Definition
Condensation is what happens when water vapor, an invisible gas floating in the air, touches a cold surface and turns into liquid droplets. It happens more often when the gap between warm, humid indoor air and cold outdoor air is large and the humidity is high.
Why do droplets form only on cold surfaces?
On a sweltering summer day, set a glass full of ice on the table. After a little while, beads of water appear on the outside. The glass isn't leaking through tiny cracks. Water vapor that was hiding in the air touched the cold glass wall and turned into liquid.
Air can only hold a certain amount of water vapor, and that amount depends on temperature. Warm air is like a sponge that can soak up plenty of vapor. Cold air can only hold a little.
When warm, humid air touches a cold window or wall, its temperature drops in an instant. The air can't hold everything it was carrying, so the extra vapor spills out. This spilling out of vapor as liquid droplets is condensation.
Why mold grows on your walls in winter
In winter, the heat is on and your home is warm. Showers, cooking, and drying laundry indoors push the humidity in the air very high. Meanwhile, balcony windows and room corners that touch an outside wall are exposed to the cold winter wind and get very cold.
When warm, damp indoor air hits these cold walls, droplets keep forming. The temperature at which water vapor in the air starts to turn into droplets has a technical name: the dew point (the boundary temperature where vapor turns into water).
If you leave the condensation on the wall, the inside of the wallpaper and the insulation get soaked. A damp wall is a great place for mold to grow. It can stain and ruin the wallpaper, and mold spores can spread into the indoor air.
Smart ways to prevent and fix condensation
To be more precise, condensation doesn't happen just because the weather is cold. It comes from the indoor-outdoor temperature gap and indoor humidity working together. So the most basic and powerful way to stop it is to manage humidity and ventilate properly.
The easiest prevention is to open the windows wide for 10 minutes or more, two or three times a day, to let the moisture trapped indoors out. Winter air outside is very dry, so even a short airing can bring indoor humidity down a lot.
Also, furniture pushed tight against an outside wall blocks air movement and makes condensation worse. Leave a gap of at least 10 centimeters between the wall and the furniture. When a building is constructed, insulation has to be installed carefully with no breaks, so the wall's surface temperature doesn't drop. Good insulation work is essential.
Common misconceptions
Condensation is a leak, caused by a burst pipe inside the wall or a crack that lets rain in.
A leak is outside water getting in through a physical gap. Condensation is a natural physical process in which a gas in the air (water vapor) turns into droplets because of a temperature difference.
Where you meet it
A packed bus in winter, with the windows fogged white and water trickling down the glass.
After a hot shower, the whole bathroom mirror is covered in droplets.
Your glasses fog up when you step from the cold outdoors into a warm room.
In one sentence
When warm, humid air meets a cold surface, the water vapor in the air turns into droplets.