🗓 Updated September 15, 2026 · EcoBack editorial team
If you have moved into a flat in Germany, Austria, Switzerland or most of Central Europe, your windows tilt inwards at the top as well as swinging open — a tilt-and-turn window, Dreh-Kipp-Fenster. In summer that tilt position is genuinely useful, and it is how most people vent a portable air conditioner hose. In winter it is the single setting most likely to put mould around your window frame.
It pulls moisture from the air into its tank — drier air feels cooler and helps prevent mould.
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Compiled from public tests & customer reviews — not tested by us. Prices vary; check the current price on Amazon. Illustrations, not product photos. One thing first: what matters is the litres/day rating plus a humidistat — it switches off at the target humidity, which is where the electricity saving is. Amazon.de ships to most EU countries, with site and checkout available in English.
Comfee MDDF-20DEN7
Proven dehumidifier for living spaces.
✓ Proven, good value
✕ Audible in operation
MeacoDry Arete One 20L
Manufacturer rating 20 l/day, HEPA filter and laundry mode; public tests praise it mainly for quiet running.
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These are the models US reviewers keep picking — compiled from public tests, not tested by us. Prices change; check the current price on Amazon. Illustrations, not product photos.
Midea Cube 50-Pint
The recurring #1 across US tests — pulls rooms from damp to dry faster than the field, and the cube folds for storage.
Frigidaire FFAP5034W1
RTINGS' basement pick: 50-pint with a built-in pump, so it drains uphill to a sink without bucket duty.
The cold corner decides, not the room
A hygrometer on your shelf reads the air in the middle of the room. Mould does not grow there. It grows on the coldest surface in the room — the window reveal, the lintel, the corner of an exterior wall, the patch of wall behind a wardrobe.
The reason is that relative humidity is relative to temperature. The same air, cooled, is closer to saturation. So warm room air that reads a perfectly comfortable value in the middle of the room can arrive at a cold surface already in the danger zone:
- Room air at 20 °C and 50 % relative humidity — comfortable, nothing on the hygrometer to worry about — reaches about 73 % at a wall surface of 14 °C.
- Spores germinate from roughly 70–80 % relative humidity at the surface, which that wall has just crossed.
- At 20 °C and 60 %, the dew point is about 12 °C: any surface colder than that collects liquid water, not just damp.
This is why the tilted window is such a reliable mould machine. It does not flood the room with moisture; it quietly holds the masonry around the opening near outdoor temperature for hours at a time, and then ordinary indoor air does the rest against that cold strip. The visible result — black speckling that follows the frame — is the shape of the problem.
Cold outside air dries your flat
The common objection is that it is cold and wet outside, so letting that air in must make things damper. It does the opposite, because cold air physically cannot hold much water.
| Outside | Water it actually carries | Same air once it is 20 °C indoors |
|---|---|---|
| −5 °C at 90 % RH | ≈ 3.1 g/m³ | ≈ 16 % RH |
| 0 °C at 80 % RH | ≈ 3.9 g/m³ | ≈ 21 % RH |
| 5 °C at 80 % RH | ≈ 5.4 g/m³ | ≈ 30 % RH |
| 10 °C at 80 % RH | ≈ 7.5 g/m³ | ≈ 42 % RH |
Figures are the Magnus formula for saturation vapour pressure, the standard approximation used in building physics. Relative humidity is the ratio of the water vapour actually present to what the air could hold at that temperature, so warming air without adding water lowers the ratio: 0 °C air at 80 % keeps its vapour pressure and lands near 21 % once it is 20 °C indoors. The grams-per-cubic-metre column is what that outdoor air physically carries. You can check the arithmetic yourself — how we work.
That "room to absorb" is the whole mechanism. Winter air comes in dry, picks up the moisture your shower, cooking, laundry and breathing put into the flat, and you throw it back out at the next airing. A window on tilt does a weak, continuous version of this while cooling the structure — the worst combination of the two.
Check your own cold spot
Point a cheap infrared thermometer at the coldest place you can find — the window reveal, the corner behind furniture on an exterior wall — and put the three numbers in here.
Will it condense on that wall?
Runs in your browser; nothing is sent anywhere. It answers one question — whether today's room air is safe against that particular surface. It cannot tell you why the surface is cold, which may be a thermal bridge or missing insulation rather than anything you did.
The four numbers worth holding on to
- 40–60 % RH in the room: the green band.
- 60–70 %: grey zone, worth watching, especially first thing in the morning.
- Repeatedly above 70 %: act — more airing, less moisture at source, and a dehumidifier if the first two are not enough.
- Below 16 °C in a bedroom: relative humidity climbs into the critical range even with no extra moisture source, which is why an unheated bedroom with the door shut is a classic mould room.
Going too far the other way is also possible: a hygrometer sitting under 40 % through a frost period means dry air, not safety. We put the running cost of fixing that on the humidifier electricity page (German).
The routine that replaces the tilt
- Twice a day, 5–10 minutes, sash fully open. Two windows on opposite sides at once (Querlüften) does it fastest.
- Turn the radiator down while the window is open, back up when you close it. A thermostatic valve left running will otherwise heat the street.
- Air immediately after showering, cooking and drying laundry — those are the big moisture events, and the moisture is easiest to remove before it spreads.
- Leave a gap behind furniture on exterior walls. A wardrobe flat against a cold wall makes its own cold spot; we go through that failure mode on why mould keeps coming back (German).
Honest limit: none of this fixes a cold surface that is cold for structural reasons. If one patch stays far colder than the rest of the same wall, no airing schedule will save it, and that is a conversation with whoever owns the building rather than a shopping problem. We are not building surveyors and this page is not legal advice about a tenancy.
The same window, six months earlier
If you arrived here in the other half of the year, the tilt position stops being the villain: it is how you get a portable air conditioner hose out of a window that was never designed for one. That is a different job with different hardware — venting a portable AC through a tilt-and-turn window.
What actually helps, if anything
Ad In order of how often it is the answer:
- A hygrometer with a min/max memory — before anything else. It catches the overnight peak you are never awake to see, and it is the difference between knowing and guessing. Hygrometers with min/max on Amazon.de →
- An infrared thermometer for the surface reading the calculator above asks for. Infrared thermometers on Amazon.de →
- A dehumidifier with a hygrostat, only if airing and source control are not enough — it holds the room under a set value and keeps working at night when opening a window is not an option. Size it by room: 20 m² · 30 m² · 40 m².
Note the order. A dehumidifier bought before a €10 hygrometer is a machine running against a problem nobody has measured, and in a room whose real fault is a 12 °C wall it will run more or less permanently.
Questions people actually ask
Should I leave my window tilted in winter?
Not all day. A tilted sash exchanges air slowly while cooling the frame and the wall around the opening for hours, and a cold surface is where mould starts. Open the window fully for 5 to 10 minutes instead, twice a day, ideally two windows at once so the air crosses the flat.
Does airing in winter make the flat damp, since outside air is cold and wet?
No, the opposite. Cold air carries very little water. Outdoor air at 0 degrees Celsius and 80 percent humidity holds about 3.9 grams of water per cubic metre; warmed to 20 degrees indoors that same air sits at roughly 21 percent relative humidity, so it has plenty of room to pick up moisture and carry it out at the next airing.
My hygrometer says 50 percent, so why is there mould in the corner?
Because a hygrometer reads the middle of the room and mould grows at the coldest surface. Room air at 20 degrees Celsius and 50 percent relative humidity reaches about 73 percent when it touches a wall at 14 degrees, which is already in the range where spores germinate. The number that decides is the humidity at the cold spot, not the room average.
How long should I open the window, and does the flat get cold?
Five to ten minutes with the sash fully open is enough to swap the air in a normal room. Walls, floor and furniture hold their heat over that time and warm the fresh air back up quickly, which is why brief full airing costs less heat than a window left on tilt for hours. Turn the radiator down while the window is open and back up afterwards.