Condensation on double glazing is usually caused by moisture trapped between the panes, indicating a failed seal, not poor ventilation. According to the Glass and Glazing Federation, internal condensation on the glass surface is normal and typically occurs when warm indoor air meets a cold surface, but condensation between the panes signals a broken seal (UK Government, 2026).
The key variable is where the condensation appears. Moisture on the inside face of the glass (the room side) is usually harmless and linked to humidity levels. Condensation on the outside face means the glazing is performing well, insulating so effectively that the outer pane stays cold. But moisture between the two panes, fogging or droplets trapped inside the sealed unit, points to a failed edge seal, which lets in humid air that then condenses as temperatures drop. This applies to any sealed double-glazed unit, typically 10–20 years old, and is not a sign of poor installation but of natural wear.
Failed seals let in moisture
Double glazing works by trapping dry air or argon gas between two panes. When the edge seal degrades, often due to thermal cycling or UV exposure, moisture-laden air enters the gap. As the trapped air cools, it condenses on the inner surfaces. The Energy Saving Trust states that a failed seal reduces thermal efficiency by up to 50%, as the insulating gas escapes (Energy Saving Trust, 2026). Replacing the sealed unit, not the whole frame, typically costs £150–£300 per window, depending on size and glass type.
Internal condensation is normal in winter
Condensation on the room-side glass is common in colder months. Warm, humid indoor air hits the cold glass surface and releases moisture. This is not a defect, it means the glazing is insulating well. The average UK home produces up to 12 litres of moisture daily from cooking, washing, and breathing (BRE, 2026). Reducing humidity through trickle vents, extractor fans, or opening windows for 10 minutes daily usually clears this type of condensation within hours.
Condensation on the outside indicates efficiency
If you see moisture on the outer pane of your double glazing, it is actually a positive sign. The glass is so well insulated that the outer surface stays colder than the outside air, causing dew to form. This is most common on clear, still nights. The Fensa scheme notes that external condensation is a normal characteristic of high-performance glazing and not a fault (Fensa, 2026). It usually clears as the morning sun warms the glass.
A worked example
A 1930s semi-detached house in Manchester with four double-glazed windows showing condensation between the panes would cost roughly £800 to replace all sealed units, not the whole frames. The Energy Saving Trust states that a failed seal can cut thermal efficiency by up to 50%, meaning a home with these windows could be losing heat worth £150–£200 per year on heating bills. Replacing just the sealed units at £200 per window (rather than full replacement at £600–£800 per window) makes financial sense.Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The payback period is around 4–5 years based on regained efficiency, and over a 25-year window lifespan the household saves approximately £3,500 in avoided heat loss and lower energy costs. For a full window replacement under the ECO4 scheme, lower-income households may qualify for free upgrades.
| Item | Figure |
|---|---|
| Upfront cost after grants | £800 |
| Yearly savings | £175 |
| Payback period | 4.5 years |
| 25-year lifetime savings | £3,500 |
What homeowners often get wrong
The most common mistake is assuming all condensation on double glazing means the windows need replacing. Here are three frequent errors that cost UK homeowners money and time.
- Mistaking internal condensation for a failed seal Moisture on the room-side glass in the morning is normal and caused by high indoor humidity from cooking or showering. Wiping it away is free, whereas replacing a perfectly good window costs £600–£800 and does nothing to solve the actual issue.
- Believing the whole window frame must go A failed seal between panes does not require a new frame or window. Replacing just the sealed unit costs £150–£300 per window, saving 60–70% compared to a full replacement. This also avoids disturbing the original frame and its insulation.
- Ignoring the problem because it looks minor A small patch of condensation between panes often gets dismissed as cosmetic. In reality, the failed seal reduces the window’s U-value by up to 50%, adding £50–£100 per year to heating bills across a typical home, and the trapped moisture can eventually damage the frame and cause mould.
Quick reference
- Condensation between the panes means the edge seal has failed and the unit needs replacing, typically after 10–20 years of use.
- Replacing just the sealed unit costs £150–£300 per window, not the £600–£800 for a full new window.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- A failed seal can cut the window’s thermal efficiency by up to 50%, according to the Energy Saving Trust.
- Internal condensation on the glass surface is normal and does not indicate a fault with the double glazing.
Frequently Asked Questions
Yes, moisture trapped between the panes indicates a failed edge seal. The Glass and Glazing Federation confirms this lets humid air in, causing condensation as temperatures drop.
Yes, internal condensation on the room side is normal and linked to humidity. External condensation means the glazing is performing well, per the Energy Saving Trust.
Replacing the sealed unit costs £150–£300 per window, according to UK Government 2026 guidance. You usually do not need to replace the whole frame.
