Do you wake up to fog and water droplets on your windows? Condensation on windows is common, especially in winter, and it almost always comes down to humidity meeting cold surfaces. Here is why it happens and how to get rid of it.
Short answer: Condensation on windows forms when humid indoor air meets the cold glass and the water vapour cools into droplets. It is most common in winter. Lower your indoor humidity, improve ventilation and let air circulate freely at the window, and the fogging will decrease.
Why do windows get condensation?
Condensation forms when warm, humid air meets a surface that is colder than the air's dew point. Window glass is often the coldest surface in the house, so that is where moisture appears first. The higher the indoor humidity and the colder the glass, the more condensation. That is why it is most noticeable on cold winter nights.
Condensation on the inside, outside or between the panes?
Where the condensation sits tells you what is causing it:
- On the inside: a sign that the indoor air is too humid or that air is not circulating at the window. This is the most common case and the one you can influence yourself.
- On the outside: usually harmless and actually a sign that the window is well insulated, since the outer pane stays cold. It clears as the day warms up.
- Between the panes: indicates that a sealed glazing unit has failed. Ventilation will not help here, and the unit usually needs to be replaced.
How to get rid of condensation on the inside
If you have condensation on the inside, the goal is to lower the humidity and get the air moving. Try this order:
- Air out briefly and effectively by opening the windows for a few minutes so humid air is exchanged for drier outdoor air.
- Check the ventilation. Make sure the extract air in the kitchen and bathroom works and that supply vents are not blocked.
- Let air circulate at the window. Pull heavy curtains aside and move furniture away from the window so warm room air reaches the glass.
- Lower the humidity. If the relative humidity is above 60 percent, a dehumidifier can bring it down. Keep an eye on it with a hygrometer and aim for 30 to 60 percent.
Common sources of indoor moisture
To understand the condensation, it helps to know where the moisture comes from. A typical household adds several litres of water to the indoor air every day through everyday activities:
- Cooking and boiling water that release steam, especially without a lid and extractor fan.
- Showers and baths that quickly raise humidity in the bathroom and nearby rooms.
- Laundry drying indoors, which can add a lot of moisture in a small space.
- Many plants and even the breath of the people at home.
When this moisture is not ventilated away it builds up in the air and settles on the cold window. That is why condensation often shows up first in the bedroom in the morning.
When is condensation on windows a problem?
Occasional fogging is rarely harmful, but recurring condensation is a sign that humidity is too high. If moisture lingers it can lead to damage in frames and walls. Read more about moisture in the home and how to measure moisture at home to keep track of the levels.
How to prevent condensation
The key is to keep humidity at a steady, moderate level and to have good air exchange. Air out regularly, use the extractor fan when cooking, do not dry laundry in already humid rooms and keep vents open. Keeping humidity within 30 to 60 percent reduces the risk of both condensation and mould.
Frequently asked questions
Is condensation on windows dangerous?
Occasional fog is rarely harmful. But recurring condensation on the inside is a sign of too high humidity, and if moisture lingers it can cause damage in frames and walls.
Why do I get condensation on the outside of the window?
Outside condensation is usually harmless and suggests the window is well insulated. The outer pane stays cold, so morning moisture settles there and disappears as it warms up.
Does a dehumidifier help against window condensation?
Yes, if the condensation is caused by too high indoor humidity. A dehumidifier lowers the relative humidity. Aim for 30 to 60 percent and follow the level with a hygrometer.
What humidity avoids condensation?
A relative humidity of 30 to 60 percent usually keeps condensation away. The colder the glass, the lower the humidity needs to be to avoid fogging.
