Humidity controlled natural ventilation without auxiliary energy supply.

As a consequence of measures required for reducing the heating energy consumption in residential buildings , there have been more and more complaints in the last few years on the appearance of mould in dwellings . In most cases, it is retrofitted or renovated old buildings which are affected [1]. Mould growth is frequently the result of a severe reduction in the natural air change rate in old buildings following the installation of airtight windows, while user habits remain the same as before. Each day, an average amount of 8 to 15 liters of moisture is generated in dwellings , which is usually conveyed to the outside through window joints . However, airtight windows and insufficient ventilation cause indoor air humidity to rise . This may lead to surface humidity on cold external walls , e.g. at thermal bridges , thus providing ideal conditions for mould growth. The effect is enhanced unless the insulation level of the external wall is greatly improved so that the surface temperature of the exposed areas is increased. According to [2], mould growth is influenced by the following parameters: nutrient availability , temperature, ph-value of the substrate and, in a decisive manner, the amount of water in the substrate . According to [3], in one third of all cases, the damage is obviously caused by user - related , high indoor air humidity (see Table 1 ) . This is the result of tests performed in 300 old buildings , where several examinations were carried out in different dwellings. Besides, structural deficiencies such as thermal bridges or insufficient humidity protection of the building envelope were identified as being responsible for the remaining cases of damage. It can therefore be concluded that, supposing the construction is sound, it is the user-related indoor moisture load that should be reduced to prevent humidity damage and resulting mould growth.