Condensation on new windows: why it happens and how to fix it

New A-rated windows seal your house far more tightly than the units they replaced. Old draughty windows quietly ventilated the house 24 hours a day; when they leave, the moisture that a family of four generates — cooking, showering, drying laundry, breathing — has nowhere to go. It condenses on the coldest surface it can find, which is usually the new glass on a winter morning.

This is not a window fault. It is a symptom of a house that is now airtight and needs mechanical or trickle-vent ventilation to compensate.

Condensation on the inside of the glass — the room-facing surface — means humid indoor air is meeting cold glass. Solution: more ventilation, especially in bathrooms and kitchens.

Condensation on the outside of the glass — the surface facing the garden — is a badge of honour. It only happens on well-insulated A-rated windows on clear autumn mornings, because the glass is now colder than the surrounding air. It clears within an hour of sunrise.

Condensation inside the sealed unit, between the two panes, means the seal has failed and the argon has escaped. The unit is faulty and must be replaced under warranty.

Open trickle vents. They are there for exactly this problem. Closing them defeats the point of the Part F regulation. Run bathroom and kitchen extractors for 15–20 minutes after showering or cooking. Dry laundry outside or in a vented tumble dryer, not on a rack in a bedroom. Consider a whole-house Positive Input Ventilation (PIV) unit if condensation persists — it is a straightforward retrofit that solves the problem permanently.