No, double glazed windows should not get condensation on the inside face of the glass under normal conditions, as this indicates a failed seal or excessive indoor humidity. The Energy Saving Trust confirms that condensation forming between the panes is a sign of a broken seal, while condensation on the internal surface is usually a ventilation issue (Energy Saving Trust, 2026).
The answer depends on where the condensation appears. If it is between the two panes of glass, the sealed unit has failed and needs replacement. If it is on the room-side surface, it is caused by high humidity levels inside your home, not the windows themselves. In winter, even high-performance double glazing can show temporary misting if indoor moisture is high and outdoor temperatures are very low, but this should clear quickly once the room warms up or ventilation improves.
Condensation between the panes means seal failure
Condensation trapped between the two glass panes indicates that the airtight seal around the double glazed unit has been breached. This allows moisture-laden air to enter the cavity, where it condenses as temperatures drop. GGF (Glass and Glazing Federation) guidelines state that a failed sealed unit should be replaced to restore thermal performance (GGF, 2026). Replacement typically costs £150–£400 per window depending on size and glazing type. If the seal fails within the manufacturer’s warranty period (usually 5–10 years), you may be entitled to a free replacement.
Internal condensation is a ventilation problem
Condensation on the room-side surface of double glazing is caused by high indoor humidity, not the windows themselves. Activities like cooking, showering, and drying clothes indoors generate moisture that condenses on the coldest surfaces, which are often windows. The average UK home produces up to 10 litres of water vapour per day from normal activities (UK Government, 2026). Improving ventilation, by using extractor fans, opening trickle vents, or running a dehumidifier, reduces internal condensation without replacing your windows.
Modern double glazing is built to resist condensation
Modern double glazed windows with low-E glass and argon gas fill have improved thermal performance, keeping the internal surface warmer and reducing the likelihood of condensation. Building Regulations Part L requires new windows to have a U-value of 1.6 W/m²K or lower for replacement installations (UK Government, 2026). Even with high-performance glazing, some condensation may occur in very cold weather if indoor humidity exceeds 60%. A hygrometer can help you monitor levels, keeping humidity below 60% minimises condensation on any glazing.
A worked example
A typical 1930s semi-detached house in Manchester with three failed double glazed units (one large living room window and two bedroom windows) would cost roughly £800 to replace the sealed units only, not the full frames. Under the ECO4 scheme, if your household income is below £31,000 or you receive certain benefits, you could get the replacement fully funded. Without grants, you pay £800 upfront but save around £110 per year on heating bills because the new low-emissivity glass and warm-edge spacers reduce heat loss. The payback period is just over seven years, and over a 25-year lifetime, the total savings reach £2,750. The Energy Saving Trust confirms that replacing failed units with A-rated double glazing cuts heat loss by up to 50 per cent compared to standard double glazing. 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).
| Item | Figure |
|---|---|
| Upfront cost after grants | £0 (ECO4 eligible) or £800 (self-funded) |
| Yearly savings | £110 |
| Payback period | 7.3 years |
| 25-year lifetime savings | £2,750 |
What homeowners often get wrong
The most common mistake is assuming any condensation on double glazing means the window needs replacing, which leads to unnecessary expense and missed solutions. Here are the three biggest errors homeowners make
- Confusing internal condensation with seal failure Many people panic when they see mist on the room-side glass in winter and call a glazier. The right answer is to check the humidity first, if the mist wipes away easily and reappears only in cold weather, you simply need better ventilation or a dehumidifier.
- Ignoring trickle vents or leaving them closed Homeowners often seal trickle vents shut to stop drafts, not realising this traps moisture inside. The right answer is to keep trickle vents open year-round, especially in kitchens and bathrooms, or you risk mould growth on walls and window frames that costs hundreds to treat.
- Believing a failed seal always requires full frame replacement Many assume a misted window means ripping out the entire frame. The right answer is that only the sealed glass unit needs replacing, which costs £150–£400 per window rather than £600–£1,200 for a full new window.
Quick reference
- Condensation between the glass panes means the sealed unit has failed and needs replacing to restore thermal efficiency.
- Internal surface condensation on modern double glazing is almost always a humidity problem, not a window defect.
- The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing.
- Keeping trickle vents open and using extractor fans reduces internal humidity by up to 40 per cent according to the Energy Saving Trust.
- A failed sealed unit voids your window warranty if left unrepaired, as moisture can eventually damage the frame and cause rot.
Frequently Asked Questions
Condensation on the inside face is usually caused by high indoor humidity. The Energy Saving Trust advises improving ventilation and using extractor fans to reduce moisture levels.
It means the sealed unit has failed. The Glass and Glazing Federation states the airtight seal is broken. This allows moisture in, and the unit should be replaced.
Replacement typically costs £150–£400 per window. According to GGF, check if the unit is still under the manufacturer's warranty (usually 5–10 years) for a free replacement.
