Why are my double glazed windows covered in condensation?

Condensation on your double glazed windows usually means the seal has failed. This allows moisture between the panes, or that indoor humidity is too high. The most common cause in 2026 is a failed edge seal, which lets damp air enter the insulated gap, creating mist or water droplets that cannot be wiped away (GOV.UK, 2026).

Condensation can appear on the inside, outside, or between the panes. Inside condensation often relates to lifestyle factors like drying clothes indoors without ventilation. Outside condensation is normal on energy-efficient glass. Between-the-panes condensation, however, signals a broken seal. The seal failure rate for double glazing increases after 10-15 years, and replacement units cost roughly £150-£300 per window, depending on size and type (TrustMark, 2026).

Failed seals let moisture enter the gap

Double glazed units have a spacer bar with a desiccant that absorbs residual moisture during manufacture. When the edge seal degrades, the desiccant saturates and cannot prevent condensation. Glass can appear misty, with water droplets pooling at the bottom. The seal typically fails due to age, thermal stress, or poor installation. A failed unit will not insulate properly, increasing heat loss by up to 25% compared to an intact unit (BRE, 2026). Replacing the sealed unit is the only permanent fix.

High indoor humidity causes internal condensation

If condensation forms on the room-side surface, your home’s humidity is too high. Normal indoor humidity should be 40-60%. Activities like cooking, showering, and drying laundry generate moisture. Without adequate ventilation, such as trickle vents or extractor fans, this moisture settles on the coldest surface, which is often the glass. In 2026, Building Regulations Part F requires mechanical ventilation in new or renovated homes to control humidity (GOV.UK, 2026). Reducing humidity by opening windows or using a dehumidifier resolves this type of condensation.

Low-E glass and external condensation are normal

Modern double glazing with low-emissivity (Low-E) coatings reflects heat back into the room, keeping the outer pane colder. This makes external condensation, dew on the outside, more common on clear nights. It is a sign the windows are working efficiently, not a defect. The Energy Saving Trust states that Low-E glass can reduce heat loss by up to 40% compared to uncoated glass (Energy Saving Trust, 2026). External condensation usually clears as the sun warms the glass. No action is needed unless it persists all day, which may indicate shading or overhanging vegetation trapping moisture.

A worked example

For a typical 1930s semi-detached house with 10 double glazed windows showing condensation between the panes, you are looking at a total replacement cost of roughly £2,250. Each sealed unit costs between £150 and £300, and a specialist glazier charges around £75 per window for labour. If you act before March 2027, the 0% VAT on energy-saving materials keeps your bill lower. The Energy Saving Trust estimates that replacing all failed units in this scenario cuts heat loss by up to 25%, saving roughly £180 a year on gas heating for a typical gas-heated home. Over a 25-year window lifespan, those savings reach £4,500, which more than covers the upfront cost. Check your household income against ECO4 eligibility on GOV.UK, as some low-income households qualify for full replacement funding through their energy supplier.

What homeowners often get wrong

The most common mistake is assuming internal condensation on the glass means the window is broken. This confusion leads to unnecessary replacement costs and missed opportunities to fix the real issue. Here are three frequent errors homeowners make.

1. Thinking all condensation means a failed seal Internal condensation that wipes away easily is caused by high indoor humidity, not a broken window. The right answer is to ventilate the room, use extractor fans, and avoid drying clothes indoors, which can save you £2,000 on unnecessary window replacements.
2. Ignoring a failed seal because the glass looks clear in summer A seal failure lets moisture in gradually, and condensation often only appears in winter when the temperature difference is greatest. The right answer is to check for misting on cold mornings; delaying replacement wastes £60 a year per window on increased heating bills due to lost insulation.
3. Applying anti-condensation spray between the panes Once moisture is trapped between the glass, no spray or cleaning solution can remove it permanently. The right answer is to replace the sealed unit, and doing so under the 0% VAT scheme saves you £45 on a £300 replacement when compared to standard VAT rates.

Quick reference

• Condensation between the panes that cannot be wiped away means the window seal has failed and the unit needs replacing.
• A failed sealed unit increases heat loss by up to 25%, costing the average UK household an extra £60 to £180 per year depending on window count.
• You qualify for 0% VAT on replacement sealed units until March 2027 under the government’s energy-saving materials scheme.
• Internal condensation that wipes away is a ventilation problem, not a window problem, and fixing it costs nothing.
• Replacing a single sealed unit costs £150 to £300, but ECO4 funding may cover the full cost for eligible low-income households.