• “Summary on proper ventilation:

  • Ventilate at least once a day
  • Open at least two windows/doors simultaneously for cross-ventilation
  • Fully open windows while ventilating, not just slightly ajar
  • Do not leave windows open for too long, especially during cold temperatures
  • Set heating between levels 3 and 4
  • The warmer the indoor temperature compared to the outside, the more frequent ventilation is needed
  • Avoid drying laundry inside the apartment
  • When drying damp items, ventilate at least every 2 hours
  • Ventilate after showering, cooking, and waking up”

Mold forms where moisture and warmth provide a breeding ground for mold spores. Mold spores are naturally present in the air in small quantities. However, in the case of high mold infestation, not only can the building structure be compromised, but the health of the residents can also be affected. To prevent mold formation, it is sufficient to eliminate one of the three factors (moisture, warmth, mold spores). Therefore, to avoid mold, moisture on walls and ceilings should be prevented.

We have now determined that mold in apartments always occurs where a surface is moist. So, if mold forms on a wall, it must have been moist there. But where does the moisture come from?

Moist walls and ceilings can have various causes. A leaking roof, cracks in the masonry, a burst pipe, or even a neighbor’s broken washing machine above us can lead to moisture on ceilings and walls. However, all of this is rather rare. In the vast majority of cases, the moisture is created as condensation from the ambient humidity.

This leads us to the next question: What is condensation, and how is it formed? Once we answer that, we’ll only need to clarify how condensation on walls and ceilings can be prevented

We’ve all surely observed that a glass fogs up when we breathe on it. Why does this happen? When we breathe on something, the air comes from our lungs and thus has a temperature of approximately 37°C. A windowpane, depending on the outside temperature, is much colder. So, we can establish that water condenses when the air is warm and a surface (window, wall, ceiling, etc.) is colder. But how does temperature relate to humidity?

Our ambient air always contains a more or less high proportion of water vapour. How much water vapour can be absorbed by our ambient air depends on the temperature. The following applies: The warmer the air, the more water it can absorb (exponentially more). Thus, at a temperature of 0 °C, 1 litre of air can absorb a maximum of 5 g of water, at 10 °C 9 g, at 30 °C already 30 g of water and at 60 °C over 100 g of water. Absolute humidity indicates how many grams of water are in one cubic metre of air. Relative humidity is the ratio of the actual mass of water vapour in the air to the maximum possible mass; or in other words, the ratio between the absolute humidity and the maximum humidity.

Assume 1 litre of air with 5 g of water vapour at 0 °C. This results in an absolute humidity of 5g/1l and a relative humidity of phi=absolute humidity / maximum humidity = (5g/1l)/(5g/1l) = 1 = 100%. If we heat the air to 10 °C without adding any more water, we still have an absolute humidity of 5g/1l, but a relative humidity of phi=absolute humidity / maximum humidity = (5g/1l)/(9g/1l) ~ 56%. If we continue to heat to 30 °C, we are left with a relative humidity of 17%. Conversely, if we cool down 1 litre of water at 30 °C with 70% relative humidity (i.e. with 21g/1l absolute humidity), the relative humidity keeps increasing. At approx. 23 °C, a relative humidity of 100% is reached and water vapour begins to condense.

From the previous physical principles it can be concluded that high humidity and cold walls and ceilings should be avoided. The relative humidity should be between 40% and 60%.

Particularly high humidity can always be found in rooms where there is a water connection (bathroom, kitchen) or where the humidity increases due to breathing air or sweat (bedroom). In order to reduce the humidity, “cross-ventilation” should be carried out, i.e. at least 2 windows or doors should always be opened so that there is a draft. This allows the moisture contained in the indoor air to be adjusted to the humidity outside as quickly as possible. The entire apartment should be ventilated at least once a day. Additionally after showering, cooking and in the morning after getting up.

If the outside temperature is colder, you should not ventilate for too long (max. 2 minutes) so that the walls do not cool down and water vapor can condense. If a window has been open for a while, the heating should be turned on after closing it and the room door should remain closed until the room has the same temperature as the rest of the rooms in the apartment. [Otherwise, warm air (22 °C; 50% relative humidity; approx. 10 g water per m^3 air) flows from the other rooms into the cold room. Assuming the walls are cooled to 10°C, the warm air near the walls cools down and the relative humidity increases. At 10 °C the rel. Humidity exceeds 100% and water condenses on the walls.]

Laundry should not be dried in apartments. [The rooms in our apartments have a volume of ~40 m^3 = 40 l. At 22 °C and a relative humidity of 50%, 400 ml of water is enough to raise the humidity in the entire room to 100%.] However, it is not always avoidable, e.g. when towels dry after showering. Then you should air it regularly every 2 hours as described above until the laundry is dry.

Chlorine bleach (sodium hypochloride) is usually used to remove it. Hydrogen peroxide can be used for disinfection, but it does not bleach the affected areas as well as chlorine. When handling agents containing chlorine, it is important to remember that they must not be mixed with other chemicals as toxic chlorine gas can be formed.