Mould on the inside of external walls, in corners, behind wardrobes and around window frames is usually condensation on a surface that runs colder than the rest of the room. In double-brick homes the cold surface can be inside the wall cavity itself; in framed and steel-framed buildings it is often a thermal bridge, where framing, a slab edge or an aluminium frame carries cold straight through the insulation. The lasting fix is to reduce indoor moisture and warm that surface, not to keep cleaning it.
How cold surfaces turn into mould
Air holds less water as it cools. When indoor air touches a surface below its dew point, water condenses there, and repeated wetting is all mould needs. The Australian Government’s YourHome condensation guide explains this and notes condensation can also form inside walls, ceilings and floors (interstitial condensation), leading to mould, mildew and decay. Showering, cooking, drying clothes and simply breathing all add moisture to the air, so the amount of condensation depends as much on how the house is used as on how it is built.
Double-brick cavities
A double-brick wall has an outer leaf, an air gap and an inner leaf that carries the internal plaster. The cavity was meant to stop rain crossing from outside to inside, and it does that well. But in cool weather the outer leaf is cold, and warm moist air that leaks into the cavity through cracks, cornice gaps and service penetrations can condense on it. Moisture can then collect on the metal wall ties that bridge the cavity, pool at the base of the wall above the damp-proof course, and track inward where ties, lintels or mortar droppings bridge the gap.
Weep holes, the open vertical joints in the bottom course of the outer brickwork, let water drain out of the cavity. When they have been mortared, rendered or buried under garden beds, water sits at the base of the wall.
Signs inside: damp patches and mould low on external walls and in corners, paint bubbling on a wall that has not been wet from outside, and a musty smell in rooms with external walls during cold months. Signs outside: white salts on the brick face, dark staining on the lower courses, and a bottom course with no open weep joints.
Thermal bridges
A thermal bridge is a path through floors, walls, windows or roof where heat and cold bypass the insulation, and YourHome notes these can lower internal surface temperatures enough to reach dew point. Because the bridge follows the structure, so does the mould, which makes the pattern a useful clue:
| Mould pattern | Likely thermal bridge |
|---|---|
| Evenly spaced vertical stripes on a wall | Studs or steel framing behind the plasterboard |
| A band at skirting level on external walls | Slab edge losing heat to outside |
| Corners, floor to ceiling | Clustered framing in the corner, plus poor airflow |
| Around every window | Aluminium frames without a thermal break |
| A line at the top of the wall | Top plate, often with gaps in ceiling insulation |
| Patches around columns or beams | Concrete or steel elements in apartments |
Older brick veneer homes with little or no insulation, single-skin brick homes, steel-framed apartments and slab homes without edge insulation are the usual candidates. Escarpment-side suburbs that run cooler than the coastal strip tend to show it more.
Getting the diagnosis right
Cold-wall mould is often confused with rising damp, which rises from the ground and needs a completely different repair; rising damp vs condensation covers how to tell them apart. A mould inspection and testing visit should include:
- Moisture meter readings at several heights on the affected walls
- Thermal imaging, ideally on a cold morning, which shows bridges and cold zones clearly
- Temperature and humidity readings to judge how close surfaces sit to dew point
- An external check of weep holes, brick faces, render and ground levels
- A borescope through a small hole into the cavity only if the non-invasive results are unclear
Fixes that work and fixes that do not
| Approach | Verdict |
|---|---|
| Bleach or anti-mould paint on its own | Cosmetic. The surface stays cold and damp, so mould returns |
| Lowering indoor humidity: wet-area fans ducted outside, no indoor clothes drying, heating | The most important step for both cavity condensation and thermal bridges |
| Clearing and reopening blocked weep holes | Lets the cavity drain; helps where water pools at the base |
| Moving furniture off cold external walls | Simple and effective for wardrobe and bedhead mould |
| Insulated internal lining (insulated plasterboard, or battens with insulation and new board) | Breaks the bridge on the worst walls; penetrations must be sealed to keep the layer continuous |
| Double glazing in thermally broken or uPVC frames | Removes frame and glass condensation; YourHome lists these as ways to keep surfaces above dew point |
| Slab edge insulation | Practical mainly when the outside is being rendered, re-clad or landscaped anyway |
| External waterproof coating on brickwork | Only relevant if rain penetration, not condensation, is the cause |
The habit side and equipment choices are covered in ventilation upgrades for mould prevention, and the wardrobe pattern specifically in mould behind wardrobes and furniture.
If an inspection finds badly corroded wall ties or a failed damp-proof course, that is structural work for a builder experienced in masonry, not a mould job. Once the cause is dealt with, existing growth on plaster and in cavities is handled through mould remediation; see hidden mould behind walls for what happens when growth is inside the wall.
Frequently asked questions
Is cavity wall condensation the same as rising damp?
No. Rising damp is ground moisture wicking up through the base of a wall, while cavity condensation comes from moist indoor air condensing on cold surfaces inside the wall. Both mark internal plaster, which is why moisture readings at different heights matter.
I have mould in regular vertical stripes on my wall. Is that thermal bridging?
Stripes spaced like wall studs are a classic sign that the framing is running colder than the insulated bays between it. A thermal imaging scan in cold weather will show the cold studs, and insulated internal lining is the usual fix.
Why does my corner keep growing mould even though I ventilate?
External corners combine more framing (so more cold) with less air movement from the room. Ventilation helps, but if the surface stays below dew point the mould returns; warming the corner with insulation and keeping furniture clear of it addresses the cause.
Does anti-mould paint fix thermal bridging?
It slows regrowth for a while but does not warm the surface, so condensation continues. Treat it as the finish after the cause is addressed, not the fix.
How do I know if my weep holes are blocked?
Look at the bottom course of external brickwork. You should see regularly spaced open vertical joints; if the whole course is solid mortar, render or buried under soil, the cavity cannot drain.