The assumption in most share houses is that mould means somebody isn’t cleaning properly. It almost never does. Mould in apartments and shared homes is far more often a product of how the building was designed than of how the people inside it are living.
Framed that way, this stops being an argument about whose turn it was to wipe down the bathroom and becomes a question about airflow — how much moisture five people generate in a day, and how little of it a single-aspect flat can physically get rid of.
Why Mould in Apartments Follows Different Rules
A single-aspect apartment — one with windows on only one side — has far less natural cross-ventilation than a house with windows on multiple walls, which means moist air has fewer ways to escape.
Add a share house’s usage pattern on top: multiple showers a day, several loads of washing a week, more bodies breathing and cooking in the same enclosed volume of air, and often more people using less ventilation because nobody’s specifically responsible for running the exhaust fan after their shower. None of that is any one person’s fault. It’s simply more moisture being produced, in a building with fewer ways to get rid of it, shared among people who didn’t design the ventilation and can’t change it.
The Balcony Door Problem
Sliding glass doors onto balconies are one of the more consistent trouble spots in apartment blocks.
The Building and Plumbing Commission explains condensation simply: it forms when moisture-laden air meets a cold surface and cools enough for that moisture to turn back into liquid water, and window frames are one of the most common places it happens. An aluminium sliding door frame conducts cold straight through from outside, making the frame and glass a magnet for exactly that on winter mornings — condensation that collects at the base of the track and often goes unnoticed because nobody thinks to check there. Left long enough, that trapped moisture at the sill supports mould growth in the track and the surrounding wall lining, in a spot most people never look at until the smell gives it away.
Whose Problem Is It, Really?
In a shared or rented apartment, working out who’s responsible follows the same logic as any rental, which makes it worth knowing even before something goes wrong.
Consumer Affairs Victoria notes that responsibility depends on cause: a renter may be responsible if their own actions contributed — not using an exhaust fan, for instance — while a rental provider is responsible where a building defect, like a faulty window frame, allowed moisture in. In a share house, that same distinction gets more tangled, because five housemates’ habits are harder to pin down than one tenant’s, and a building’s single-aspect layout can be the dominant cause regardless of anyone’s habits, which is exactly the kind of situation an independent assessment is built to untangle.
Why “It’s Not My Room” Doesn’t Actually Hold Up
Mould doesn’t stay confined to the room it started in, which matters in a household where everyone assumes the bathroom is someone else’s concern.
Airborne spores travel through open doors and shared hallways, and a bathroom with a chronic ventilation problem affects the air quality of the whole apartment, not just whoever showers there. That’s the same reason a musty smell is worth investigating even when nothing is visible yet — moisture-laden air moves through wall cavities and shared plumbing risers in high-density buildings, so a source in one unit can occasionally show up as a symptom in another, which is one more reason it’s worth treating as a shared issue rather than one bedroom’s problem.
What Actually Helps in a Space You Don’t Control
You can’t renovate someone else’s building, but a few things make a real difference within what you do control.
Run the exhaust fan for the full length of every shower and for twenty minutes after, not just while the water’s running — and be aware that an underpowered or poorly ducted fan may not be clearing much regardless of how long it runs. Wipe down the balcony door track and sill after a cold night rather than letting condensation sit there for days. Where a bedroom has only one small window, cracking it slightly even in winter — for short periods, security allowing — does more for humidity than any amount of heating alone. And if the household genuinely can’t agree on shared responsibility for ventilation, a written house agreement about who does what after using the bathroom is a low-effort fix for a problem that otherwise falls to whoever notices it first.
When the Building Itself Is the Problem
Sometimes no amount of housemate coordination fixes it, because the building’s ventilation is genuinely inadequate for the number of people and the moisture they produce.
That’s a structural condition, not a behavioural one, and it’s worth having assessed rather than assumed. Thermal imaging can reveal moisture patterns that aren’t visible on the surface, which is exactly what identifies whether the recurring damp behind a wardrobe or around a balcony door track is a building-design issue or something else entirely — the kind of evidence that’s also useful if you ever need to make the case to a landlord or body corporate that it’s genuinely not down to how the tenants are living.
Conclusion
Mould in a share house or high-density apartment is rarely about one person doing something wrong — it’s usually density, layout and shared habits combining in a building that wasn’t given enough ventilation for how many people are actually living in it. If you’ve tried the habits above and it’s still coming back, booking a mould inspection is a sensible way to find out whether the building itself is the real cause.
Frequently Asked Questions (FAQs)
Why does my apartment get mouldy even though I clean regularly?
Cleaning removes visible mould but doesn’t address the moisture producing it, and in a single-aspect apartment with limited cross-ventilation, humidity from showering, cooking and breathing has fewer ways to escape than in a detached house. If the underlying ventilation is genuinely inadequate, regular cleaning will keep removing the same regrowth rather than solving it.
Who is responsible for mould in a share house — the tenant who found it or all the housemates?
Responsibility generally follows the same rules as any rental: it depends on whether the cause was tenant behaviour or a building defect, not on who happened to notice it first. In a share house, that’s often genuinely shared, since ventilation habits and building layout both contribute.
Why does mould keep coming back around my balcony door?
Aluminium-framed sliding doors conduct cold efficiently, which makes the frame and track a common condensation point on cold mornings. If that moisture isn’t wiped away regularly, it can support mould growth in the track and surrounding wall lining over a winter.
Can mould in one apartment spread to a neighbouring unit?
It’s uncommon but possible, particularly where moisture or airflow moves through shared wall cavities or plumbing risers in high-density buildings. More commonly, similar conditions — the same building design, the same aspect, the same climate — simply produce mould independently in multiple units for the same underlying reason.