Why is there condensation in my windows?

28th July 2026

Condensation forms when a surface is cold enough to turn the moisture in the surrounding air into water. That temperature is called the dew point. When the interior glass temperature drops below the dew point of the room's air, you get condensation.

Before diagnosing the cause, it helps to know which type you have. There are three, and they mean very different things.

• Exterior condensation appears on the outside of the glass, usually on cold mornings. It is a sign that the window is performing well thermally because the glass is cool and the warm air inside is being retained. No action is needed.
• Interior condensation appears on the inside face of the glass. This is a humidity and ventilation problem, not a window fault. Warm, moisture-laden air is contacting cold glass and releasing water.
• Between-pane condensation appears inside the glazing unit itself, between the panes. This means the sealed unit's gas fill has failed and moisture has entered. The glazed unit needs replacing.

A household of two people can produce up to 24 pints of water a day from normal activities including cooking, bathing, and drying clothes, and all of that ends up in the air.

A comfortable indoor relative humidity sits around 40 to 55%. When it climbs above 60%, according to Dorset Council's guidance on damp and condensation, the risk of condensation and mould problems rises sharply [1].

How Do You Remove Condensation from The Inside of Windows?

Interior surface condensation, the mist that appears on the inside face of the glass on a cold morning, can be wiped away. The underlying problem will keep producing it until the humidity issue is addressed.

Once moisture has entered a sealed glazing unit, the only fix is unit replacement. Wiping the glass will not clear between-pane condensation.

Persistent interior condensation that goes unchecked leads to mould growth. Mould carries real health risks, particularly for children, elderly residents, and anyone with a respiratory condition or asthma. That is the reason to act quickly rather than tolerate it.

How Do You Prevent Condensation from Forming Inside Windows?

Ventilation is the most direct fix. Extractor fans in bathrooms and kitchens, combined with regular window opening to exchange humid air for fresh air, make a significant difference. For properties where moisture is a persistent problem, a dehumidifier will help.

Temperature matters too. Rooms that cool significantly overnight create a larger gap between the warm moist air and the cold glass surface. Maintaining adequate background heat reduces the risk, though this has to be weighed against running costs.

Curtains and blinds are worth watching. Thick window coverings trap humid room air directly against cold glass overnight, creating ideal conditions for condensation to form. Keeping them open during cold nights or high-moisture periods improves air circulation across the glass.

If you are specifying windows for a project, thermal performance is where the work gets done at source. Approved Document L sets a maximum U value of 1.6 W/m²K for replacement windows in existing homes, with a notional target of 1.2 W/m²K for new builds. Triple glazing comfortably exceeds that regulatory minimum. The better a window's U value, the warmer the interior glass surface stays, and the less likely condensation is to form [2].

Aluminium windows with a polyamide thermal break, the technology used across Alimax's manufactured range, create a barrier between the cold outer frame and the warm inner surface. That reduces the temperature differential that causes condensation to form in the first place. Bespoke aluminium windows manufactured to a high thermal specification address the root cause, not just the symptoms.

For period properties where single-glazed or poorly fitted original windows are driving the condensation problem, the MAXi4 heritage range replicates Victorian steel window profiles with B-rated thermal performance as standard. It is suited to listed buildings and conservation areas where the frame profile has to match the original.

For more on the specific causes of condensation on aluminium frames and how to address them, see our guide to stopping condensation on aluminium windows.

Specify Alimax Aluminium Windows for Your Next Project

All Alimax windows are tested against the BS 6375 series, the British Standard for window and door performance published by the British Standards Institution, and carry a minimum B energy efficiency rating as standard [3].

From the MAXi2 flush casement to the MAXi5 vertical sliding, every window we manufacture is made to measure at our Hampshire factory and delivered to site fully assembled. Lead times are as little as five days on prime product lines. If you want to see the range in person before specifying, the Fordingbridge showroom is open Monday to Friday. You can also browse the full range of windows to see every product we make.

Call us on 01425 205465 or get in touch with Alimax with your requirements.

External Sources

[1] GOV.UK, Ministry of Housing, Communities and Local Government (MHCLG), Conservation of Fuel and Power: Approved Document L (2023): https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l [2] GOV.UK, Dorset Council, How to Control Damp, Condensation and Mould in Your Home: https://www.dorsetcouncil.gov.uk/documents/35024/282001/How+to+prevent+damp+mould+and+condensation+in+your+home.pdf/52c52240-9d1b-39c5-4401-e665cc6e5a69 [3] British Standards Institution (BSI), BS 6375 — Performance of Windows and Doors: https://landingpage.bsigroup.com/LandingPage/Series?UPI=BS+6375