Why do windows fog up
Window dew occurs when warm, moist air indoors meets the cold surface of the glass. Water condenses in the form of small drops – similar to a cold glass in the summer. The most common causes are:
- high humidity in the apartment,
- poor ventilation,
- low surface temperature of the glass or frames,
- unprofessional installation or thermal bridge around the windows.
⚠️ If condensation appears between the panes of insulating glazing, it is no longer ordinary condensation – the insulating glass is probably damaged and needs to be replaced.
How to ventilate properly
The simplest and most effective way to reduce condensation on windows is to air the apartment or house regularly and briefly. Instead of opening it for a long time, it is enough to open the window wide for 3-5 minutes, ideally three times a day, especially after showering and cooking or in the morning after waking up. The air is exchanged, but the walls are not cooled down enough.
If you have new, well-sealed windows, consider installing a window ventilation grille or automatic humidity sensors – they help keep fresh air in with no need for manual opening.
How window shading helps
Properly selected window shading has a greater effect on condensation than you might think.
Exterior blinds, exterior roller shutters or screen roller shutters help keep temperature stability of the glass – in summer they protect against overheating, and in winter they reduce heat loss and thus the risk of the glass cooling down.
👉 Find out more about contemporary shading that also protects against humidity – for example, ISOTRA exterior blinds or screen roller shutters.
How to reduce indoor humidity
The ideal humidity in an apartment is between 40 and 50%. Higher values mean a greater risk of condensation and mould formation.
Here are some simple tips to keep it under control:
- use a kitchen extractor hood or fan while cooking,
- do not dry laundry in the apartment with no ventilation,
- keep furniture away from walls,
- equip your windows with suitable shading,
- monitor humidity with a hygrometer,
- use a dehumidifier if necessary.
If humidity remains high despite these measures, the problem may be insufficient air exchange or insulation of the building.
When to become attentive and call an expert
You should put yourself on alert if the windows get foggy between the panes, or if dark spots or mould appear on the frame. This usually means a faulty seal or poor installation. In such a case, it is advisable to contact experts who will check the condition of the frames, insulating glass and tightness of joints.
Window dewing is not just a minor aesthetic defect, but a signal that the room is too humid or there is a lack of proper air exchange. The basic solution is simple: air regularly, keep optimal humidity, do not obstruct the flow of warm air around the windows, and purchase suitable window shading that improves thermal comfort and reduces energy losses.
FAQs
1. Why do windows fog up and when is it a problem?
Why do windows fog up? The most common cause is high humidity and insufficient air exchange in the apartment. Warm, moist air condenses on cold glass, where condensation forms. If fogging occurs frequently or for a long time, it can lead to mould around windows and damage to frames.
2. How to prevent fogging windows in winter?
Dewy windows in winter are a common result of poor ventilation or blocked heat flow from the radiator. Short, intensive ventilation three times a day, proper heating, and free access of warm air to the windows will help. A long-term solution can be suitably selected shading or a window ventilation grille, which ensures a continuous supply of fresh air even when the window is closed.
3. How to reduce humidity in the apartment and prevent mould around the windows?
The ideal indoor humidity is 40–50%. Higher values increase the risk of condensation and mould around the windows. It helps to use a kitchen extractor hood and ventilate after showering, do not dry laundry in the apartment without an open window, maintain proper temperature around the windows and use a dehumidifier or ventilation of the apartment as needed. Proper air circulation is the most effective prevention of mould.
4. What does condensation between window panes mean?
If condensation appears between the window panes, it is not about humidity in the room, but about malfunction of the insulating glass. The seal is probably damaged and the inner space between the panes has lost its insulating capabilities. In such a case, only professional window maintenance and service or replacement of the glazing will help.
5. How often should windows be maintained and serviced concerning moisture?
It is recommended to check the condition of seals, fittings and drainage holes at least once a year. Regular maintenance and service of windows extends their lifespan and reduces the risk of condensation and mildew. Properly adjusted windows and timely inspection are the key to effective mould prevention and healthy climate in the apartment.
