When mould comes back after treatment, the usual reason is that the moisture feeding it was never fixed, not that the product or technique failed. As the US EPA’s guide to mould and moisture puts it, the key to mould control is moisture control. The other common causes are growth left inside porous materials, an untreated area nearby, no containment during the work, painting before things dried, and seasonal condensation returning.
Working out which applies is the starting point for a redo that holds.
1. The moisture source was never fixed
Mould is a symptom. Cleaning it while the cause is still active is mopping with the tap running. The sources most often cleaned around and left in place:
- a slow leak at a shower base, waste connection or tap that never pools visibly
- winter condensation on cold, poorly ventilated walls, with nothing about the room changed
- blocked, undersized or missing subfloor vents and poor drainage under the house
- a leaking balcony flashing, window seal or roof junction feeding a wall cavity
A proper job starts with a moisture investigation: meter readings, the likely water pathway, and what needs repairing before or alongside the mould work. If your treatment skipped that step, recurrence is no surprise. An independent mould inspection can establish why the first attempt did not hold.
2. The growth was not fully removed
Wiping or spraying the surface of porous material often leaves growth inside it. The EPA notes that absorbent materials may have to be thrown away if they become mouldy, because mould can grow into them and be hard to remove completely. Typical examples:
- plasterboard sprayed with a biocide and repainted instead of being cut out
- timber framing treated on the face where growth had gone into the grain
- carpet or underlay dried and spot-treated rather than replaced
See mould removal vs remediation for where a surface clean ends and remediation begins.
3. Another mouldy area was missed
If an adjacent cavity, roof void or subfloor still has growth, it can keep recontaminating the treated room. A bathroom ceiling cleaned while the roof space above it is mouldy (often because the exhaust fan vents into it) is a classic case. Hidden mould behind walls is another. A thorough inspection maps the whole extent before work starts, not just the visible patches.
4. There was no containment
Disturbing mould releases spores. Professional remediation seals the work area and uses negative air pressure so they do not travel through the house. If a meaningful area was cleaned with no containment, the work itself may have spread the problem to new spots.
5. It was painted or sealed too soon
Paint or sealer over a damp surface traps moisture beneath it, and over surviving growth it simply hides it. The EPA’s advice is direct: do not paint or caulk mouldy surfaces. The order that works is: fix the source, remove or clean the affected material, confirm with a moisture meter that it is dry, then prime and paint. An anti-mould paint over a damp wall still fails; see our review of anti-mould paints and primers.
6. Seasonal condensation came back
Sometimes the treatment was fine but the conditions returned: winter condensation on a cold external wall, or a humid, poorly ventilated bathroom. That is a building condition to manage, not a one-off fix. The EPA’s prevention advice is to reduce humidity and increase ventilation or air movement; in practice that means a properly ducted exhaust fan, opening up rooms, moving furniture off cold walls, and warming or insulating the cold surface where practical. Our guide to ventilation upgrades for mould prevention covers the options.
How to tell which one applies
| What you are seeing | Most likely reason |
|---|---|
| Back in the same spot within weeks or months, area still feels damp | 1 or 2 |
| Same spot every winter, dry in summer | 6 |
| New spots in rooms that were not treated | 3 or 4 |
| Bubbling or spotting under fresh paint | 5 |
| Musty smell persists even where surfaces look clean | 3 |
What a proper redo looks like
- Independent inspection: moisture readings, full extent mapping, source identified.
- Fix the source first: leak, drainage, waterproofing or ventilation, by licensed trades where required.
- Check adjacent cavities, the roof void and subfloor for missed growth.
- Contained removal of affected porous materials.
- Clean and HEPA vacuum the remaining surfaces.
- Confirm dry with moisture readings before any sealing or painting.
- Verify before containment comes down; see post-remediation verification.
A redo done properly costs more than the first attempt, and it should be the last. A redo done the same way as the first gets the same result. Enquiries through this site go to an independent local remediation professional who quotes in their own name; see mould remediation for how a job is scoped, or send photos and a description through the contact page.
Frequently asked questions
Why does mould keep coming back in the same spot?
Usually because the moisture feeding it is still there: a slow leak, a cold surface that condenses every winter, or poor ventilation. Growth left inside porous material such as plasterboard is the next most common reason.
My painter said I used the wrong paint. Is that why it came back?
Rarely on its own. Anti-mould paint can help where conditions are marginal, but if the wall stays damp or the growth was not removed first, no paint will stop it returning.
Is it normal for mould to return every winter?
It is common in the Illawarra but not inevitable. Seasonal mould in the same place is usually condensation, and the fix is reducing indoor humidity, improving ventilation in that room, or warming the cold surface.
How can I tell if my original treatment was done properly?
Ask whether moisture was measured before and after, whether the source was identified and fixed, whether affected porous material was removed rather than sprayed, whether the area was contained, and whether you received a written completion or clearance report. Mostly “no” means it was a surface clean.
Can I do the redo myself?
Small areas on hard, non-porous surfaces, yes. Anything involving plasterboard removal, wall cavities, the roof void or subfloor is better handled by a professional with containment, because the risk of spreading spores is real.