The short answer
- Most significant mould problems have no smell at all, and nose blindness sets in within days of living with one. The honest test is the first 10 seconds after returning from a few days away.
- Given the right conditions, mould can begin growing in around 48 hours. A water event dried within hours often develops nothing; one left damp for 3 days already has.
- Colour identifies nothing. The same species appears black, green, grey or pink on different surfaces in the same room, some are effectively transparent, and identification requires a laboratory.
- Building signals beat sightings: condensation returning to the same window each morning, blistering paint in a patch, warped skirting, cupped floorboards, rust on fixings, a door sticking in one room only.
- Relative humidity regularly above 65 percent is the finding worth acting on, and most of that moisture comes from showering, cooking, drying laundry indoors and breathing, not from outside.
The mould people find is rarely the mould that matters. A dark patch on a bathroom ceiling is visible, reachable, on a non-porous surface, and largely a housekeeping-scale job. The problems that quietly damage a building and its contents are usually behind, under or inside something, and they announce themselves in ways that have nothing to do with seeing a spot.
The visible patch is the easy version
Surface growth on paint, tiles, glass or a window reveal can genuinely be removed with a cloth, a plain detergent and real physical effort. No spray does the work for you. If that is all a home has, and the moisture driving it gets addressed, it is a manageable situation.
The harder cases sit inside wall cavities, under floor coverings, behind cabinetry and in subfloors. Before hunting for spots, it helps to understand why so many Brisbane homes stay damp in the first place, because the conditions come first and the growth follows.
Smell is a weak signal in both directions
Most significant mould problems produce no odour at all. A house can smell perfectly neutral and still have active growth inside a wall cavity, so the sentence "I can't smell anything" carries almost no weight as evidence.
Working in the other direction, a musty smell with nothing visible is worth taking seriously rather than dismissing. Something is producing it.
There is also the nose blindness problem. Olfactory adaptation happens within days, so anyone living in a home stops registering its background smell entirely. This is why the most honest test most people have costs nothing: go away for 4 or 5 days, then pay attention to the first 10 seconds after you open the front door, before you put the bags down. That short window is the only time your nose is telling you the truth. Failing that, ask a visitor who has never been to the house, and ask them to be rude about it.
Condensation is data, not just a nuisance
Windows fogging up most mornings means the indoor air is carrying more moisture than the coldest surfaces in the room can hold. Wiping it off is worth doing, because standing water on a sill soaks into timber and paint. It just does not address why the moisture is there.
Almost all of it is generated inside: showering, cooking, boiling a kettle, drying laundry on a rack in the lounge room, unflued gas heating, and simply breathing overnight in a closed bedroom. Outdoor humidity adds to the load in a Brisbane summer, but the indoor sources are the ones you can actually change.
A healthy home generally sits somewhere around 40 to 65 percent relative humidity. An inexpensive hygrometer in 2 or 3 rooms will tell you more about your risk than any amount of squinting at ceilings. If the reading in the main bedroom is 75 percent every morning, that is the finding.
What the building itself is telling you
Materials move when they get wet, and they stay moved. These are the signals worth walking around and looking for:
- Paint bubbling, blistering or flaking in a patch rather than evenly across a wall
- Plaster that feels slightly soft, chalky or powdery when pressed
- Skirting boards that have warped, lifted at a join, or pulled away from the wall
- Floorboards cupping (edges higher than the centre) or a springy spot in vinyl or laminate
- A tide mark or faint horizontal line low on a wall, especially in a room on a slab
- A door that has started sticking in one room only, while the rest of the house is fine
- Rust on nail heads, screws, hinges or fixings, particularly in wet areas and subfloors
- Grout or silicone discolouring along a line rather than in a patch
- Wallpaper lifting at an edge or seam
None of these prove mould. All of them prove moisture, and moisture is the part that decides whether mould happens.
The room that always feels damp
Most homes have one. It is cooler than the rest, the air feels heavier, and clothes stored in there smell faintly different from clothes stored anywhere else. Leather shoes, camera bags and handbags in that room develop a bloom the others do not.
Usually it is the room with the least air movement, on the shaded side, against a retaining wall, under a deck, or over a poorly ventilated subfloor. Occupants tend to explain it away as "that room's just always been like that". Being long-standing does not make it normal. It makes it a longer exposure.
Old water damage counts, even years later
Given suitable conditions, mould can begin growing in around 48 hours. That timeframe is why the response to a water event matters more than anything applied afterwards: dried within hours, and often nothing develops at all; left damp for 3 days, and something has already started even if the surface looks fine.
So the relevant history is anything that ever put water where it should not be. A burst flexi hose under a vanity. A washing machine that overflowed while nobody was home. A roof leak that was properly repaired but where the cavity was never dried out. Storm water that came under a door and was mopped up, with the carpet underlay left in place. Repaired does not mean dried.
It is also worth saying plainly that dead and dormant mould still matters. Whether growth is currently alive is not the measure of whether it is a problem, and it is not a reason to leave old material sitting in a wall.
The still places nobody looks
Mould needs moisture, a food source and time. Dust is food. Clutter and overstuffed storage block airflow, keep surfaces still and cool, and hide the early signs for months. Cleaning does not solve a moisture problem, but it removes the food and keeps the evidence visible, which is not a small thing. Walking through a home, this is roughly the route worth taking:
- Behind and underneath the fridge, including the drip tray at the back
- The back corner of the under-sink cabinet, and the underside of the cabinet base where a slow leak collects
- Behind the toilet cistern, and the wall surface it sits against
- The silicone line where the bath or shower base meets the wall, and the ceiling directly above the shower
- The rubber door seal and the detergent drawer of a front loading washing machine
- Inside the linen cupboard, particularly on the wall it shares with a bathroom
- The back of the wardrobe where it meets an external wall, and shoes or leather sitting at floor level
- Under beds and behind bedheads pushed hard against a wall, which are also the corners most standard cleans skip
- Window reveals, and the bottom track of aluminium sliding windows and doors
- Air conditioner filters, blades and drip trays, plus the wall below a wall-mounted unit
- Behind large mirrors, artworks and headboards mounted on external walls
- The subfloor, and the ground where downpipes discharge next to the house
- Inside the dehumidifier itself, which needs regular cleaning and maintenance or it becomes a moisture and microbial source in its own right
New homes are not exempt
The idea that mould is an old-house problem is comfortable and wrong. Older homes have their failure modes, and a raised timber house has a specific set worth knowing about if you live in one, covered separately in what to watch for in a Queenslander.
New builds have different ones. They are sealed tighter and insulated better, so the same shower steam and laundry moisture has fewer routes out. Exhaust fans sometimes vent into the roof space rather than outside, which relocates the problem instead of solving it. And in steel-framed construction, condensation can form inside a wall cavity where there is nothing to see from either side until the situation is well advanced. A 4 year old house is not automatically a dry house.
Colour tells you very little
"Black mould" is a colour, not a species. The same organism can present as black, green, grey or pink depending on the surface it is growing on, and 2 patches in the same room can look completely different while being identical. Some species are effectively transparent.
Identifying what is actually growing requires a laboratory, not a photograph and not a colour chart.
Which is mostly liberating. You can stop calibrating how alarmed to be against the shade of a stain, and get on with the useful question: where is the water coming from.
What to do with what you find
The goal is physical removal, not killing. This is the point most product marketing gets backwards. On a non-porous surface such as tile, glass, painted plasterboard or laminate, a cloth, a plain detergent and sustained effort genuinely remove growth. On porous materials the growth extends below the surface, and the right approach depends on the material and the extent. Sometimes the material has to come out.
Bleach is a poor choice on porous surfaces. It removes the colour while leaving the growth in place, so the area looks resolved and is not, and it is mostly water, which adds moisture to a situation defined by moisture.
A small patch in a bathroom is a reasonable thing to handle yourself. Anything large, anything inside a wall or ceiling, and anything following a flood is specialist remediation work, and disturbing it badly makes things worse. We are not remediators, and we will say so.
A formal mould and moisture assessment is its own discipline, carried out by trained assessors. That work involves moisture readings taken across building materials, thermal imaging to find temperature differences behind surfaces, and air or surface samples sent for laboratory analysis, comparing the indoor fungal profile against outdoor air. In a healthy home those 2 profiles broadly resemble each other. Belinda is currently studying in that field (unit NAT11429003) and does not carry out assessment, testing, sampling or moisture readings, so where a home clearly needs that level of investigation, the job is to say so plainly and point you towards an assessor rather than guess.
What we do is clean surface mould where it is safe to do so as part of a clean, and during a Home Health Review walk the whole home and flag where moisture and mould risk is developing. The wardrobe on the shaded wall. The bathroom fan that is not actually moving air. The subfloor with no cross-ventilation. The room reading 72 percent every morning. Much of what turns up is addressable with ventilation, faster drying and a few changed habits, and we would rather tell you that than sell you something.
Scope note: this article is about buildings, not people. Nothing here is medical advice or a diagnosis. Damp, mould-affected buildings are associated with higher rates of respiratory symptoms among occupants, a position held by the World Health Organization and reflected in Queensland Health guidance, but that is a population-level association and says nothing about any individual's health. If anyone in your household has health concerns, please see your GP or another appropriate health practitioner.
Common questions
Can you have mould in your house without smelling it?
Yes, and this is the normal situation rather than the exception. Most significant mould problems produce no detectable odour at all, particularly when growth is inside a wall cavity, under flooring or in a subfloor. Nose blindness compounds it: olfactory adaptation sets in within days, so anyone living in a home stops registering its background smell. The absence of a musty smell is not evidence that a home is dry.
What are the first signs of mould in a house?
Usually building signals rather than visible growth. Condensation that returns to the same windows most mornings, paint blistering or bubbling in a patch, skirting boards that have warped or lifted, a door that sticks in one room only, floorboards cupping, rust appearing on fixings, or one room that always feels cooler and damper than the rest. All of these indicate moisture, and moisture is what determines whether mould develops.
How quickly can mould grow after water damage?
Given the right conditions, growth can begin in around 48 hours. That is why the speed of the response matters far more than what gets applied afterwards. Drying a water event within hours rather than days is the single most useful action available. A leak repaired years ago still counts if the affected materials were never properly dried at the time.
Does black mould mean it is dangerous?
Colour tells you almost nothing. Black mould is a colour, not a species. The same organism can appear black, green, grey or pink depending on the surface it is growing on, 2 patches in the same room can look entirely different while being identical, and some species are effectively transparent. Identifying what is growing requires laboratory analysis, not a photograph. The more useful question is where the water is coming from.
Should I use bleach to kill mould?
Killing mould is the wrong goal, because dead and dormant mould still matters. Physical removal is the goal. Bleach is a particularly poor choice on porous surfaces: it removes the colour while leaving the growth in place, so the area looks resolved when it is not, and it is mostly water, which adds moisture to a problem caused by moisture. On non-porous surfaces a cloth, a plain detergent and genuine physical effort remove growth properly.
Do new houses get mould?
Yes. New builds are sealed more tightly and insulated better, so the moisture generated indoors by showering, cooking and drying laundry has fewer routes out. Exhaust fans that vent into the roof space instead of outside are common. In steel-framed construction, condensation can form inside a wall cavity with nothing visible from either side until the situation is advanced. The age of a house is not a reliable indicator either way.