The short answer
Condensation mould appears on the coldest surfaces — ceilings, corners, behind furniture, around windows — spread across a room, with dry material behind it. Leak mould is concentrated around one source, often with staining or a tide mark, and the material behind it is wet. Moisture and dew point readings tell them apart.
How condensation works
Air holds water vapour. When it meets a surface colder than its dew point, that water condenses onto the surface. The Australian Building Codes Board’s Condensation in buildings handbook explains how indoor humidity, surface temperature and the way walls and roofs are built combine to cause it.
Indoor humidity comes from people and how they live: showers, cooking, drying clothes inside, unflued gas heaters, fish tanks and pot plants. In a sealed, modern home that moisture has fewer ways out.
The patterns
| Clue | Condensation | Leak |
|---|---|---|
| Where | Coldest surfaces — external corners, ceilings near the roof, behind wardrobes, window frames | Around one point — under a window, below a bathroom, at a roof penetration |
| Spread | Diffuse spotting across a wide area | Concentrated, often with a tide mark or stain |
| Behind the surface | Usually dry | Usually wet, with readings highest near the source |
| Timing | Worse in winter mornings or when air-conditioning runs | Worse after rain, showers or when a fixture is used |
| Weather link | Cold nights, still air | Storms, wind-driven rain |
The readings that settle it
- Surface temperature vs dew point — if the mouldy surface sits at or below the room’s dew point, condensation is happening.
- Moisture in the material — a pin or non-invasive meter shows whether the plaster, timber or frame behind the mould is wet.
- Gradient — readings that climb towards one spot point to a source.
- Humidity logging — a logger over several days shows whether humidity spikes with showers and cooking or sits high all the time.
Why it matters
The fix is completely different: ventilation, heating and insulation for condensation; a plumber or roofer for a leak. In a rental it also decides responsibility — the RTA puts structural causes on the owner and tenant-caused damp on the tenant. In Queensland homes it is often both, which is exactly why measuring beats guessing.
Questions people ask
Why do I get condensation in a hot, humid climate?
Warm humid air condenses on anything cooler than its dew point. In Queensland that includes air-conditioned rooms, cold wall cavities behind air-conditioned spaces, and roofs on clear nights.
Will a dehumidifier fix condensation mould?
It can help in a closed room, but it treats the symptom. Exhaust fans that vent outside, airflow behind furniture and fixing cold spots address the cause.
References
- [1]Condensation in buildings handbook — Australian Building Codes Board
- [2]Mould and damp — Queensland Government
- [3]Minimum housing standards — Residential Tenancies Authority (Queensland)
Checked against these references on 2 November 2026.