Mould inside a wall cavity is fed by a moisture source you cannot see: a leaking shower or pipe, a failed window seal, rain getting in through cladding or flashings, condensation on cold masonry, or rising damp. Because the growth is out of sight, the signs are indirect: a musty smell that airing does not clear, mould that returns to the same spot, bubbling paint, or mould that suddenly spreads. Mould that is spreading fast almost always means the moisture supply has increased, so the priority is finding the water, not cleaning harder.
Signs mould is hiding in a wall
- The same patch keeps coming back. If a cleaned spot returns in the same place, it is being fed from behind the plasterboard. Why mould came back after treatment covers this pattern.
- A musty smell that ventilation does not shift. Active mould gives off musty-smelling gases that travel through plasterboard joints, gaps around fittings and floor gaps. Airflow can carry the smell to a different room from the source; musty smell in the house helps trace it.
- Bubbling or blistering paint on a wall that should be dry, especially an internal wall.
- Soft, swollen or discoloured plasterboard that gives slightly when pressed.
- Mould on the back of furniture against an external wall, showing the wall behind is cold and damp.
- Mould on a wall next to a bathroom, laundry or kitchen, often from a leaking shower, tap or pipe on the other side.
- A damp patch that appears after rain, away from any ceiling stain or window, pointing to water entering the wall from outside.
- Symptoms that are worse at home. That is a question for a GP, not a builder, but it is a reason to look for a source.
When mould spreads fast
A patch that sits unchanged for months usually means a stable, low-level problem such as a shower that steams up. Mould that visibly grows week to week, or turns up in a second room, means something is now supplying more water: a new or worsening leak, a gutter overflowing into a wall, a subfloor that has gone from damp to wet, or a fan that has failed. Where it shows tends to point at the cause:
| Where and how it is spreading | Likely source | Next step |
|---|---|---|
| Ceiling, near a bathroom or laundry | Roof or flashing leak, or an exhaust fan ducted into the roof | Check the fan outlet and roof; inspect the roof space |
| Low on walls from skirting level, floors cool or springy | Subfloor moisture or rising damp | Subfloor and moisture inspection |
| Worse after each rain, drier in between | Water getting in through roof, gutters, windows or cladding | Fix the entry point, then dry and treat |
| Suddenly after a storm or burst pipe | Water damage still drying inside walls or floors | Drying response; see mould after storms |
| Several rooms, no obvious pattern | Not yet identified | Full moisture investigation before any treatment |
What to do now:
- Photograph it today with a wide shot and a close-up, and again in a few days. The comparison is useful evidence.
- Note the pattern: after rain, one room, along a ceiling, rising from the floor.
- Cut the moisture you control: run exhaust fans, air out on dry days, stop drying clothes indoors near the area.
- Do not paint over it or seal it in. Paint over active mould fails and hides the extent from whoever inspects it.
Small, fresh mould on hard surfaces such as tiles or glass is often reasonable to clean yourself while you fix the cause; NSW Health’s mould factsheet gives household cleaning advice and notes that extensive contamination, particularly in absorbent materials, may need professional cleaning or replacement. Once mould has spread through plasterboard, timber, carpet or insulation, or into another room, cleaning the surface does not reach what is behind it.
Why cavity mould goes unnoticed
- Mould can grow on the cavity-side paper of plasterboard for a long time before staining comes through the painted face.
- Brick veneer and double-brick walls have an air gap where water can run, pool and wet the framing without showing inside.
- Water entering high, at a window head or roof junction, can travel down and sideways before it shows anywhere.
- Cold external walls condense on the inside of the cavity; see cavity wall condensation.
How an inspection finds it without opening the wall
A mould inspection and testing visit starts non-invasively:
- Moisture meters read how wet the plasterboard, timber or masonry is at and just behind the surface.
- Thermal imaging shows temperature differences; damp areas often read cooler. A cold patch can also be missing insulation or a pipe, so the camera points to where to measure rather than proving mould.
- Pattern and history: where the smell is strongest, what is on the other side of the wall, and whether it worsens after rain.
High moisture readings and a cold anomaly in the same spot, alongside a smell or recurring mould, make hidden mould likely. Confirming it, and the extent, means looking inside: a borescope through a small hole, or a small inspection opening. That is justified when readings are clearly elevated, when a leak is confirmed, when a remediation scope needs real extent rather than an estimate, or when an insurer wants the damage documented.
When the wall is opened
Mould remediation for a cavity involves fixing the moisture source first (plumber, waterproofer, roofer), containing the area, removing materials that cannot be cleaned (mouldy plasterboard and insulation, and badly affected timber), treating the remaining framing and masonry, drying, and checking the result before the wall is closed. The size of the job depends on how far the water travelled and for how long.
Frequently asked questions
Can mould behind a wall make you sick?
NSW Health says mould can cause respiratory irritation, congestion and eye and skin irritation in sensitive people, and that people with asthma, allergies, weakened immunity or chronic lung disease are most at risk. If you have symptoms, see your GP; mould smell, headaches and fatigue covers what to discuss.
Why is my mould suddenly spreading so much faster?
Almost always because more moisture is reaching it, not because the mould has changed. A new leak, an overflowing gutter, a failed fan or a wetter subfloor after rain are the usual triggers.
Does mould spread faster after rain?
It can. Rain can open up an intermittent leak through a roof, gutter, window or wall that feeds mould, which is why “worse after rain, better in between” is a strong clue to water getting in rather than condensation.
Does cavity mould always mean removing the wall?
Not always. Mould on masonry inside a cavity can sometimes be treated without removing the lining if it can be reached and the source is fixed, but mouldy plasterboard and saturated timber generally have to be removed.
My paint is bubbling but there is no smell. Should I worry?
It is worth checking. A small or recently established colony may not smell much yet, and bubbling paint means moisture is reaching the surface either way. A moisture reading behind the bubbling is a sensible first step.