Condensation on windows – causes and cures

From time to time, and during certain months of the year, we receive emails and calls from customers who are concerned about condensation on their windows. In this article we look at the causes and cures for condensation.

What is condensation?

Condensation is the process of a gas or vapour changing into a liquid. This happens when the temperature of the gas or vapour falls below its ‘dew point’. Morning dew on the grass is caused by this – on cold nights the ground radiates heat skywards and, as a result, the temperature of the ground drops below the air temperature, causing water vapour to condensate on it as dew.

This is why you can see your breath in cold weather – the water vapour in the air you breathe out is at a higher temperature than the air you’re breathing into, so it condensates and becomes mist-like.

The higher the air temperature the more water vapour it can hold (which is why hot countries tend to be more humid).

What causes condensation on windows?

When the colder weather arrives we all enjoy retreating to our cosy, insulated, central-heated homes. These modern comforts may help to keep out the cold, but they are the main cause of condensation on your windows.

How condensation forms

Condensation forms when water vapour inside your home is unable to escape. The amount of water vapour builds up in the air and, when it can hold no more, it liquifies on a colder surface (usually your windows). This starts off looking like a mist on the windows but, if allowed to continue, it will eventually form droplets of water.

Every time you boil a kettle, have a shower, use your tumble dryer, water your houseplants or even breathe out, more water vapour is added to the atmosphere in your home.

Modern homes are so well insulated and energy-efficient, our rooms are warmer with less ventilation. In times gone by draughtier homes had far fewer condensation issues as the air was constantly moving and water vapour could escape up chimneys, through gaps in window frames or under doors.

External condensation

Condensation doesn’t just happen on internal surfaces. With the latest glazing technologies we have achieved amazing insulation for our homes, but this can lead to external condensation. Because the outer pane of glass is never heated, in cold, fresh spring or autumn days external condensation can gather due to a combination of humid air conditions and the glass having a temperature below the dew point.

Why is window condensation a problem?

While the immediate effect of condensation (misty windows) could be considered just a nuisance, it can lead to more serious problems if it isn’t dealt with quickly:


In any areas of untreated damp caused by condensation, black mould can grow. As well as being unsightly and damaging the surfaces it forms on, it can also become a health hazard.

According to the NHS, “if you have damp and mould in your home you’re more likely to have respiratory problems, respiratory infections, allergies or asthma. Damp and mould can also affect the immune system.” Further information from the NHS.

Damage to walls

Damp patches can appear on walls around the affected window. As well as becoming a breeding ground for mould, this can discolour paint and cause wallpaper to peel.

Left untreated this damp can seep through to the plasterwork and cause it to disintegrate.

Damage to soft furnishings

Curtains and other soft furnishings can also be damaged by the damp caused by condensation. They can become mouldy and take on the musty smell of the room.

External condensation

Fortunately, external condensation on your windows is very unlikely to cause damage as it is dried off by warmth and/or air flow during the day. Therefore, neither the windows nor the surrounding area stays damp long enough to cause any problems.

Are some windows more prone to condensation than others?

The design of windows and conservatory/orangery glazing is constantly evolving to create more and more energy-efficient solutions. This not only keeps your home warm, but it saves on your heating bills and reduces your carbon footprint. However, this doesn’t necessarily stop condensation.

Single glazed windows

Single glazed windows are the biggest cause of internal condensation. As there is only one pane of glass it doesn’t warm up with the room temperature and will instead be almost as cold as it is outside.

Less energy efficient double glazing

Ironically, double glazing which isn’t constructed to the most energy saving specification is less likely to result in condensation.

The internal pane of glass will be warmed by the heat of the room, therefore won’t provide such a cold surface for the water vapour to accumulate on. The less effective insulation values mean that the external pane of glass will also be slightly heated by the warmth escaping, so external condensation is also less likely.

However, the trade-off is a colder home or higher heating bills.

Energy-efficient double or triple glazing

The most sophisticated energy-efficient glass is the biggest culprit when it comes to external condensation.

While the internal pane of glass will warm up with the room, thereby minimising internal condensation, none of this warmth will reach the outer pane of glass.

This means that it will be colder and more prone to external condensation when the outside humidity levels are high and/or the temperatures are particularly cold.

How to stop condensation on windows

It’s important to prevent internal condensation and deal with any existing problems as quickly as possible. But this doesn’t mean sacrificing the snugness of your home.

There are a few ways in which you can reduce condensation, but the main thing to remember is MORE VENTILATION = LESS CONDENSATION!

Internal condensation

This is generally caused by the room not being double or triple glazed, not being sufficiently heated and/or being shut up so the air doesn’t move around enough. The upshot is too much water vapour in the room and windows which are cold enough to attract it.

Keep rooms well ventilated

The obvious way to do this is to open the windows a little or use a trickle vent if one is fitted. Try to open a window in each room every day to move the air around.

If the room was originally constructed with a fireplace, but this has since been blocked up, check that adequate ventilation has been installed, which usually means wall vents or air bricks.

Pay close attention to the ventilation in rooms where steam is produced, ensuring that bathrooms are fitted with extractor fans where possible. In kitchens, use extractor hoods or cooktop extractors to deal with steam from cooking, particularly if you’re in an open plan room, as you don’t want the moisture to travel and settle in living areas.

After a shower or bath close the bathroom door and open the window to allow the moist air to escape outside, not into the rest of your home.

Try to dry clothes outside when possible to avoid hanging them in the house and causing more moisture to form in the air.

If necessary, a dehumidifier can help to remove moisture from the air, though this isn’t as effective as good ventilation. You might find a dehumidifier of particular use in a conservatory or orangery over winter.

Replace single glazing with double glazing

Single glazed windows will inevitably cause condensation over the colder months as the glass cannot warm up. Replacing single glazing with double or triple glazing will give the window an internal layer of glass, insulated from the temperatures outside, which can warm up with your room and reduce condensation. This will also help to keep the room warmer, enabling the air to hold onto more moisture.

PVCu windows

Keep your home warm

Slightly increasing the temperature in your home will help the air retain water rather than allowing it to condensate. You still need to ventilate the rooms well, which will allow the water vapour to escape.

Installing radiators under your windows can also be useful as it will help to keep the inner pane of glass warm and prevent water vapour condensating on it. The same applies to heated towel rails in bathrooms.

Don’t forget unoccupied rooms

Condensation can become a big issue in rooms you don’t use very often if you turn the heating off in that room and shut the door. Water vapour has nowhere to go and the glass is being kept cold, so condensation will inevitably occur. And, because you’re not going in there regularly, it can cause damage before you’ve even realised it’s happening.

Consider keeping the heating on in that room, perhaps installing thermostatic radiator valves so you can control the temperature separately to the rest of the house. You should also ensure that the room is well ventilated, perhaps by keeping the windows open a little or installing a dehumidifier.

External condensation

There is little you can do to prevent external condensation. You will probably find that it’s worse on some windows than others because of the aspect of that window and/or how sheltered it is. It’s also likely to be a bigger problem near to rivers or lakes where the humidity will be naturally higher.

You should find that external condensation only lasts for a few hours in the morning before the warmth of the sun or air movement from the wind dries it.

For conservatories, you can significantly reduce external condensation by using specialist glass, such as Pilkington’s ActivTM self-cleaning glass. This has a hydrophilic coating that repels water, minimising the effects of condensation.

Condensation between window panes

In standard double or triple glazing this should not occur as the unit should be hermetically sealed to prevent any water vapour getting in.

So, if you do have condensation between your window panes, this is a sign that the seal has failed and the unit should be replaced.

If you have secondary glazing installed alongside single glazed windows, the unit will not be hermetically sealed, and therefore some condensation in the cavity may occur.

More help with condensation

If you are repeatedly finding issues with condensation despite ventilating and heating your room well, we’d be happy to help investigate possible causes and discuss options for remedying the problem.

Pop into our showroom for a chat or call us on 01444 45 71 45 to speak to one of our experts.