In Germany, tenants of rented housing must follow some rules concerning the efficient heating of their apartments. International residents are sometimes puzzled to learn that this is also the case for proper ventilation – which in German buildings means allowing fresh air to flow through their rooms. If these ventilation steps are skipped, mould may form in corners or around the windows, which can pose serious problems.
Where does mould come from?
Mould forms wherever there is too much moisture in the air and thus on walls, but the underlying causes for this vary. Among the main reasons are insufficient insulation, leaks or cracks in the walls, roofing or windows, or incorrect heating and ventilation. All this may lead to the condensation of humidity on the edges of windows and in the corners of the room.
Altbau buildings, meaning houses built roughly between 1870 and 1940, are very common in Hamburg. Altbau buildings are particularly prone to mould due to poorly insulated walls and windows. But if their windows have been swapped for more modern, leak-proof ones, mould is especially prone to forming around windows and in the corners of the room if the masonry is leaky.
Big, bulky pieces of furniture standing too close to exterior walls are the perfect breeding ground for mould, as they are likely colder than walls separating rooms, attracting condensation. When large pieces of furniture are against them, the flow of fresh air is blocked, creating the perfect, moist breeding ground for mould. That is why, if possible, furniture should be placed against inside walls and there should always be a space of 2–4cm (about .8–1.5 inches) between the piece of furniture and the wall.
Legal issues
Mould is ugly but mostly harmless. However, in larger quantities, it may cause allergies or even serious illnesses. That’s why mould should be removed as soon as it is spotted. According to German housing law, mould is considered a damage to the rental object, so the landlord is responsible for removing it, or the tenant is entitled to rent reduction.
However, the landlord can try to blame the tenant for failing to heat and ventilate the apartment correctly. So the tenant must keep the apartment at a certain temperature and ventilate regularly to keep mould at bay. Otherwise tenants can even be made responsible for the damage.
Please note: Our editing staff at hamburg.com are not legal experts and cannot be held accountable for information on German housing law.
Proper ventilation in German buildings
As a general rule, the lower the temperature of the room, the more frequently it should be aired. This is because hotter air can bind more humidity; if the temperature is lower, the more humidity condensates. Read more on room heating here.
Ventilating properly means opening the windows completely for a short period of time (German term: Stoßlüften). The greater the difference in temperature between inside and outside the room, the shorter this period needs to be for a complete exchange of the air in the room. In winter, 3–5 minutes is usually enough to prevent the interior walls and furniture from cooling down too much.
Querlüften – meaning that the windows in rooms at different sides of the building are fully opened at once – is even better, as it creates a diagonal airflow through the rooms. Kipplüften, on the other hand – which means tilting the windows open slightly for a longer time – is very inefficient, as the air cannot stream out as effectively. Additionally, Kipplüften may cool the walls and furniture down, causing humidity to condensate.
Tips for effective ventilation
According to the Deutscher Mieterbund (DMB, German Tenants’ Association), the rooms of an apartment should be aired more than just once a day. Especially rooms in which people spend a lot of time during the day need to be ventilated more frequently, as humans actually produce a lot of humidity just by breathing. In the evening, before going to bed, the whole apartment including the bedroom should be ventilated.
Pay special attention to bathrooms and other rooms in which great amounts of humidity are produced, for example by cooking, or hanging or ironing the laundry. To keep such rooms properly dry, doors should be kept closed and windows should be fully opened after use.
After having a shower in an interior bathroom (i.e. without outdoor windows), open the door and fully open the nearest window to the outside; keep the other doors closed to prevent the hot, humid air from entering the other rooms.
Even on rainy days, ventilation is important. Unless the rain is entering through the window, the cold air outside is still dryer than the hot air inside the room.
Of course, if there is no one inside the apartment during daytime, ventilation is not possible, but it is also not necessary. In this case, it is enough to air the apartment thoroughly in the morning and in the evening.
Ventilation and Mould Prevention
