Condensation, Mould and the $3,000 Deep Clean: Is It the Building, or the Way It's Being Lived In?

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• Mould is a moisture problem wearing a cleaning problem's clothes. Until the moisture source is identified, any clean is temporary.
• Responsibility follows the cause, not the person's status. Occupant habits, lot fixtures and common property each produce mould, and each points the bill at someone different.
• Condensation on windows and glass doors usually means airborne humidity, which can come from habits, from the building's design, or both.
• A deep clean without a diagnosis is the most commonly repeated mistake in strata mould disputes.

The scenario comes up in every strata community sooner or later. Mould starts appearing through an apartment. Photos show condensation running down the inside of closed sliding balcony doors. Someone recommends a $3,000 deep clean. The owner suspects the occupants never open a window and run the dryer with everything shut; the occupants suspect the building is damp. Ten years without a complaint, and now this.

Both sides usually believe they are obviously right. Both are sometimes wrong. What settles it is not opinion but a diagnosis, and the diagnosis is cheaper than the argument.

Where the water actually comes from

Mould needs moisture, something organic to grow on, and mild temperatures. In an apartment the last two are always present, so the only question that matters is where the moisture comes from.

An ordinary household releases around eight litres of water into its indoor air every day: cooking, showers, kettles, breathing, indoor plants. A single load of washing dried indoors, or run through an unvented dryer, adds several litres more in one hit. If that vapour has nowhere to go, indoor humidity climbs, and when humid air touches a cold surface — single-glazed windows and aluminium-framed sliding doors are the coldest surfaces in most Australian apartments — it condenses. Water runs down the glass, pools on sills and tracks, soaks into frames, carpet edges and curtain hems, and mould follows.

That is one pathway. The others have nothing to do with anyone's habits: a failed waterproofing membrane in the bathroom above, a leaking pipe inside a wall, deteriorated façade sealing, blocked balcony drainage, a roof defect, rising damp, or a bathroom exhaust fan that vents into the ceiling cavity instead of outside. These produce mould in an apartment ventilated perfectly.

The two pathways look similar on a wall. They look very different to an inspector with a moisture meter.

What to look for: the pattern tells the story

Before commissioning anything, read the mould like evidence. The pattern and location usually point to the source.

Signs the moisture is airborne

Condensation-driven:

• Condensation visibly forming on windows and glass doors, worst on cold mornings
• Mould concentrated around window frames, sills, door tracks and the wall areas beside them
• Mould in still-air spots: room corners, behind wardrobes and beds pushed against external walls, inside built-in robes
• It appears in winter or humid weather and eases when the weather changes
• Spotty, surface-level growth across paint, grout and silicone rather than a single saturated patch

Signs the moisture is coming through the building

• A defined damp patch or stain that persists regardless of season or ventilation
• Mould returning within weeks of cleaning in the same location, even with windows open and humidity managed
• Bubbling paint, lifting plaster, swollen skirting boards or a musty smell from carpet
• Damp that tracks weather: worse during and after rain
• One wall or ceiling section consistently cold or damp to touch while the rest of the room is dry
• Mould in a bathroom whose exhaust fan is dead, disconnected, or ducted nowhere

Signs the building's design is the problem

• Single glazing and uninsulated external walls that turn cold in winter (thermal bridging), attracting condensation even at moderate indoor humidity
• No exhaust ventilation in bathrooms or the laundry, or windows that were built sealed
• Apartments that face away from sun and breeze and never dry out

The third category is the awkward one, because the occupants' behaviour and the building's fabric are both contributing. A 1970s unit with single glazing and no exhaust fans demands more ventilation discipline from its occupants than a new build with ducted extraction — and there is a point at which the demand becomes unreasonable and the fabric, not the habits, is the operative cause.

Who is responsible: follow the cause, not the person

Whether the apartment is occupied by its owner, a family member or a paying tenant changes the paperwork, not the principle. Responsibility for mould follows the moisture source.

If the source is occupant behaviour

Dryer run daily with every window shut, no exhaust fans used, steam left to settle: the cleanup belongs to the occupants, along with the habit change. No amount of building maintenance fixes moisture that is generated indoors and never released.

If the source is within the lot

A leaking flexi hose under the sink, an internal pipe, an air conditioner never serviced, an unvented dryer installed without thought: it belongs to the lot owner.

If the source is common property

Membranes, the roof, façade sealing, shared plumbing, balcony drainage, and in Queensland building format plans the windows and doors on the lot boundary: it belongs to the body corporate or owners corporation, which in every Australian jurisdiction carries a duty to maintain common property. The occupant's habits are irrelevant to a failed membrane.

If the source is original design

No mechanical ventilation, single glazing, sealed windows: the position is genuinely contested. Bodies corporate must maintain the building to its original standard, but are generally not obliged to improve it. In a tenanted unit, though, the calculus shifted: Queensland's minimum housing standards (fully in force since September 2024) require a rental property to be free of damp and mould at the start of the tenancy and functionally ventilated throughout it — while expressly excluding damp or mould "caused by the tenant", the regulation's own example being a failure to use an installed exhaust fan. The standard bakes the behaviour-versus-building question directly into the law.

For a tenanted Queensland unit specifically, the process is well worn: the party that spots the problem notifies the other, a Notice to Remedy Breach (Form 11) follows if nothing happens, then the RTA's free conciliation service, then QCAT. Tribunals have decided these cases in both directions. QCAT has dismissed tenant claims where the evidence showed mould growing around windows the tenants never opened; NCAT has ordered a landlord to refund months of rent and pay $15,000 in decontamination costs where the real cause was rising damp and the "just ventilate" defence collapsed under expert evidence. The pattern across the cases is consistent: the side with the moisture evidence wins.

Why the deep clean keeps failing

A deep clean removes mould. It does not remove moisture. If the moisture source is still active — behavioural or structural — regrowth typically begins within weeks, which is how a scheme ends up paying for the same clean twice a year and calling it maintenance.

The sequence that works runs the other way around:

1. Diagnose first. A specialist mould and moisture inspection ($400–800) maps humidity, takes moisture readings through walls, checks exhaust ducting, and identifies the source. If the cause turns out to be common property, that cost is generally recoverable. If it turns out to be habits, $600 has just saved a $3,000 clean from being an annual subscription.
2. Fix the source. Repair the leak, reseal the membrane, duct the fan outside, vent the dryer — or change the routine and add a dehumidifier. Whichever it is.
3. Then remediate. Small areas (under about one square metre) are a household job: detergent or a vinegar solution, microfibre cloths, discard the cloths afterwards. Bleach is the persistent myth here — it strips the colour so the surface looks clean, but on porous surfaces the growth structure survives and returns. Larger or recurring growth, mould inside wall cavities, or anything following a significant leak is professional territory: containment, HEPA filtration and safe disposal, not a wipe-down.
4. Verify it stays gone. A $20 hygrometer answers the ongoing question. Indoor humidity persistently above 60% means the moisture problem is still live, whoever's fault it is.

Prevention, on both sides of the wall

For whoever lives in the apartment: open windows for a few minutes daily even in winter, and always when drying clothes; run exhaust fans during and after showers and cooking; vent the dryer to the outside or use a condenser model; keep furniture a hand's width off external walls; wipe visible condensation off glass before it soaks the sills; use air conditioning on dry mode or a dehumidifier in humid weather.

For the owner and the committee: confirm exhaust fans actually duct to the outside and actually work; take recurring condensation in multiple lots as a building-level signal worth an expert report rather than a lot-by-lot cleaning bill; keep membranes, sealing and drainage on the maintenance schedule, because every year of deferral converts more "behaviour" problems into fabric problems; and treat glazing or ventilation upgrades as what they are — capital improvements that permanently lower the building's mould baseline.

The evidence habit

Every mould dispute, whoever the parties are, is eventually decided on documentation. Date-stamped photos of the mould and the condensation. Humidity readings over weeks, not one afternoon. The inspection report. Records of when the problem was reported and what was done. Ten years without a complaint is itself a data point — something changed, and the file should show what.

The cases that go badly at tribunal, on either side, are the ones argued from certainty instead of evidence. The cases that settle early and cheaply are the ones where somebody spent a few hundred dollars finding out what the water was actually doing.

Climate and building type change the baseline

A Melbourne south-facing brick walk-up in July is a different moisture machine from a Brisbane high-rise in February, or a sealed 2010s tower with tilt-and-turn windows that nobody opens.

Cool-climate winters

Melbourne, Canberra, Hobart, highland NSW produce the classic condensation pattern: indoor air at 18–22°C and 60–70% relative humidity meeting single-glazed glass that is close to outdoor temperature. The glass is the condensing surface. Growth sits on sills, tracks and the cold corners of external walls. Occupant habits matter more here because a few litres of extra vapour, trapped overnight, is enough to wet the glass.

Warm-humid summers

Brisbane, Gold Coast, Darwin, north-coast NSW produce a different pattern. Outdoor air is already wet. Air-conditioning cools surfaces below the dew point. Mould appears in wardrobes, behind furniture and in rooms that stay shut. The useful habit is dehumidification and airflow, not "open the windows all day" when outdoor humidity is 80%.

Sealed or poorly exhausted buildings

These sit on top of both climates. Bathroom fans that dump into the ceiling cavity, dryers that vent indoors, and apartments with no openable window in a wet area will grow mould even if the occupants are careful. That is a fabric problem. A committee that treats every such lot as a lifestyle complaint will keep paying for the same clean.

If several lots on the same elevation, the same stack or the same winter all report the same pattern, stop treating it as one household. That is a building-level signal: glazing, thermal bridging, balcony drainage, or a shared exhaust system.

What a useful inspection actually does

A $3,000 clean is not an investigation. A useful moisture inspection is. Ask the report to do these things, in writing, before anyone books it:

• Measure indoor relative humidity and temperature over time, not one afternoon snapshot
• Take moisture-meter readings through the affected walls, ceilings, sills and skirtings, and compare them with a dry control wall in the same lot
• Check whether bathroom and laundry exhaust fans actually move air, and where the duct terminates (outside, roof space, or nowhere)
• Note window type, frame material, seals, and whether the glass is single or insulated
• Photograph the pattern against a floor plan
• Say, in plain language, whether the moisture is airborne condensation, a leak, or both
• Identify who is likely responsible: occupant, lot fittings, or common property

Thermal imaging is useful when a leak is suspected. It is less useful for condensation, which is a surface-temperature problem you can often see with your eyes on a cold morning.

If the inspector will not name a source, get a second opinion before you spend on remediation. A report that only says "mould present, professional clean recommended" has not answered the only question that matters.

Who owns the window, the fan and the wall

Responsibility follows the moisture source, but the source is attached to a legal object. That object is not the same in every state.

New South Wales

External windows and doors in most schemes registered after 1974 are common property, including frames, sashes and seals, unless the strata plan or a by-law says otherwise. Window cleaning is usually the lot's job if the glass can be reached safely. Failed seals, rotten sills and leaking balcony doors are typically an owners corporation item under the strict duty to maintain common property. Internal plaster and paint are lot property. If common-property water damaged them, the corporation still has to fix the source and, if it delayed, can be ordered to pay for the consequential damage.

Queensland

On a building format plan, the body corporate generally maintains windows, doors and the waterproofing on the boundary of the lot. Standard format (townhouse) schemes treat more of the structure as lot property. Check the plan before you argue.

Victoria, WA and the others

Start with the registered plan and the maintenance schedule, not a rule of thumb from a NSW Facebook group. Victoria also overlays rental minimum standards: rooms must be free of mould and damp caused by or related to the building structure, and habitable rooms plus wet areas must have adequate ventilation.

A dead exhaust fan that is a lot fixture is the lot owner's problem. A fan that was never ducted outside is often an original-construction defect sitting in the common fabric. Those two facts produce opposite invoices.

The three-party problem: occupant, landlord, corporation

Most messy files involve three parties, not two.

The occupant

The occupant controls daily vapour: showers, cooking, drying clothes, whether fans run, whether furniture sits hard against a cold wall. If they generate the moisture and trap it, they own the clean and the habit change.

The lot owner or landlord

They own lot fittings and, in a rental, the duty to provide a habitable, ventilated dwelling. Queensland's minimum housing standards (in force for all rentals since September 2024) require the property to be free of damp and mould at the start of the tenancy and functionally ventilated during it, while excluding mould "caused by the tenant". Victoria requires rooms to be free of structure-related mould and damp, and adequate ventilation in wet areas. NSW rental fitness standards include adequate airflow to prevent dampness. A landlord cannot hide behind the owners corporation if the fan is dead, the dryer is unvented, or they never reported a leak.

The owners corporation

The owners corporation or body corporate owns common property and the duty to maintain it. It does not become the cleaner of last resort for every black spot. It does become liable when the moisture comes through the fabric, or when it sits on a known defect.

The useful sequence in a tenanted lot:

1. Occupant tells the landlord or agent in writing, with photos and dates
2. Landlord inspects promptly and, if the source is unclear, commissions a moisture report
3. If the source is common property, the landlord (or the occupant, if the landlord will not) notifies the corporation
4. Nobody should wait for the other two parties to finish arguing before stopping active water

Tribunals have gone both ways because the facts go both ways. The file that wins is the one that shows humidity readings, fan tests and a source, not the one that asserts "they never open a window" or "the building is damp".

What insurance usually will not do

Strata building insurance is built for sudden and accidental damage: a burst pipe, a storm, a fire. Gradual condensation, long-term leaks, poor ventilation and wear-and-tear mould are routinely excluded. A $3,000 clean billed to the insurer is likely to be declined, and the declined claim still costs the scheme an excess and a loading at renewal if it was lodged carelessly.

Contents insurance may cover some damaged belongings after a sudden event. It will not usually fund a lifestyle clean. Health insurers and public health units deal with medical effects; they do not decide who pays for the wall.

Do not delay a moisture inspection waiting for an insurer to accept a claim that was never covered. Fix the source first.

What the $3,000 should actually buy

Small, surface growth on hard finishes after the source is fixed is a household job: detergent or a vinegar solution, disposable cloths, dry thoroughly. Bleach on porous paint or grout is cosmetic. It does not kill the growth structure in the material.

A professional remediation invoice should describe containment, HEPA filtration, removal of unsalvageable porous material, and clearance. "Deep clean" with no method is a mop. Typical ranges, as a sense-check rather than a quote:

• Moisture inspection: roughly $400–800
• Localised professional clean of a small room after the source is fixed: often $400–1,200
• Multi-room remediation with plaster or carpet replacement: several thousand, sometimes well above $3,000
• Membrane or leak repair behind the mould: the large number, and the one that actually stops recurrence

If a contractor quotes $3,000 without asking where the water comes from, they are selling a clean, not a solution. Get the diagnosis first. Then scope the clean against the diagnosis.

A committee playbook when more than one lot reports mould

• Log every report with lot, elevation, room, date and photos
• Ask whether condensation is visible on glass, and whether it tracks rain or cold mornings
• Test or inspect common exhaust and balcony drainage on that stack before writing "lifestyle" letters
• Fund one independent moisture report on a representative lot if the pattern repeats
• Minute the cause and the responsible party so the next committee does not relitigate it
• Put failed membranes, seals and fans on the capital works or maintenance plan instead of treating each clean as an admin surprise
• Do not issue by-law breach notices for "failure to ventilate" until you can show the lot has working, correctly ducted extraction and openable windows

A by-law about drying clothes on balconies does not make a failed membrane the occupant's problem. Conversely, a corporation is not required to upgrade single glazing just because one occupant runs an unvented dryer. Follow the water.

Evidence checklist

Keep, in one place:

• Date-stamped photos of the mould, the condensation, and the same spots after any clean
• A cheap hygrometer log for two to four weeks (morning and evening is enough)
• The inspection report and moisture readings
• Proof the exhaust fan works, or photos that it does not
• Written reports to the agent, landlord or secretary, and their replies
• Invoices for any clean or repair already done
• Medical notes if anyone in the household is affected (you do not publish these to the building; you keep them for a tribunal or insurer)

Ten years without a complaint is a data point. So is a new occupant, a new dryer, a new sealed window, or a roof repair next door. Record what changed.

How UnitBuddy helps

UnitBuddy's maintenance and defects module keeps the mould file in one place: the incident report with photos, the moisture inspection, the responsibility determination, quotes, the remediation record and the follow-up readings. When mould recurs — or appears in a second lot — the building has a documented history instead of a memory, and the pattern across lots that distinguishes a habits problem from a building problem becomes visible in the data.

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Related reading

• Mould in Your Apartment: Your Legal Rights When the Owners Corporation Won't Act: the escalation pathway when the cause is common property
• Water Leaks From Above: Who Pays and How to Force a Repair
• Waterproofing Bathrooms and Showers in Strata
• Apartment Heat and Ventilation in Strata Buildings

Further reading

• Mould in a rental property — who is responsible? — Residential Tenancies Authority (QLD)
• Minimum housing standards — Residential Tenancies Authority (QLD)
• Mould and damp — Queensland Government
• Maintenance problems in a body corporate: a step-by-step guide — Unit Owners Association of Queensland
• Tenants affected by mould — Tenants Queensland

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Last updated: 14 August 2026. UnitBuddy publishes general information for Australian strata owners and committees. It is not legal, financial, or medical advice. For advice specific to your scheme or your health, consult a qualified building consultant, your owners corporation's professional advisers, or a medical practitioner.

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