Humidity and mould growth in living areas

A pleasant room climate is essential for a sense of well being in the home. Room humidity has a major impact on the quality of the room climate. A relative humidity of 40 % to 65 % is generally considered to be a comfortable and healthy room climate. The relative humidity is mostly a function of the following factors:

  • Everyday domestic activities in the home
  • Use of showers and baths as well as the natural transpiration of people, animals and plants
  • Temperature and weather conditions

The right room climate is also essential with regard to health and for maintaining the building fabric of the living areas. An appropriate room climate can effectively prevent the mould growth in colder spots on outside walls or in wet-rooms. To avoid mould growth it is therefore necessary to keep humidity at a good level for the residents and for the building, by adopting a ventilation behaviour that suits the structural conditions.

 

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Relationship between temperature, humidity and mould growth

The amount of moisture in one cubic metre of air varies as a function of the air temperature. Warm air can absorb more moisture than cold air. At a room temperature of 20° C and a relative humidity of 60 % one cubic metre of room air holds 10 g of water. If the room temperature is reduced to 8° C the air in the room can only absorb half the amount of moisture. This means that 50 % of the moisture escapes and precipitates as condensation water, mostly on the cooler outside walls. The risk of mould growth therefore increases at these points.

 

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Dewpoint and mould-critical temperature

When the relative humidity reaches the threshold value of 100 % excess moisture starts to condense and forms condensation water. The temperature at which condensation occurs is called the dewpoint. As the relative humidity is a function of the air temperature the dewpoint varies depending on the respective situation. For example, the dewpoint at a room temperature of 20 °C, with a relative humidity of 50 %, is 9.3 °C.

We know today that mould can already start to grow at a room humidity of 80 %. This means that in the given example, mould can start to grow at a surface temperature of 12.6 °C or less on walls, especially in corners and niches.

As already mentioned, humidity can be regulated by correct ventilation behaviour and kept at a pleasant level. Due to modern building standards and growing requirements for energy efficiency in new buildings, more and more importance is being attached to the topic of living area ventilation