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What causes condensation on windows?

Internal condensation forms where humid indoor air meets a cold surface, usually indicating too much moisture generation and too little ventilation. Condensation between panes means the sealed unit has failed and needs replacing.

Condensation on the room side is a ventilation and moisture balance problem. Drying clothes indoors, unvented showers and blocked wall vents are the usual causes, and the window is simply the coldest surface where it shows first.

Condensation on the outside face of the glass in the morning is a sign of a well-performing window, not a fault — the outer pane stays cold because little heat escapes through it.

Reducing internal condensation

  • Extract at source in bathrooms and kitchens
  • Keep background ventilators open
  • Avoid drying laundry on radiators
  • Maintain a more even background temperature

Sources

Last reviewed 28 August 2026. General information only — not legal or planning advice.

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