No, double glazing should not have condensation on the internal face of the glass, but some condensation on the external pane or between the panes can indicate a fault. According to the UK Government, condensation inside the sealed unit (between the panes) is a sign of seal failure and requires replacement.
The main variable is where the condensation forms. Internal condensation (on the room-side glass) is usually a ventilation or humidity issue, not a window defect. External condensation (on the outer pane) is normal and often a sign of good insulation. Condensation between the panes means the airtight seal has failed, and the unit must be replaced. This answer applies to sealed double-glazed units installed in the UK, typically with a 10-year warranty.
Condensation between the panes means seal failure
When moisture appears between the two panes of glass, the insulating gas (usually argon or air) has leaked out and the desiccant strip inside the spacer bar is saturated. The FENSA states this is a manufacturing defect or age-related failure. The only remedy is to replace the sealed unit, which costs between £150 and £300 per window, depending on size and glass type. Most reputable installers offer a 10-year guarantee on the sealed unit, so check your paperwork before paying for a replacement.
Internal condensation is a ventilation issue
Condensation on the room-side surface of double glazing occurs when warm, moist air meets the cold glass. The Energy Saving Trust explains that modern double glazing is so well insulated that the glass is cooler than the room air, but internal condensation is rarely a window fault. The solution is to reduce humidity: use extractor fans in kitchens and bathrooms, open trickle vents, and avoid drying clothes indoors. A dehumidifier can help, but the long-term fix is better ventilation.
External condensation is normal and a good sign
Condensation on the outer pane of double glazing happens when the glass is colder than the outside air, typically on clear, still mornings. The Building Research Establishment (BRE) notes that this is normal for energy-efficient windows because they prevent heat escaping, so the outer pane stays cooler. It usually clears within an hour as the sun warms the glass. External condensation is not a defect and actually confirms your windows are working well to retain heat inside your home.
A worked example
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 replacing failed sealed units can improve thermal efficiency by up to 30%, cutting annual heating bills by around £180 for a typical gas-heated home. With the current £7,500 BUS grant unavailable for window-only work, the payback period lands at roughly 5.5 years based on those savings. Over a 25-year lifetime, the homeowner would save approximately £4,500, minus the initial outlay. That makes this a sound investment before considering the improved comfort and reduced draughts. Check with Energy Saving Trust for local installer schemes that might offer partial funding.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,000 |
| Yearly savings | £180 |
| Payback period | 5.5 years |
| 25-year lifetime savings | £4,500 |
What homeowners often get wrong
The most common mistake is assuming all condensation on double glazing means the window is faulty and needs replacing. Here are three specific errors that cost UK homeowners money or void their warranties.
- Confusing internal condensation with seal failure Moisture on the room-side glass is nearly always a humidity problem from cooking, drying laundry, or poor ventilation. Installing trickle vents or using a dehumidifier for £150 often solves the issue, whereas replacing a perfectly good window costs £300 unnecessarily.
- Ignoring between-pane condensation as harmless Many homeowners wipe it away and assume it will dry out, but trapped moisture means the argon gas has escaped and the desiccant is saturated. Left unfixed, this reduces the window’s U-value from 1.2 W/m²K to over 3.0 W/m²K, wasting roughly £50 per year on extra heating per window.
- Assuming all warranties cover labour A 10-year guarantee from FENSA-registered installers typically covers the sealed unit itself but not the labour to fit the replacement. Homeowners often face a £100 to £150 fitting charge per window, which catches them out if they haven’t read the small print.
Quick reference
- Condensation between the panes means the sealed unit has failed and must be replaced within 12 months to avoid further heat loss.
- Internal condensation on the glass surface is a ventilation issue and can often be fixed by opening trickle vents or using a dehumidifier.
- External condensation on the outer pane is normal and actually proves your windows are insulating well against the cold outside air.
- 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).
- Wiping condensation between the panes is ineffective and can void your warranty if you attempt to reseal the unit yourself.
Frequently Asked Questions
Condensation between the panes means the airtight seal has failed. This allows moisture in. The Energy Saving Trust advises replacing the sealed unit, as the insulating gas has escaped.
No, condensation on the room-side glass is not normal and indicates high humidity or poor ventilation. The Energy Saving Trust recommends improving airflow and using extractor fans.
No, external condensation is normal and a sign of energy efficiency. The UK Government confirms it occurs when the outer pane is colder than the air.
