Condensation on windows: why it forms and how to prevent it
You open the curtains in the morning and the glass is fogged up, drops are running down it and there is water standing on the window sill. Condensation on windows is a common sight as soon as the heating season starts. Occasional window sweating is not a fault, but if water collects every morning, wet window reveals, black dots in the corners and mould soon follow.
This guide explains why windows fog up, what it means when the water is inside, outside or between the panes, how to prevent condensation and when the problem is the window itself or how it was installed. At the end there is a checklist for the tradesperson and a sample request.
Condensation on windows: why it forms
Indoor air always contains water vapour. Warm air can hold much more moisture than cold air. When warm, humid air touches a cold surface, it cools down and the extra vapour turns into droplets. The temperature at which this happens is called the dew point. The more humid the air, the sooner it reaches the dew point, even on a surface that is only slightly cold.
In winter the glass is usually the coldest surface in the room. That is why vapour settles on the window first, just like on a cold glass in summer or a bathroom mirror after a shower.
There is more moisture in a home than you might think. According to VELUX, a family of four produces roughly 10 to 15 litres of water vapour a day through cooking, showering, drying laundry and breathing. That moisture does not disappear on its own. If it is not aired out, it ends up on the glass, the frame and the wall around the window.
Condensation usually gets worse with:
- drying laundry in the room or on the radiator,
- cooking without lids or an extractor hood,
- long showers with the bathroom door open,
- many people or many plants in a small room,
- rooms that are barely heated, for example a bedroom with the radiator turned off,
- heavy curtains and deep window reveals, because warm air cannot reach the glass.
Where the water is and what it means
First check which side of the glass the water is on. It tells you almost everything.
| Where the condensation is | What it means | What to do |
|---|---|---|
| On the inside of the glass | Too much moisture in the air or cold glass | Air the room, reduce moisture, heat evenly |
| Only along the bottom edge of the glass | Cold glass edge, often an old aluminium spacer | Air the room; when replacing, ask for a "warm edge" |
| Between the panes | The sealed glass unit has failed | Replace the glass unit, the frame usually stays |
| On the outside, in the morning | The glass insulates well, the outer pane is cold | Nothing, it clears when the air warms up |
Dew on the outside is not a fault
Droplets on the outside are most common in autumn and spring, in the morning after a clear, windless night. Good insulated glass keeps the room's heat inside, so the outer pane stays cold and dew forms on it, just like on grass or a car. With triple glazing this happens even more often than with double glazing. Wiping does not help; the dew clears once the sun warms the glass. If it bothers you, lower the shutters in the evening.
Water between the panes means a fault
An insulated glass unit is hermetically sealed. When the edge seal fails, moisture gets into the gap and the glass loses part of its insulation. You cannot wipe this fog away from either side, and airing will not fix it. The glass unit is replaced, and the window frame usually stays. If your windows are fairly new, check the warranty. Glass manufacturers give a warranty on the seal, some for up to 10 years from production.
How to prevent condensation on windows
If you are wondering how to prevent condensation on windows, start with your habits. They cost nothing and solve most cases.
- Air the room briefly and fully. Open the window wide for 5 to 10 minutes several times a day. Cross-ventilation with windows on opposite sides of the home works best. The air is replaced quickly and the walls do not have time to cool down.
- Do not leave the window tilted for hours. It cools the reveals around the window, wastes heating and barely replaces the air.
- Track humidity. Buy a cheap hygrometer and keep indoor humidity roughly between 40 and 60%. If it stays above 60%, condensation and mould are only a matter of time.
- Heat all rooms. A cold room is a damp room. VELUX advises keeping your home above 18 °C.
- Reduce moisture at the source. Cook with lids on and the hood running, keep bathroom and kitchen doors closed and do not dry laundry in living spaces if you can avoid it. If you have no choice, use a dehumidifier.
- Let warm air reach the glass. Do not draw curtains down over the radiator at night and keep large plants and clutter off the sill.
- Wipe the water straight away. Use a cloth or a window squeegee in the morning, especially on the frame and sill, so the water does not soak the seals and the wall.
- Use micro-ventilation. Many newer window fittings have a handle position that opens the sash just a few millimetres. It is handy at night in the bedroom.
If your air conditioner has a dehumidifying mode (usually "dry"), that helps too. What an air conditioner service and cleaning cost before winter is explained in our guide air conditioner service cost. To learn how moisture from the air ends up on your walls, read our guide to damp and mould on walls. If the radiator under the window only heats halfway, see radiator not heating.
Sweating PVC windows and condensation on new windows
A common story: you replace old wooden windows with new ones and that is exactly when your PVC windows start sweating. The problem is usually not a bad window. Old windows leaked air through tiny gaps, so the home "breathed" on its own, at the cost of a higher heating bill. New windows seal well, so the moisture that used to escape stays in the room. That is why condensation on new windows usually means you need to air the room more often than before.
In new builds and after major works, walls, screed and plaster keep releasing moisture for a while. According to VELUX, new houses usually dry out after a few years, and condensation then decreases.
Still, the air is not always the only culprit. If water and mould appear on the reveal and in the corner next to the frame while indoor humidity is normal, suspect the installation:
- A poorly sealed joint between window and wall. Window manufacturers state that a gap of just 1 mm over 1 m, not sealed with a vapour-tight tape on the inside, lets about 360 g of water condense in the wall in 24 hours. That is why RAL installation uses a vapour-tight tape inside and a vapour-permeable tape outside.
- An unfinished reveal. Expanding foam fills the gap, but on its own it neither insulates nor seals. The reveal is insulated with a thin board, reinforced with mesh and only then finished.
- Worn seals and a badly adjusted sash. Cold air gets in through hard or cracked seals and cools the frame further. Replacing the seals and adjusting the fittings often helps.
- Old aluminium windows without a thermal break. The metal frame conducts cold, so droplets form on the frame itself, not just on the glass.
If you are planning new windows, ask about glass with a "warm edge" spacer and a lower Ug value (the lower it is, the warmer the inner pane), about RAL installation and about trickle vents. Trickle vents are small openings in the frame that let in a little fresh air all the time, and humidity-controlled ones let in more when the air is damper.
Condensation on roof windows
Condensation on roof windows is common because warm, humid air rises towards the roof, and sloped glass cools down faster on the outside than a vertical window. In an attic room the same rules apply, only more strictly: air the room several times a day, do not dry laundry in the attic and keep a steady temperature.
For roof windows, VELUX also advises:
- place the radiator under the window so warm air flows along the glass,
- the interior lining around the window should not be at a right angle to the glass but as open as possible, horizontal at the top and vertical at the bottom,
- roof insulation needs a vapour barrier, otherwise moisture gets into the roof structure.
If droplets only appear on the roof window and the lining is built like a box around it, ask a tradesperson to take a look.
Window condensation or damp in the wall?
Condensation on windows is a problem of air and cold surfaces. It follows the heating and cold days and disappears in summer. Damp coming from the wall looks different:
- a stain under the window grows after rain: water is probably getting in from outside, through the drip edge, the window joint or the facade,
- a dark band and salts at the bottom of the wall: rising damp from the ground,
- a wet stain near pipes, even in summer: a leaking installation.
In those cases wiping the glass and airing the room will not help. How to tell them apart and how to remove mould is covered in our guide to damp and mould on walls. For cold exterior walls the lasting fix is insulation, and approximate prices are in our guide to facade insulation cost per m². For a list of autumn jobs around the house, from gutters to heating, see preparing your home for winter.
When to call a tradesperson, and which one
You can usually handle condensation on the inside yourself. Call a tradesperson when the problem does not go away even with good habits.
| Problem | Who you need |
|---|---|
| Fog or droplets between the panes | Window specialist: glass unit replacement |
| Water and mould on the reveal, indoor humidity is normal | Window specialist: check installation, seals and reveal |
| Draughts, the sash sticks, the seals are hard | Window specialist: seal replacement and fitting adjustment |
| A stain under the window after rain | Window specialist or facade contractor: seal the joint and drip edge |
| Cold exterior walls, mould every winter | Facade contractor: thermal insulation |
| Mould is cleaned, the wall needs restoring | Painter: repair, skim coat, paint |
How to write a good request on Gigron
The clearer your request, the faster you get a reply. Include:
- where the water is: on the glass inside, between the panes, on the frame or on the reveal,
- which room and how many windows,
- what kind of windows: PVC, aluminium or wood, roughly how old, double or triple glazing,
- since when it has been happening and whether it started after the windows were replaced,
- what humidity your hygrometer shows, if you have one,
- what you have already tried: airing, a dehumidifier, cleaning the mould,
- city, neighbourhood and a time that suits you.
Add close-up photos of the glass, the frame and the reveal.
Sample request (made up): "In my flat in Split (Bačvice), two PVC windows in the bedroom are constantly fogged up between the panes and I can't wipe it off from either side. The windows are about 15 years old. The other rooms don't have this problem. I need an assessment and a quote for replacing the glass units. Next week in the afternoon suits me."
On Gigron, check the windows category. You can find window specialists in Zagreb, Split, Osijek and Šibenik. For mould and the wall around the window, see painting and decorating and painters in Zagreb, and for cold exterior walls, facades. If you need better heating or an air conditioner with a dry mode, there are heating and air conditioners.
Find a tradesperson on Gigron
On Gigron you post a request and local tradespeople get back to you with offers. It is free and you do not have to call ten numbers.
Download the app:
Frequently asked questions
Is it normal for windows to fog up in the morning?
Occasional light condensation is normal, especially on cold mornings. Dew on the outside is even a sign that the glass insulates well. It is a problem if water collects on the inside every day, runs onto the sill and soaks the frame and the wall.
Does a dehumidifier help?
Yes, especially if you dry laundry indoors or the rooms are small. Still, it does not replace airing. A dehumidifier removes moisture from the air, while airing also replaces stale air.
Do I need new windows because of condensation?
Usually not. If the water is on the inside of the glass, change your habits first and track humidity. The glass unit is replaced if there is fog between the panes, and the whole window only if it is worn out, warped or seals very poorly.
In short: condensation on windows means there is too much moisture in your home for the cold glass. Air the rooms briefly and fully, keep humidity between 40 and 60% and heat all rooms evenly. Dew on the outside is not a fault, but fog between the panes is, and then the glass unit needs replacing. If water and mould keep coming back on the reveal even though you control humidity, call a tradesperson to check how the window was installed.