Every winter we get called to the same job. A ceiling corner in a bathroom, the top of a window reveal in a bedroom, or the wall behind a wardrobe on the south side of the house. Black speckling that was painted over last year and has come straight back.
Canberra produces this problem more reliably than most Australian cities, and painting over it almost never fixes it. Here is what is actually happening and what has to change before a repaint will hold.
Why Canberra homes get it worse
Mould needs moisture, and our climate manufactures moisture indoors in a very specific way.
Canberra winters are genuinely cold. Overnight minimums sit below zero for weeks at a time, and that chills the inside face of external walls, ceilings and window frames. Meanwhile the household is generating warm, wet air: showers, cooking, drying washing indoors because it is too cold to hang it out, gas heaters and unflued heating, and simply breathing in a closed-up house.
When warm moist air touches a cold surface, it condenses. That is dew forming on your wall. Do it every night for three months and you have a permanently damp surface, which is exactly what mould needs.
The pattern is predictable. South-facing rooms get it first because they see the least winter sun. Corners and ceiling junctions get it because they are thermal bridges where the structure conducts cold through. Behind furniture pushed against an external wall is a classic, because there is no air movement to carry the moisture away.
Painting over mould is a holding pattern, not a repair
If you roll paint straight over mould, three things happen. The spores are still there under the film. The moisture that fed them is still arriving every night. And the new paint gives them a fresh food source, because most of the binders and additives in a modern coating are organic.
It usually looks fine for a few months, then reappears through the new paint over the following winter. Homeowners often assume the paint was faulty. It was not. The paint was asked to solve a building physics problem, which is not a job paint can do.
Anti-mould or mould-resistant paints are useful and we use them, but they buy time and reduce severity. They are not a substitute for removing the moisture.
What actually has to happen first
Kill it, do not just clean it. Wiping mould with a cloth spreads spores and leaves the root structure in the surface. The affected area needs treating with an appropriate fungicidal wash, left to dwell, then removed and allowed to dry fully. Bleach is a common home remedy and it is a poor one on porous surfaces: it lightens the colour so the mould looks gone while the growth remains below the surface.
Find the water. This is the part people skip. If the moisture is condensation, ventilation and heating habits are the fix. If it is a leak, a failed shower seal, a blocked gutter overflowing behind a fascia, or rising damp, no amount of painting or ventilation will help until the leak is repaired. A mould patch that is sharply defined and appears in the same spot regardless of season is usually a leak. Diffuse speckling that worsens in winter and improves in summer is usually condensation.
Let it dry properly. Substrates hold moisture long after the surface feels dry. Coating a damp wall traps water behind the film and causes blistering and adhesion failure later.
Then seal and coat. A stain-blocking sealer prevents residual staining from bleeding through, followed by a quality topcoat, using a mould-resistant product in wet areas.
The ventilation changes that make the difference
Most Canberra condensation problems improve dramatically with habits rather than building work.
Run the bathroom exhaust fan during the shower and for twenty minutes afterwards, not just while you are in there. Check that the fan actually vents outside rather than into the roof space, because a surprising number do not, and one that discharges into the ceiling cavity simply moves your mould problem somewhere you cannot see it.
Use the rangehood every time you cook. Crack a window for ten minutes a day even in winter, ideally on opposite sides of the house to get cross-flow. Pull furniture five centimetres off external walls so air can move behind it. Dry washing outside or in a vented dryer rather than on an airer in the lounge room, because a single load of wet washing releases a couple of litres of water into your house.
If the house has unflued gas heating, be aware that it produces water vapour as a combustion product. Every hour of running adds moisture to the air.
When it is worth getting someone in
Small, seasonal condensation mould in a bathroom is a job most people can manage themselves with a fungicidal wash and better ventilation.
Get help when the affected area is large, when it keeps returning in the same place after treatment, when it appears on ceilings away from wet areas, or when there is any staining that suggests water is coming from above or behind. Those are building problems wearing a paint costume, and the repaint is the last step rather than the first.
For anyone in the household with asthma or respiratory sensitivity, it is also worth acting sooner rather than living with it through a Canberra winter.
What we do on these jobs
We treat the growth, identify whether we are looking at condensation or a leak, and tell you honestly if the answer is a builder or a plumber rather than a painter. If it is condensation, we prepare properly, seal the stains and use mould-resistant coatings in the areas that need them, and we tell you what has to change in the room for it to stay gone.
What we will not do is roll two coats over an active mould problem and take your money knowing it will be back next winter.
Common questions
Can I just paint over mould on my ceiling?
You can, but it will almost always come back. The spores remain under the paint film, the moisture that caused the growth is still arriving, and the fresh coating gives the mould organic material to feed on. The growth has to be killed and the moisture source addressed before painting.
Does mould-resistant paint actually work?
It helps and we use it in bathrooms, laundries and other wet areas. But it slows regrowth rather than preventing it. If condensation keeps wetting the surface every winter night, a mould-resistant coating will eventually be overwhelmed.
Why does mould keep coming back in the same corner?
Corners and ceiling junctions are thermal bridges where the building structure conducts cold from outside, so they run colder than the surrounding surface and condensation forms there first. South-facing rooms and areas behind furniture are affected for the same reason.
Is bleach a good way to remove mould before painting?
Not on porous surfaces like plasterboard or render. Bleach lightens the pigment so the mould appears to have gone while the growth remains below the surface. A proper fungicidal wash that is left to dwell is more effective.
How do I know whether it is condensation or a leak?
Condensation mould is usually diffuse speckling that worsens through winter and improves in summer, often on south-facing walls, corners and window reveals. A leak tends to produce a sharply defined patch in a consistent spot regardless of season, often with staining.
Read next: Why a ceiling patch never matches →
Premier Painting & Maintenance ACT is a veteran-owned painting business servicing Canberra, Queanbeyan, Jerrabomberra, Googong, Bungendore and surrounds. Rated 5.0 on Google. Free quotes.
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