Who and what causes mildew, and how can you prevent it?

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Mildew does not only occur in old walls. Mildew can also strike in new buildings. Where there is moisture, the furry fungus can sprout. Are the inhabitants to blame for not airing out the building enough? Architects who plan incorrectly? Or negligent craftsmen?

The corner of the room begins to darken and the stain grows rapidly. It smells, irritates the respiratory tract, is poisonous and endangers health. The building fabric also suffers: Mildew can do great damage. Whose fault is it?

Clearly, the residents breathe, sweat, shower, cook and wash. In a nutshell, they constantly release moisture that has to settle somewhere. "But that's all perfectly normal," states Mag. Brigitte Tassenbacher. The microbiologist and environmental technician is concerned with energy-efficient construction and building services engineering and has many years of experience as a certified mildew expert. She is often consulted as an expert witness in court cases.

If temperature and humidity are measured in the middle of a room, the values are usually within the normal range and are therefore not hazardous: The room temperature is around 20 °C, the relative humidity is about 50-55%. On the other hand, if the temperature is measured on the surface of a building component, for example in an outside corner of the building, the temperature can easily drop to 12 °C and lower. The relative humidity at this point is then 80% and higher. This creates the conditions for mildew to grow.

What the planner can do

The planner can prevent this by taking into account the specific needs of future residents. Sounds easy enough, but it is not – especially when old factory premises are converted into residential lofts. The geometry of a new building can also pose a danger: According to Brigitte Tassenbacher, sharp corners and cantilevered components can promote the growth of mildew later on. Brigitte Tassenbacher also cites windows that cannot be opened or situations in which ventilation causes noise and the risk of burglary. "If no comfort ventilation is planned," she says, "the moisture which arises naturally, cannot be sufficiently removed." Thermal bridges and interior insulation are further keywords in her catalogue. The details of the connections must therefore be carefully planned.


Sharp corners and cantilevered components can promote mildew growth.

As an example, think of a home in which parents, a baby and a toddler sleep in a 10 m2 bedroom. The room-high window element that only has double glazing remains closed at night, as it would open onto a busy road. "High humidity, condensation on the panes and mildew growth are unavoidable," says Brigitte Tassenbacher. In another house, a room has a balcony on both sides. The floor in between cantilevers as a bay with windows. The room never gets warm, let alone the structural envelope that surrounds it.

The craftsman can do this

The quality of construction supervision and execution is just as important after planning. According to Brigitte Tassenbacher, mistakes occur particularly frequently when the building envelope is airtight. If the electrician or installer does not seal the openings, pipelines, cables, sockets and connections, or does not carefully seal them airtight, then humid, warm room air can penetrate into the components. "This creates the conditions for moisture accumulation, building damage and fungal growth."

An example from Brigitte Tassenbacher's rich practical experience: A wooden beam on a canopy was infested with mildew. Her investigations led to the airtight installation of an LED luminaire that illuminated a niche in the shower. The fact that the mildew does not develop at the "scene of the crime" but elsewhere is quite common and can be attributed to different pressure conditions. Even if the bulk material added to build up the soil structure is too wet, this can add moisture to a building and lead to mildew later on.

This can be done by the building administration and the housekeeper

Maintenance and regular servicing help to preserve the value of the building fabric. And, as is so often the case, the solution here is "information and communication". If the residents are sensitized to how they can take care of the building, much has already been achieved. Brigitte Tassenbacher adds the observation that nowadays apartments or houses are no longer ventilated as often during the day as they used to be, as both parents are often out at work. From her expert point of view, this would also suggest the installation of a comfort ventilation system, which regularly renews the air and also regulates the humidity.

In all her professional years, Brigitte Tassenbacher has also learned time and again that the blame for mildew is placed on the landlords and that they are sometimes even deliberately intimidated. It is important for all parties involved to remain objective, she says, and to find and eliminate the causes together and to remove the mildew professionally.

Andrea Schädler

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