No, double glazed windows should not steam up on the inside of the sealed unit. If you see mist or condensation trapped between the two panes of glass, it indicates a failed seal, and the unit will need replacing (GOV.UK, 2026).
Condensation on the room-facing surface of double glazing is normal in cold weather, especially in kitchens and bathrooms. The key difference is location: internal surface condensation is temporary and manageable, while condensation between the panes is a manufacturing defect that reduces thermal performance. The Energy Saving Trust notes that a failed seal can cut the insulating value of a window by up to 50% (Energy Saving Trust, 2026).
Internal condensation is normal and manageable
Steam on the inside of your double glazing happens when warm, humid air meets a cold glass surface. This is not a fault. In a typical home, relative humidity of 60-70% can cause surface condensation when the outside temperature drops below 5°C. Trickle vents, extractor fans, and opening windows for 10 minutes each morning can reduce this. The average cost of installing a whole-house mechanical ventilation system is around £2,500 to £4,500 (GOV.UK, 2026).
Between-pane mist means a broken seal
If you see condensation between the two glass panes, the edge seal has failed. This allows moisture-laden air to enter the argon-filled cavity. The U-value of a typical sealed unit rises from around 1.2 W/m²K to over 3.0 W/m²K after seal failure, meaning heat loss doubles. Replacing a single sealed unit costs £150 to £350, including labour, and should be done by a FENSA-registered installer (FENSA, 2026).
Blown units are not covered by standard warranties
Most double glazing manufacturers offer a 10-year seal warranty, but this typically covers only manufacturing defects. If the seal fails due to accidental damage, poor installation, or age, you will need to pay for replacement. TrustMark-registered installers must provide a written guarantee for at least 2 years on repair work (TrustMark, 2026). Claims for seal failure should be made directly to the original installer or manufacturer.
A worked example
A typical 1930s semi-detached house in Manchester with 10 double glazed units showing between-pane mist would cost roughly £2,100 to replace all affected sealed units. This scenario assumes the frames are still in good condition and only the glass units need swapping. Without replacement, the failed seals cut the windows’ insulating value by up to 50%, adding an estimated £180 per year to heating bills according to the Energy Saving Trust. 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). If the householder is on a low income and receiving certain benefits, the ECO4 scheme may cover the full cost of replacement. Payback on the investment comes from regained energy efficiency and comfort, not direct savings, so the financial return is measured over the remaining life of the window frames rather than a traditional payback period.
| Item | Figure |
|---|---|
| Upfront cost after grants | £2,100 |
| Yearly savings | £180 |
| Payback period | 12 years |
| 25-year lifetime savings | £4,500 |
What homeowners often get wrong
The most common mistake is mistaking normal surface condensation for a failed sealed unit and replacing windows unnecessarily. Here are three frequent errors that cost UK homeowners money and time.
- Confusing surface mist with between-pane condensation Many homeowners panic when they see moisture on double glazing in winter. The right test is to wipe the glass, if the condensation returns on the room-facing surface, it is normal humidity. If it reappears between the panes, the seal is broken and replacement is needed. Wasting money on new windows for surface condensation alone can cost £3,000 to £8,000 unnecessarily.
- Assuming all mist means the whole window must go Some believe a misted unit requires a full new frame and installation. The right approach is that only the sealed glass unit needs replacing, not the frame, which cuts the cost by roughly 60%. Replacing an entire window when only the glass is faulty can waste £500 per window.
- Ignoring the problem and hoping it dries out A failed seal will not fix itself and the trapped moisture often contains minerals that etch the glass permanently. The right action is to replace the sealed unit promptly, delaying more than six months can lead to permanent glass staining that voids the warranty on the replacement. Ignoring a failed seal for a year can reduce the window’s insulation value by 50%, adding £90 to annual heating costs.
Quick reference
- Condensation trapped between the two panes of double glazing always means the edge seal has failed and the unit must be replaced.
- A failed seal reduces the window’s thermal performance by up to 50%, according to the Energy Saving Trust.
- 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).
- Surface condensation on the room-facing side of double glazing is normal at humidity levels above 60% and does not indicate a fault.
- Ignoring a misted sealed unit for more than six months can cause permanent glass etching that voids any warranty on replacement.
Frequently Asked Questions
It means the edge seal has failed, letting moisture into the argon-filled cavity. The Energy Saving Trust says this can reduce thermal performance by up to 50%.
Yes, on the room-facing surface it is normal when humidity is 60-70% and temperatures drop below 5°C. Use trickle vents or extractor fans to manage it, advises GOV.UK.
Replacing a standard sealed unit typically costs £150–£400 per window. A whole-house mechanical ventilation system costs £2,500–£4,500, according to GOV.UK.
