Mould on walls: condensation, rising damp or a leak?

Mould on a wall is a sign that the wall is wetter than the room around it. On most walls that happens for one of three reasons: the wall is cold enough for the room’s moisture to condense on it, water is rising into it from the ground, or water is leaking into it from a pipe, a shower or the outside. They look different, they are fixed by different trades, and in a rental they land on different people. Working out which comes before any cleaning.

Three sources, three patterns

The Better Health Channel’s list of common causes covers all of them: leaking roofs and walls, faulty plumbing, condensation from cooking, showering and clothes drying, and areas with poor air circulation — cupboards, corners and furniture against uninsulated outside walls (Better Health Channel (Victorian Department of Health), Mould and your health).

Condensation: the wall behind the wardrobe

An outside wall can be among the coldest surfaces in a room, and a wardrobe or bedhead pushed against it keeps the air behind still. Moist air from breathing, showering, cooking and drying clothes meets that cold, still surface and condenses, and mould follows. It tends to appear in a shape that matches the furniture.

The fix is about moisture and air, not chemicals: move the furniture a little off the wall, run exhaust fans when showering and cooking, open windows when the weather allows, and dry clothes outside or in a vented dryer. If it keeps returning on one wall only, that wall may be uninsulated or have a cold bridge, which is a building question rather than a housekeeping one.

In new building work, the ABCB Housing Provisions control condensation inside external walls too, with rules on membranes and drained cavities (ABCB Housing Provisions Standard 2025, 10.8.2(2)), and require kitchen, bathroom and laundry exhaust to discharge to outdoor air; mould on the ceiling sets out the exhaust rules.

Rising and lateral damp

NSW Health describes rising damp as ground moisture rising up a brick or stone wall, worsened by poor sub-floor ventilation or moisture under the floor, and fixed by installing a new dampcourse or waterproof barrier in the wall (NSW Health, Mould factsheet). It also says to make sure the weep holes and air vents at the base of the home are uncovered (NSW Health, Mould factsheet), which is the cheapest check there is: garden beds, paving or a raised path built up against a wall can cover them.

Consumer Affairs Victoria’s checklist for buyers says damp brick walls can indicate rising damp or salt damp, and that mouldy walls, lifting tiles, peeling paint or pools of water in wet areas can indicate excessive moisture (Consumer Affairs Victoria, Inspect properties before you buy). So rising damp is a pre-purchase question as much as a mould one; a building inspection that flags it is worth following up with a specialist before exchange. NSW Health’s advice is that an experienced building consultant can check the damp course and recommend the fix. Cleaning mould off a wall with rising damp treats the symptom only.

A leak inside the wall

Mould on a wall that backs onto a shower, bath, laundry or kitchen is a leak until shown otherwise. The NCC’s performance requirement for wet areas is that water is prevented from penetrating behind fittings and linings and into concealed spaces (National Construction Code 2022, Volume Two, H4P1 (Wet areas)); a failed membrane or a cracked grout line breaks exactly that. Signs that the water is inside the wall rather than on it: skirting boards swelling, tiles drummy or lifting, paint bubbling on the far side of the wall, and a moisture meter reading high deep in the wall rather than only at the surface. The leaking shower guide covers the waterproofing rules and the building warranty, and burst pipes covers supply leaks.

A leak in a wall cavity is also where mould can be growing out of sight. The Better Health Channel notes that contamination may be in cavities; a musty smell in a room with nothing visible is the usual clue, and musty smell covers finding it.

Cleaning, repairing, and the rental question

For a painted wall, NSW Health’s routine method is mild detergent or vinegar diluted four parts vinegar to one part water, and diluted bleach where mould is not readily removed. Wallpaper, plasterboard that has softened, and anything else that cannot be cleaned and dried completely falls under its rule: discard material that cannot be cleaned and dried completely (NSW Health, Mould factsheet). WA Health guidelines treat no greater than 1 m² as a simple job (WA Department of Health, Guidelines for Managing Mould and Dampness Related Public Health Risks in Buildings).

In a rental, the three sources tend to fall on different sides. Rising damp and a leaking pipe are the building; NSW law requires floors, walls and ceilings to be not subject to significant dampness (Residential Tenancies Act 2010 (NSW) s 52(1B)(b)), and WA’s Commissioner lists damp walls from plumbing as a landlord failure (Government of Western Australia, Commissioner’s blog: Mould in rental properties). Condensation from how the room is used is where the tenant may carry responsibility. Mould in a rental property sets out each state’s rules, and mould inspection covers getting the source documented.

Common questions