The warmer and moister the air indoors, the higher the vapour pressure.
When a building component or a layer of a component separates two rooms with different temperatures and humidity, there is varying water-vapour partial pressure on both sides. Water vapour moves through building material with this difference in pressure. This is known as water vapour diffusion. The path and direction are always from warm to cold. A much greater part of humidity escapes through leaks due to air currents. Whereas only 0.3 litres of water escape through 1m² of wall or ceiling during the heating period, up to 30 litres of water pass into the construction through a 20 cm long and 2m wide crack.
Where does water vapour come from and where does it go?
The house occupants produce moisture in the rooms every day: through breathing, sleeping, cooking, showering… A household of 4 people can emit up to 10 litres of moisture into the room’s atmosphere in the course of a day. Part of this is removed through airing but the water vapour left in the air, similarly to the warmth, tends to seek its way via the exterior construction components to the outside. Warm air can retain more moisture than cold air (e.g. at a temperature of 20°C 17.3g/m³ and only 2.14g/m³ at -10°C.) If warm air cools too quickly it produces moisture and condensation occurs. By the way: anyone who has ever taken a bottle of beer out of the refrigerator is familiar with this principle.