No, double glazing should not get condensation on the inside face of the glass in normal use, though it can occur temporarily in certain conditions. The Energy Saving Trust confirms that modern double glazing is built to reduce condensation by keeping the inner pane warmer than single glazing (Energy Saving Trust, 2026). If you see persistent internal condensation on sealed double-glazed units, it usually indicates a problem with the seal or the room’s ventilation.
The key variable is where the condensation forms. Internal condensation on the glass surface, the side inside your room, is often a ventilation issue, not a window fault. Condensation inside the sealed gap between the two panes (mist or fogging) means the seal has failed and the unit needs replacing. External condensation on the outer pane is normal and even a sign of good insulation.
Internal condensation on the glass surface
This happens when warm, moist indoor air meets a cold glass surface. Even with double glazing, if your home has high humidity from cooking, showering, or drying clothes, and the room is poorly ventilated, the glass can be the coldest surface. Opening trickle vents or using extractor fans reduces moisture levels. The British Fenestration Rating Council notes that double glazing with a low-emissivity coating and argon gas fill keeps the inner pane warmer, lowering condensation risk (FENSA, 2026). If condensation appears only occasionally in cold weather, it is usually not a defect.
Condensation between the panes
Mist or water droplets trapped inside the sealed unit is a clear sign of seal failure. The gap between the panes should be dry and hermetically sealed. When the seal breaks, moisture enters, and the thermal performance drops. A failed unit may also show a U-value increase from around 1.2 W/m²K to over 3.0 W/m²K, according to the Building Research Establishment (BRE, 2026). Replacement is the only solution, repairing the seal is not cost-effective. Most modern double-glazed units come with a 10-year guarantee; check your paperwork.
External condensation on the outer pane
Seeing condensation on the outside of your double glazing is actually a positive sign. It means the glass is so well insulated that little heat escapes. This allows the outer surface to cool to the dew point. This occurs on clear, still nights when the air is humid. According to the Energy Saving Trust, this is more common with high-performance windows and indicates energy efficiency (Energy Saving Trust, 2026). It should clear once the sun warms the glass. No action is needed.
A worked example
A typical 1930s semi-detached house in Manchester with 12m² of double-glazed windows that have failed seals would cost roughly £3,200 to replace all sealed units, but a broken seal on a single large 2m² window costs around £250 to replace. The Energy Saving Trust notes that replacing failed units restores thermal efficiency and can save around £115 per year on heating bills for a typical semi (Energy Saving Trust, 2026).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). Payback on a single unit replacement is under three years, and over 25 years the total savings exceed £2,800, assuming energy prices rise 5% per year. A BUS grant of £7,500 does not apply here as it is for heat pumps, not glazing.
| Item | Figure |
|---|---|
| Upfront cost after grants | £250 |
| Yearly savings | £115 |
| Payback period | 2.2 years |
| 25-year lifetime savings | £2,875 |
What homeowners often get wrong
The most common mistake is assuming all condensation on double glazing means the window has failed and needs replacing. Here are three key misunderstandings that cost UK homeowners time and money.
- Thinking internal condensation means a broken seal Condensation on the room-side glass surface is usually a ventilation issue, not a window fault. Mistaking it for seal failure leads to unnecessary replacement costs of £200-£600 per window when opening trickle vents or running an extractor fan would solve the problem for free.
- Ignoring condensation between the panes Mist or fogging inside the sealed gap confirms the seal has failed and the unit is no longer insulating properly. Leaving it untreated wastes roughly £50-£100 per year in extra heating costs because the argon gas has escaped and the pane acts like single glazing.
- Believing external condensation is a defect Water on the outside of your windows on a cold morning is a sign of efficient double glazing, not a fault. It means the glass is so well insulated that the outer pane stays cold enough to condense moisture from the outside air, and it proves your windows are working correctly.
Quick reference
- Condensation on the room-side glass surface is caused by high indoor humidity and poor ventilation, not a window defect.
- A failed seal shows as persistent mist or fog between the panes and requires the whole sealed unit to be replaced.
- External condensation on the outer pane is normal and confirms your double glazing is insulating effectively.
- Opening trickle vents and using extractor fans in kitchens and bathrooms can eliminate internal condensation without any window work.
- Replacing a failed sealed unit costs roughly £250 for a standard 2m² window and pays back in under three years through lower heating bills.
Frequently Asked Questions
Yes, condensation between the panes means the seal has failed. The Energy Saving Trust advises replacing the sealed unit to restore insulation.
Yes, temporary internal condensation can occur in cold weather if indoor humidity is high. The British Fenestration Rating Council states this is usually not a defect.
Reduce humidity by using extractor fans and opening trickle vents. The Energy Saving Trust recommends improving ventilation to keep the inner pane warmer.
