Window condensation – where the water vapour comes from

Recognising and controlling the sources of water vapour inside a home can help to reduce the likelihood of condensation forming on windows and doors.

Flowers in a vase on a window sill

Water vapour and condensation

Condensation happens when a gas or vapour changes to a liquid. In the home, this occurs when water vapour condenses onto surfaces such as windows, doors and mirrors.

When air saturated with water vapour meets a surface – such as a window pane – which is cooler than itself, it will shed surplus water vapour on that surface, first as a fine mist or, if in great enough quantities, as droplets of water.

One way to reduce the amount of condensation that occurs in the home is to provide adequate ventilation for moist air. Another is to make efforts – where practicable – to reduce the formation of water vapour in the first place.

Sources of water vapour inside a home


Two sleeping adults produce approximately one litre of moisture in eight hours.


Steam clouds from heating water in saucepans and kettles. The gas in a cooker contains some water and average gas cooker can produce approximately one litre of moisture per hour.

Washing up

Through vapour clouds, given off by the hot water.

Bathing, laundry, and wet outer clothing

These are common major sources of water vapour in the home.


A flueless gas heater can produce up to 350cc of moisture per hour.

Indoor plants

The life processes of plants are a significant source of water vapour.

New properties

The bricks, timber, concrete and other materials in an average three bedroomed house absorb about 7,000 litres of water during construction. Much of this is dissipated indoors later during the drying out period.

Sources of water vapour outside a home

Water vapour is always present in the outside air and levels are dependent upon atmospheric conditions (temperature and humidity).

A typical example is the formation of condensation on the entire surface of a car, including the glazing, when left in an exposed area. This condensation would typically be removed using both the wipers blades and a squeegee.

As double and triple glazing becomes more efficient, heat is less able to pass through the window and warm the outer pane. This means that the outer pane surface remains cold creating the conditions that may allow condensation to form.

For more advice and information about reducing condensation in your home, get in touch with a local GGF Member company.

Related topics

What is condensation and why does it occur on windows?
Where condensation can form on a window and how to reduce it
How double or triple glazing helps reduce window condensation
Top tips on reducing condensation room by room
A guide to home ventilation