Mould in an older house means moisture is getting in or staying in, and the fix is to find that moisture rather than to clean the wall. The visible patch is the symptom. A renovation is the cheapest point to track the cause down, because the linings are off, the framing is visible and the roof, wet areas and subfloor can all be checked while access is easy.
The NSW Health advice, as published on the South Eastern Sydney Local Health District mould page, says mould is a fungus that grows indoors where there is too much moisture, and that it is often caused by external leaks or flooding, excess condensation on walls and windows, and reduced airflow inside the home. That gives three things to rule in or out.
Is it a leak?
A leak is the first thing to rule out, because it will keep feeding the mould whatever you do to the surface. Look for a stain that has a defined edge or that grows after rain, a patch below a roof valley or chimney, or one under a bathroom, kitchen or laundry.
- Roof and gutters. Cracked or slipped tiles, rusted flashing, and gutters that overflow against the wall can all send water into the ceiling or the top of a wall.
- Windows and doors. Failed flashing or sealant around a window lets rain into the frame, and the stain shows up beneath it.
- Plumbing. A weeping joint, a leaking shower base or a tap that has dripped for months can soak framing and sheeting. The advice is to repair leaks from the roof, pipes or taps straight away to stop dampness spreading.
- Ground levels and drainage. Garden beds, paths or paving built up against an outside wall, or downpipes that discharge beside the house, push water towards walls and the subfloor.
If a leak has gone on for long, the damage may not stop at the plasterboard. Timber that has stayed wet can soften and need replacing, and it is far easier to see and replace while the wall is open than after it is closed.
Is it condensation?
Condensation forms when warm, moist air meets a cold surface. In an older house it shows up on windows, on the cold side of external walls and in the corners of rooms, and it comes from everyday living: showers, cooking, drying clothes indoors and breathing.
The same NSW Health page gives the practical steps. Use exhaust fans during showering, cooking and laundry, wipe condensation away, dry clothes outside where you can, and heat rooms evenly in winter to avoid cold damp ones. It also advises limiting humidifiers and avoiding gas heaters, which release moisture into the room.
The renovation side of this is extraction. A bathroom or laundry with no fan, or a fan that discharges into the roof space, only moves the moisture to a different place. Fans should be ducted to the outside, a point we cover in adding a second bathroom or ensuite.
Is it airflow?
Rooms that stay shut and still hold moisture. Older houses with small rooms, few openings and doors that stay closed are the usual culprits, and the fix is usually a path for air rather than a piece of equipment. Our post on ventilating an older house goes through openings, high-level vents and louvres over internal doors.
Under the floor is part of this. A house on piers or stumps relies on vents in the subfloor wall to let air through, and those can be blocked by garden beds, stored items or soil that has built up against the wall. A damp, stagnant subfloor can feed moisture upward into floors and walls. Checking that the vents are clear is a cheap first step.
What order should the fixes go in?
The order matters more than any single repair.
- Find and stop the moisture. Fix the leak, redirect the drainage, or add the extraction.
- Dry the area. Let it dry out properly before anything is closed in or painted. NSW Health advises drying water-damaged materials thoroughly and discarding anything that cannot be completely cleaned and dried.
- Replace what is damaged. Soft or stained framing, sheeting and insulation that stayed wet come out and are replaced.
- Clean the mould that remains. The NSW Health page points to its Got Mould? guide for how to clean it.
- Finish the surface. Lining, paint and fittings go on last, over a surface that is dry and has a reason to stay that way.
Doing it in the reverse order, painting first and finding the cause later, is how the same patch comes back within a season.
When is it more than a small job?
A patch the size of a hand from a shower with no fan is a ventilation fix. Mould across a wall, a ceiling that has stained and sagged, or any area that stays wet after the likely cause has been fixed points to a bigger source, and it needs someone to open it up and look. If the house is old enough to have been built before the late 1980s, check for asbestos before cutting into linings, which our post on asbestos in older Illawarra homes explains.
NSW Health also says to seek medical advice if you develop health problems after being exposed to mould, since mould can make asthma and allergies worse.
Where this fits in a renovation
Damp is one of the things we look for on the first visit to an older house, because it changes the order of the job. If framing needs replacing, or a wet area needs waterproofing and extraction, that work comes before the new linings and finishes. Our home renovations page covers what we take on, and if you have a patch of mould you cannot explain, send the details through the contact page or call 0434 037 851 and we will look at where the moisture is coming from.
Common questions
- What causes mould in an older house?
- Too much moisture. NSW Health lists external leaks or flooding, excess condensation on walls and windows, and reduced airflow inside the home as the usual causes. In older houses the source is often a roof or plumbing leak, a bathroom or laundry without proper extraction, or a room that never gets air through it.
- Will painting over mould stop it coming back?
- No. Mould returns when the moisture does. Cleaning and painting remove what you can see, but the cause has to be found and fixed first, then the surface dried and cleaned. Painting over a damp wall also tends to blister the new paint.
- Should I fix damp before or during a renovation?
- Before the linings go back on, ideally during the renovation itself. Walls and ceilings are open, so leaks, rotted framing and missing extraction can be seen and fixed once rather than hidden under new plasterboard.
Want a hand with this at your place? Get a free quote or call 0434 037 851.