong solution.

Double glazing raises the internal glass temperature, which lifts it above the dew point in normal conditions. But if the moisture load in the room is high enough, the dew point rises to meet even a warm pane. Plenty of homes fit new windows and find the condensation simply migrates to the next coldest surface, typically the window reveal, the corner of an external wall, or the cold spot behind a sofa.

There is one exception worth knowing. Condensation between the panes is not a humidity problem at all. It means the sealed unit has failed and the desiccant inside is saturated. No amount of ventilation will fix that, and the unit needs replacing.

Why the Living Room Is Often the Worst Room in the House

It is usually the bedroom or bathroom people blame, but the living room quietly stacks up more risk factors than either.

It typically has the largest area of glazing in the house, so it has the most cold surface for moisture to find. It is occupied for the longest continuous stretch in the evening, by the most people, all of them breathing. It is frequently where the airer goes when the weather is bad. It often contains the majority of the household's plants. And in older properties it is where you find a disused chimney, either draughty and cold or blocked and unventilated, neither of which helps.

It is also the room with the least tolerance for mould, because it is where the furniture, the artwork and the fittings you actually care about live.

If your living room sits stubbornly above 60 per cent humidity through the winter and the ventilation basics have not shifted it, an appliance is the honest answer. Dutch specialist Luchtontvochtigerwinkel sizes its dehumidifiers for living rooms by room volume and notes running noise for each model, which matters far more here than in a utility room, since you will be sitting next to it all evening.

Do Houseplants Cause Condensation?

They contribute, though they are rarely the main culprit, and the honest answer is more interesting than a simple yes or no.

Plants release moisture through transpiration, and roughly speaking most of the water you pour into a pot ends up back in the room air. A few small succulents make no measurable difference. A dozen large leafy plants in a single room, all watered generously, genuinely can lift humidity by several percentage points, and a big specimen with a lot of leaf area will out-transpire a small one considerably.

That does not mean getting rid of them. It means being aware of the total moisture budget of a room. If you have a heavily planted living room with condensation every morning, you can water less often and more thoroughly, move the thirstiest specimens to a brighter, better ventilated spot, group them away from the coldest window, or mix in artificial pieces where the greenery is doing a decorative job rather than a horticultural one. Faux plants carry the same visual weight in a corner or on a shelf without adding a drop of water to the air, which is a genuinely useful trick in a north facing room that never quite dries out.

Should You Wipe Condensation Off a Window?

Yes, and it is the most underrated five seconds in the whole routine.

Water sitting on the glass will run down into the frame, soak the seal and the timber or the plaster below, and feed the mould at the reveal. Wiping it away removes that water from the house entirely. Leaving it simply lets it evaporate back into the room, so the same water condenses again tomorrow night.

Use a window vacuum or a cloth, and wring the cloth out into a sink. Not onto a radiator, which puts every drop straight back into the air.

How Do You Stop Condensation Forming on Windows?

The full checklist, in order of impact:

• Ventilate in short, deliberate bursts. Ten minutes of windows fully open, twice a day, exchanges the air far more effectively than a window on the latch all day and loses much less heat.
• Run extractor fans properly, and for a few minutes after you finish cooking or showering, with the door closed rather than open.
• Stop drying laundry indoors without ventilation. A single load releases around two litres of water into the room.
• Keep a steady background temperature rather than short blasts of heat, so the masonry stays warm rather than just the air.
• Move furniture a few centimetres off external walls so air can circulate behind it.
• Buy a hygrometer. Aim for 40 to 60 per cent. You cannot manage what you are not measuring.
• Add a dehumidifier where the moisture source cannot be removed.

What Condensation Does to Lamps and Fittings

A brief word from our own corner of the subject, because window sills and side tables are exactly where table lamps live.

Persistent moisture at a sill will tarnish brass and copper bases, work its way under powder coating wherever it has been chipped, and encourage corrosion at the lamp holder inside an enclosed shade, which is a common and frequently misdiagnosed cause of flickering. Fabric shades absorb ambient moisture and hold onto that musty smell long after the room itself has dried out. Timber bases and woven rattan move with humidity and do not always move back.

So if you are choosing a lamp for a bay window or a north facing room that runs damp, favour a solid metal or ceramic base over veneered timber, and an open shade over a fully enclosed one so air can move around the holder. Our vintage table lamps and floor lamps are a good place to start, and if you simply want to soften a cold looking corner, a warmer bulb from our vintage bulbs range does more for the feel of a room than another 100 watts ever will.

Conclusion

Condensation on your windows is not a fault in the glass. It is the visible edge of your home's moisture balance, and the glass is simply the coldest place for that moisture to show itself. Get the balance right through ventilation, steady heat, and dehumidification where the source cannot be removed, and the water on the sill disappears along with the mould at the reveal.

Then you can go back to worrying about the far more enjoyable question of how to light the room properly.