Short answer first: you prevent condensation on the inside of your windows by doing two things at once. Putting less moisture into the house (extractor fan on while cooking, bathroom door shut while showering, no washing dried indoors) and ventilating better (trickle vents open, mechanical ventilation running, windows thrown open briefly several times a day). Aim for a relative humidity between 40 and 60 percent, and measure it with a hygrometer.
And take note: condensation on the OUTSIDE of your modern double or triple glazing is not a problem. It’s actually a sign that your glass is insulating well. It clears on its own once the sun comes up.
Below you’ll read exactly why condensation forms, the difference between inside and outside condensation, how much moisture your household produces in a day, and what you do step by step to stay ahead of misted-up windows and mould.
Why does condensation form on your windows?
Condensation is nothing more than water vapour turning back into liquid.
Warm air can hold a lot of moisture. Cold air almost none. When warm, damp indoor air meets a cold window, that air cools down. At a certain point it can no longer hold the moisture, and it settles on the glass as droplets. That point is called the dew point.
Put simply: warm damp air + cold glass = a misted-up window.
That’s why you mainly see condensation during the heating season. It’s cold outside, your glass is cold, and indoors you’re heating air that’s full of moisture from showering, cooking and breathing. Mornings are often worst, because that’s when the glass is coldest and the moisture has had all night to build up.
Condensation inside or outside: the crucial difference
This is the part most people mix up. And it makes a world of difference.
| Where is the condensation? | What does it mean? | Action needed? |
|---|---|---|
| Inside of the glass | Too much moisture in the house, too little ventilation, cold glass | Yes, tackle the moisture and the ventilation |
| Outside of the glass | Your double or triple glazing is insulating excellently | No, this is normal and good |
| Between the panes | The unit has failed, the sealed glazing is broken | Yes, get the glass replaced |
Condensation on the inside is the one to watch. That’s the moisture you produce yourself in the house, settling on your cold glass. You want to prevent it, because over time it turns into mould.
Condensation on the outside of double or triple glazing is actually good news. Because your glass insulates so well, hardly any heat reaches the outer pane from inside. So that outer surface stays cold. On a damp, clear night it drops below the dew point and the outside air settles on your window. Glazing specialists say this is a sign your glazing is doing precisely what it’s meant to do. It clears by itself as soon as the sun hits it or the outside temperature rises. So leave it alone.
Condensation between the panes is a different story. That means the edge seal around your sealed unit has failed and moisture has got into the cavity. You can’t wipe that away and you can’t ventilate it away. The unit is broken and needs replacing.
How much moisture does a household produce in a day?
More than you think. And that’s exactly why condensation is so stubborn.
An average family produces 10 to 15 litres of moisture a day indoors. That comes from cooking, showering, the washing, plants, and simply from breathing and sweating. Source: this figure is widely quoted by ventilation and damp specialists in the Netherlands.
To give you a picture of where that moisture comes from:
- Showering: a solid burst of steam each time, certainly with several people
- Cooking: steaming pans without lids give off a lot of moisture
- Drying washing indoors: a wet load releases litres of moisture into the room
- Breathing and sweating: a person gives off moisture just lying asleep
- Plants and a fish tank: a small, constant source that’s easy to forget
All that moisture has to go somewhere. Ventilate too little and it stays hanging in the house, the humidity climbs, and it settles on the coldest spot: your window pane.
What is the ideal humidity in the house?
Aim for a relative humidity between 40 and 60 percent. Around 50 percent is ideal.
If you’re consistently above 60 percent, the odds of condensation and mould are high. Below 40 percent the air gets too dry and you end up with dry eyes, irritated airways and cracked lips.
How do you know where you stand? Measure it. A hygrometer costs a few pounds and shows the humidity as a percentage, just as a thermometer shows the temperature. Put one in your living room and one in the bedroom. Seeing readings above 60 percent and your windows misting up? Then you know enough: ventilate more and make less moisture.
Preventing condensation on windows: tackle the moisture at the source
The smartest approach is to catch the moisture where it’s created. Stop it spreading through the whole house. Here we go:
1. Switch on the extractor fan while cooking. And put a lid on the pan, that saves a lot of steam. Cooking with fat or oil? Then leave the extractor running for another hour afterwards. Advice from the Dutch consumer organisation Milieu Centraal.
2. Bathroom door shut, ventilation on the highest setting. Keep the bathroom door closed during and after showering, so the steam doesn’t spread through the house. Put the mechanical ventilation on the highest setting or open the window. Ventilating for 20 to 30 minutes afterwards is enough. Then wipe the walls and floor dry.
3. Dry the washing outdoors or in a ventilated space. A wet load drying indoors releases litres of moisture into the room. Hang it outside if the weather allows, or use a tumble dryer that vents outside. Has to be indoors? Then do it in one room with the window open, not spread through the whole house.
4. Cook with lids on your pans. Little effort, saves a lot of steam.
Preventing condensation by ventilating properly
Catching the moisture is half the job. The other half is getting the damp air that’s there anyway out of the house. In other words, ventilation.
An important distinction: background ventilation is not the same as airing out.
- Background ventilation is continuous, round the clock. Trickle vents open, mechanical ventilation running, top-hung windows on the latch. Day and night, all year, winter included. Milieu Centraal puts it literally: always ventilate.
- Airing out is throwing your windows wide open now and then. Best done briefly and several times a day, rather than once for a long stretch. Certainly after cooking, showering or DIY work.
Practical tips:
Open your trickle vents. Those things above your windows are there for a reason. Vents screwed shut are a classic cause of condensation and mould.
Let the mechanical ventilation run. If you have a ventilation unit, don’t put it on the lowest setting to save money. That’ll cost you more in mould and repairs later than you save on electricity.
Get cross ventilation going. One open window only half helps. Open windows on two sides of the house and the damp air really does get pulled through. Internal doors open helps the air flow.
Bedroom: window or vent on the latch. Two people breathe out moisture all night long. A top-hung window or open vent keeps your panes and walls dry.
Heating against condensation: steady and even
This is where people slip up during the heating season.
Letting the room go completely cold and only then turning the heating on is asking for condensation. Cold glass and cold walls catch the most moisture. Warm air, on the other hand, can hold more moisture before it settles.
Better to heat steadily and evenly at a comfortable temperature, instead of letting your house cool right down and then blasting it back up. Rooms you never heat (spare room, hallway) are exactly where condensation and mould strike first. The Dutch public health service GGD advises keeping your house at a minimum of 18 degrees during the day, so moisture evaporates rather than settles.
Another classic mistake: curtains pulled tight across cold glass or in front of the radiator. Thick curtains hanging floor-length in front of a cold window trap the layer of cold air against the glass. Moisture settles there all the more easily. And curtains in front of the radiator send your heat up behind the curtain instead of into the room. So keep curtains clear of the glass, and leave the radiator free.
Does a dehumidifier help against condensation?
Sometimes, but it’s a plaster, not a cure.
An electric dehumidifier can bring the humidity down in a problem room (cellar, poorly ventilated bedroom). Handy as a stopgap.
But take note: those little moisture-trap tubs with crystals barely do anything in a normal living room. Milieu Centraal points out that products like that can’t do their job properly in larger spaces. They pull out a fraction of your 10 to 15 litres of daily moisture.
The real fix stays the same: make less moisture and ventilate better. You bring in a dehumidifier as a supplement, not as a replacement.
Better glass: from single glazing to double or triple
Still got single glazing and your windows mist up constantly? Then the heart of the problem is that your glass is stone cold.
Single glazing is so cold on the inside that moisture almost always settles on it. Replace it with insulated glazing (double or triple) and the inside surface of the glass stays far warmer. The moisture then settles less readily, because the glass sits further away from the dew point.
That’s an investment, and not something for a quick Saturday. But it kills two birds with one stone: less condensation and a lower energy bill. And yes, you may then suddenly see condensation on the outside. That, as you now know, is exactly the proof that your new glass is doing its job.
Removing mould from your window frame safely
Too late, and there’s already black mould on your sealant or frame? Then here’s how you tackle it, following the approach of the Dutch public health service GGD:
1. Dampen the surface first. Sounds odd, but scrubbing dry mould sends it straight into the air. Wetting it keeps the spores down instead of you breathing them in. Never use a dry brush.
2. Use an approved mould treatment product. You can recognise an approved product by the authorisation number on the packaging (in the Netherlands, for instance, a code starting with ‘NL’ or ‘EU’, or ending in ‘N’). In practice a lot of people use a solution of soda crystals in water, but with stubborn mould that doesn’t always do the job.
3. Never mix different products. No mixing soda crystals, bleach, chlorine and vinegar. That can give off dangerous fumes.
4. Protect yourself and ventilate. Wear gloves and open the windows while you clean. Sensitive lungs or asthma? Let someone else do it.
5. Is the mould IN the material? Then it has to come out. On smooth glass and smooth frames you wipe mould away easily. But if the timber, sealant or plaster is affected and the mould is inside it, cleaning won’t help and that material has to be replaced. Renting? Talk to your landlord first.
And after that: deal with the cause. Otherwise the mould is back in no time. The GGD sums it up in four steps: remove it, limit the moisture, ventilate and heat. You have to do all four, they work together.
Frequently asked questions
Is condensation on the outside of my windows harmful?
No. Condensation on the outside of double or triple glazing is normal, and it’s actually a sign that your glass is insulating well. The outer pane stays cold because hardly any heat comes through from inside, and on a damp night the outside air settles on it. It disappears by itself when the sun comes up or the outside temperature rises. There’s nothing you need to do.
What is the ideal humidity in the house?
Between 40 and 60 percent, with around 50 percent as the ideal. Above 60 percent the odds of condensation and mould rise sharply. Below 40 percent the air gets too dry. Measure it with a hygrometer, which only costs a few pounds.
How much moisture does a household produce in a day?
An average family produces 10 to 15 litres of moisture a day through cooking, showering, the washing, plants, breathing and sweating. All that moisture has to get outside through ventilation, otherwise it settles on your cold windows.
Does a window on the latch help against condensation?
Yes. A window or trickle vent on the latch lets the damp air escape. Throw the windows wide open briefly several times a day (cross ventilation works best) and alongside that keep your vents and mechanical ventilation running continuously. In the bedroom especially, a window on the latch helps against misted-up panes in the morning.
Why do I get more condensation since my house was better insulated?
Because a well insulated, draught-proof house doesn’t let the moisture escape on its own so easily. In an old, draughty house the damp air leaked away everywhere. Insulate and seal everything up, and you have to start ventilating deliberately. Open your vents and let your mechanical ventilation run, and this problem is solved.
Can I get rid of condensation between the panes?
No. If there’s condensation BETWEEN the two panes of your sealed unit, the unit has failed and the insulation is gone. You can’t wipe it away or ventilate it away. The glass has to be replaced.
Condensation, ventilation and insulation all tie back to how smartly you heat. Get your moisture and ventilation under control and insulate properly, and you keep a healthy house and a lower energy bill. Planning an extension, a renovation or a garden room where the insulation and ventilation need to be right from day one? Then have a quick look at Fred’s DIY Plans. Explained plainly, do it yourself, exactly the way Fred would go about it.