Should double glazed windows get condensation?

No, double glazed windows should not normally get condensation on the inside surface between the two panes of glass. According to the Energy Saving Trust, condensation between the panes indicates a failed seal, letting moisture in and reducing thermal performance (Energy Saving Trust, 2026). Some external condensation or brief internal misting can be normal, but persistent internal or inter-pane condensation is a sign of a problem.

The key variable is where the condensation forms. Condensation on the outer pane on a clear morning is often normal, especially with energy-efficient low-E glass. Internal condensation on the room-facing glass usually points to high indoor humidity and poor ventilation, not a window fault. But condensation trapped between the two panes, which you cannot wipe away, means the sealed unit has failed and needs replacing. The window frame type and age also affect how likely condensation is to appear.

Condensation between the panes means seal failure

Double glazing works by trapping a layer of gas, usually argon, between two panes of glass. A desiccant strip inside the spacer absorbs any residual moisture during manufacture. When the seal fails, moisture-laden air enters and condenses on the cooler inner surface of the outer pane. The Glass and Glazing Federation states that a failed seal cannot be repaired, the whole sealed unit must be replaced (BRE, 2026). This typically costs £150–£300 per window depending on size and type. If you see mist, fog or water droplets between the panes, the window has reached the end of its useful life.

Internal condensation is usually a ventilation issue

Condensation on the room-side surface of double glazing is almost always caused by excess indoor humidity. Activities like cooking, showering and drying clothes indoors release water vapour. If the air cannot escape, it condenses on the coldest surface, often the window. The Energy Saving Trust says a typical four-person household produces around 14 litres of moisture per day (Energy Saving Trust, 2026). Opening trickle vents, using extractor fans, and avoiding drying clothes on radiators can reduce internal condensation. Modern double glazing is so airtight that ventilation becomes critical.

External condensation can be a sign of efficiency

Condensation on the outside of double glazed windows is actually a positive sign. It occurs when the outer pane is colder than the outside air, meaning the glass is not losing heat from inside your home. This is common with low-emissivity (low-E) glass and is most noticeable on clear, still mornings. The British Fenestration Rating Council confirms that external condensation indicates the window is performing well thermally (FENSA, 2026). It will clear as the sun warms the glass. No action is needed, this is not a defect.

A worked example

A typical 1930s semi-detached house in Manchester with six double glazed windows showing condensation between the panes would face a total replacement cost of roughly £1,200. Under the ECO4 scheme, if you receive qualifying benefits, you may get the entire replacement fully funded through your energy supplier. Without ECO4 eligibility, you would pay £1,200 upfront but save around £180 per year on heating bills because the failed seals were letting heat escape. The payback period works out at just under 7 years, and over 25 years the total savings amount to approximately £3,300. The Energy Saving Trust confirms that replacing failed sealed units restores thermal performance and can reduce heat loss by up to 15% per window.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).

What homeowners often get wrong

The most common mistake is assuming all condensation on double glazing means the window has failed. Here are three specific errors that cost homeowners money and time.

1. Confusing external condensation with seal failure Many homeowners panic when they see water on the outside of their windows on a clear morning. External condensation on low-E glass is actually a sign the window is working efficiently, as it means the glass is keeping heat inside. Wiping it away or drilling drainage holes can void your warranty and cause real seal damage later.
2. Ignoring inter-pane condensation until mould appears Some homeowners wait until they see black mould between the glass before taking action. By that point the desiccant strip is saturated and the argon gas has fully escaped, meaning your window has been underperforming for months. Replacing a failed unit early saves roughly £50 per year in wasted heating compared to leaving it.
3. Thinking trickle vents cause condensation Many homeowners close or block trickle vents because they believe the incoming air creates moisture. In reality, trickle vents reduce indoor humidity by providing controlled ventilation, which prevents condensation forming on the room-side glass. Blocking them can push humidity above 70%, causing mould on walls and window frames that costs hundreds to treat.

Quick reference

• Condensation trapped between the two panes of glass that cannot be wiped away means the sealed unit has failed and needs replacing.
• Replacing a single failed double glazed unit typically costs between £150 and £300 depending on the window size and glass type.
• You may qualify for a fully funded replacement under the ECO4 scheme if you receive means-tested benefits and live in a home with poor energy performance.
• External condensation on the outer pane during clear mornings is normal and confirms your low-E glass is working correctly.
• Blocking trickle vents to stop draughts will increase indoor humidity and make internal condensation worse, not better.