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The whole house

Why does mould keep coming back in the bathroom and wardrobe?

The water behind bathroom and wardrobe mould is the air’s own: Victoria’s health department names condensation from showering and clothes drying among the common causes of mould indoors, and Homes Tasmania sets out the mechanism, moist air meeting a colder surface such as a mirror, a tile or a window. Cleaning the mould off without moving that air on is the step that fails, and the trace below follows one evening’s shower through the sources to show where it goes.

General information from official health, building and housing sources, not advice about your home, your health or your tenancy. The page behind most of this one is Condensation, from YourHome.

Water from a leaking roof, reaching the ceiling: NSW Health lists a leaking roof and a blocked rain gutter among the water leaks and plumbing problems to repair.Water from a gutter fault, into the top of the outside wall: Consumer Protection WA’s list of what a landlord should do to avoid mould developing includes cleaning and maintaining the gutters, so rainwater stays out of the roof cavity.Where a roof or ceiling leak shows: NSW Fair Trading’s own inspection list says to check for damp or mould along skirting boards, walls and ceilings.Where a gutter fault shows: a discoloured or bulging patch that may be cold or damp to touch, in the wall or roof cavity (Consumer Protection WA). Shower steam, and the bathroom exhaust fan the building code sizes: the 2025 National Construction Code sets a minimum exhaust flow rate, 25 litres a second for a bathroom or sanitary compartment.Signs in a wet area: the SA Government’s inspection list says to look for mouldy walls, lifting tiles, peeling paint or pools of water. Moist air reaching cold window glass: the national home design guide says condensation forms when humid air meets a cooler surface, such as a window in winter.Condensation and mould on the window: the national home design guide says aluminium frames form a thermal bridge into the home and are prone to condensation.Moist air reaching the outside wall behind a wardrobe: one of Homes Tasmania’s tips for a dry, well-aired home is to allow space between furniture and the wall so air can flow, and it calls this very important with wardrobes.Mould on the outside wall behind the wardrobe: one of Homes Tasmania’s tips is not to stack boxes or other items against outside walls, especially walls that do not get a lot of sun.A small patch on a bedroom wall: NSW Health says it is good to remove mould as soon as it appears. Water from a concealed pipe leak, down inside the wall: SA Water says damp, warped or discoloured walls, floors or carpets can suggest a concealed leak indoors.Where a concealed leak shows: Icon Water says to look in the kitchen, bathrooms and laundry for dripping taps, washing machine and dishwasher connections, and damp patches on the walls. Flood water in the home (the line marks no depth): Victoria’s health department says flooding can cause excessive mould growth, which has to be cleaned up before moving back in.A floor joist with staining or decay: the Queensland timber guide says a surface stain that scrapes off to sound timber underneath is not of concern, while wood that is soft and spongy or easily penetrated has decayed. Damp rising from the ground into the base of the wall: NSW Health says rising damp is ground moisture rising up a brick or stone wall.Where rising damp shows: the base of the wall and the skirting, where Consumer Affairs Victoria says damp brick walls can indicate rising damp or salt damp.Damp ground under the floor: Homes NSW says rising damp is moisture from the ground travelling up into a home through the bricks or stone in its walls.A damp subfloor: NSW Fair Trading’s inspection list says to lift carpet and check for rotting floorboards or damp. Water from a leaking roof, reaching the ceiling: NSW Health lists a leaking roof and a blocked rain gutter among the water leaks and plumbing problems to repair.Where a roof or ceiling leak shows: NSW Fair Trading’s own inspection list says to check for damp or mould along skirting boards, walls and ceilings.Water from a gutter fault, into the top of the outside wall: Consumer Protection WA’s list of what a landlord should do to avoid mould developing includes cleaning and maintaining the gutters, so rainwater stays out of the roof cavity.Where a gutter fault shows: a discoloured or bulging patch that may be cold or damp to touch, in the wall or roof cavity (Consumer Protection WA). Shower steam, and the bathroom exhaust fan the building code sizes: the 2025 National Construction Code sets a minimum exhaust flow rate, 25 litres a second for a bathroom or sanitary compartment.Signs in a wet area: the SA Government’s inspection list says to look for mouldy walls, lifting tiles, peeling paint or pools of water. Moist air reaching cold window glass: the national home design guide says condensation forms when humid air meets a cooler surface, such as a window in winter.Condensation and mould on the window: the national home design guide says aluminium frames form a thermal bridge into the home and are prone to condensation.Moist air reaching the outside wall behind a wardrobe: one of Homes Tasmania’s tips for a dry, well-aired home is to allow space between furniture and the wall so air can flow, and it calls this very important with wardrobes.Mould on the outside wall behind the wardrobe: one of Homes Tasmania’s tips is not to stack boxes or other items against outside walls, especially walls that do not get a lot of sun.A small patch on a bedroom wall: NSW Health says it is good to remove mould as soon as it appears. Water from a concealed pipe leak, down inside the wall: SA Water says damp, warped or discoloured walls, floors or carpets can suggest a concealed leak indoors.Where a concealed leak shows: Icon Water says to look in the kitchen, bathrooms and laundry for dripping taps, washing machine and dishwasher connections, and damp patches on the walls. Flood water in the home (the line marks no depth): Victoria’s health department says flooding can cause excessive mould growth, which has to be cleaned up before moving back in.A floor joist with staining or decay: the Queensland timber guide says a surface stain that scrapes off to sound timber underneath is not of concern, while wood that is soft and spongy or easily penetrated has decayed. Damp rising from the ground into the base of the wall: NSW Health says rising damp is ground moisture rising up a brick or stone wall.Where rising damp shows: the base of the wall and the skirting, where Consumer Affairs Victoria says damp brick walls can indicate rising damp or salt damp.Damp ground under the floor: Homes NSW says rising damp is moisture from the ground travelling up into a home through the bricks or stone in its walls.A damp subfloor: NSW Fair Trading’s inspection list says to lift carpet and check for rotting floorboards or damp. Bathroom, Bedroom wall and wardrobeSee the house

One shower, traced

  1. The shower

    Victoria’s health department names condensation from cooking, showering and clothes drying among the common causes of mould indoors, and Homes Tasmania walks the same ground: moisture in the air comes from cooking, washing and drying clothes inside, and when that air meets a colder surface, a mirror, a tile, a wall or a window, it turns into water droplets. Homes Tasmania adds where the droplets collect: warm air rises, so they gather on ceilings and towards the top of walls, and they can form on furniture too. The wet side of a shower is built for water, up to a point: the 2025 National Construction Code requires building elements in a wet area to be protected with a waterproofing system that is either waterproof or water resistant.

  2. The fan and the window

    The fan is the part of the room built to carry that vapour out. The 2025 National Construction Code sets a minimum exhaust flow rate for an exhaust system installed in a kitchen, bathroom, sanitary compartment or laundry, at 25 L/s for a bathroom or sanitary compartment, and it requires the air to discharge to outdoor air rather than into a roof space. Where a bathroom exhaust is not run continuously and the room is not ventilated by openable windows, the same part of the code requires the fan to be interlocked with the room’s light switch. Western Australia’s health department gives the everyday version: use the exhaust fan when showering, keep it working, and shut the bathroom door until the vapour has cleared. Victoria’s Better Health Channel, on rented homes with mould, puts the same habit on the tenant: use the extractor fans when they are available, and air the home regularly where possible.

  3. The wet-area signs

    Where the moisture has been sitting long enough, the surfaces say so. The SA Government’s inspection list, written for people looking over a property, names mouldy walls, lifting tiles, peeling paint and pools of water in wet areas, under its heading for signs of structural problems. The Queensland building commission’s guide adds the reason a wet area is harder to dry than it looks: waterproofing under a tiled or vinyl floor is there to stop water, and it also holds moisture in the timber floor beneath it, so the structure dries out slowly.

  4. The cold wardrobe wall

    The wardrobe is the same air meeting a colder surface, with no fan behind it. Homes Tasmania’s tips for a dry home include leaving space between furniture and the wall so air can move, and it calls that one very important with wardrobes; it also says not to stack boxes or other things against outside walls, especially walls that do not get much sun. Western Australia’s health department lists behind furniture among the common places for mould growth. Victoria’s health department points at the same spot: poor air circulation, in cupboards, in corners, and behind furniture set against an uninsulated outside wall. In a cool climate the wall itself is the coldest surface in the room. The national home design guide says a thermal bridge, a metal frame or stud that heat passes through quickly, can pull a surface below the temperature at which the air’s moisture condenses, and that windows and doors are common places for it.

Why mould cleaning alone fails here

The clean is not what fails. The air is. NSW Health’s own instruction is to remove the mould and to treat the cause as well, because the mould is likely to return if the cause is left alone. Victoria’s removal page opens with the same logic: the moisture source has to be removed or controlled, or the problem will reoccur. Consumer Protection WA’s landlords bulletin gives the bathroom version in two lines: scrub the mould off the hard surface with soapy water, because the physical action is what prevents regrowth, then dry the surface, and find where the moisture comes from and fix it.

What that means in this room is that a scrub and a bleach wipe buy time, and the fan, the window and the space behind the wardrobe are what decide whether the time is kept. WA Health’s guidelines put the same thing the other way round: fixing the source of the moisture has to happen at the same time as the cleaning, or the mould regrows.

Keeping the air moving

  • Run the exhaust fan when showering and leave the bathroom door shut until the vapour clears, rather than leaving both open at once.
  • Use the exhaust fan when cooking too, put a lid on saucepans so the steam stays in the pot, and do not leave a kettle boiling.
  • Dry clothes outside where that is possible; where it is not, exhaust a vented dryer outdoors, and ventilate the room a condensing dryer sits in.
  • Pull furniture a little away from the wall so air can pass behind it, and leave cupboard doors open now and then to air them.
  • Keep documents, furniture and cardboard boxes out of damp spaces and away from cold outside walls and foundations, since they hold moisture where there is little air movement.
  • Point garden sprinklers so they do not soak the wall or the foundations, and keep the roof’s rainwater runoff at least 1 metre away from the foundation.
  • Keep the air conditioner’s drip pans clean and its drain lines clear.
  • Use heat, insulation and draught sealing in the colder months so the inside faces of walls and windows stay above the dew point; the national home design guide says insulated walls and ceilings stay warmer. The national energy guide splits ventilation into its two kinds, natural air moving through openings such as windows and doors, and mechanical fans or systems bringing outside air in and stale or damp inside air out, and says effective ventilation can maintain air quality, add thermal comfort and reduce humidity and the potential of mould.
  • Where a room has no window to open, the code’s answer is mechanical. The 2025 National Construction Code requires ventilation to be provided to a bathroom, shower room, laundry and other occupied rooms, and allows an exhaust fan or other mechanical ventilation for a bathroom, sanitary compartment, laundry or kitchen where the contaminated air discharges to outdoor air. The code’s own objective for these rules is the reason the habit matters: it aims to reduce the likelihood of condensation or water vapour build-up causing illness, injury or loss of amenity for a building’s occupants.
  • Where there is an air conditioner in a sealed room, the national energy guide is direct about the effect: it dries the air, which can be effective in managing mould. The same page warns of the reversal, that switching it off and opening the windows lets humid air inside, and suggests keeping the room closed before opening it up to the evening breeze.
  • After the shower, the habit that carries most of it: wipe down the surfaces and windows that are wet, because Homes Tasmania says droplets that are left turn into mould or patches of damp, and can damage the home.

If you rent

In Western Australia the ventilation question is written into who pays. Consumer Protection WA puts the mould that poor ventilation caused on the landlord, with windows that cannot be securely opened and wet areas left without exhaust fans as its examples, and the same page puts the mould from a lack of maintenance or damage, leaking gutters or windows among it, on the landlord too. The tenant’s side of the same rule is use: the landlords bulletin says a tenant is responsible for mould they have caused, such as not using a provided, working exhaust fan, leaving surfaces wet for a long time, or not ventilating or cleaning the home.

Western Australia’s maintenance page lists three duties for the tenant: keep the home clean and tidy, use fans and windows to stop mould, and tell the landlord when something needs maintenance or repair. The other states that draw the line are set out state by state, with each state’s own rules, in the renting guide.

Who fixes mould in a rental in your state?

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