How to Stop Condensation on Windows

Condensation on windows is a common problem in UK homes, but it signals excess humidity or poor ventilation. Here's why it forms and how to fix it.

Why Does Condensation Form on Windows?

Condensation happens when warm air, which holds more water vapour, meets a cold surface. Windows are the coldest part of most rooms in winter because glass conducts heat away quickly. When warm indoor air contacts cold window glass, it cools. Once it drops below the dew point (the temperature at which air can no longer hold all its moisture), water vapour condenses into visible droplets.¹

Single-glazed windows are worst affected because the inner pane is almost as cold as the outdoor air. Double glazing helps by trapping an air gap, keeping the inner pane warmer. But if indoor humidity is high (common in UK homes due to cooking, showers, and drying clothes indoors), even double glazing can show condensation.

Is Condensation on Windows Normal?

Light condensation on single-glazed windows in winter is common and not usually a concern. It forms overnight when heating is off and indoor temperature drops, then disappears when rooms warm up in the morning.

Heavy condensation on double-glazed windows, or condensation that persists all day, is not normal. It signals one of two problems:

• Excess indoor humidity. Indoor relative humidity should sit between 40-60% RH. Above 60%, condensation risk rises. Above 70%, mould risk is high.²
• Poor ventilation. Moisture from cooking, showers, and breathing builds up faster than it escapes. Building Regulations Part F requires continuous background ventilation (trickle vents or MVHR) plus intermittent extract in kitchens and bathrooms.

Condensation between double-glazed panes means the sealed unit has failed. The gap should contain dry air or inert gas. If you see moisture inside the unit, the glazing needs replacing.

How Do I Stop Condensation on Windows?

Three levers: reduce indoor humidity, improve ventilation, and upgrade window insulation.

Reduce Indoor Humidity

• Use extractor fans in kitchen and bathroom during and for 15-20 minutes after cooking or showering.
• Dry clothes outside or in a vented tumble dryer, not on radiators indoors.
• Lid pans when cooking to trap steam.
• Keep lids on fish tanks and limit indoor plants (soil releases moisture).
• Fix leaks (dripping taps, roof leaks) that add hidden moisture.

Improve Ventilation

UK Building Regs Part F 2021 requires all habitable rooms to have either trickle vents in windows or continuous mechanical ventilation.³ If your windows have trickle vents, keep them open. Closing them saves a small amount of heat but traps moisture, raising condensation and mould risk.

Mechanical ventilation with heat recovery (MVHR) is the best solution for airtight, well-insulated homes. MVHR extracts stale, moist air and supplies fresh air, recovering 85-95% of heat in the process. It keeps humidity low without heat loss. Installation costs £4,000-£8,000 for a typical 3-bed home.⁴

Upgrade Windows

Better-insulated glazing keeps the inner pane warmer, reducing condensation. Double glazing with low-emissivity (low-E) coating performs better than standard double glazing. Triple glazing is even better but costs 30-50% more than double glazing and may not be cost-effective unless you're building new or replacing windows anyway.

When Does Window Condensation Cause Mould?

Mould spores are always present in air. They germinate when a surface stays damp for 48 hours or more. Persistent condensation on window frames, sills, or surrounding walls provides ideal conditions. Black mould (Stachybotrys chartarum) appears first in corners, on window sealant, and behind furniture against external walls.

Mould is a health risk, particularly for people with asthma or allergies. Fix the condensation source before cleaning mould. Wiping mould away without fixing ventilation or humidity means it returns within weeks.

Healthy Indoor Humidity

40-60% RH (year-round)

Mould Growth Threshold

Surfaces damp for 48+ hours

Double Glazing Benefit

Inner pane 10-15°C warmer than single glazing

Part F Ventilation Requirement

Background: 8,000 mm² per habitable room

Last reviewed: 20 May 2026 | Word count: 1,247 | Reading time: 6 minutes