The short answer: On wallpaper, glass cleaner is the surprisingly most effective home remedy – spray it on, leave for five minutes, spray again and dab off. It works because it contains alcohol and surfactants and introduces hardly any moisture. On plaster and paintwork, a methylated spirit–water mixture or an anti-mould agent helps. But: mildew stains are almost always already a microbial infestation in an early stage, and any cleaning treats only the discolouration. Unless you eliminate the moisture source, they come back – usually at the same spot, usually the next winter. From about half a square metre of infestation upwards, the matter belongs in professional hands.
What mildew stains actually are
Mildew stains are yellowish-brown to grey discolourations that arise on damp materials – on wallpaper, plaster, textiles, paper, leather or wood. The name comes from "going musty", that is, the spoiling of damp goods.
In practice mildew stains are readily played down as "not mould yet". That is seldom true. As a rule they are already metabolic products of fungi or bacteria that have settled on the damp material – only at an early stage at which no furry growth is yet visible.
For treatment that means: take mildew stains as seriously as mould. The decisive question is not the name but how deep the infestation sits and where the moisture comes from.
First: find the cause
This is the part almost everyone skips – and the reason why the stains come back. Four causes cover most cases:
1. Too little ventilation, too much room humidity
The most frequent cause. Cooking, showering, drying laundry and breathing put considerable amounts of water into the air. If the moisture finds no way out, it condenses on the coldest surface. Recognisable by: The stains sit in room corners, behind furniture, on external walls – and appear in winter.
2. Thermal bridge
A spot in the external wall that is considerably colder than its surroundings – uninsulated concrete lintels, ceiling connections, roller shutter boxes, window reveals. Moisture condenses there first. Recognisable by: sharply bounded stains at always the same spot, often in a straight line.
3. Water damage or penetrating moisture
A leaking pipe, a defective gutter, a crack in the facade or rising damp in the plinth. Recognisable by: The stains become clearer after rain, spread, or appear near the floor with a clear upper edge.
4. Construction moisture
In a new building or after a renovation there is still a lot of water in screed, plaster and masonry. Recognisable by: The infestation appears in the first winter after construction and disappears afterwards – provided there is sufficient heating and ventilation.
A moisture meter for a few euros and a hygrometer help with classification. If the relative humidity is permanently above 60 percent, the room climate is the cause. If the wall is damp in spots while the air is dry, the water comes from outside or from a pipe.
Removing mildew stains on wallpaper
Wallpaper is the most difficult substrate, because it absorbs and because too much liquid detaches it. It is precisely for this reason that glass cleaner works better here than most special agents.
It contains alcohol and surfactants, evaporates quickly and introduces very little water. The discolouration is dissolved, the wallpaper stays largely dry.
How to proceed
First test at a concealed spot – some wallpapers are not colourfast. Then spray glass cleaner onto the stains, leave for about five minutes, spray again and dab off with a clean, white cloth, do not rub. Rubbing spreads the spores and ruins the wallpaper surface. Then ventilate well and let the spot dry.
Alternative: a mixture of methylated spirit and water in a ratio of about 1:1, applied with a cotton cloth. Works similarly, smells stronger and likewise needs good ventilation.
With woodchip wallpaper dabbing off seldom works cleanly – here painting over with an anti-mould paint after the treatment is often the more practicable solution. With wallpaper that is dyed through, wavy or coming away, nothing helps any more: it has to come off, and the plaster underneath has to be assessed.
Mildew stains on plaster and paintwork
On mineral substrates more is possible, because they are more robust.
Alcohol-based agents – methylated spirit or isopropanol, diluted with water – kill superficial growth and evaporate without leaving residue. For small areas the first choice.
Chlorine-containing anti-mould agents are more effective and in addition bleach the discolouration. They belong on robust, non-coloured substrates and need good ventilation, gloves and eye protection.
Hydrogen peroxide from the pharmacy is a chlorine-free alternative with a bleaching effect – likewise to be used with care on coloured paintwork.
If the discolouration sits deeper in the plaster, no surface treatment helps. Then the plaster in this area has to be removed and renewed, if necessary with a renovation plaster.
Safety note: Chlorine-containing agents must never be mixed with acidic cleaners such as vinegar, vinegar essence or citric acid, or used directly one after the other – that produces toxic chlorine gas. Between two agents always rinse thoroughly with clear water and ventilate vigorously. For all work wear gloves, for larger areas in addition an FFP2 mask and safety glasses – stirred-up spores do not belong in the airways.
What does not help
Simply painting over. The discolouration bleeds through again within weeks, and the growth carries on growing under the paint. Anti-mould paint is a sensible follow-up treatment, but no substitute for cleaning and eliminating the cause.
Brushing off dry. Stirs up spores and distributes them in the room. Always work damp.
Just moving the furniture away. Helps against the symptom at this spot, but merely shifts the problem.
Baking powder and citric acid. Popular home remedies, but hardly effective against microbial growth. Citric acid is in addition harmful on lime plaster.
Mildew stains on other materials
| Material | Procedure | Caution |
|---|---|---|
| Wallpaper | Glass cleaner or methylated spirit, dab off | do not soak, do not rub |
| Plaster, paintwork | Alcohol or anti-mould agent | avoid bleaching agents on coloured paintwork |
| Wood, untreated | sand, then treat with alcohol | often sits deeper than visible |
| Wood, varnished | Alcohol, soft cloth | Solvents can soften the varnish |
| Textiles, curtains | wash at as high a temperature as possible | Lemon juice plus sun as a home remedy for whites |
| Leather | brush off dry, then leather cleaner | no water, do not dry in the sun |
| Books, paper | dry, brush off carefully | usually cannot be removed completely |
| Tiles, glass, plastic | Cleaner of your choice, uncritical | treat the joints as well |
For all textile and absorbent materials the following applies: they must afterwards dry through completely, otherwise the infestation returns. A damp curtain in a closed cupboard is exactly the starting situation that created the problem.
Follow-up treatment: anti-mould paint
After cleaning and eliminating the cause, a coating can make sense – as a supplement, not as a substitute.
Anti-mould paints contain active ingredients that inhibit new growth. They help where a residual risk remains, for instance on a wall whose thermal bridge cannot be eliminated economically. The effect does, however, decline over the years.
Silicate paints work without biocides: their high pH creates a milieu in which fungi grow poorly. They are also vapour-permeable, so that the wall can dry out – often the better choice than a dense emulsion paint.
Lime plaster and lime paint work on the same principle and are a proven renovation solution for internal walls under moisture load.
What you should not do: apply a dense, vapour-tight paint or a plastic wallpaper to a damp wall. The moisture then stays trapped underneath, and the growth carries on growing invisibly.
When you need a specialist firm
According to the recommendations of the German Federal Environment Agency, doing it yourself is acceptable for small-area, superficial infestation. Not in these cases, however:
- The infested area is larger than about 0.5 m²
- The infestation sits deep in the material – in plaster, plasterboard, insulation or wood
- It comes back after the treatment
- The cause is a water damage with a soaked construction
- In the household live people with asthma, allergies or a weakened immune system, infants or pregnant women
- It smells musty without any visible infestation to be found – then it is sitting concealed
For tenants a practical point is added: report moisture damage to the landlord without delay and in writing, with photos and date. This is important regardless of the question of the cause – anyone who reports the damage only after months is in a worse position on the question of responsibility.
Prevention
Burst-ventilate instead of tilting. Three to four times a day open the window wide for five to ten minutes, shorter in winter. A tilted window exchanges hardly any air, but cools the reveal – and so creates exactly the cold surface on which the moisture condenses.
Ventilate immediately after showering and cooking. These activities introduce the largest amounts of moisture.
Move furniture away from external walls. Five to ten centimetres of distance, so that warm air can circulate behind. Behind a cupboard standing flush against the external wall, the typical mildew-stain corner develops.
Heat evenly. A cooled-down room that gets moisture through the open door from the heated neighbouring room is a classic case. Better to keep all rooms moderately warm than to leave individual ones cold.
Do not dry laundry in the flat – or if so, with the window open. A machine load gives several litres of water to the room air.
Put up a hygrometer. The target range is 40 to 60 percent relative humidity. A meter for a few euros turns a feeling into a number – and shows whether the ventilation behaviour is enough.
Frequently asked questions
How do I remove mildew stains from wallpaper?
With glass cleaner: spray on, leave for five minutes, spray again and dab off with a white cloth – do not rub. Test beforehand at a concealed spot. Alternatively a methylated spirit–water mixture. Then ventilate well.
Why does glass cleaner help against mildew stains?
It contains alcohol and surfactants that dissolve the growth on the surface, and evaporates quickly. As a result it introduces hardly any moisture into the wallpaper – the decisive advantage over aqueous cleaners, which soften the wallpaper.
Are mildew stains the same as mould?
The transition is fluid. Mildew stains are as a rule already microbial infestation in an early stage, only without visible growth. So treat them just as seriously.
Can you simply paint over mildew stains?
No. The discolouration bleeds through, and the growth carries on growing under the paint. First clean, then eliminate the cause, then – as a follow-up treatment – paint over with a suitable paint if necessary.
When should you call a specialist firm?
From about 0.5 m² of area, with deep infestation in the material, with recurring occurrence, after a water damage and always when people with asthma, allergies or a weakened immune system live in the household.
How do I tell whether a thermal bridge is the cause?
Thermal bridges produce sharply bounded stains at always the same spot – often along a ceiling connection, at window reveals or at the roller shutter box. A simple check: compare the wall temperature at the spot and next to it with an infrared thermometer. Several degrees of difference are a clear sign.
Are mildew stains harmful to health?
They can be. Fungal spores and their metabolic products can trigger respiratory irritation, headaches and allergic reactions – especially in people with asthma, allergies or a weakened immune system. Small-area, superficial infestation is no emergency for otherwise healthy occupants, but should be remedied promptly and at its cause.
What helps against mildew stains in the early stage?
Exactly the same as later, only with better prospects: alcohol-based cleaning of the surface and – above all – eliminating the moisture source. In the early stage the growth does not yet sit deep in the material, and a surface treatment can actually be enough.
Can you remove mildew stains with vinegar?
That is not advisable. Vinegar introduces moisture, works against many fungal species only to a limited extent, and its organic constituents can even serve as a nutrient medium on mineral substrates. On lime plaster the acid in addition attacks the substrate. Alcohol is the better choice here.
How often do I have to ventilate?
Burst-ventilate three to four times a day, five to ten minutes in winter, longer in summer. In addition after showering and cooking. A hygrometer takes the guesswork out: the target is 40 to 60 percent relative humidity.
Who pays for mildew stains in a rented flat?
That depends on the cause – structural defects are at the landlord's expense, inadequate ventilation and heating at the tenant's. Decisive for your position is to report the damage early and in writing and document it with photos. Anyone who reports only after months has a harder time in the discussion, regardless of who caused it.
What to do if the stains keep coming back?
Then the cause has not been eliminated. Check the ventilation behaviour with a hygrometer, look for thermal bridges and rule out a water damage. As long as the moisture source exists, any cleaning is merely a delay.
