A winter morning, sub-zero outside, and a row of droplets has gathered along the bottom edge of the glass. From October to March this is the question our team hears most often: is condensation on windows a sign that the windows are faulty?
As a manufacturer, the profitable answer would be: yes, blame the windows, order new ones. The truth is less convenient. In our experience, roughly seven times out of ten condensation on windows is not a window defect at all - it is a mirror of the room's humidity and ventilation. Only one type of condensation actually means something is wrong with the window itself.
What follows is the physics without the formulas: why windows stream, how to tell harmless condensation from a genuine fault, and what to change so the glass stays dry.
Condensation on windows: three types, only one is a fault
Before you call anyone or start pricing up replacements, establish one thing: which side of the glass is the water on? The answer tells you immediately what you are dealing with.
| Where is the water? | What it means | What to do |
|---|---|---|
| On the room side of the glass | The most common case: it speaks about humidity and ventilation, not about window quality | Hygrometer, a ventilation routine, a clear path for warm air from the radiator |
| Between the panes, inside the sealed unit | The only genuine fault: the unit has lost its seal. No cloth will wipe this away | Replace the glazed unit only, not the whole window |
| On the outside, street side | Good news: the window holds heat so well that the outer pane stays cold, and morning dew settles on it | Nothing: the sun comes up and the dew is gone |
Dew point: the physics without formulas
Air always carries water vapour, and warm air carries more of it. The dew point is the temperature at which air can no longer hold its moisture, so the moisture turns into droplets on the nearest cold surface. In a home, the coldest surface is almost always glass.
A couple of numbers worth knowing. At 20°C indoors and 50% relative humidity, the dew point is about 9.3°C: any glass surface colder than that will start to mist. Push the humidity to 60% and the threshold rises to 12.0°C. The damper the air, the warmer the glass has to be to stay dry.
Two practical conclusions follow. First: condensation is governed by two factors - air humidity and the surface temperature of the glass. Second: windows that never get condensation do not exist in physics. At 80% indoor humidity and -20°C outside, any glass will mist over, including the most expensive one. A manufacturer promising otherwise is simply hoping you have forgotten your school physics.
The comfortable indoor range in winter is 40-60% relative humidity. In a hard frost we suggest staying closer to 40-45%: comfortable for people, safe for windows. A hygrometer that shows you this number costs about £10 and answers more questions than any window advert.
Condensation on the room side: a mirror of your home
Droplets or misting along the bottom edge of the glass on the room side mean there is more moisture in the air than your current ventilation can carry away. The window is only the messenger: physics shows up first on the coldest surface.
Why the bottom edge specifically? The edge of a glazed unit is always colder than its centre, because that is where the spacer bar joins the panes. Add thick curtains and a deep windowsill, and the air behind them barely moves, so the lower glass never gets washed with warm air from the radiator.
What to do? Three steps in the right order: measure the humidity with a hygrometer, sort out your ventilation rhythm (there is a dedicated section below), and clear the path for warm air from the radiator to the window. In most homes that is the whole fix, and nobody needs to be called out.
Misted units: condensation between the panes
A sealed glazed unit is exactly that - sealed: dry gas between the panes, usually argon, and a desiccant inside the spacer bar that mops up any remaining moisture. When the edge seal ages or gets damaged, outside air carrying moisture creeps in. The desiccant absorbs it for a while, but once saturated, the moisture starts settling on the inner faces of the glass.
The sign is unmistakable: fog or droplets that cannot be wiped off from either side. You will often see chalky white marks too, where the water has dried. In the UK trade this is called a misted unit, blown unit or failed double glazing - all names for the same thing.
The good news: it does not mean a new window. Only the glazed unit needs replacing; the frame stays - a timber frame is typically in fine shape decades on. We have explained how units differ on our glazing options page. If your condensation is between the panes, send us a photo of the window and rough dimensions: we will tell you what unit is in there and what a replacement costs.
Dew on the outside: a sign your windows work brilliantly
On a clear, cool night the outer pane radiates its heat to the sky and drops below the dew point of the outside air. By morning there is dew on it, exactly as on a car windscreen or the lawn. With an energy-efficient window, the room's heat simply never reaches the outer pane - which is why it stays cold.
Old windows never did this, for a sad reason: they were heating the street, so the outer glass stayed warm. Dew on the outside is not a defect; it is confirmation that the insulation is doing its job. Nothing needs doing: the sun comes up and the dew is gone within a couple of hours.
Why new windows stream when the old ones never did
The most common call we get every January comes from homeowners who replaced draughty twenty-year-old windows in autumn and are now waking up to condensation for the first time. Are the new windows worse? No - they are tighter, and in almost every case they are working exactly as intended.
Old windows with their gaps ventilated the house continuously: cold air came in, moist air went out. You paid for that "ventilation" through your heating bill, because the warmth left through the same gaps. New windows close the gaps, the heat stays in - but so does the moisture that used to drain away by itself.
The conclusion is not "bring back the draughts" but to take ventilation into your own hands. Airtight windows and a deliberate ventilation routine are a pair that only works together. How to set that routine is two sections down.
Where 10 litres of water a day comes from
A family of four releases roughly 10 to 15 litres of water into the air every day. That is not an exaggeration; it is ordinary life:
- breathing and skin - about 1-2 litres per person per day;
- cooking - 1-2 litres a day;
- a shower or bath - 1-2 litres per wash;
- drying laundry indoors - 1.5-2 litres per load;
- houseplants - 0.5-1 litre a day.
Three loads of laundry dried indoors a week is a full bucket of water that has to end up somewhere. If ventilation does not carry it out, it settles on the coldest surfaces - the glass first, then the cold corners of external walls.
How to ventilate in winter: three working modes
Purge ventilation is the winter workhorse: windows fully open for 5-10 minutes, two to four times a day. The air in the room changes quickly, but walls and furniture have no time to cool down, so the heat loss is modest. Non-negotiable after a shower, after cooking, and before bed in the bedroom. Building Regulations treat purge ventilation seriously enough to require that openable windows give you at least 1/20 of the floor area for it.
Background ventilation is the steady trickle: trickle vents in the frame, or the night-vent position on the handle leaving a few millimetres of gap. Comfortable in autumn and spring. In a hard frost, be careful with the night-vent position: a constant cold stream chills the rebate zone and that is exactly where the glass can mist or frost over. Below zero, short and intensive purge ventilation is the safer habit.
Mechanical ventilation with heat recovery (MVHR) changes the air continuously and recovers 80-90% of the outgoing heat. In new builds and deep renovations it is the most sensible route today: fresh air without an open window and with far smaller heat losses.
Trickle vents and the 2022 rules
Since June 2022, Approved Document F in England has required that replacement windows do not leave a room with less ventilation than before - and in practice that usually means fitting trickle vents, with guidance values of 8,000 mm² equivalent area for habitable rooms and 4,000 mm² for bathrooms, unless the home has a mechanical ventilation system. Plenty of homeowners dislike how they look; as window makers we understand. But the rule exists precisely because of the paradox above - tight new windows in a home with no ventilation plan reliably produce condensation and, further down the line, mould.
Mould is no longer treated as a cosmetic nuisance in the UK. Awaab's Law, in force for social landlords from October 2025, sets hard legal deadlines for investigating and fixing damp and mould. If you rent and the streaming windows come with a mouldy wall, you now have considerably more leverage than a few years ago.
What a timber frame changes in the condensation equation
Remember: condensation forms on cold surfaces, so the frame material plays a direct part. Timber's thermal conductivity is about 0.13 W/(m·K) - better than uPVC at roughly 0.17, and in a different league from aluminium, which without thermal breaks is a genuine cold conductor. That is why the room-side surface of a timber frame stays warm, and condensation on the frame itself is rare.
The second thing we recommend checking on any window, after 12+ years of making them: the warm-edge spacer. It is a composite strip along the edge of the glazed unit replacing the old aluminium one, and it lifts the temperature of the glass edge by several degrees. Where does misting start first? Exactly there, along the bottom edge. A warm-edge spacer largely neutralises that weak spot.
The third factor is the unit itself: the better its insulation, the warmer the inner pane and the later dew appears on it. Whether triple glazing is enough or a fourth pane earns its keep, we have set out in our triple versus quadruple glazing comparison.
Mould around windows: where the health risk starts
Condensation by itself is just water. The problem starts when surface humidity stays above 70-80% for days on end: mould begins to grow on the seals, the reveal or the wallpaper. That is no longer cosmetic - mould spores aggravate asthma and allergies, particularly in children.
Where the mould grows tells you where to look for the cause. Black spots along the bottom edge of the glass or on the silicone joint mean regular condensation collecting and running down: the fix is humidity and ventilation. Mould on the reveal, on the wall around the window or in room corners tells a different story - that is usually a thermal bridge: an under-insulated reveal or a poor installation joint. Ventilation will soften the symptoms there but will not cure the cause: the installation needs looking at. It is also why we treat window restoration and installation with the same care as the windows themselves.
Five habits that make windows stream
- Curtains to the floor and deep sills over the radiator. Warm air never reaches the glass, the glass cools, and the dew point does the rest. Leave warm air a path from the radiator up past the window.
- Drying laundry indoors with no ventilation. Two litres of water go into the air within hours. If you must dry indoors, do it in a room with background ventilation open, or purge-ventilate afterwards.
- Blocked or sealed-up air vents in the kitchen and bathroom, "to stop the cold". Background extraction is the only permanent moisture outlet most homes have: close it, and the moisture has nowhere to go.
- Furniture and heavy curtains tight against the window. In still air the glass and reveal cool faster, and that is where both dew and mould appear first. A few centimetres of circulation gap is the cheapest fix there is.
- Fighting blind, with no measurements. Without a hygrometer you cannot tell whether your problem is 70% humidity or a cold glass edge. A £10 device turns guessing into a diagnosis.
When you genuinely need a professional
Most condensation questions you will solve yourself with a hygrometer and a ventilation routine. You need a professional when you see one of these:
- condensation or chalky marks between the panes - the unit has failed and needs replacing;
- water running down the frame, or a wet reveal - the installation joint and waterproofing need checking;
- mould on the reveal keeps returning although indoor humidity is normal - suspect a thermal bridge;
- an old single-glazed timber window mists over every morning - consider restoration with upgraded glazing, keeping the original frame.
And if you are planning replacements and want to see how glazing choices move the price, try our window configurator: pick the sizes, profile and glazing, and the estimate lands with us together with your questions.
Frequently asked questions
Does condensation on windows mean the windows are bad?
Usually not - in most homes the condensation comes from indoor humidity, not from a faulty window. Dew on the room side speaks about air humidity and ventilation; dew on the outside is actually a sign of good energy efficiency. Only condensation between the panes points to a window fault - the sealed unit has failed and needs replacing.
When should I worry about condensation on the inside of windows?
When it stops being an occasional morning mist and becomes standing water every day, when paint or seals start staining, or when mould appears. Persistent condensation despite reasonable humidity (40-60%) and daily airing deserves a proper look at ventilation and the glass edge temperature.
Should you wipe condensation off windows?
Yes - wiping protects seals, paint and gaskets from sitting in water. But treat it as first aid, not the cure. If you are wiping every morning, the real fix is humidity control and a ventilation routine, not a better cloth.
What do I do about condensation between the panes?
Replace the glazed unit - not the whole window. In a timber window the unit is held by glazing beads, and a fitter swaps it within hours. Send us a photo of the window and rough sizes: we will tell you what unit you need and what it costs.
What indoor humidity is right in winter?
40-60% is the comfort zone; in a hard frost aim nearer 40-45%. At 20°C and 50% humidity, glass only mists once its surface drops below 9.3°C - rare with modern glazing. A cheap hygrometer tells you instantly whether your problem is in the air or in the window.
Is it the same with uPVC windows?
Yes - the physics does not care about the frame material: dew point, humidity and cold glass behave identically on uPVC and timber. The difference is the frame itself: timber stays warmer, so dew settles on it less often. Every piece of advice in this article applies to uPVC windows too.
Do trickle vents have to be fitted with new windows?
In England, since June 2022, replacement windows generally must include background ventilators - 8,000 mm² equivalent area for habitable rooms, 4,000 mm² for bathrooms - unless mechanical ventilation covers the room. Scotland, Wales, Northern Ireland and Ireland have their own building rules, so check locally. The principle is the same everywhere: airtight new windows without a ventilation plan are a reliable recipe for condensation.
If you want a specific answer about your own windows rather than general advice, write to us or call: we will tell you what to change first in your situation. And if you are weighing up window materials more broadly, our comparison of wooden versus uPVC windows and their real cost is the next read. And once the windows are in, our guide to timber window maintenance keeps them dry and working for the next 50 years.
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