Most mould that appears in Australian homes over winter is not a leak. It is condensation — moisture generated inside the house that has nowhere to go. Spring is when it becomes visible, and it is also the best time of year to fix the cause rather than the symptom.
The pattern is familiar: black spotting in the corners of bedrooms, along the top of skirtings on south-facing walls, behind wardrobes and furniture, on window reveals, and inside cupboards on external walls. Wiping it off works for a fortnight. It returns because the conditions that produced it have not changed.
Why it happens
A household generates a surprising amount of water vapour — showering, cooking, drying clothes indoors, and simply breathing. In winter that vapour meets cold surfaces, and where a surface is below the dew point, the moisture condenses out of the air onto it.
Cold surfaces are predictable: single-glazed windows and their frames, uninsulated external walls, the wall behind a wardrobe where air cannot circulate, and the corners of rooms where two external walls meet. That is precisely where the mould appears.
Three things have made this more common. Homes are better sealed than they were, which is good for energy use but reduces incidental ventilation. Clothes dryers and indoor drying racks are used more. And heating patterns have changed — heating one room while others stay cold creates strong temperature differences within the house.
How to tell condensation from a leak
This distinction decides everything that follows.
Condensation appears on surfaces rather than within them, is worst in winter and in unventilated spots, is often symmetrical or widespread, and typically shows on cold external walls, windows and inside cupboards.
A leak is usually localised, has a defined edge or tide mark, may worsen after rain regardless of season, and often stains rather than spots. Damaged plaster or swollen skirting suggests liquid water rather than vapour.
Rising damp shows as a horizontal tide mark at the base of a wall, sometimes with salt deposits, and does not follow the seasons in the same way.
If it is a leak or rising damp, ventilation will not fix it and you need the source addressed.
Fixing the cause
Condensation is a balance between moisture produced, moisture removed, and surface temperature. Change any of the three and it improves.
Remove moisture at the source. Exhaust fans in bathrooms and kitchens should vent outside — not into the roof cavity, which simply relocates the problem into your roof space. This is a very common fault in Australian homes and worth checking directly. Run the fan during and for a period after showering.
Stop drying clothes indoors, or vent the dryer outside. A single load of washing releases litres of water into the house.
Ventilate deliberately. Opening windows on opposite sides of the house for a short period each day moves far more air than leaving one window ajar all day, and costs less heat.
Let air reach cold surfaces. Pull furniture a little off external walls, and leave wardrobe doors open occasionally.
Raise the surface temperature. Insulation and draught sealing reduce cold spots. Heating the whole house gently is more effective against condensation than heating one room hard.
Dealing with what is already there
Clean it while the weather is warm enough to dry surfaces afterwards. Ventilate while cleaning, wear gloves and eye protection, and avoid dry brushing, which sends spores into the air.
Do not paint over mould, and treat anti-mould paint as a supplement rather than a solution — it does nothing about the moisture. If mould covers a large area, keeps returning after the causes are addressed, or is inside wall cavities or carpet underlay, get professional advice. Extensive mould can affect health, particularly for people with asthma or respiratory conditions.
Why spring is the moment
Warmer, drier air makes surfaces dry properly after cleaning, so the job holds. It is also the sensible window for the structural fixes — checking that exhaust fans vent outside, adding subfloor or roof ventilation, sealing draughts, topping up insulation — because none of it is urgent in October and all of it is unpleasant to organise in July.
Do it now and next winter is materially different.
Where to go next
Ventilation and moisture control also protect against rot and termite activity. Read next: Preventative Maintenance.
August 15, 2026