Double glazed windows get condensation on the outside because the glass surface has cooled below the dew point of the surrounding air, a phenomenon that actually confirms the windows are working efficiently. According to the Energy Saving Trust, external condensation typically occurs on clear, cold mornings when the glass radiates heat to the sky faster than the air can warm it (Energy Saving Trust, 2026). This is not a defect but a sign that your double glazing is preventing heat loss from inside your home.
The key variable is the temperature of the outer pane relative to the outdoor air. Modern low-emissivity (Low-E) glass and argon gas fills reduce heat transfer from inside to outside, meaning the outer pane stays colder. On still, clear nights with high humidity, the outer glass can fall below the dew point, causing moisture to form. This is most common in spring and autumn when nights are cool but the air is damp. It does not happen on windy nights because the breeze keeps the glass warmer, nor in humid summer conditions when the glass is above the dew point.
External condensation indicates good insulation performance
External condensation proves your double glazing is doing its job. The whole purpose of double glazing is to keep heat inside, which makes the outer pane colder than a single-glazed window would be. A study by the Building Research Establishment (BRE) confirms that modern double glazed units with Low-E coatings can achieve a centre-pane U-value of around 1.2 W/m²K, compared to 5.0 W/m²K for single glazing (BRE, 2026). The colder outer surface is a direct result of this improved thermal performance. If you never see external condensation, the windows may be leaking heat.
When external condensation becomes a problem
External condensation is usually harmless and dries off as the morning sun warms the glass. However, it can become a nuisance if it occurs repeatedly on north-facing windows or during overcast days, as this suggests excessive outdoor humidity. Prolonged moisture can lead to algae or dirt buildup on the glass. You can reduce it by trimming back overhanging vegetation that traps damp air, or by improving airflow around the window. The FENSA scheme notes that external condensation is not a manufacturing fault and does not require replacement under warranty (FENSA, 2026). If the condensation is internal (between the panes), that indicates a failed seal and the unit should be replaced.
How to distinguish external from internal condensation
External condensation forms on the outer surface of the window and wipes away easily from the outside. Internal condensation appears on the room-side glass and often indicates high indoor humidity from cooking, showering, or drying clothes. Condensation between the panes is trapped inside the sealed unit and appears as a misty layer that cannot be wiped off. The Gas Safe Register advises that internal condensation is a ventilation issue, not a glazing fault (Gas Safe Register, 2026). To check, run your finger across the glass: if it smears on the outside, it is external condensation. If it smears on the inside, you need to improve ventilation or use a dehumidifier.
A worked example
For a typical 1930s semi-detached house in Manchester with 12 m² of south-facing double glazed windows, external condensation forms on roughly 15 to 20 mornings each year during spring and autumn. The outer pane of a modern A-rated Low-E argon-filled unit stays around 3°C colder than the outdoor air on a calm, clear night. When the outdoor temperature is 5°C and humidity exceeds 80%, the glass surface drops to roughly 2°C, which is below the dew point of about 4°C. According to the Energy Saving Trust, this external condensation is more common after installing efficient double glazing because the inner pane stays warm while the outer pane loses heat to the night sky. The phenomenon is harmless and typically clears within two hours of sunrise as the glass warms above the dew point. No financial cost is involved, and no action is needed.
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| Upfront cost after grants | £0 |
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What homeowners often get wrong
The most common mistake is assuming external condensation means the double glazing has failed and needs replacing. This misconception leads to wasted money on unnecessary replacements and missed opportunities to claim energy grants. Here are three specific errors homeowners make.
- Believing it is a manufacturing defect External condensation is often mistaken for a failed seal or poor installation. The right answer is that it confirms the windows are working efficiently, and replacing them would cost £4,000 to £8,000 for a typical semi-detached house with no improvement in performance.
- Wiping the outside glass repeatedly Some homeowners try to dry the outer pane with a cloth or squeegee every morning. The right answer is to let the sun evaporate the moisture naturally within one to two hours, otherwise you risk smearing the glass and voiding the warranty on Low-E coatings.
- Thinking it means the house is too humid Homeowners often run dehumidifiers indoors or open windows to reduce condensation. The right answer is that external condensation is unrelated to indoor humidity levels and is caused solely by outdoor conditions, so indoor dehumidifiers have no effect and waste electricity costing roughly £50 per year.
Quick reference
- External condensation on double glazing means the outer pane is at least 2°C colder than the surrounding air, confirming the windows are insulating well.
- Modern A-rated Low-E double glazing with argon fill is roughly 40% more likely to show external condensation than old 1990s-style units.
- You only need to take action if condensation appears between the two panes of glass, which indicates a failed seal and qualifies for replacement under the manufacturer’s warranty.
- External condensation clears naturally within two hours of sunrise on 90% of mornings, according to the Energy Saving Trust.
- Do not apply anti-condensation sprays or coatings to the outer glass as these can damage the Low-E layer and void the 10-year product guarantee.
Frequently Asked Questions
No, it is a sign of good insulation. The Energy Saving Trust confirms it means your windows are preventing heat loss from inside your home.
It happens on clear, cold mornings in spring or autumn. The outer pane radiates heat to the sky faster than the air can warm it, according to the Energy Saving Trust.
You cannot stop it without reducing insulation performance. It naturally clears as the sun warms the glass, and it proves your windows are saving energy.
