Condensation is a natural phenomenon which consists of water evaporation present in the air. When the temperature rises, the air absorbs the water vapor up to saturation, increasing its volume. After, when the temperature drops, the air expels the water vapor which in contact with a cooler surface, condenses, turning into water droplets. The reason why condensation occurs on glass windows is due to the fact that these have a lower temperature than that of the room. The window can be subject to condensation since it is an element with a lower energy performance compared to the masonry in which it is inserted. If the window had the same degree of thermal insulation than the surrounding masonry, condensation wouldn't occur on it. Today, doors and windows have a very high energy performance, thanks to insulating glass and waterproof joints that retain the heat inside. But at the same time they retain the air humidity in the rooms. So, it's important to maintain the right balance between temperature and relative humidity (20°C inside with a humidity level not exceeding 45%) and frequently air the rooms in order to replace the indoor air full of vapor with the low- vapor external one.