Prevention of mould infestation

It is important that the moisture generated by the activities in the room (e.g. moisture release by people, showers, cooking, washing) is removed to the outside by regular ventilation.

Humidity and ventilation

The possibility of removing moisture from the room by ventilation is based on the fact that air can absorb different amounts of water vapour depending on the temperature. Warm air contains much more water than cold air at the same relative humidity. Cold outside air in winter contains little water, even if its relative humidity is high.

Water transport during ventilation

Cold outside air that enters the interior during ventilation absorbs moisture during heating, which is removed outwards again with the heated air. With cold outside air, drying out can be achieved in the interior, even in rainy weather, by ventilation. The colder the air, the more water it can absorb when heated. Therefore, more moisture can be removed from a room in winter than in summer by ventilation with cold outside air. Air is therefore able to absorb more water when heated. This can be used to remove moisture from a room by exchanging air.

For example, if steam-saturated outside air is introduced into a room at a temperature of 5 °C and transported out again at a temperature of 20 °C and a relative humidity of 60 %, 8.7 – 5.5 = 3.2 g of water per kg of air is transported outwards. Here you can see the possibility of reducing the humidity in a room by ventilation.
The air exchange required to remove moisture can be calculated if it is known how much water is produced. In a three-person household, about 10 ± 4 kg of water is released daily by the release of water vapour (30 to 100 g/h per person) from showers, washing, laundry drying, cooking, plants, aquariums and other sources of moisture!
From a hygienic point of view, an air exchange rate of 0.5-1.0/h is often considered sensible for normal living space use. However, there is still no binding definition of minimum air exchange standards. Higher air exchange rates may be necessary for high room occupancy, e.g. in school classes.

Why drying can be achieved by ventilation and why condensation can form on cold walls.

If warm air saturated with water is cooled, the air must separate water as condensation or mist. We know this process from nature. In the apartment, condensation can occur on cold walls where the warm room air is cooled down, or very high relative humidity can be achieved, which makes mould growth possible.
The worse the thermal insulation of the outer walls is or the more structural faults have been made in the building construction (e.g. in the form of thermal bridges) and the worse the outer walls are heated by circulating room air, for example behind cabinets or wall cladding, the lower the surface temperature of these outer walls in winter. This increases the relative humidity on the inner wall surface and the risk of condensation.

Therefore, no tightly fitting furniture, pictures or heavy curtains should be set up or hung up on external walls, especially if there is insufficient thermal insulation. A minimum distance of approx. 10 cm can be regarded as a guideline. In basement rooms, the wall temperature is often low even in summer. However, since the absolute humidity of the outside air is often high in summer, frequent ventilation with outside air to “dry off” would then be wrong, because more and more moisture is introduced into the room and condenses on the cold walls.
In cellars, which only serve as storage and are not intended for the longer stay of persons, mold fungus infestation is often accepted. The only remedy would be better thermal insulation, heating or drying the room air. Cellars in which mould growth is not prevented, however, should not have a direct connection to the rest of the building, for example through stairs, shafts or unsealed openings in the cellar ceiling.

Especially in older buildings, windows often have poorer thermal insulation than the walls. The advantage of this is that condensation first occurs at the window, indicating that more heating or ventilation is required. With tightly closing windows with better thermal insulation than walls, it is no longer the windows but the outer walls (especially the outer corners) that are the coldest places and thus the places with the highest relative humidity. Any condensation water that may occur there is usually not detected as quickly as with “fogged up” windows.

For tightly closing, well thermally insulated windows, ventilation should therefore be increased as a precautionary measure. In buildings with tightly closing windows, ventilation must be increased in order to remove moisture from the room and thus prevent possible mould fungus problems. Landlords should inform their tenants absolutely about the consequences of the sealing and thermal insulation measures.

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