To Lüft or Not to Lüft

In Germany, you will find strong opinions on Lüften.Lüften German, verb/noun: the practice of opening windows wide to fully exchange the air in a room. Stoßlüften: a brief, full-throttle airing. Kipplüften: leaving the window tilted open over a longer stretch. Stoßlüften vs. Kipplüften, how long to air out, how often, and whether the heating should stay on while the window is open.

We sleep with the window open year-round for reasons that I will explore in this and later blog posts. This behaviour has sparked the occasional comment from non-Lüfting friends, mostly around the dangers of mould. Luckily, we’ve been tracking our apartment temperature for a while with indoor and outdoor weather stations (from Netatmo). After 180 winter days and 51,372 measurements, the short version:

Cold air is dry air. Opening the window in winter pulls room moisture out, which is why we only spend ca. 1.5% of time in mould risk.

To be more scientific, we have included in our analysis the data from 8 additional bedrooms.Data from Winterberger, An, Biallas & Paice (2024), Smart Home environment data across 4 European countries. We use 8 bedroom-tagged smart-home stations across Denmark, Switzerland and Portugal, chosen for best data coverage. CC-BY 4.0.

The mechanism

There are two ways to talk about humidity. Relative humidity (RH) is a share of the maximum water the air can carry. Absolute humidity is grams of water per cubic metre. Warm air can carry far more water than cold air: about 7 g/m³ water at 5°C, about 23 g/m³ water at 25°C.

Our balcony averages 4.9°C and 77.8% RH in winter, or 5.41 g/m³. Bring that same air inside and warm it to bedroom temperature, and the relative reading would drop from 77.8% to about 35% because the warmer room can hold so much more water. Opening a window in winter dries the room out rather than damping it.

This is why our bedroom shows the highest RH of the indoor rooms (44.7%) while carrying the least water in absolute terms (6.89 g/m³):

Two of the eight European bedrooms carry noticeably more water than the rest. Portuguese bedroom 33 sits cold and damp at 13.5°C and 78.6% RH, looking unheated. Swiss bedroom 6 hits the same corner from the other side, a normal 21.2°C with a moderately high 51.9% RH, which adds up to the most water of any bedroom in the dataset, 9.67 g/m³.

The simple threshold for mould

The Umweltbundesamt, Germany’s federal environment agency, publishes the Schimmelleitfaden, the country’s reference guide on indoor mould.Umweltbundesamt (2024), Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden. Its simplest advice is: keep relative humidity below 60% and the room is in the clear.

Across 180 winter days of bedroom readings, our room spent only 42 hours above 60% RH, or 0.97% of the time. Above 70%, 80%, or 90% RH the count is zero.

The two outliers from Chart 1 now diverge. Portuguese bedroom 33 spends 100% of winter above 60% RH, deep inside the red mould-risk band. Swiss bedroom 6 spends 0% above 60%.

The threshold that actually matters

The 60% room-air rule from the UBA is just a ballpark. Mould grows on cold surfaces, not in the air, and germinates once surface RH stays above 80% for several days.Sedlbauer, Krus et al. (2003), Mould growth prediction with a new biohygrothermal method and its application in practice. Germination times per species depend on surface RH, temperature, and substrate. What matters is the humidity and temperature on the cold wall by the window, not the room itself.

The temperature of the wall is a weighted blend of indoor and outdoor air, set by a coupling factor called fRsi: 1 means perfect insulation (surface stays at room temperature), 0 means none (surface tracks the outdoor air). At fRsi = 0.5, with a 20°C bedroom and 0°C outside, the surface sits at 10°C. Modern walls run fRsi 0.85 to 0.92; DIN 4108-2 sets the legal floor at 0.7.

We don’t have a measured fRsi for our flat (it’s a renovated Altbau) or a thermometer on the wall, so we apply the worst case, fRsi = 0.5. The result: at that worst case the modelled surface spent only 66.1 hours above 80%, or 1.629% of the time. Not a problem at all.

For the European bedrooms we use the same conservative fRsi = 0.5 and the same 80% surface-RH threshold, with outdoor temperature taken from Open-Meteo’s capital-city series (Zurich, Copenhagen, Lisbon) since the Zenodo dataset anonymises location to country level. In this view three bedrooms spend too much time in mould-growing conditions: Swiss 4 and 6, and Portuguese 33. The failure modes split into two patterns:

• Cold & damp (PT 33): an unheated bedroom at 13.5°C. Cold air can’t hold much water, so room RH sits at 78.6% on its own. The wall is barely warmer than the outdoor 13°C, which means surface RH stays just as high as room RH.
• Warm & wet (CH 4 and 6): a normally heated bedroom around 21.2°C with room RH that looks safe (51.9% in CH 6). The trap is what happens when humid warm room air meets a cold wall. The same water content reads as a much higher RH at the lower wall temperature. CH 6 carries 9.67 g/m³, the most water of any bedroom in the dataset, and the cold Zurich wall pushes its surface RH well over 80%.

For the two Swiss bedrooms at least, fRsi 0.5 is a harsh assumption. At a DIN-compliant fRsi 0.7 both drop back to normal levels: CH 4 would spend 0.09% of winter above the 80% surface-RH line, down from 26.89%; CH 6 would spend 6.85%, down from 77.98%. PT 33 barely moves, still at 44.3%, because when the bedroom is already as cold as the outdoor air no amount of wall insulation can save the surface.

Back to the open window

So yes, our bedroom is cold but rarely gets humid enough for mould. The cold winter air outside is dry, and opening the window pulls room moisture out rather than adding it.