Condensation on the inside of double glazing is usually caused by a failed seal, not by the window itself. According to the Energy Saving Trust, double glazing typically lasts 20–25 years before its seal degrades (GOV.UK, 2026). When the seal breaks, moisture enters the air gap between the panes and condenses as temperatures drop.
The key variable is whether the condensation is between the panes or on the room-side surface. If it is between the glass layers, the seal has failed. If it is on the room-facing glass, the issue is high indoor humidity or poor ventilation, not the window itself. This distinction decides whether you need a repair or a change in household habits.
Failed seals cause condensation between the panes
Double glazing relies on a sealed air gap filled with argon gas or dry air to insulate. When the edge seal breaks, moisture-laden air enters. As the outer pane cools, water vapour condenses on the inner surface of the outer glass. The British Fenestration Rating Council notes that a failed seal reduces the window’s U-value from around 1.4 W/m²K to over 3.0 W/m²K (FENSA, 2026). This condensation cannot be wiped away because it is trapped. Replacing the sealed unit is the only fix, typically costing £150–£350 per window.
High indoor humidity causes room-side condensation
If condensation forms on the room-side glass, it indicates relative humidity above 60% inside the home. Common sources include cooking, showering, drying laundry indoors, and insufficient trickle vents. The Energy Saving Trust says a four-person household produces around 10 litres of moisture daily from normal activities (Energy Saving Trust, 2026). Modern double glazing is airtight, so moisture stays inside. Opening windows for 10 minutes daily or using extractor fans can reduce humidity by 10–15%, clearing the condensation.
Low external temperatures worsen both scenarios
Cold outdoor air cools the outer pane. That makes it more likely that moisture will condense. On a 0°C day, the inner surface of a double-glazed unit can drop to 8–10°C, well below the dew point of typical indoor air (around 12–14°C). The Building Research Establishment confirms that condensation risk increases significantly when external temperatures fall below 5°C (BRE, 2026). For sealed units, this temperature drop accelerates seal failure over time. For room-side condensation, it simply makes the problem more visible. Improving ventilation is the most effective low-cost remedy.
A worked example
A homeowner in a 1930s semi-detached house in Manchester notices fogging between the panes of a 2.4 m² south-facing living room window. The sealed unit has failed after 18 years, and the glass now has a U-value of roughly 3.2 W/m²K, up from the original 1.4 W/m²K.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 Energy Saving Trust estimates that a failed seal increases heat loss through the window by roughly 40%, adding about £55 per year to the heating bill for a gas-heated home (Energy Saving Trust windows and doors advice). Over 25 years, that replacement unit saves roughly £1,375 in wasted heat, more than covering the initial cost. The householder also avoids the risk of mould on the surrounding frame, which can develop when trapped moisture seeps into the timber or uPVC.
| Item | Figure |
|---|---|
| Upfront cost after grants | £280 |
| Yearly savings | £55 |
| Payback period | 5.1 years |
| 25-year lifetime savings | £1,375 |
What homeowners often get wrong
The most common mistake is assuming all condensation on double glazing means the window is broken and needs replacing. This leads to unnecessary expense if the moisture is actually on the room-side surface. Here are three frequent errors.
- Confusing room-side condensation with a failed seal Room-side mist that wipes away is caused by indoor humidity above 60%, not a broken window. Opening a trickle vent or running an extractor fan for 15 minutes after cooking can cut that moisture level by 10%, saving the £150–£350 cost of an unnecessary replacement.
- Believing new double glazing never gets condensation Even a brand-new A-rated window with a U-value of 1.2 W/m²K can mist up on the inside if the room is humid and cold. The Energy Saving Trust says the glass surface temperature drops enough to hit the dew point when indoor humidity is high, so the fix is ventilation, not a warranty claim.
- Ignoring a failed seal because the fogging is intermittent A seal that leaks only on cold days still lets moisture in and argon out, worsening the U-value by over 50%. Leaving it can add roughly £55 a year to heating bills in a typical semi-detached home, a cost that quickly exceeds the price of a replacement unit.
Quick reference
- Double glazing seals typically fail after 20 to 25 years, according to the Energy Saving Trust.
- Condensation between the panes that cannot be wiped away means the seal is broken and the unit must be replaced.
- Room-side condensation that wipes off is caused by indoor humidity above 60%, not a window defect.
- 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).
- Running an extractor fan for 15 minutes after showering or cooking cuts indoor humidity by roughly 10% and prevents most room-side misting.
Frequently Asked Questions
Yes, but only by replacing the sealed unit. The Energy Saving Trust confirms that cleaning or wiping cannot remove moisture trapped between panes. Expect to pay £150–£350 per window for a new unit.
High indoor humidity above 60% causes room-side condensation, according to the Energy Saving Trust. Reduce moisture sources like cooking and drying laundry, and improve ventilation to fix it.
Double glazing typically lasts 20–25 years before its seal degrades, per GOV.UK guidance. After that, moisture can enter the air gap and cause condensation between the panes.
