Why the mould comes back three weeks after you clean it

You cleaned the wall. It looked perfect. Three weeks later it's back in exactly the same spot.

This isn't a sign you did a bad job. It's a sign the job was never a cleaning job.

Mould is a moisture problem wearing a cleaning problem's clothes

The single thing that separates a remediation tech from someone with a spray bottle is this: you don't treat mould, you treat the reason it's there.

Bacteria and fungi feed on the organic material buildings are made of, and they do it under moist conditions. Take away the moisture and growth stops. Leave the moisture and it resumes, regardless of what you sprayed on it.

Routine cleaning does control microbial dust in a dry environment, which is why normal housekeeping keeps most homes fine. But once water gets in, microbial amplification happens fast, and cleaning the surface stops addressing the actual driver.

The numbers that matter

This is where it gets useful, because there are specific thresholds rather than vague advice about "damp."

66 to 70% relative humidity is where xerophilic moulds thrive — the dry-loving group that includes Penicillium and Aspergillus. These are the common indoor ones, and they can release allergens and toxins. You don't need a flood for these. You need a room that sits humid.

70% relative humidity and above promotes fungal growth generally, and raises the risk of odour and secondary damage.

20% moisture content in timber is the point where wood becomes vulnerable to wood rot fungi.

Notice how low those numbers are. A bedroom in a Melbourne winter with the window shut and a clothes horse in the corner will sit above 70% quite comfortably. Nothing has leaked. There's no flood. The conditions are simply right.

Where the moisture is actually coming from

In Australian homes it's usually one of a short list.

Condensation. Warm moist indoor air meeting a cold surface. Single-glazed windows, uninsulated external walls, south-facing rooms that never see sun. Drying washing indoors makes it worse, and so does an unflued gas heater.

Poor ventilation. Bathrooms with an extraction fan that vents into the roof cavity rather than outside, or none at all. Wardrobes and cupboards against external walls with no air movement.

Leaks. Roof, gutter, plumbing, failed shower waterproofing. Often small and slow, which is worse than a burst pipe because nobody notices for months.

Damp subfloors and rising damp. Especially in older housing stock.

Trapped humidity after wet weather. A run of humid days with a closed-up house.

Different causes, same result. And each one needs a different fix, which is why "clean it and see" is guesswork.

Why bleach isn't the answer

Two reasons, and neither is the one people usually give.

The first is that it doesn't address the moisture, so you're back to the start.

The second is more technical. Biocides are ineffective in the presence of organic matter. Spraying a product onto a surface that's still got a film of grime and biological material on it wastes the product — the pre-clean has to happen first or the chemical is doing very little.

And not every situation warrants a chemical at all. That's a judgement call, not a default.

There's a third thing worth knowing: treatment kills active growth but doesn't always remove the stain. Mould that's penetrated paint, plaster, grout or silicone can leave marks permanently. Repainting, regrouting or replacing silicone is sometimes the only way the surface looks right again, and that's after the mould problem is solved, not instead of solving it.

What actually stops it returning

Find the moisture source and deal with it. Genuinely find it — not "the bathroom's a bit damp" but which specific mechanism is putting water into that wall.

Get the humidity down and keep it down. Ventilation, extraction that vents outside, heating patterns, drying washing somewhere else.

Fix the physical faults. A leaking gutter or failed waterproofing won't be resolved by any amount of treatment.

Then deal with the mould, in that order.

When it's beyond a clean

Some situations aren't a surface problem.

If material is porous and can't be effectively cleaned, it comes out rather than gets treated. If mould keeps returning to the same spot after the moisture has genuinely been addressed, there's usually something happening inside the wall or under the floor. If there's a lot of it, disturbing it puts spores into the air and spreads the problem — which is why proper remediation uses containment, negative pressure and HEPA filtration rather than a sponge.

That's a different job with different training behind it, and knowing where the line is matters more than being able to do everything.

The short version

Mould on a wall is information. It's telling you where water is getting in or where air isn't moving.

Clean it and you've removed the message. Fix the moisture and you've fixed the problem.