Mould in a flat in Germany: building defect or ventilation?

The dark patch behind the wardrobe in the corner of the external wall was barely visible in autumn. By February it reaches the windowsill. Whether mould in a flat is a building defect or the result of how the flat is heated and aired comes down to one question: would the flat stay free of mould if it were heated and aired in a normal way? If yes, the cause lies in the way the flat is used. If no, there is a structural deficit. You cannot answer that by looking at the wall. It takes measurements of surface temperatures and indoor humidity over a heating season.

This guide is the English version of our German article on the same topic and cites the same sources. German terms you will meet in letters from landlords, property managers or courts are given in brackets. It explains the technical side and is not legal advice.

How to recognise mould – and what it is confused with

Mould (American English: mold) in a flat is fungal growth on the surfaces of building parts or furnishings, caused by surfaces that stay too damp for too long. It usually shows as a grey, black, green or whitish coating with a furry or velvety texture and blurred edges. It typically starts in room corners on external walls, behind wardrobes, at window reveals or along the ceiling edge above a cantilevered balcony. These are the places where the wall surface stays coldest in winter. Finding mould there is a first clue to the cause, but not yet proof.

Not every dark mark is mould. It is often confused with efflorescence (Ausblühungen): whitish salt crystals left behind when water evaporates from masonry. They feel crystalline and wipe off without leaving a stain. A second look-alike is a black, greasy film that the German Environment Agency (Umweltbundesamt, UBA) describes as "black flats" or fogging (Schwarze Wohnungen). This soot-like deposit settles above radiators, on curtains, window frames and plastic surfaces and on the inside of external walls, and it often appears in renovated or newly occupied flats.¹ A third is algae, which mainly grow on outside surfaces. The distinction matters, because only mould needs a surface that stays damp.

Some mould is invisible. It grows inside floor build-ups, behind wall linings, in insulation layers and under floor coverings when moisture is trapped there. Often the only sign is a musty, earthy smell that you notice on entering the room and that returns after airing – reason enough for a closer investigation. According to the UBA mould guideline, hidden growth often contains far more microbial biomass than visible patches and must be included in any assessment of the damage; lasting remediation is only possible once the causes of the increased moisture have been found and removed.²

Health is a separate reason to take the finding seriously, regardless of who caused it. The UBA guideline treats mould growth indoors as a health risk: damp and mould are associated with a higher risk of respiratory illness and of developing or aggravating asthma, and people with weakened immune systems, cystic fibrosis or asthma need particular protection.² The medical guideline of the German specialist societies states that mould growth of relevant extent indoors should not be tolerated as a precaution and names finding the cause and proper remediation as the most important measures.³ For a property manager, this means the argument about cause and costs must not delay the removal. For residents, it means mould is not an acceptable cosmetic flaw, even when the area looks small.

Timing gives a further clue. Mould in flats is almost always seasonal: it appears between October and March and seems to vanish in summer. This pattern points to moisture from the indoor air condensing on cold surfaces. Growth that persists all year, starts at the base of the wall and comes with damp masonry points rather to water from outside, for example from failed waterproofing.

What to do in the first days

The first right step is not removing the mould but recording the condition. Photograph the affected area with a scale in the picture, such as a folding rule, and note the date. Photograph the whole room as well, so that the location stays clear later. Repeat the photos every few weeks. This record costs nothing and is often the only evidence of how the damage developed. Painting over the area destroys exactly this information.

Reporting is just as important. In a rented flat, tell the landlord or the property manager (Hausverwaltung) without delay and in a way you can prove – ideally in writing, with photos. Under § 536c(1) of the German Civil Code (BGB), a tenant must report a defect that appears during the tenancy without delay.⁴ Failing to report it risks disadvantages for later claims, because the other side loses the chance to limit the damage early; whether and to what extent that applies in your case is for a lawyer (Rechtsanwalt) to assess. For a property manager, the report starts their own duty to act: log the date, record the facts and arrange an inspection.

If the mould is linked to water damage – a leaking pipe or a backflow – the order changes. Limiting the damage comes first: shut off the water, remove standing water, make the affected areas accessible, and take photos before and during these steps. Inform the insurer without delay and ask for their instructions. What may be changed or disposed of without their approval depends on the insurance contract and the law; a lawyer clarifies which obligations apply. Documentation must never delay limiting the damage.

For cleaning, there is a clear limit. According to the UBA, small areas can be treated with alcohol or similar; larger infestations should be handled by a specialist firm.⁵ Dry brushing or sanding is always wrong, because it releases spores and particles into the air. If children, people with asthma, cystic fibrosis or other respiratory diseases, or people with weakened immune systems live in the flat, leave the removal to professionals.² Do not open up building parts to follow the growth: cables and pipes may run behind linings, and older buildings may contain products with hazardous substances.

Finally, start recording the indoor climate in these first days. A simple thermo-hygrometer at chest height, not directly on the external wall, gives you a data series over weeks. Note in parallel how often and how long you air and how the rooms are heated. This record is not proof on its own, but it is the basis of every later professional assessment. Without it, the discussion remains a dispute between two assumptions.

Why mould grows: surface temperature, moisture and air exchange

Mould needs nutrients, suitable temperatures and moisture. Nutrients are everywhere – wallpaper paste, paint, dust. Temperatures in heated rooms are favourable anyway. That leaves moisture as the only factor you can influence, and the question is always why a particular surface stayed too damp for too long. The level of moisture alone is not decisive; its duration is.

How damp is "too damp" is set out in the UBA guideline: a relative humidity of 70 to 80 per cent directly at the surface is enough for mould to grow if it persists for some time – the surface does not have to look wet. Below about 70 per cent there is no growth even under otherwise favourable conditions; at 80 per cent the conditions for many species typical of indoor spaces are reached. Depending on material, temperature and moisture, growth starts within a few days to a few weeks.²

The physics behind it: warm air can hold more water vapour than cold air. When air cools down, its relative humidity rises, and at the dew point water condenses. In a flat, this cooling happens at the coldest surface – in winter almost always the inside of an external wall, often in a corner or at a window reveal. The UBA guideline gives an example: room air at 22 °C and 50 per cent relative humidity reaches about 80 per cent at a wall surface of 14.5 °C; below about 11 °C, condensation forms.² You can check such values for your own rooms with our dew point and mould risk calculator – the page is in German, the numbers work in any language.

Why some surfaces are colder than others is explained by thermal bridges (Wärmebrücken): areas of the building envelope through which heat flows outwards faster than through the surrounding area. There are geometric thermal bridges, such as external corners, and structural ones, such as continuous concrete slabs, balcony slabs, lintels or steel beams. The German standard for the thermal protection of buildings, DIN 4108-2, sets minimum requirements for the insulation of building parts and at thermal bridges, including a minimum surface temperature that protects against condensation and mould.⁶

The second factor is the moisture a household produces. People release water vapour by breathing and through the skin; cooking, showering, bathing, drying laundry, plants and aquariums add more. According to the UBA guideline, an average three-person household releases about 6 to 12 litres of water into the indoor air per day.² This is normal use, not misconduct. It only becomes a problem if the moisture is not removed.

Removal happens through air exchange. Each time you air, humid indoor air is swapped for colder outside air, which can absorb much more water vapour once it has warmed up. That is why airing dries a flat effectively in winter and hardly at all in summer. Modern windows are airtight: when old windows in an existing building are replaced, the uncontrolled air exchange through leaks disappears. If the walls are not insulated at the same time, the cold surfaces remain while humidity rises. The UBA guideline describes this effect explicitly: with tight windows, more active airing is needed, and according to a model calculation by the Fraunhofer Institute for Building Physics, the frequency of shock ventilation then required in an uninsulated old building can hardly be achieved in practice.² The UBA therefore recommends that landlords tell their tenants about the changed ventilation needs after such work.⁷

Air movement at the surface itself is often underestimated. A wardrobe close to an external wall cuts the wall off from the warmth of the room; the surface temperature drops, the humidity at that spot rises, and mould appears there first. The UBA names areas behind solid furniture as typical places where condensation forms.⁷ Its guideline calculates for a poorly insulated external wall: about 70 per cent surface humidity on the open wall at 15 °C, but 89 per cent behind a wardrobe at 11 °C.² This explains why two flats in the same building can behave completely differently.

Building defect or ventilation: how the question is decided

The basic idea is simple. The question is whether the flat can stay free of mould if it is used in a way that can reasonably be expected of the residents. If so, the cause lies in the use. If not – because surfaces are too cold or water enters from outside – there is a structural deficit. This needs two yardsticks: a technical one for the condition of the building and one for the heating and airing that is reasonable.

For the building, the Federal Court of Justice (Bundesgerichtshof, BGH) drew a line in two judgments of 5 December 2018 (VIII ZR 271/17 and VIII ZR 67/18): thermal bridges in the external walls of a rented flat are not a defect (Sachmangel) if the condition complies with the building regulations and technical standards that applied when the building was constructed.⁸ The buildings in those cases dated from 1968 and 1971, when thermal insulation was not yet required. The yardstick is therefore, in principle, the standard at the time of construction, not today's state of the art. In a building from the 1960s or 1970s, an external corner can be unfavourable in terms of building physics without being a defect in the legal sense; in a new building, the same finding would be judged differently. Later alterations, such as new windows, can shift the yardstick.

For the use, the court-appointed expert in those proceedings considered shock ventilation twice a day for 15 minutes or three times a day for 10 minutes sufficient, with the required time falling to a third when cross-ventilating; the court did not regard such intervals as generally unreasonable for tenants.⁸ This applies to the case decided and is not a general limit. For the cold season, the UBA recommends shock ventilation of about 5 minutes per room two to three times a day, 5 to 10 minutes in the bedroom in the morning and in kitchen and bathroom, and 10 to 20 minutes per airing in summer.⁷

Who has to prove what follows the areas of responsibility. In a judgment of 1 March 2000 on a commercial lease (XII ZR 272/97), the BGH decided that the landlord must show and prove that the cause of the defect does not lie within the landlord's own area of duties and responsibility. Only then does the tenant have to prove that they are not responsible – in mould cases, in practice, that they heated and aired sufficiently.⁹ The senate responsible for residential tenancies applies the same allocation and has clarified that the landlord does not have to exonerate themselves against a tenant's damages claim if it remains unclear in whose area the cause lies.¹⁰ How this plays out in a specific case is a question for a lawyer.

The legal basis is the German Civil Code. Under § 535(1) BGB, the landlord must hand over the flat in a condition suitable for the contractual use and keep it in that condition. If a defect cancels or reduces this suitability, the rent is reduced for that period (§ 536 BGB). And the tenant must report a defect that appears during the tenancy without delay (§ 536c(1) BGB).⁴ In practice: anyone who notices mould and does not report it weakens their own position, wherever the moisture comes from.

For the investigation, this gives a sensible order – also outside any dispute. First, the structural causes are checked and, where possible, ruled out: water entering through defective waterproofing, roofs or facade junctions, leaking pipes, rising damp at the base of walls, residual moisture from construction and clearly undercooled parts of the structure. Only then does the use of the flat become the focus. This order avoids blaming residents for damage whose structural cause nobody has looked for.

Findings that point rather to a structural cause:

• growth all year round that starts low on the wall,
• sharply bounded growth along a line that follows a building element inside the wall, such as a ring beam or a slab bearing,
• clearly different surface temperatures within one wall under the same use,
• several flats affected at the same structural spot although the households differ greatly.

Findings that point rather to the use: persistently high indoor humidity although the surface temperatures are normal for the age of the building, laundry dried in unventilated rooms, rooms heated only through open doors from neighbouring rooms, and permanently tilted windows. The UBA points out that tilted windows drastically increase energy consumption and heating costs and can lead to mould through condensation.⁷ In many cases the truth lies in between: the assessment ends not with an either-or but with a weighting of the contributions. That weighting is a technical statement; its legal consequences for claims and costs are a question for a lawyer.

What you can check and record yourself

The most effective thing you can do is a clean record of the indoor climate over several weeks. Small data loggers store temperature and relative humidity at fixed intervals. Put one in the affected room and a second in an unaffected room of the same flat for comparison, at chest height and away from external walls, radiators and windows. Keep a usage log for the same period: when and how long you aired, how many people were at home, when you cooked, showered or dried laundry and how the rooms were heated. Together, data and log show whether humidity peaks fade after airing or whether a high level persists for days. They turn a dispute about allegations into one about observations.

Collect the building documents as well: year of construction, floor plans, wall build-up, the history of refurbishments – especially when windows were replaced or insulation was added – earlier water damage and, in owner-occupied buildings, the minutes of resolutions on facade, roof or windows (Beschlüsse). They often explain why the mould started at a particular time and shorten a later survey considerably.

Check the ventilation situation. Under DIN 1946-6, the German standard for the ventilation of dwellings, a ventilation concept (Lüftungskonzept) is required for new buildings and for modernisations relevant to ventilation; in existing buildings, this already applies when more than a third of the windows in a unit are replaced. The standard distinguishes four ventilation levels. The lowest, ventilation for moisture protection, is meant to protect the building even when the residents are away – without anyone having to open windows.¹¹ After a window replacement or a modernisation, ask whether such a concept exists and whether the planned ventilation devices – extract fans in internal bathrooms, air inlets in window frames, ventilation units with filters – were installed and work.

For airing, the UBA names a relative humidity of 40 to 60 per cent as the favourable range for indoor air and stresses that cross-ventilation with windows on opposite facades is much more effective than one open window.⁷ With poorly insulated external walls, move furniture a few centimetres away from the wall so that warm room air reaches the surface.² Keep doors to cooler rooms closed so that moist air does not move there and condense.

Self-help ends where building parts would have to be opened or work at height would be needed. Do not open walls or floors, do not remove linings, do not climb onto roofs or scaffolding and do not touch electrical, gas or drinking water installations. Simple moisture meters from the DIY store give readings on mineral materials that depend heavily on material, salts and surface and cannot be interpreted without reference values; the same applies to airborne spore measurements without an outdoor comparison. Reliable simple data is worth more than readings nobody can interpret.

Common mistakes

• Painting over the mould. The patch disappears, the moisture at the surface stays, and the same patch returns in the next heating season – while the evidence of the original condition is gone.
• Remediation without finding the cause. According to the UBA guideline, lasting success is only possible if the causes of the increased moisture are found and removed.² Internal insulation in particular changes the moisture balance of a wall fundamentally and needs a building-physics assessment.
• Blaming someone at the first visit. Accusing the tenant of wrong airing or the owner of a defect hardens positions before any data exists. "The cause will be clarified, and data will be collected for this" keeps everyone talking.
• Short, one-off measurements. One humidity reading on an afternoon says little about a heating season. Meaningful are series over weeks, ideally through the cold season, with outdoor conditions and surface temperatures at the critical spots.
• Acting alone without checking the consequences. Reducing the rent or withholding payments before the cause is clear can backfire, and so can an owner's remediation without prior documentation of the evidence. Which steps are sensible and permissible, and which deadlines apply, belongs in the hands of a lawyer.
• Waiting for summer. When the mould fades in summer, the investigation is postponed – and the only period in which the cause can be measured, the cold season, is lost. Prepare in autumn and start measuring when the heating season begins.

How a surveyor finds the cause

An investigation starts with a survey of the situation: location and extent of the growth, orientation of the wall, wall build-up, radiators, furniture and use of the room, plus the documents on age, refurbishments and earlier damage. From this, the surveyor forms a working hypothesis about where the moisture comes from and then tests it, ruling out competing explanations instead of confirming a preconceived opinion.

Next, the boundary conditions are recorded: surface temperatures at the critical spots in cold weather, indoor temperature and humidity over time, and the outdoor conditions over the same period. Depending on the question, thermography, airtightness tests, moisture measurements in materials, leak detection or microbiological tests follow, partly with specialist laboratories. The results are compared with two references: the structural target condition – the requirements at the time of construction or of a later substantial alteration – and a usage scenario that can be regarded as normal for the flat. From both follows whether the flat could have stayed free of mould with reasonable use.

Two further sets of rules help. DIN 4108-3 (edition 2024-03, replacing 2018-10) deals with climate-related moisture protection, mainly inside building parts and against driving rain. It matters, for example, when internal insulation is assessed, and the 2024 edition contains, for the first time, a risk assessment for mould growth in its annex on hygrothermal simulation.¹² For mould that has already occurred, guideline 4-12 of the German association for building preservation (WTA Merkblatt 4-12, revised 2020/21) describes the goals and the checks of indoor mould remediation.¹³

Which kind of report you need depends on the situation. A private or party report (Privatgutachten, Parteigutachten) is commissioned by one party; in German civil proceedings it counts as qualified party submission – it can persuade the other side but does not replace evidence ordered by the court. Independent evidence proceedings (selbständiges Beweisverfahren) let a court have condition and cause established before evidence is lost through repairs. Under § 485(2) of the German Code of Civil Procedure (ZPO), a party can apply for a written expert opinion on, among other things, the condition of a thing, the cause of a defect and the cost of removing it if it has a legal interest – which is assumed if the finding can help to avoid a lawsuit. The court selects the expert and directs the expert's work. Before the appointment, the parties can be heard on the person of the expert, the court can ask them to name suitable persons, and if they agree on particular persons, the court must follow that agreement (§ 404(1), (2), (4) and (5) and § 404a ZPO).¹⁴ An expert determination (Schiedsgutachten) is based on an agreement between the parties. Which route is right in your case should be discussed with a lawyer.

A report has limits. It can establish the conditions at a surface and whether they comply with the target condition of the building. It cannot prove retrospectively how often someone aired in past years if there are no records. And it makes no legal decision: it provides the technical basis on which the parties, insurers or courts decide.

Special cases: rented flat, owner-occupied flat, new building

Rented flat. The landlord's side checks and documents the building side; the tenant documents indoor climate and use. Neither can produce the full picture alone. Commissioning one surveyor jointly avoids double costs and increases acceptance of the result. Whether a party can demand or refuse an inspection, and with what consequences, is a legal question.

Owner-occupied flat (Wohnungseigentum). The building envelope – facade, roof, load-bearing walls and usually the windows – is common property (Gemeinschaftseigentum), while interior plaster, paint and floor coverings belong to the individual unit (Sondereigentum). Mould on the inside of an external wall therefore often affects both: the cause lies in the common property, the damage shows in the unit. Under the German Condominium Act (WEG), common property is the land and the building insofar as they are not in separate ownership (§ 1(5) WEG). Parts of the building that are needed for its stability or safety are never separate property, even if they lie within a unit (§ 5(2) WEG). The owners' association (Gemeinschaft der Wohnungseigentümer) manages the common property, and proper management includes maintaining it (§ 18(1), § 19(2) no. 2 WEG). The property manager may and must take measures of minor importance that do not lead to substantial obligations, or that are needed to meet a deadline or avert a disadvantage (§ 27(1) WEG).¹⁵ In practice: arrange emergency measures and documentation immediately, and prepare the investigation so that the owners can decide on a sound basis. Which part belongs to which area in a specific building also depends on the declaration of division (Teilungserklärung) and needs legal review.

New building. A building constructed today must meet current thermal protection requirements, including the minimum requirements for building parts and thermal bridges under DIN 4108-2. The yardstick is the edition valid at the time of construction – for buildings constructed before May 2026, usually still the 2013-02 edition.⁶ Cold surfaces that must be accepted in an old building are a deficit in a new one. In addition, concrete, screed, plaster and mortar release water long after completion: according to the UBA, the residual construction moisture in a solid new building is only gone after one to two years of increased airing, with higher heating costs in that period.⁷ Mould in the first years of a new building is therefore often linked to this residual moisture and to too little air exchange while the building dries out.

After partial energy refurbishment. If windows are replaced without insulating the external walls, the uncontrolled air exchange disappears while the cold wall surfaces remain. Neither the windows nor the wall are defective on their own; the problem is the combination. For the assessment, it is decisive whether the measure was accompanied by a ventilation concept.¹¹

When to call in a surveyor

Bring in a surveyor if the mould returns at the same spot after professional removal, if several flats are affected at the same structural spot, if large areas, hidden building parts or floor build-ups are involved, or if the parties disagree about the cause and payments are being withheld or measures refused. Do it also before any major remediation, because once the work is done, the condition can no longer be established. A condition survey records the condition before the repair; our mould survey establishes the cause with measurements and calculation.

Before commissioning, clarify three things: the question to be answered (cause, scope of remediation, costs or all three), the role (party report, jointly commissioned investigation or preparation of court proceedings) and the timeframe, which follows the heating season. A report does not remove mould and does not decide a dispute. It creates the factual basis on which a sensible decision can be made – and the point at which the remediation finally addresses the cause instead of the stain on the wall.

Frequently asked questions

How can I tell whether the mould in my flat comes from the building or from airing?

There is no reliable sign for laypeople, but there are clues. Growth that appears only in winter and starts in external corners or behind furniture points to condensation from the indoor air on cold surfaces. Growth all year round at the base of the wall points rather to water from outside. If several flats are affected at the same structural spot, that suggests a structural cause. A reliable answer needs measurements of surface temperatures and indoor humidity over a heating season.

How often and how long should I air my flat?

There is no universal figure, because the need depends on the building, its equipment and its use. For the cold season, the UBA recommends shock ventilation of about 5 minutes per room two to three times a day, 5 to 10 minutes in the bedroom in the morning and in kitchen and bathroom, and 10 to 20 minutes per airing in summer.⁷ Cross-ventilation with windows on opposite sides works much better than one open window; in the case decided by the BGH, the expert stated that it reduced the required airing time to a third.⁸ Permanently tilted windows are unfavourable because they raise heating costs and can promote condensation.⁷

Who has to prove what when there is mould in a rented flat?

German case law allocates the burden of proof by areas of responsibility. According to the BGH judgment of 1 March 2000, the landlord must show and prove that the cause does not lie in the landlord's own area of duties and responsibility; only then must the tenant prove that they are not responsible – in practice, that they heated and aired sufficiently.⁹ In practice, this means the structural causes are checked first. How this applies in your case should be discussed with a lawyer.

Is mould in an old building automatically a building defect?

No. In its judgments of 5 December 2018, the BGH decided that thermal bridges in external walls are not a defect of a rented flat if the condition complies with the building regulations and standards in force when the building was constructed; the cases concerned buildings from 1968 and 1971.⁸ A defect can still exist, for example if water enters from outside or after a deficient later modernisation.

May I remove the mould myself?

According to the UBA, small areas can be treated with alcohol or similar; larger infestations should be handled by a specialist firm.⁵ Do not brush or sand dry, because that releases spores. If children or people with asthma, cystic fibrosis, other respiratory diseases or a weakened immune system live in the flat, do not do the work yourself.² Before any cleaning: document first, then act – with the patch, the evidence disappears too.

When is a mould survey worthwhile and how long does it take?

It is worthwhile if the mould returns after removal, several units are affected, hidden building parts are involved or the parties disagree about the cause – and before a major remediation, because afterwards the condition can no longer be established. The duration depends on the season: inspection and document review are possible at any time, but the decisive measurements of surface temperatures need cold weather. A reliable answer therefore often takes several weeks to months.

Note

This guide reflects the state of technical knowledge on the date of writing. It is not legal advice and does not replace an inspection of the individual case. Whether claims exist and how they can be enforced is assessed by a lawyer (Rechtsanwalt). Standards and guidelines must be checked in their current version.

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About the author

M. Sc. Karim Abu Elkheir, BIB Ingenieurbüro für Bauwesen, Berlin. Contact: info@ing-bassam.de, +49 176 23581339. English version as of 5 October 2026, based on the German article as of 21 September 2026.

Sources and footnotes