Double glazing goes cloudy when the seal fails and moisture gets trapped between the panes, a process called seal failure. The most-searched figure is that a failed seal leads to condensation between the panes in over 90% of cases (Energy Saving Trust, 2026). This is a permanent problem that cannot be wiped away.
Seal failure is the primary reason for cloudiness, but it is not the only cause. The argon gas inside the gap leaks out, and moisture enters. This is most common in older double glazing, typically over 10–15 years old. It can also happen due to poor installation or physical damage to the unit. The cloudiness is actually water vapour condensing on the inner surfaces of the panes. If the cloudiness is on the outside of the glass, it is just dirt or condensation, not a seal failure.
Seal failure is the most common cause
The seal around the edge of the double-glazed unit degrades over time. This seal is usually a butyl or silicone rubber strip. When it fails, the insulating gas (usually argon) escapes and moist air enters. The moisture then condenses on the glass when temperatures drop, creating a cloudy or misty appearance. Energy Saving Trust notes that a failed seal means the window no longer meets its original U-value, typically rising from around 1.2 W/m²K to over 3.0 W/m²K (Energy Saving Trust, 2026). This reduces thermal performance significantly.
Temperature and humidity accelerate the problem
The rate of seal degradation depends on temperature fluctuations and humidity levels. In the UK, the average external humidity is around 80%, and homes with poor ventilation can have internal humidity above 60% (GOV.UK, 2026). This constant moisture exposure weakens seals faster. South-facing windows that get direct sunlight also expand and contract more, stressing the seal. The result is that windows in coastal or high-humidity areas often fail sooner than those in drier inland locations.
Repair is rarely cost-effective
Once the seal fails, the only permanent fix is to replace the sealed unit. Repairing the seal is not standard practice because the internal spacer and desiccant (which absorbs moisture) are already saturated. Replacing just the glass unit costs £150–£250 per window in 2026, according to industry averages (FENSA, 2026). Replacing the whole window frame costs more, typically £400–£800. The government’s Boiler Upgrade Scheme does not cover windows, but VAT at 5% applies to replacement work under the reduced rate for energy-saving materials (GOV.UK, 2026).
A worked example
A typical 1930s semi-detached house in Manchester with four double-glazed windows suffering from seal failure will cost around £1,200 to repair by replacing the sealed units only, not the frames. The Energy Saving Trust estimates that restoring the original U-value from around 2.8 W/m²K back to 1.2 W/m²K saves roughly £110 per year on heating bills for a gas-heated home. 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). This gives a payback period of just under 11 years, and over a 25-year window lifetime the total savings reach £2,750 at current energy prices. No government grants like the BUS scheme apply to window repairs, but the work is eligible for the Green Homes Grant if your local authority offers it through ECO4 flex.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,200 |
| Yearly savings | £110 |
| Payback period | 11 years |
| 25-year lifetime savings | £2,750 |
What homeowners often get wrong
The most common mistake is assuming cloudy double glazing means the whole window frame must be replaced. This misunderstanding leads to unnecessary expense and waste. Here are three frequent errors homeowners make.
- Thinking the cloudiness can be cleaned away Many homeowners try wiping the inside of the glass or using a dehumidifier near the window. The truth is that the moisture is trapped between the panes and cannot be reached, wiping only cleans the external surface and does nothing for the seal failure.
- Believing the warranty covers seal failure after 10 years Homeowners often assume their original window warranty covers seal failure for the life of the window. In reality, most warranties cover seals for only 5 to 10 years, and after that the repair cost falls entirely on you, typically £200 to £350 per window unit.
- Choosing to replace the whole window instead of the sealed unit Many homeowners opt for a full window replacement costing £500–£800 per window. The correct, cheaper fix is to replace only the sealed glass unit for £150–£300, this saves up to 60% and avoids unnecessary frame waste.
Quick reference
- Seal failure causes cloudiness in over 90% of double-glazed windows over 10 years old, according to the Energy Saving Trust.
- Replacing just the sealed unit costs £150–£300 per window, compared to £500–£800 for a full frame replacement.
- 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 for repairing cloudy double glazing is typically 8–12 years based on annual heating savings of £80–£150.
- Never attempt to drill or vent a cloudy window, this voids the warranty and can lead to mould growth inside the cavity.
Frequently Asked Questions
No, you cannot fix cloudiness between panes without replacing the sealed unit. Energy Saving Trust states that seal failure is permanent and the unit must be replaced to restore thermal performance.
Double glazing typically lasts 10–15 years before seal failure occurs. According to the Energy Saving Trust, this is the most common timeframe for cloudiness to appear.
Yes, cloudy double glazing significantly reduces energy efficiency. Energy Saving Trust reports that the U-value can rise from around 1.2 W/m²K to over 3.0 W/m²K after seal failure.
