January 22, 2020, 7:14 am | Read time: 5 minutes
What is common in the apartment—regularly airing out—is rarely done in the basement. Yet it’s especially important there, as mold can quickly develop otherwise.
Many basements have a musty smell because they are ventilated incorrectly. Simply opening the windows is not the solution. Fresh air from outside can, depending on the circumstances, accelerate mold growth. So, how do you do it right?
Ventilate Basements Differently in Summer and Winter
The problem: “Because basement rooms are surrounded by cool earth, the interior surface temperature of the exterior walls is lower than in other rooms,” explains Alexander Lyssoudis from the Bavarian Chamber of Civil Engineers. This means that you should ventilate the basement completely differently in winter and summer.
It’s actually easiest in winter: When the outside air is cold and dry, basements can be ventilated without issues. “The moist basement air is exchanged for the cold winter air through open windows,” explains Lars Beckmannshagen from the Hamburg Center for Energy, Building, Architecture, and Environment (ZEBAU).
Prolonged Ventilation Promotes Mold Growth in the Basement
“However, you shouldn’t ventilate for too long, or the building components will cool down,” adds the expert. Otherwise, the moist air will condense on the cold walls. The resulting water film is an ideal breeding ground for mold.
In summer, you shouldn’t open the windows whenever it suits you. “The outside air is warm and humid. When it hits the cold surfaces of the walls in the basement, condensation forms, which can lead to mold growth,” explains Robert Kussauer from the Federal Association for Mold Remediation. This means that frequently ventilating the basement in this way poses a greater risk of mold growth.
Therefore, he advises for the warm season: Ventilate only in the early morning or late evening hours when it’s cooler outside. Keep all windows closed for the rest of the day. This summer condensation is exacerbated if damp laundry is dried in the basement. “In summer, that’s not a good idea; it would only bring additional moisture into the basement,” says Beckmannshagen. It’s better to hang laundry outside, where it dries quickly in the sun and wind.
In winter, however, drying laundry indoors is quite possible because the moisture can easily escape through open windows and be absorbed by the cold outside air. This even means: You must always ventilate the room well.
Also interesting: How to Avoid Mold in Your Home During Winter
Windows Open, Heating Off
During ventilation, the heating should be completely turned off; otherwise, it quickly reaches maximum throughput because it constantly heats against the incoming cold air. The result: Heating costs rise. “After ventilation, the heating should be reset to the previous temperature,” adds Kussauer. Otherwise, the same problem threatens here: If the surrounding walls are too cold, the risk of condensation forming on the walls increases.
Ventilating Occupied and Unoccupied Basements
There is a difference between whether the basement rooms are occupied or unoccupied. Basements used only as storage rooms are usually cooler than occupied rooms because they are generally not insulated or heated. Here, there is a high risk that on summer days, warm outside air flows in and cools down on the walls. Then, moisture settles on the walls and especially in the lower corners. Regular ventilation is advised here, at night in summer and during cooler outside temperatures in winter, recommends Kussauer.
Occupied basement rooms are usually insulated and heated to about 68 degrees Fahrenheit, while the surrounding earth is only about 54 degrees. But: “With insufficient or improper insulation, room air can condense on the cooler components,” explains Kussauer. If older basement rooms are converted into living spaces, owners should seek advice from an expert familiar with building physics to calculate the necessary measures.

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What Time of Day You Should Ventilate in Spring
Radioactive Gas Radon in the Basement
In some places, ventilating the basement is important not only because of the risk of mold: The radioactive gas radon can accumulate in the basement. It originates in the soil and can enter indoor spaces through cracks, joints, or pipe penetrations. This is especially possible in areas of the house that have ground contact–like the basement. Radon is not visible, smellable, or tastable. It is considered a health risk because, according to the Federal Office for Radiation Protection (BfS), radon is the second leading cause of lung cancer after smoking.
The advice from the Federal Office for Radiation Protection: Ventilate regularly so that the entire indoor air is exchanged within a few minutes. This is achieved with cross-ventilation. By opening opposite windows simultaneously, a draft is created. A tip for the often-forgotten basement during ventilation: Use the reminder function on your smartphone. However, since ventilation is not always sufficient, the radon concentration in the building should be measured. If necessary, seal any leaks in the basement and ground floor.
Note: Radon is not a problem everywhere. The concentration in the soil varies greatly by region. According to the BfS, it tends to be higher between central and southern Germany and in the far north, specifically in most low mountain ranges, the Alpine foothills, and areas with glacial moraines from the last ice age.