December 14, 2024, 2:06 pm | Read time: 3 minutes
It is a widespread assumption that mold grows more quickly in thermally insulated houses – because the walls can no longer “breathe.” Is this really true? A professional explains.
Mold in the home can not only damage the fabric of the building but also your health. There is a widespread belief that houses with thermal insulation are more prone to mold growth. The walls need to “breathe” so that air exchange can take place. Find out what is behind this assumption and what influence insulation has on mold growth here.
Mold Due to Thermal Insulation? What an Expert Says
A common myth is that thermal insulation promotes the formation of mold because walls can supposedly no longer “breathe.” “However, this is a misconception,” says Zukunft Altbau, an information program sponsored by the Baden-Württemberg Ministry of the Environment. “If a wall does let air through, it is dilapidated,” says Frank Hettler from Zukunft Altbau. According to the expert, intact walls are always airtight and windproof. Air is only exchanged through ventilation, leaky windows, or ventilation systems.
Where does the myth come from?
The myth of the breathing wall has its origins in a scientific error dating back to 1858 when the researcher Max von Pettenkofer wrongly concluded that brick walls were permeable to air. The basis for this error was an experiment in which Pettenkofer came to the conclusion that brick walls were breathable. According to Zukunft Altbau, however, he was very wrong.
At the time, Pettenkofer sealed all the joints between the windows and walls in an office space and then took an air exchange measurement. The result: the measurement data did not differ significantly from the result before the sealing.
However, he had overlooked the stove and its smoke outlet to the outside, according to Zukunft Altbau. The ceiling was probably also leaking, allowing air to escape. A few decades later, the physicist Ernst Raisch refuted this theory. He proved that the exchange of humid indoor air does not take place via the walls. However, the myth that walls breathe persists to this day.
How Thermal Insulation Actually Affects Mold
Thermal insulation actually plays a crucial role in preventing mold. Insulated exterior walls have a higher surface temperature, which prevents moisture from condensing in the room air.
According to Hettler, continuous thermal insulation reduces the risk of mold enormously. This is because it raises the temperature on the inside of the exterior walls – which prevents moisture from the air from condensing on them. According to Zukunft Altbau, insulation is, therefore, a very effective strategy against mold.

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What Really Helps Against Mold
In old or new buildings that are not sufficiently dry, insufficient ventilation can encourage mold on the exterior wall. Professional insulation significantly reduces this risk by increasing the wall temperature and preventing condensation.
However, the best and simplest strategy against mold is regular air exchange. According to the Federal Environment Agency, windows should be opened wide for five to ten minutes several times a day. Alternatively, a ventilation system can effectively transport moist air to the outside.
With material from dpa