Window fogging is a common problem in the cold season. Many people associate it only with humidity, but in practice one of the main causes is insufficient or poorly distributed heating.
Insufficient heating when the cold sets in
When it starts to get cold outside while the heating is not yet on or is working weakly, glass units cool down quickly. The indoor air at this time stays relatively warm and contains moisture.
When warm, moisture-laden air meets cold glass, condensation forms — the windows “fog up”. In even colder conditions this can even lead to the glass freezing and ice forming on the inner surface.
💧 Important: Condensation usually does not mean the glass unit’s seal is damaged. In most cases it is a heat-and-moisture balance problem in the room — not a window defect.
When the heating is on but the heat does not reach the glass
Even with the heating on, the condensation problem can persist. The reason is that the heat does not reach the glass surface. The glass stays cold and moisture from the air keeps condensing on it.
Before — in old wooden windows:
- There were gaps and micro-cracks
- Air came in naturally
- Spontaneous ventilation occurred
In modern PVC, wooden and aluminium windows:
- Complete airtightness
- Multi-chamber glass units
- Seals block the air intake
This happens in the following cases, when heat is blocked before reaching the glass:
- Wide windowsills block the flow of warm air from the radiator — the heat cannot rise up to the glass
- Heavy curtains, especially floor-length ones, block the circulation of heat at the window
- Fully closed blinds create a barrier between the glass and the warm room air
- Cluttered windowsills — flowers, decor, objects — obstruct the airflow at the radiator
- Decorative radiator covers reduce heat output and worsen convection
As a result, the glass stays cold and condensation keeps forming despite the heating being on.
Why constant window fogging is a serious problem
Condensation on windows is not just an aesthetic nuisance. If left unaddressed, it can lead to more serious consequences:
- Mould growth — moisture on reveals, wall corners and windowsills creates an ideal environment for fungus
- Damage to reveals and windowsills — constant moisture damages plaster, paint and wooden structures
- Worsening indoor climate — high humidity affects air quality and wellbeing
- Energy waste — cold glass means heat loss through the window, which increases heating costs
How to stop windows fogging up
To avoid condensation on glass units, it is important to ensure free access of warm air to the glass and to control the humidity in the room:
- Do not cover radiators with curtains or furniture — leave a free circulation strip of at least 10–15 cm
- Do not use decorative radiator covers, or choose ventilated models
- Do not clutter windowsills with objects — especially important above radiators
- Air the rooms regularly, especially after cooking, laundry and showering
- Install air supply valves — they provide constant air exchange without heat loss
- In the transitional period (September–October), turn the heating on in good time so the glass does not fog up
- Leave blinds and curtains open enough at night so heat can reach the glass
* Free consultation and specialist inspection — in Riga only. Outside Riga, transport costs apply.
When is the glass unit itself to blame?
There are cases where condensation indicates a glass unit problem — not heating:
- Condensation between the panes (inside the glass unit) — a clear sign that the seal is damaged and moist air has got in
- Condensation on one window but not others in the same room — may indicate a local defect
- Condensation regardless of heating or airing — it is worth checking the glass unit quality
In such cases we recommend replacing the glass unit — a more economical solution than fully replacing the window.
Summary
Window fogging is most often related not only to humidity but also to the glass temperature. If a glass unit stays cold due to insufficient heating or blocked air circulation, condensation is inevitable.
Proper heating, free access of heat to the windows and regular airing are the key to solving the condensation problem without expensive repairs.