Steamy Windows: What You Can Do About window Condensation

We asked the experts at VELFAC, makers of modern aluminium/wood window, to explain how and why condensation occurs... and what you can do to control it.
Condensation on glass window panes is a natural phenomenon that can't be completely avoided, no matter how well insulated your house or how efficient your double-glazing. But there's are things you can do to help the situation. Let's dive into the how and why of condensation.

Why is there condensation on my windows?

Condensation occurs when warm, moisture-laden air meets a cool surface such as glass – the change in temperature causes the moisture to condense onto that surface in the form of water droplets. The greater the air humidity, and the colder the temperature of the surface, the greater the condensation. This is why the problem is worse in cold weather.

Is condensation harmful?

Although the condensation itself is not harmful, it is unattractive and can damage windows. Condensation running down onto wooden window frames can encourage mould growth, and the continual build up of water will eventually rot the frame. Condensation is also evidence of a humid atmosphere which is, in itself, unhealthy and can exacerbate respiratory problems, allergies and other conditions. (I've paraphrased the original text here but I would be careful about giving any health related advice unless it's fully verified)

How can I prevent condensation?

The key to preventing condensation is to control humidity and temperature. The ideal humidity level is between 40-50% when air temperature is 20oC, but it is very easy to increase humidity levels without even realising. If condensation is starting to become a problem, try the following:

Increase ventilation: You want to replace warm moist air with cool dry air, so air rooms for five to 10 minutes, several times a day; airing a house in this way results in minimal heat loss.

Make sure to air the house evenly – don't just open kitchen or bathroom windows, as this will simply move the air with the highest humidity into (rather than out of) the house. Air your house even if it is raining - the warm internal air will still hold more moisture than the cold air outside.

If your windows have trickle ventilators – narrow slits placed at the top of the frame – then keep these open all day (it is possible to retrofit trickle ventilators into VELFAC windows if necessary). Make sure other air vents – such as air bricks – are large enough, and are not blocked.

Ventilate well when cooking and bathing: If you have an extractor hood in your kitchen, or fan in your bathroom, then turn these on as soon as you start cooking or bathing and keep them running for 10 to 15 minutes after you have stopped. If you don't have such extractor fans, then these rooms have to be aired more frequently.

Dry clothes outside: Whenever possible, avoid drying clothes inside the house. If you use a tumble drier, make sure the warm air is vented outside.

Install an air recuperation system: If condensation remains a serious problem, then an air recuperation system could be installed, to maintain a continuous flow of dry air.

It's also important to remember that there are other, less obvious sources of humidity which have to be managed:

People, pets and houseplants: Every living thing within a building adds to the overall humidity; for example, a single adult will produce around two litres of water vapour a day. Ventilation should take account of the number of people, pets and houseplants living together.

Lower night time temperatures and unheated rooms: Cold air can't hold as much moisture as warm air, so if temperatures drop – when heating is turned off – then the cooling air will deposit moisture, which is why morning condensation is common.

An empty house: If your house has been unoccupied for any length of time (while you are on holiday, for example) then walls, furniture, carpets and so on will have absorbed moisture while the house was unheated, especially if the weather was damp when you were away. For up to 10 days after you return, keep windows open 24 hours a day while raising the internal temperature by 4-5'C.

If condensation has suddenly become a problem

It's not unusual for windows which have been condensation free to suddenly mist up on a regular basis. If you have experienced such a change, then it might be the result of one – or a combination – of the following causes:

You have new windows: New low energy glazing improves insulation and so you have to ventilate your house more regularly.

You have recently extended part of your house, or moved into a new build property: New plaster, cement, concrete and paint all contain a lot of moisture which takes time to evaporate completely – possibly up to a year. As a result, extended or new build houses have to be aired more frequently than older properties.

Onset of winter: Condensation is at its worst when autumn turns to winter. Temperatures start to fall, especially overnight, and heating is turned on indoors; the building structure, unheated all summer, is relatively cool but as it heats up it emits moisture into the air. To combat condensation, air the building more frequently until it has adapted to the changing temperature.

Underfloor heating: Condensation can result if you have recently replaced radiators with underfloor heating, as radiators cause warm air to rise and circulate, encouraging better ventilation. Again, you need to air more frequently.

Deeper windowsills or new curtains: If you have installed windows with deeper sills, or hung thicker curtains, then heat from nearby radiators may not be able to reach and warm the glass pane, thereby encouraging condensation. The answer, again, is to air more frequently, or open curtains overnight.

Other concerns

If you have condensation outside your windows: this is nothing to worry about and demonstrates the excellent insulation properties of your double-glazed units. Traditional double-glazing still transmits heat through both panes making the outer pane too warm for external condensation to occur – low energy glazing keeps this heat inside the room.

If you have condensation between the panes: this means that the double-glazed seal is broken and moisture has entered the space between the panes. The insulating properties of the window are not compromised, but the resulting thin grey film of condensation is unattractive – it prevents light from entering and you can't see out properly. Check your window manufacturer's guarantee if this occurs.

VELFAC offers high quality, low maintenance composite windows and doors that combine low energy performance with elegant and contemporary design. Find out more at

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