Around mid-September the heating in most Melbourne houses goes off for the year, the windows start opening, and the winter mould gets found. Behind the wardrobe. Along the bottom of the bedroom curtain. In the corner of the ceiling above the shower. On the underside of the windowsill in the room that never gets sun. And the question that follows is nearly always the same: now that the weather is warming up, will it just go away?
Quick answer: No — mould that grew over winter does not die when the weather warms. It dries out, stops spreading and goes dormant, and it comes straight back the next time the surface is damp for long enough. Spring is when most of it gets found, because houses are opened up and cleaned, but the warmer weather has not removed anything.
It will not. Mould that grew over winter does not die when the house warms up. It dries, stops spreading and goes dormant, and it waits. Understanding what is actually happening on that wall makes the difference between dealing with it once and meeting it again every September.
What mould does when the surface dries out
Mould needs a surface that stays damp for long enough, food (which almost any building material or the dust on it provides), and a temperature it can grow at. Through a Melbourne winter the damp is the easy part: cold external walls, closed windows, showers, cooking and breathing all put moisture into the air, and it condenses on whichever surface is coldest. Growth spreads while those conditions hold.
When the surface dries, growth stops. The patch may go powdery, flatten out or fade a little in colour, and that is where the idea that it has gone comes from. But the mould is still there, in a dormant state, and the spores it produced are still on and around it. Dry mould is if anything easier to disturb than damp mould, because nothing is holding the spores to the surface. Vacuuming it with a household vacuum or brushing it down spreads it through the room.
The next time that surface is damp for long enough, the same patch restarts. It does not need to be reintroduced from anywhere. This is why a house can have a patch in the same corner year after year without anyone ever seeing it arrive.
Why spring is when it gets found, not when it goes
The timing is about the house, not the mould. Through winter the curtains are closed for months, the wardrobe is not moved, the spare room is shut, and everyone is inside with the lights on. Once the days lengthen and the heating goes off, rooms get aired, furniture gets shifted, and the light comes in at a different angle. Growth that was there in July becomes visible in September.
The other thing spring does is expose the smell. A closed, heated house has a uniform stale background that people stop noticing. Open it up, let cooler outside air through, and the musty note in one particular room stands out against the fresh air in the rest of the house. A musty smell with no visible growth is a strong sign there is mould somewhere that cannot be seen — inside a wardrobe, behind a bedhead, under carpet on a cold floor, or in a wall cavity.
The cold-wall pattern, and why furniture makes it worse
Almost all winter mould in a Melbourne home follows one rule: it grows on the surface that is coldest and stillest. External walls lose heat to the outside all night. Moisture from the room reaches those walls and condenses on them, exactly as it does on a cold window. Wherever the air cannot move, that condensation stays.
Furniture against an external wall creates the perfect version of this. The strip of wall behind a wardrobe or a bedhead cannot be warmed by the room, cannot be reached by any air movement, and cannot dry. The same wall a metre along, out in the open, is a few degrees warmer and drying constantly. The result is that the mould grows in the boxed-in section and nowhere else, which is why it so often surprises people when furniture is moved.
The same logic explains the other usual sites: the bottom of curtains and blinds against a cold window, the corner where two external walls meet, the ceiling above a shower with no working exhaust, and the inside of a built-in wardrobe on an external wall. Each is a cold surface with no airflow.
Why wiping it and moving on does not end it
Wiping is a reasonable response on a hard, non-porous surface. Tile, glass, gloss paint and laminate hold the growth on the surface, and a proper wipe with the cloth bagged afterwards removes it. The problem is that most of the surfaces mould grows on in a bedroom are porous. Plasterboard, matt paint, bare timber, the back of a wardrobe and carpet all let the growth get into the material rather than sit on top of it. Wiping those smears the visible part around and leaves the rest.
Painting over it is worse. A coat of paint seals living growth under a fresh surface, and because the wall is still cold and still damp in winter, it comes through the new paint. Anti-mould paints slow this down but do not change the conditions underneath.
The other thing wiping cannot do is address the reason the surface was damp. If a bathroom has no working exhaust, a bedroom wall has no insulation, a window leaks, or a gutter has been soaking the wall from outside, the surface will be damp again by next winter and the growth restarts. This is the reason mould keeps coming back to the same spot in so many houses.
What to actually do with it now
Spring is a good time to deal with winter mould properly, because the surfaces will dry afterwards and stay dry for a while, which gives any treatment a chance to hold.
- Find all of it first. Move furniture off external walls, lift the bottom of curtains, open built-in wardrobes and look at the back and the floor, and check ceiling corners in wet rooms. Follow any musty smell to its source rather than airing the room and hoping.
- Do not dry-brush or vacuum it. Dormant mould sheds spores easily. If it must be touched, damp it first, work with the window open and bag the cloth.
- Clean hard surfaces; treat or remove porous ones. Tile and gloss paint can be cleaned. Plasterboard, timber and carpet with growth in them need proper treatment, and where the growth is well established, removal and replacement of the affected section.
- Fix what made the surface wet. A working exhaust fan in every wet room, furniture pulled off external walls, windows opened daily, and a look at whether the roof, gutters or a leak have been wetting the wall from outside.
- Look at the cold walls themselves. A wall that is cold enough to condense on every winter night will grow mould every winter. Insulation in that wall or ceiling changes the surface temperature, which is the only thing that changes the pattern permanently.
When it is a remediation job rather than a clean-up
A single patch on a tiled shower wall is a cleaning job. Growth on plasterboard, growth over more than a small area, growth that returns to the same spot after cleaning, a musty smell without a visible source, or anyone in the house with symptoms that ease when they are away — those are signs the growth is in the material or being fed by something nobody has found.
That is the point at which mould remediation is a different job from cleaning. It starts with finding the moisture source, removes the affected material safely without spreading spores through the house, treats the surrounding area, and leaves the room in a state where the growth does not simply restart in June. Doing that in spring, while the house is dry and open, is the version of this that does not repeat next year.
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Frequently asked questions
If the mould has dried out and gone flat, is it still a problem?
Yes. Dry mould is dormant, not dead, and a dry patch sheds spores more readily than a damp one because there is nothing holding them to the surface. Brushing or vacuuming it with an ordinary vacuum spreads it into the room air. The growth also restarts from the same patch the next time the surface stays damp, which in a Melbourne house is next winter at the latest and often sooner in a bathroom or a bedroom on a cold wall.
Why is it always behind the wardrobe and not on the open wall?
Because that section of wall is both the coldest surface in the room and the one with the least air movement. An external wall loses heat to the outside all night; the part of it that is boxed in behind furniture cannot be warmed by the room and cannot dry, so moisture from the room condenses there first and stays. The open wall next to it is the same wall, but the room air keeps it slightly warmer and drier. Pulling furniture a hand-width off external walls is the single most useful thing a household can do about it.
Can I just wipe it off and paint over it now the weather is better?
Wiping removes the visible growth from a hard surface and is a reasonable first step on tile, glass or gloss paint, provided the cloth is bagged afterwards rather than rinsed and reused. On plasterboard, bare timber or matt paint the growth is in the surface rather than on it, and wiping smears it rather than removing it. Painting over it seals living growth under a fresh coat and it comes through again, usually within a season. What ends it is removing the affected material or treating it properly and, critically, dealing with why the surface was damp.
When does winter mould become a job for a remediation company rather than a cleaning job?
When it is on a porous surface such as plasterboard or timber, when it covers more than a small area, when it keeps returning to the same spot after cleaning, when there is a musty smell with no visible growth, or when anyone in the house has respiratory symptoms that ease when they are away. Any of those points at growth that is either inside the material or being fed by a moisture source nobody has found yet. A remediation visit finds the source, removes the affected material safely and treats the surrounding area, which is a different job from cleaning what can be seen.
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