What is indoor air outdoor air


The maximum possible water content in the air depends on the temperature. The higher the temperature, the more moisture can be stored in the air. At 20 ° C air can absorb a maximum of 17.3 g / m³ of water, at 0 ° C it is only 4.8 g / m³. This is known as the dew point. In the cold winter air there is significantly less water vapor than in the warm indoor air. If the indoor air cools on the way outside, the excess water precipitates in liquid form.

Moisture balance
The air humidity prevailing in a room is determined by:

  • the moisture production in the room
  • the exchange of air with the outside air (air exchange) and, if necessary, neighboring rooms as well as their temperature and humidity
  • the sorption properties of the room enclosing surfaces as well as the furniture or other objects in the room
  • the moisture transport through external components
The calculation is carried out with the help of the moisture balance across the room. If one neglects the small amount of moisture transport through external components as a result of diffusion and the complex sorption processes, the relative room air humidity in a room under steady-state conditions results from the equation given above (Fig. 1).

Explanation of symbols:
  • π i: Relative humidity of the room air
  • π a: Relative humidity of the outside air
  • pSat: Saturation vapor pressure of the outside air
  • pSR: Saturation vapor pressure of the room air
  • mR.: Moisture production in the room
  • R.D.: Gas constant of water vapor
  • TR.: Air temperature in the room
  • V.R.: Air volume of the room
  • n: air exchange of the room

Expertise on the subject

Building physics


Diffusion is a compensation of different gas concentrations that occurs without any external influence. A gas concentration ...

Building physics

Moisture and condensation

Air always contains moisture - different amounts depending on temperature and air pressure. The relative humidity denotes ...

Planning basics

Indoor air quality

A certain air exchange is necessary for a comfortable indoor climate; An air requirement of 25-30 cubic meters per hour (m³ / h) per person is to be used as a guideline.

Building physics


With sorption, water vapor penetrates into materials as a result of diffusion if the room air humidity is higher than the ...

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