Quick answer Find the hidden source first — it is usually behind a wardrobe, under a bed, inside a wall cavity, under a sink or in carpet underlay. Stop the moisture, treat the growth, and document everything. If the mould covers more than about 10 sq ft, is inside the wall, or keeps returning after you treat it, that is a landlord repair, not a tenant cleaning job.

On this page

  1. What the smell tells you
  2. Finding the hidden source
  3. Stopping the moisture
  4. Treating the growth safely
  5. When it is the landlord's job
  6. Health signs you should not ignore
  7. Keeping it gone

What the smell tells you

Mould smell is a specific, earthy, damp-cardboard odour. It is produced by microbial volatile organic compounds — gases the mould releases as it metabolises. Two things follow from that.

First, you can smell it before you can see it. The compounds diffuse through a room long before the growth is large enough to notice on a wall. A persistent mouldy smell with no visible growth almost always means the growth is hidden.

Second, dead mould still smells. This is the mistake almost everyone makes. They treat a patch, the growth dies, and the smell stays because the dead material and spores remain in the surface. Killing the growth is only half the job; removing the residue is the other half.

Finding the hidden source

In an apartment the growth is usually where air does not move and a cold surface meets warm damp air. Work through this list in order — it is arranged by how often each one turns out to be the answer.

LocationWhy mould grows thereHow to check
Behind and on top of wardrobesCold exterior wall, zero airflowPull it out 30 cm and look at the wall
Under the bed and mattress baseBody moisture, no airflow beneathLift the mattress; look at the slats
Under the kitchen and bathroom sinkSlow drip feeding constant dampFeel the cabinet floor and the pipe joints
Carpet and underlaySpills or overspill trapped beneathLift a corner and smell the underside
Window reveals and sillsCondensation on the coldest surfaceLook at the corners, not the middle
Inside a wall cavity / behind skirtingLeak, rising damp or a bridged damp courseLook for paint blistering, soft plaster, tide marks
Ceiling below a bathroom aboveEscaping water from the flat aboveLook for staining and a ring outline
Air conditioning or extract ventsDust plus condensation in the ductSmell at the vent while it runs
Where mould hides in an apartment
The wall test tells you the scope Press a dry paper towel against the suspect wall and hold it there for ten seconds. If it comes away feeling damp, the moisture is coming from inside the wall — that is a structural problem and a landlord issue. If the wall feels dry and cold and the towel stays dry, the moisture is condensing from the room and you can address it with ventilation and drying.

Stopping the moisture

There are only two possible sources and they need different responses.

Source 1: condensation from the room

The flat is producing more moisture than it can get rid of, and it condenses on the coldest surfaces. This is the common case in apartments. Fix it with:

  • Air exchange. Cross-flow ventilation for 10–15 minutes, twice a day, even in cold weather. Short bursts beat windows left ajar all day.
  • Humidity control. A dehumidifier holding the flat at 45–50%. This is the step most renters skip and the one that actually resolves the smell.
  • Behaviour fixes. Dry laundry with the door closed and a window open, always use the extractor over the hob, and never dry towels on radiators.

Source 2: water ingress

Water is getting in from outside or from another flat. This is not solvable with a dehumidifier, and treating the visible growth will not help because it will simply return. The signs are damp walls, tide marks on plaster, blistering paint, and a smell that returns within days.

This is a repair, and in almost every jurisdiction it is the landlord's responsibility. Do not spend money fixing a building defect yourself.

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Treating the growth safely

If the affected area is small and on a hard surface, you can treat it yourself. The safety rules are not optional: mould spores are an allergen and some species produce irritants.

  1. Protect yourself. N95 or better mask, gloves, eye protection. Do not use a dusting brush on dry mould — that aerosolises spores. Wipe, never brush.
  2. Seal the area. Close the door and tape a plastic sheet over the doorway if you are treating a lot of growth, so spores do not spread to the rest of the flat.
  3. Treat the surface. A commercial mould cleaner or a diluted white vinegar spray. Bleach works on non-porous surfaces like tile and glass, but never on plaster, timber or fabric.
  4. Remove the residue. Wipe with a damp cloth and dispose of the cloth in a sealed bag. This step is what removes the smell.
  5. Dry the area completely with airflow and a dehumidifier before you put furniture back.
  6. Replace porous items you cannot clean. Carpet underlay, a mattress and a badly affected sofa are usually not recoverable, and treating them repeatedly wastes money.

When it is the landlord's job

Renters spend a lot of money treating mould that was never theirs to fix. The line is reasonably clear.

SituationWho fixes itWhy
Small patch on tile or window frameYouCondensation, and easily treated
Mould covering more than ~10 sq ftLandlordBeyond routine cleaning; health risk
Mould inside a wall cavity or behind plasterLandlordStructural defect
Any sign of water ingress or a leakLandlordBuilding repair, not tenant behaviour
Mould under carpet or floorboardsLandlordRequires lifting the floor
Mould that returns within days of treatmentLandlordEvidence of an ongoing defect
Mould after you have ventilated and dried properlyLandlordThe property has inadequate ventilation
Who is responsible for mould in a rental

Put everything in writing. Photograph the growth with something for scale, log the dates, note your humidity readings, and keep a copy. A written report that shows you have ventilated and dried the flat — with numbers — is far more effective than a phone call, because it removes the landlord's usual defence that the tenant caused it.

In the UK, the Housing Health and Safety Rating System gives local authorities the power to require a landlord to act on serious damp and mould. Several US states have similar implied-warranty habitability provisions. If written requests are ignored, a council environmental health team or a tenants' rights organisation is the next step.

Health signs you should not ignore

Mould exposure affects people differently, and some are far more sensitive than others. Seek proper medical advice if anyone in the flat experiences:

  • A persistent cough, wheeze or shortness of breath that improves when you are away from the flat.
  • New or worsening allergic rhinitis — sneezing, itchy eyes, blocked nose — indoors.
  • Skin irritation or recurring rashes.
  • Frequent headaches or unusual fatigue that clears when you leave.

Anyone with asthma, a weakened immune system, or an infant in the home should treat mould as a priority rather than a nuisance, and should not do the removal work themselves. This guide is not medical advice — if symptoms are significant, see a doctor.

Keeping it gone

The fixes that actually hold are boring and cheap.

HabitTargetFrequency
Check humidity40–50% RHDaily in winter
Cross-flow ventilation10–15 minutesTwice daily
Wipe window condensationDry sills and framesEvery morning in winter
Squeegee the showerDry walls and screenAfter every use
Pull furniture off exterior walls2–3 inch gapPermanent
Check under sinks for dripsDry cabinet floorsMonthly
Air the wardrobeLeave doors open to dryWeekly
The habits that prevent mould returning

Frequently asked questions

How do I get rid of moldy smell in my apartment?
Find the hidden source — usually behind a wardrobe, under a bed or beneath carpet — stop the moisture, treat the growth, then remove the dead residue. Dead mould still smells, so cleaning up is as important as killing it.
Why does my apartment smell moldy but I can't see mold?
Mould smell travels before the growth becomes visible. The growth is usually behind furniture, under a mattress base, inside a wardrobe, under a sink or in carpet underlay, where airflow is low and surfaces are cold.
Is moldy smell dangerous?
Mould spores are a common indoor allergen and can trigger respiratory symptoms in sensitive people, particularly those with asthma. If anyone in the home has persistent symptoms, treat it as a priority and seek medical advice.
Does a dehumidifier get rid of moldy smell?
It stops the smell being produced if the moisture is condensation from inside the flat, and it prevents regrowth. It cannot help if water is entering through the building fabric — check the suspect wall with a paper towel test first.
Should my landlord fix the mold?
If the mould covers more than roughly 10 sq ft, is inside a wall, is under carpet or floorboards, involves any water leak, or keeps returning after you treat it, it is normally the landlord's repair. Put it in writing with photos and humidity readings.
Does vinegar kill mold?
White vinegar kills surface mould on hard surfaces and neutralises much of the odour. It does not penetrate porous materials, so it will not fix mould that has grown into plaster, timber or fabric.
Bottom line Moldy smell means the growth has usually moved into something porous, which is why treating the visible patch rarely works. Find the hidden source, stop the moisture, remove the dead residue rather than just killing it, and document everything — because in an apartment a large, hidden or recurring mould problem is a repair, not a cleaning job.
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