Condensation appearing on window panes is a common occurrence, though it often causes concern. Water droplets can affect not only indoor comfort but also the technical condition of the window frames, contribute to mould growth and reduce indoor air quality. In this article, we explain what causes condensation — both on the inside and outside of the glass — and what steps you can take to effectively minimise the problem.
Why do windows steam up on the inside?
When warm, moist air comes into contact with a cooler surface, part of the water vapour condenses and settles as droplets. In practice, activities such as cooking, taking a hot bath or drying laundry indoors can significantly raise humidity levels. As a result, steamed-up windows become common, particularly in bathrooms and kitchens, where water vapour forms quickly.
Poor air circulation is one of the main reasons for persistent condensation. Even the best window systems won’t handle excess humidity if the windows and doors are sealed too tightly and ventilation doesn’t function as it should. Moist air becomes trapped, and when it meets cold glass surfaces, condensation forms.
In modern buildings with highly insulated, airtight windows, proper mechanical or hybrid ventilation is essential for maintaining a healthy indoor climate.
What can you do?
- Ventilate rooms regularly, even in winter — a short but intensive airing can significantly lower humidity levels.
- Install trickle vents or use supporting devices such as air dehumidifiers and purifiers.
- After bathing or cooking, make sure to ventilate the room for a few minutes to replace moist air with fresh air.
- A well-designed ventilation system is also crucial, especially in new buildings with highly sealed windows. Proper air inlets and exhaust ducts should be planned already at the design stage.
Condensation on the outside – is it a problem?
Paradoxically, condensation on the outside of windows can actually be a sign of good thermal insulation. The outer glass surface stays cooler than the surrounding air, causing moisture in warm, humid air to condense.
High-quality double or triple glazing helps maintain a temperature difference between the inside and outside, increasing the likelihood of external condensation. This phenomenon often occurs in spring or during humid, stormy days. Though it may seem worrying, it actually indicates that your glazing is performing efficiently and preventing heat from escaping.
Simple remedies
- Wipe off excess droplets with a squeegee or soft cloth.
- Apply anti-condensation agents, which form an invisible layer on the glass and reduce water accumulation.
- Consider a special exterior film that helps minimise condensation formation.
What if condensation appears between the panes?
If fogging occurs inside the sealed glass unit, it’s usually due to a loss of airtightness. Moisture enters where inert gas (usually argon) should be contained. Unfortunately, this issue can’t be resolved with DIY methods — it indicates a permanent seal failure, leading to the loss of the window’s insulating properties.
What can be done?
- Replace the glazed unit, or in more severe cases, the entire window.
- Check the condition of seals and frames to identify potential leaks.
- Keep in mind that damaged double- or triple-glazed units cannot be repaired effectively — replacement is necessary.
How to prevent window condensation – proven tips
- Control humidity levels – Keep indoor humidity between 40–60%. Use dehumidifiers if necessary.
- Maintain air circulation – Besides regular ventilation, install trickle vents, use exhaust fans in bathrooms and ensure ventilation ducts are clean and functional.
- Proper heating – Rooms that are too cold cause glass surfaces to cool rapidly, increasing the risk of condensation. Maintain indoor temperatures between 18–21°C.
- Use specialist products – Anti-condensation sprays can help if you want consistently clear glass (e.g. for a living room overlooking the garden). Regular application ensures better visibility.
- Opt for anti-condensation coatings – Glass with a special surface coating reduces condensation on the exterior side, especially on cool, humid mornings.
Modern aluminium windows – a long-term solution
For a long-term fix, consider installing highly insulated aluminium windows. By choosing advanced systems from Reynaers Aluminium, such as SlimLine 38, MasterLine 8 or MasterLine 10, you can significantly reduce condensation risk and say goodbye to water streaks on your window sills. With proper use — including regular ventilation and balanced heating — condensation on your windows will become a thing of the past.
