Tracing a finger across a foggy window is popular childhood pastime, but steamed-up, wet windows can cause damp and unsightly mould. Poorly ventilated, cold and cluttered homes are prone to this problem. During cold weather, or when temperatures drop overnight, the water vapour in moist indoor air condenses on cold window panes. Keeping your home free of foggy windows involves heating your home effectively, reducing moisture levels indoors and improving ventilation.
Condensation on foggy windows is water vapour that's cooled down when it came into contact with cold window panes, turning it to liquid water. Indoor air is usually warmer than air outside but window panes in poorly heated homes are often as cold as outdoor temperatures. This is why condensation problems are worse at night and during winter. Keeping your home heated to between 18 and 21 degrees Celsius (65 and 70 degrees Fahrenheit), so that windows stay warmer, helps prevent condensation.
Excessive moisture in indoor air can cause wet, foggy windows. Homes contain moisture from washing, cooking, drying clothes, people breathing, and bottled gas heaters, if used. Drying clothes indoors adds 5 litres (9 pints) of water to the air, and cooking and using a kettle adds 3.5 litres (6 pints). Leaking pipes, roofs and gutters also raise moisture levels. Some moisture in indoor air is unavoidable but householders can reduce the amount of water released into the air, and increase ventilation to help move it outside.
Homes with poor ventilation and air movement are prone to foggy windows. In cold weather, it's tempting to keep doors and windows closed to prevent heat escaping, but this allows moisture to build up. Other common sources of poor ventilation in homes are blocked air bricks, extractor fans, air vents and trickle ventilators. Rooms cluttered with lots of furniture, and wardrobes and cupboards filled with items, also prevent air moving freely and can lead to condensation on windows.
We can't prevent low outdoor temperatures, but changing a few habits can help stop windows becoming foggy and wet. Kitchens and bathrooms are the main sources of water in homes, so keep doors closed and extractor fans on, or windows slightly open, after cooking or washing. When cooking, keep lids on pans. Drying clothes indoors adds lots of moisture to the air, so always dry outside whenever possible or put wet washing in the bathroom with the door closed and window open. Move furniture 3 or 4 inches away from walls to allow air to circulate, and reduce cluttering.