Yes, it is normal for double glazed windows to get condensation on the outside or inside surface, but condensation between the panes signals a failed seal requiring replacement. According to the Energy Saving Trust, modern double glazing typically lasts 20 years or more before seal failure occurs (Energy Saving Trust, 2026).
The type of condensation determines whether it is a problem or just a sign of normal household humidity. External condensation on the outer pane is common on clear, cold mornings and indicates the glazing is working efficiently. Internal condensation on the room-side glass usually results from high indoor moisture from cooking, showering, or drying clothes. Only condensation trapped between the two panes, visible as mist or water droplets that cannot be wiped away, indicates a broken seal and means the unit needs replacement.
External condensation is normal and a good sign
Condensation on the outside of double glazed windows happens when the outer pane is cooler than the surrounding air, typically overnight or early morning. This is more common with low-emissivity glass and argon-filled units, which insulate so effectively that the outer pane stays cold. The Energy Saving Trust explains that external condensation means your windows are performing well, reducing heat loss (Energy Saving Trust, 2026). It usually clears as the morning sun warms the glass. No action is needed, and it should not be confused with a defect.
Internal condensation is caused by household moisture
Condensation on the inside of double glazing is not a window fault but a ventilation issue. The average UK household produces around 12 litres of moisture per day from breathing, cooking, and washing, according to the BRE (BRE, 2026). When warm, moist air hits the cooler glass surface, it condenses. Reducing internal moisture through trickle vents, extractor fans, or opening windows for 10 minutes daily usually solves this. If condensation persists despite good ventilation, check for high humidity sources like a leaking pipe or unvented tumble dryer.
Condensation between panes means the seal has failed
Mist or water trapped between the two glass panes indicates the edge seal has broken. This allows moisture in. This is not normal and means the insulating gas (usually argon) has leaked out, reducing thermal performance. The Glass and Glazing Federation states that a failed sealed unit should be replaced to restore efficiency (FENSA, 2026). Replacement costs around £150 to £300 per window in 2026, depending on size and glass type. Ignoring it can lead to further heat loss and potential mould growth on the frame. Check if your installation is still under warranty, as many carry a 10-year guarantee on seals.
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). A householder eligible for the ECO4 scheme might receive full funding for the replacement if they receive qualifying benefits, bringing their upfront cost to zero. Without grant support, the £3,200 outlay saves around £195 a year on heating bills by eliminating draughts and improving the U-value from roughly 2.8 W/m²K to 1.2 W/m²K, according to the Energy Saving Trust. The payback period on that spend is about 16 years, and over a 25-year lifespan the cumulative saving reaches roughly £4,875.
| Item | Figure |
|---|---|
| Upfront cost after grants | £3,200 |
| Yearly savings | £195 |
| Payback period | 16 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is mistaking harmless external condensation for a failed seal and ordering unnecessary replacement windows. Here are three frequent errors and the right way to handle each one.
- Wiping condensation off the outside pane This does nothing useful because external condensation is a sign your windows are working well. The right response is to leave it alone, it will evaporate as the morning sun warms the glass.
- Blaiming the windows for internal condensation Many homeowners assume the glazing is faulty when moisture appears on the room-side glass. The real culprit is indoor humidity from cooking or drying clothes, and the fix is better ventilation, opening a trickle vent or using an extractor fan, not replacing the window.
- Ignoring condensation between the panes Some people wipe the inner surface and hope the mist clears, but trapped moisture between the glass layers means the seal has failed. Leaving it risks mould growth inside the unit and voids any remaining warranty, replacement is the only solution.
Quick reference
- External condensation on a cold morning means your double glazing is working efficiently and is completely normal.
- Internal condensation is caused by household humidity from cooking, showering, or drying laundry, not by faulty windows.
- Condensation trapped between the two panes of glass indicates a failed seal and usually requires a full unit replacement.
- Modern double glazing typically lasts 20 years or more before seal failure occurs, according to the Energy Saving Trust.
- Opening trickle vents or running an extractor fan for 20 minutes after cooking reduces internal condensation without any window work.
Frequently Asked Questions
Yes, it is normal on the outside or inside surface. According to the Energy Saving Trust, external condensation is a sign the windows are insulating well, while internal condensation is caused by high indoor humidity.
Condensation between the panes is caused by a failed seal that allows moisture to enter the gap. This is not normal and means the sealed unit needs to be replaced, typically after 20 or more years of use.
To reduce internal condensation, improve ventilation by using extractor fans, opening windows, and avoiding drying clothes indoors. External condensation cannot be stopped and will clear naturally as the glass warms.
