(Kopie 2)
The vapour pressure or steam pressure is a direct function of the water content of the air. If the water content is 7.733 g/kg air the vapour pressure is 1.227kN/m2. Unlike Relative Humidity (RH), the vapour pressure does not change with temperature changes. The air temperature may be 0°C, or 50°C, , as long as the vapor content is 7.733 g/kg air, the vapour pressure will be 1.227kN/m2.
The vapour pressure is one of the causes to moisture related problems in a building construction. In winter the outdoor air contains small amounts of moisture, whereas the indoor air normally has a much higher moisture content. If the outdoor air is 0°C, and the RH is 85 %, then the moisture content is 3.821 kg/kg air and the vapour pressure 0.611 kN/m2. Say that the indoor temperature is 22°C, and the RH 50%, then the vapour pressure will be 1.402 kN/m2. The difference of pressure makes indoor vapour penetrate openings(also microscopic ones) in the outer walls and condens at he point where the wall temperature is at dewpoint. The accumulated amounts of condensation water can be considerable during a winter season.