Recurring mould in a Melbourne bathroom is one of the most common — and most preventable — moisture problems in the home. It is almost always a sign that the exhaust fan is not removing enough moisture, whether through being undersized, poorly ducted, clogged, or simply not run long enough. This guide explains why mould forms in Melbourne’s climate, how exhaust fan faults cause it, and the specific fixes that keep a bathroom dry through the damp winter months.
Why Mould Forms in Melbourne Bathrooms
Mould needs moisture, a surface to grow on, and time. A bathroom provides all three in abundance: every shower fills the room with warm water vapour, and the tiles, grout, ceiling and corners give the spores somewhere to settle. The deciding factor is how long the moisture lingers — and that is where Melbourne’s climate makes things harder.
Through the cool, damp Melbourne winter, the building fabric stays cold and the outside air is often humid, so a bathroom does not dry out on its own between uses. Warm shower moisture hits cold ceilings and walls and condenses almost immediately, then sits there. Repeated every day, this gives mould exactly the persistent moisture it needs to establish and spread. The job of the exhaust fan is to break this cycle by removing the moist air before it can condense and linger. When mould keeps coming back, the fan is failing at that job somewhere.
How Exhaust Fan Faults Cause Mould
A fan can be running every day and still allow mould, because running is not the same as extracting. The common failure points in Melbourne homes are:
- Not ducted to the outside: the fan dumps moist air into the roof cavity instead of outdoors, so the moisture never leaves the building. Causes mould in both the bathroom and the roof space. The most common fault of all.
- Restricted ducting: a long, kinked, crushed or sagging duct chokes the airflow so little air actually leaves.
- Undersized fan: a fan below the minimum, or whose ducted airflow falls short, cannot keep up with the moisture load.
- Clogged fan: dust and lint on the impeller and grille can halve the real airflow.
- Not run long enough: switched off the moment the shower ends, the fan leaves most of the residual moisture behind.
- No make-up air: a tightly sealed door starves the fan of replacement air so it cannot extract.
Diagnosing which of these applies — and there is often more than one — is the key to a lasting fix. See our troubleshooting guide for how to identify a weak fan.
Sizing and Run-Time — The Two Big Fixes
Right-sizing the fan
AS 1668.2 sets a minimum of 25 L/s for a bathroom, but reliable condensation control often needs more, because the airflow printed on the box is a free-air figure that drops once a duct is fitted. Aim for a real delivered airflow of 25 to 40 L/s in a standard Melbourne bathroom, and 40 to 60 L/s in a large bathroom or ensuite with a separate shower enclosure. Choosing a fan with margin above the minimum, and ducting it well, means it still performs after real-world losses.
Running it long enough
The single cheapest improvement most Melbourne bathrooms can make is to run the fan for 15 to 20 minutes after the shower. The room keeps releasing moisture after the water stops, and in the cool climate it condenses quickly. A run-on timer makes this automatic — the fan keeps going after the switch is off, then stops itself. A humidity sensor goes further, running the fan whenever room humidity is high. Both are inexpensive upgrades that a licensed electrician can fit, typically as a complete job from $250.
Ducting to the Outside
No fan can keep a bathroom dry if the moisture it extracts is dumped back into the roof cavity. Ensuring the fan discharges to the outside — through a roof cowl or an eave vent — is the foundation that the other fixes build on. If your fan has no visible external discharge serving it, this is the first thing to put right.
Good ducting is short, smooth, well-supported and as direct as possible, with gentle bends and no sags. Sagging flexible duct collects condensed water, which restricts airflow further and can drip. A backdraught damper at the discharge keeps cold Melbourne air and pests out when the fan is off. Getting the ducting right both stops the roof-cavity moisture damage and restores the bathroom airflow. See our venting to roof guide for how it should be done.
Habits That Keep a Bathroom Dry
Alongside a properly sized and ducted fan, a few simple habits make a real difference through a Melbourne winter:
- Run the fan during every shower and for 15 to 20 minutes afterwards (a timer makes this effortless).
- Leave the bathroom door open after showering when possible, to let the room dry.
- Wipe down the shower screen and tiles after use to remove standing water.
- Keep the fan grille and impeller clean so it delivers its full airflow — see our cleaning guide.
- Address any plumbing leaks or rising damp, which add moisture independently of showering.
When to Upgrade Your Fan
If your bathroom has recurring mould despite reasonable habits, it is time to look at the fan itself. Upgrade when the fan is undersized for the room, when it cannot be ducted properly in its current form, when it is old and weak, or when you want the convenience of a timer or humidity sensor built in. A modern, correctly sized centrifugal or inline fan, ducted to the outside and fitted with a run-on timer, is what keeps a Melbourne bathroom dry through winter.
FreshDuct can assess your bathroom, identify why the mould keeps returning, and supply and install a fan and ducting that actually solves it. Call 0431 918 137 for a quote.