Recently, the German term “lüften” has been circulating on social media and trending on Google. The term refers to the practice of opening windows and doors to replace stale indoor air with outdoor air, a longtime practice in many European homes. Americans have dubbed it “house burping” in many videos on Instagram, TikTok and YouTube.
William Bahnfleth, a professor of architectural engineering at Penn State, has personal and professional experience with lüften. He spoke to The Conversation’s Pittsburgh Editor Cassandra Stone about the science behind it and how Pittsburgh homeowners can best navigate the “house burping” trend.
What does “lüften” actually mean in Germany and other parts of Europe, and how is it different from the way Americans typically ventilate homes?
Literally, “lüften” means “to air out.” It refers to opening windows – and sometimes doors – to rapidly flush a house with outdoor air, typically at least twice daily. This improves indoor air quality and controls humidity to prevent condensation, which can damage buildings and promote mold growth.
The practice is not unique to Germany, although it may not go by that name in other regions. I spent a sabbatical leave at the Technical University of Denmark in Copenhagen, and upon arriving at the house I’d been assigned, I found instructions to air it twice a day for about 15 minutes. The stated purpose in this case was to prevent excessive humidity, because the climate in Denmark can be cool and damp. Moisture produced by bathing, laundry and cooking can raise indoor humidity to high levels if not controlled. The house also had a shower squeegee to remove water from walls and a sensor-controlled bathroom exhaust fan. Without mechanical cooling, opening windows for a bit early in the day kept the house comfortable as weather warmed.
Lüften is uncommon in the U.S., even in older, naturally ventilated homes. Americans tend to rely on HVAC systems for thermal comfort with windows closed, disconnecting indoor air quality from temperature control. The way American buildings are heated and cooled actually discourages window opening. After returning from Denmark, I started opening windows during mornings in cooling season – and leaving them open until it became too hot or humid indoors – and briefly in winter. This reduced summer cooling costs and improved indoor air quality all year – I was surprised how many days I could skip the air conditioner.
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Is there scientific evidence that short, intense “airing out” improves air quality more effectively than just cracking a window all day?
It depends on whether a “cracked” window admits a lot of outdoor air or a little. Opening windows continuously provides better ventilation than brief, 5- to 10-minute periods, but with potential downsides for thermal comfort and energy use.
The best approach today is continuous outdoor air supply at design standard levels via an energy recovery ventilator. The ventilator uses fans to bring in a reliable outdoor air supply that’s partially conditioned by exchanging heat and moisture with exhausted air, providing good indoor air quality with low energy impact and stable indoor conditions. Researchers have also investigated “smart ventilation” systems that maintain desired average ventilation rates by bringing in more outdoor air to reduce operational strain and reduce energy costs– a kind of “next-generation lüften.”
One important aspect of indoor air quality that may not be improved by lüften is indoor particle control. Small particles that come from cooking, some cleaning activities or burning candles, for example, are the most harmful contaminants in most indoor environments. In urban areas, outdoor particle levels may exceed acceptable limits, so opening windows may release indoor pollutants such as cooking fumes and lower humidity inside, but it can also let in bad air from cars and industry.
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Pittsburgh struggles with outdoor air pollution at times. How should locals think about the trade-off between bringing in outdoor air and introducing pollutants into the home?
“Fresh air” isn’t a synonym for outdoor air because it’s not actually “fresh” in many locations. It’s best to filter outdoor air to remove particles before bringing it into the house, which can be done with an energy recovery ventilator. Unit prices range from $600 to $1,500 on average, but these ventilators can reduce utility bills by 10% or more by preconditioning incoming fresh air with the outgoing air’s energy.
If that isn’t possible, portable air purifiers are a good solution with many benefits, such as a reduced risk of transmitting airborne respiratory infections, control of seasonal allergens such as pollen, resilience during wildfires, better air quality when you are using your wood-burning fireplace, and capturing emissions from cooking.
Can lüften make an older, damp home more comfortable?
It can lower humidity in the air, which reduces the potential concentration, but it doesn’t eliminate indoor sources of moisture. If a house is damp, especially if there are specific wet spots, the owner should try to identify and fix the causes. Lüften, and ventilation in general, are mitigation measures, not solutions.
Why do you think practices like lüften persist culturally?
The simplest answer is that they actually work. It’s a good place to start with taking responsibility for managing the air quality in your home. The better understanding of the causes and effects of poor indoor air quality, and the technology available to measure and control it, can beneficially update a good historical practice to obtain even more value from it.
