Bathroom Humidity: Landlord, Syndic or Tenant – Who Is Liable?

Liability depends on the technical cause. Three scenarios: a faulty VMC ventilation system (syndic, Article 9 of the 1965 co-ownership law), a poorly sized or badly installed ventilation duct (builder, under the two-year/ten-year guarantees), or thermal overloading caused by the tenant (tenant repair charges, Decree 87-712). Only an on-site measurement of actual airflow combined with a mould mapping survey can settle the question.

Why This Question Is Urgent For Landlords

Chronic bathroom humidity exposes a rental property to two major legal risks:

• Breach of decency standards (Article 6 of the 1989 law): a decent home must be « free of any water infiltration » and « free of any mould ». Persistent humidity can trigger a tenant claim for reduced rent or a reclassification of the property as unfit for habitation.
• Landlord’s civil liability: even within a condominium, the landlord cannot ignore the condition of the rented property. A defect tied to normal use (humidity from inadequate heating) engages the landlord’s liability towards the tenant.

Acting fast isn’t optional: the real cause must be identified within 4 to 6 weeks of the complaint being raised.

The Three Causes And Who Is Responsible

Cause 1: Faulty Or Undersized VMC Ventilation → Syndic Liability

Mechanical ventilation (VMC) is a common part of the building (Article 9 of the 1965 law, Decree 67-223). Its maintenance and proper operation fall under the syndic’s responsibility, funded through shared service charges.

Applicable standards:

• Order (arrêté) of 24 March 1982: a bathroom must have a continuous extraction airflow of at least 30 m³/h.
• Standard NF X 35-203: airflow measured at the extraction vent itself (not at the fan unit).

Technical signature: mould in the upper corners, on the ceiling, around the condensation zone. The wall surface is permanently cold and damp. Relative humidity exceeds 60% even after airing the room.

Landlord’s course of action: formally request that the syndic have the VMC airflow measured by an independent expert. If the flow rate is below 30 m³/h, the syndic must fund the necessary work (duct cleaning, fan replacement, or full unit replacement).

Cause 2: Ductwork Defect → Builder/Developer Liability

A ventilation duct that is poorly sized, badly sealed, obstructed, or fitted with miscalculated bends will reduce actual airflow below the required standard even if the fan unit itself is working. This constitutes a construction defect, covered by statutory guarantees:

• Two-year guarantee (Article 1792-3 of the French Civil Code): covers fixtures built into the structure for 2 years from handover.
• Ten-year structural guarantee (Article 1792 of the French Civil Code): covers defects that make the building unfit for its intended use, for 10 years.

Technical signature: abnormally low VMC airflow (below 20 m³/h at the vent) even with a brand-new fan unit. Air leaks along the ductwork. Condensation localised at specific poorly sealed corner joints.

Landlord’s course of action: bring a claim under the two-year or ten-year guarantee against the builder or developer (for buildings under 10 years old). Prerequisite: a technical expert report establishing the link between the construction defect and the humidity.

Cause 3: Tenant Behaviour → Tenant Repair Liability

Tenants must maintain the property under normal conditions of use (Article 7 of the 1989 law). In a bathroom, this means:

• Airing the room regularly (opening the window).
• Not drying laundry directly inside a closed bathroom.
• Keeping the heating at a decent level (minimum 16°C, 18-19°C recommended in bathrooms).
• Running the VMC extraction after showering (at least 30 minutes).

Excess humidity caused by thermal overloading is the tenant’s responsibility (Decree 87-712, Article 4-1: « tenant repair charges »). Restoration costs (mould cleaning, repainting) can be deducted from the security deposit.

Technical signature: diffuse mould along grout lines and at the base of walls (residual capillary moisture), but no condensation in the ceiling corners. Low relative humidity after airing (below 50%). Heating units found switched off or closed. Damp laundry drying on the floor.

Landlord’s course of action: send the tenant a letter documenting the breach and proposing corrective measures. Deduct restoration costs from the security deposit (backed by supporting invoices).

How Cassini Expertise Diagnoses On Site

Pinpointing the exact cause relies on four combined measurements:

1. Measuring Actual VMC Airflow With A Trotec BA06 Vane Anemometer

Our firm is equipped with a professional anemometer that measures volumetric airflow in m³/h directly at the extraction vent, in line with the Order of 24 March 1982. This measurement eliminates false readings – a fan unit that spins but a duct that’s obstructed, or vice versa.

• Flow rate ≥ 30 m³/h: VMC compliant.
• Flow rate 15-25 m³/h: likely syndic failure or construction defect.
• Flow rate < 15 m³/h: VMC faulty or duct fully obstructed.

2. 24-Hour Hygrometry Monitoring

Continuous readings of relative humidity (%). A healthy property stabilises below 60% after airing. A persistent level ≥ 70% signals either a ventilation failure or a pathological source of moisture (water infiltration, rising damp).

3. Mould Mapping

Precise localisation of affected areas:

• Upper corners and ceiling → condensation (inadequate VMC, tenant neglect, or insufficient heating).
• Base of walls, lower corners → capillary action (rising damp from the ground, landlord or builder liability).
• Localised stains near pipework or joints → water infiltration (landlord/syndic liability depending on the pipework concerned).

4. Inspection Of The Fan Unit And Ductwork

Visual inspection of the fan unit (condition, electrical connection), seal integrity at joints, and duct accessibility. Photographic record of the full ventilation run.

Summary Liability Grid

VMC Airflow	Humidity	Mould	Likely Cause	Responsible Party
< 20 m³/h	≥ 70%	Ceiling corners	Faulty VMC or obstructed duct	Syndic
25-30 m³/h (low end)	60-65%	Corners/joints	Tenant behaviour (insufficient airing, laundry drying indoors, low heating)	Tenant
30+ m³/h	> 70% persistent	Base of walls / capillary	Rising damp or infiltration	Landlord/Builder (guarantees)
15-25 m³/h	≥ 65%	Multiple areas	Ventilation duct defect	Builder (two-year/ten-year guarantee)

A distinction that protects you: Cassini Expertise carries out no repair work and sells no equipment (VMC units, damp-proofing products, drying systems). The firm has no financial stake in the outcome of its findings: it diagnoses the issue, identifies the source, and writes the report. A neutrality that your insurer, your syndic, or a court can rely on with confidence.

Field Experience That Makes The Difference

Cassini Expertise draws on fifteen years spent in public works and on construction sites, in project management roles. This inside knowledge of construction – reading plans, supervising execution, coordinating trades, and signing off on completed works – underpins every diagnosis: when the expert measures VMC airflow with an anemometer or maps mould patterns, he understands what he’s looking at because he has managed this kind of work himself.

FAQ: Frequently Asked Questions

Is the tenant always at fault when there’s humidity?

No. The landlord must provide a home « free of any mould » (Article 6 of the 1989 law). If the VMC is faulty or the duct badly installed, the tenant cannot fix that alone. However, neglectful behaviour (never airing the room, heating switched off) can worsen or create residual humidity for which the tenant remains liable.

Who pays for VMC maintenance in a condominium?

The syndic, through shared service charges (Article 9 of the 1965 law). This covers annual duct cleaning, replacement of a worn fan motor, and intervention if the airflow drops below standard. The landlord only pays if they are a co-owner. Otherwise, they can formally request the syndic to act.

Can rent be reduced because of humidity?

Yes, if the humidity persists despite the tenant’s efforts and the landlord’s interventions. The tenant can bring the matter before the local judicial court to have a rent reduction set in proportion to the loss of enjoyment. Case law often grants 10-25% depending on severity. It is therefore very much in the landlord’s interest to resolve the problem quickly.

What does a humidity expert assessment by Cassini cost?

The cost of a technical expert assessment (site visit, anemometer measurement, 24-hour hygrometry monitoring, detailed report) generally ranges from €500 to €1,200 excl. VAT, depending on the complexity of the case and access to the ventilation network. This investment avoids far costlier legal proceedings. Costs can be passed on to the liable party (syndic, builder, or tenant) depending on the expert findings.

Contact Us

Cassini Expertise operates across the Alpes-Maritimes (06) and the Var (83), including Antibes and the surrounding area. We appear before the relevant courts: Grasse, Nice, Toulon and Draguignan Judicial Courts, and the Aix-en-Provence Court of Appeal.

For a bathroom humidity assessment, or any independent building diagnosis:

• Phone 04 22 46 06 04 – email: [email protected]
• Areas covered: Antibes, Grasse, Nice, Draguignan, Toulon and surrounding areas.

We produce a detailed technical report, admissible in court, including anemometer measurements and photographs.

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📅 Mis à jour le 25/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).