Does opening the window really dry out a Berlin flat?

It depends entirely on the month. Outdoor air only takes water away if it carries less than the air inside. A living room at 20 °C and 60 percent humidity holds 10.4 grams of water per cubic metre. Averaged over ten complete years at the Deutscher Wetterdienst station in Berlin-Tempelhof, Berlin outdoor air holds 4.9 g/m³ in January, 8.3 in October, 9.6 in September and 11.1 in August. So a five minute airing in January strips 5.5 grams from every cubic metre it swaps, while the same airing in August puts about 0.7 grams in. Across the whole grid of twelve months by twenty four hours, 49 of the 288 hours make that room wetter and another 68 move less than half a gram.

What time of day should I air my flat in Berlin?

In summer, mid afternoon. The instinct to open up early because the air feels cool and fresh is wrong from June to August in Berlin. Moisture collects near the ground overnight and drier air mixes down through the day, so the dew point falls while the thermometer climbs. In July the air at 06:00 carries 11.3 grams per cubic metre and at 16:00 only 10.2. In August it is 11.6 at 07:00 against 10.4 at 15:00. From November to March the hour hardly matters: the whole of January sits inside a 0.2 gram band, so air whenever it suits you.

How cold does a wall have to be before mould grows on it?

Colder than you would guess from looking for water. The Umweltbundesamt's mould guide puts the growth threshold at 70 to 80 percent relative humidity at the surface of the material, sustained over time, and says explicitly that materials do not have to be visibly wet. In a room at 20 °C and 60 percent humidity, any surface below 15.4 °C sits at that 80 percent line, while droplets only appear below 12.0 °C. That leaves about 3.4 degrees in which mould has what it needs and you get no visual warning. The usual Berlin candidates are the window reveal, the outer corner of a rear-building room, and the wall behind a wardrobe pushed flat against the outside masonry.

Why did my Berlin flat get damp after the landlord fitted new windows?

Because the old windows were ventilating the flat for you. The Umweltbundesamt models exactly this case: an untouched old flat has an infiltration rate around 0.5 air changes per hour, which is twelve complete air changes a day that nobody has to remember, and its model bedroom stays clear on a single morning airing even with laundry drying in the room. Fit tight modern windows and leave the walls uninsulated, which is the most common retrofit in Berlin, and infiltration falls to roughly 0.1 per hour, about 2.4 free changes a day. The guide's own conclusion is that the airing frequency then required is barely achieved in practice.

How much water does a household put into the air each day?

The Umweltbundesamt gives 6 to 12 litres a day for an average three-person household. The components it lists are 30 to 40 grams an hour per person at light activity, about a quarter of a litre per person per night in a bedroom, and 50 to 200 grams an hour from a 4.5 kilogram drum of spun laundry, rising to 100 to 500 grams an hour if it is dripping wet. For comparison, one complete air change of a 70 square metre Berlin flat with three metre ceilings removes about 1.15 litres in January and adds about 0.15 litres in August.

Can I reduce my rent in Berlin because of mould?

Not simply because the outside walls are cold. On 5 December 2018 the Bundesgerichtshof decided VIII ZR 271/17 and VIII ZR 67/18 on two buildings from 1968 and 1971, overturning a 20 percent rent reduction and holding that thermal bridges in the outer walls are not a defect when the building matches the building codes and technical standards in force when it was built. Almost every Berlin Altbau meets the standards of its own construction date by definition. The same case is more helpful on what can be asked of you: the lower court held that airing twice a day for up to ten minutes was the fair limit, and that a tenant cannot be required to keep bedrooms below 16 °C or other rooms below 20 °C, while the court expert calculated that twice daily for about 15 minutes, or three times daily for about 10 minutes, would have prevented the mould. Take a dated photo log and hygrometer readings to the Berliner Mieterverein or another tenants' association before deducting anything.

Is tilting the window enough?

No, and in the cold months it can cause the problem it is meant to solve. The Umweltbundesamt calls wide open and cross ventilation the means of choice and says tilt airing is markedly less effective and would have to run far longer. It also warns that long spells on the tilt setting strongly cool the window reveal and the lintel, and that condensation then forms on those cooled surfaces, which is exactly where mould appears in a Berlin flat. It also wastes a great deal of heating energy. Open two windows on opposite sides of the flat fully, for five minutes, then shut everything.

What should I do about damp in a Berlin flat this autumn?

Until about mid October the window is doing under a gram per cubic metre, so work on the sources: lid on the pan, extractor on, wet laundry out of the flat, bathroom door shut with the window open while the shower is still steaming. From mid October the numbers pass two grams and climb, so build the habit of wide open cross airing for five minutes, twice a day, plus once more after a shower or a big cook. Put a ten euro hygrometer in the coldest room rather than the living room, and pull solid furniture 5 to 10 cm off the outside walls. Keep bedrooms between 16 and 18 °C with the door to the warm rooms closed.

Mould in Berlin Flats: When Airing Dries the Room and When It Soaks It

Yusuf Ucuz

Sep 17

8 min read

The first cold night of the Berlin autumn always announces itself the same way. You get up, walk into the back room, and there is a strip of water along the bottom of the glass. The heating has not come on yet. Nothing has changed in the flat. The outside just dropped below the inside.

Then the advice arrives from every direction at once, from the Hausverwaltung letter, from the neighbour on the landing, from the lease itself: lüften, three times a day, windows wide open. It is good advice in January. In the week you are actually reading this it does almost nothing, and there are hours in the Berlin summer when it makes the flat wetter than it was before you touched the handle.

I pulled ten years of Berlin's own hourly weather to find out exactly when the window works. The answer is sharper than I expected.

A wide open window in a Berlin flat looking onto the facades opposite

The move everyone recommends. Whether it dries the room depends entirely on the month and the hour you do it.

The number your window actually cares about

Relative humidity is the number on every weather app and it is the reason the advice goes wrong. Ninety percent humidity at 3 °C and sixty percent at 20 °C describe completely different amounts of water. The first is a bone-dry airstream once you warm it up. The second is a room you should be worried about.

What matters is the absolute figure: grams of water per cubic metre of air. Air can only take water out of your flat if it is carrying less than the air already inside.

A normal Berlin living room at 20 °C and 60 percent humidity holds 10.4 grams per cubic metre. Here is what Berlin outside holds, averaged hour by hour across ten complete years at the Deutscher Wetterdienst station at Tempelhof:

January: 4.9 g. Every cubic metre you swap takes 5.5 grams out of the room.
October: 8.3 g. Down to 2.0 grams removed. Still worth doing.
September, right now: 9.6 g. Under a gram. The window is working at a seventh of its January rate.
August: 11.1 g. Higher than the room. Opening up adds about 0.7 grams per cubic metre.

Across the whole grid of twelve months by twenty four hours, 49 of the 288 hours put water into that room instead of taking it out, and another 68 move less than half a gram. For roughly four months of the year, the most repeated piece of German household advice is doing nothing at all.

In summer, the afternoon is drier than dawn

The other half of the rule everyone gets backwards is the hour.

The instinct is to air early, when it feels cool and fresh. In Berlin that instinct is exactly wrong from June to August. Overnight the air near the ground collects moisture and holds it; through the day, drier air from higher up mixes down. The dew point drops while the thermometer climbs.

In July the Berlin air at 06:00 carries 11.3 g per cubic metre. At 16:00 it carries 10.2. In August the gap is wider still, 11.6 at 07:00 against 10.4 at 15:00. That is over a gram per cubic metre of difference, in the wrong direction from what your body is telling you.

In winter the hour barely matters. Through January the whole day sits inside a 0.2 gram band, so air whenever it suits you and go back to bed.

Why mould in Berlin flats starts before you see water

Condensation running down the inside of a window pane

Water on the glass is the late signal. The wall behind your wardrobe crossed the line that matters several degrees earlier.

This is the part that surprises people who think they are fine because nothing is dripping. Almost every case of mould in Berlin flats I hear about started on a surface nobody was looking at.

The Umweltbundesamt's mould guide is blunt about the threshold: 70 to 80 percent relative humidity at the surface of a material is enough for mould to grow, if it sits there long enough. Its words are that materials do not have to be visibly wet.

Run that through the same arithmetic. In a room at 20 °C and 60 percent:

Any surface colder than 15.4 °C sits at 80 percent humidity in the thin layer of air touching it. That is the mould line.
Only below 12.0 °C does water actually appear as droplets you can see.

So there are 3.4 degrees of wall temperature in which mould has everything it needs and you have no visual warning whatsoever. In a Berlin flat those surfaces are always the same short list: the window reveal, the outer corner of a room in the rear building, the wall behind a wardrobe pushed flat against the outside masonry, and the unheated back room whose door you keep shut.

That last one is the classic. The heat stays in the living room, the moisture spreads everywhere, and the coldest wall in the flat collects it.

The Berlin trap: new windows, old walls

Ornate Gründerzeit facades on Yorckstraße in Kreuzberg

About a quarter of all Berlin flats sit behind facades like these, built before 1918.

Berlin's housing stock is unusually old. Roughly a quarter of the city's flats were built before 1918, and another 13.7 percent between 1919 and 1949, according to the Investitionsbank Berlin's 2025 housing market report. Only 7.3 percent are newer than 2011.

Those buildings came with leaky windows, and the leak was doing real work. The Umweltbundesamt models exactly this case. An untouched old Berlin flat has an infiltration rate around 0.5 air changes per hour, which is twelve complete changes of air a day that nobody has to remember. Its model bedroom needs one Stoßlüften in the morning and stays clear, even with laundry drying in the room.

Then the landlord fits tight new windows and leaves the walls alone, which is the single most common energy retrofit in this city. Infiltration falls to about 0.1 per hour, roughly 2.4 free air changes a day. The guide's own verdict on the airing frequency now required is that it is barely achieved in practice.

The arithmetic is worth doing once. A three-person household puts 6 to 12 litres of water vapour into the air every day, on the Umweltbundesamt's figures: 30 to 40 grams an hour per person at rest, a quarter of a litre per person per night, and 50 to 500 grams an hour from one drum of drying laundry. In January, one complete change of a 70 m² flat with 3 metre Altbau ceilings removes about 1.15 litres. Eight to ten complete changes a day, and the old windows were quietly supplying twelve of them.

Nobody told the tenant that the retrofit moved that job onto them.

What the courts expect, and what they do not

This matters because damp in a rented Berlin flat quickly becomes an argument about whose fault it is.

The decisive ruling is from the Bundesgerichtshof on 5 December 2018 (VIII ZR 271/17 and VIII ZR 67/18), on two buildings from 1968 and 1971. The tenants had won a 20 percent rent reduction over the risk of mould from thermal bridges in the outside walls. The federal court overturned it, holding that thermal bridges are not a defect when the building matches the codes and technical standards in force when it was built.

Read that carefully, because it is the trap: almost every Berlin Altbau matches the standards of its own construction date by definition. Cold outer walls alone are not a defect you can deduct rent for.

What the same case says about you is more useful. The lower court had held that a tenant could fairly be asked to air twice a day for up to ten minutes, and no more, and could not be required to keep bedrooms below 16 °C or other rooms below 20 °C. The court-appointed expert calculated that twice daily for about 15 minutes, or three times daily for about 10 minutes, would have prevented the mould.

So there is a real ceiling on what can be demanded of you, and it is set case by case. If you are heading towards that argument, keep a dated photo log of the affected surface and a cheap hygrometer reading in the same frame, and take the file to the Berliner Mieterverein or another tenants' association before you deduct anything. I am a tour guide, not a lawyer, and a Berlin damp case turns on the specific building.

The cold rooms, and what actually helps

A Berlin courtyard with the rear building and bare trees

Rear buildings and side wings hold the coldest walls in the city. That is where the damp shows up first.

Berlin flats have a geography of cold: the Seitenflügel, the Hinterhaus, the north side of the courtyard, the Berliner Zimmer with its single window onto the yard. Those rooms are where I would look first.

What changes the outcome:

Keep the bedroom between 16 and 18 °C and the door to it closed against the warm rooms. The Umweltbundesamt is specific here: a cold room fed with warm damp air from the corridor is worse than a cool room kept to itself.
Pull furniture 5 to 10 cm off outside walls, especially anything solid-backed. The wall behind a wardrobe runs several degrees colder than the wall beside it.
Wide open and cross-ventilated beats a tilted window, which is the part of the guidance I would underline twice. The Umweltbundesamt warns that long spells on the tilt setting cool the window reveal and the lintel badly, and condensation then forms on those cooled surfaces, which is precisely where Berlin mould starts.
Get the laundry out of the flat or air hard straight after. One drum of spun washing can put half a kilo of water into the air per hour.
Buy a hygrometer. Ten euros. Put it in the coldest room, not the living room. If it reads above 60 percent when the heating is on, you have a problem the window alone will not fix this month.

My move for this autumn

A Berlin street under yellow autumn leaves

Mid October is when the window turns back into a real tool. Before that, the source matters more than the airing.

Here is the calendar I would actually work to, and it is not the one printed in the lease.

Now to early October: the window is nearly useless as a drying tool. Work on the sources instead. Lid on the pan, extractor on, laundry out, and the bathroom door shut with the window open while the shower is still steaming.

From mid October: airing starts paying again, past two grams a cubic metre and climbing. This is when the habit is worth building. Wide open, two windows on opposite sides of the flat, five minutes, then everything shut and the radiators back on. Heating season in a Berlin lease normally runs from 1 October, and if yours is not delivering, that is a separate fight I have written about in the heating season guide.

November to March: every five minute cross-airing is doing serious work, five and a half grams a cubic metre in January. Twice a day, and once more if anyone showered or cooked.

June to August: check the grid before you open anything. Mid afternoon, not dawn. If you have a cellar compartment, keep it shut on warm days. Average August air arriving in a 14 °C cellar lands at 92 percent humidity, which is well past the mould line before it has even settled.

One last thing worth knowing if this turns into a dispute with your landlord. Heating and hot water costs come back to you once a year in the Nebenkostenabrechnung, and a flat you have been underheating to save money shows up there as clearly as it shows up on the wall. If the argument has already moved on to the rent itself, the four limits that stop an increase are set out in the rent increase guide, and the timing rules for getting out are in the notice period guide.

The city you walk through on my tour is largely the same masonry these flats are made of, brick and lime plaster that has been managing moisture for a hundred and thirty years. It was built to breathe through its gaps. Most Berlin flats today are the version where the gaps have been sealed and nobody mentioned it.

Image credits

Source and licence details for the 4 photographs used in this article.

Facades on Yorckstrasse in Berlin-Kreuzberg, between Grossbeerenstrasse and Mehringdamm: JoachimKohler-HB, CC BY-SA 4.0, via Wikimedia Commons.

Condensation on the inside of a window pane: Maxxmagic, CC BY-SA 4.0, via Wikimedia Commons.

Nassauische Strasse in Berlin-Wilmersdorf in autumn: Sebastian Rittau, CC BY 4.0, via Wikimedia Commons.

Rear courtyard buildings on Wilhelmstrasse in Berlin-Kreuzberg: Gunnar Klack, CC BY-SA 4.0, via Wikimedia Commons.