What causes condensation on double glazed windows?

Condensation on double glazed windows usually happens when **warm, moist air meets a colder glass surface**. In many homes, that’s a sign of **high indoor humidity, poor ventilation, or a cooler window surface** , rather than a fault with the window itself.

Quick Scoop

• Inside the room-side pane: common in bedrooms, kitchens, and bathrooms when humidity is high and air circulation is poor.

• Between the two panes: usually means the window seal has failed , letting moisture into the sealed unit.

• Outside the window: can happen on clear, cool nights when the outer pane gets colder than the surrounding air.

Why it happens

Double glazing helps reduce heat loss, but it does not remove moisture from the air. If your home is tightly sealed, steam from cooking, showers, drying clothes, or even breathing can build up indoors and then condense on the coldest surface available, which is often the window glass.

What the location means

Where the condensation is| What it usually means
---|---
Inside surface of the room-facing pane| High humidity and not enough ventilation 135
Between panes| Failed seal or blown unit 178
Outside surface| Normal weather-related condensation on a cold outer pane 3

How to reduce it

• Improve ventilation by opening windows briefly or using extractor fans.

• Reduce indoor moisture from cooking, showers, and drying laundry indoors.

• Keep rooms a little warmer and air moving, especially at night.

• If moisture is between the panes , the unit may need repair or replacement.

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