PERTHVentilation, condensation and mould: the moisture a house can't get rid of
Ventilation and condensation under the NCC: why houses get damp inside, interstitial condensation in cavities, the exhaust and roof-space requirements
Mould on a ceiling or in a wardrobe is rarely a cleaning problem. It is a ventilation problem showing its symptoms, and where it is growing inside the building fabric rather than on the surface, it can quietly degrade the structure while it damages the air people breathe. Building and Energy has published on ventilation for this reason, and the National Construction Code now treats condensation as its own subject.
Why buildings get damp inside
A house generates moisture constantly — washing, cooking, drying clothes, breathing. That water vapour has to leave the building, and if it cannot, it condenses wherever a surface is cool enough, then mould grows where the condensation forms. The NCC puts it plainly: the design, construction and use of a building can create prolonged damp, which leads to poor indoor air quality, along with mould and gradual building degradation.
The most dangerous form is interstitial condensation — moisture condensing inside a wall or roof cavity, out of sight. It can affect the structural integrity of a building, and because it is hidden, it often goes undetected until the cost to fix it is significant.
What the Code now requires
The NCC addresses this through its condensation management and ventilation provisions, which apply particularly to residential buildings because of how much moisture domestic use generates. The requirements work on a simple principle: get the moist air out to the outside, and let cavities breathe.
Exhaust fans in wet areas such as bathrooms and kitchens have to discharge to outdoor air, not into the roof space, and a non-continuous bathroom exhaust that is not naturally ventilated has to be interlocked with the light switch and run on for ten minutes after. Where exhaust does discharge into a roof space, that space has to be ventilated to outside through openings sized to the roof — the Code sets ratios of the ceiling area depending on the roof pitch. In the cooler climate zones, roof spaces and pliable building membranes have specific vapour-permeability and ventilation requirements, because cold climates trap internal moisture more readily.
NCC 2022 went further and set a measurable target: a verification method that a Mould Index of 3 must not be exceeded. The lesson built into that change is important — a well-sealed house with poor ventilation is at higher condensation risk, while a house that is both well-sealed and well-ventilated manages it well. Sealing without ventilating makes the problem worse.
Where it shows up in a Perth house
Perth's mild climate hides this until it doesn't. The surface signs are mould in the corners of ceilings and on cold south-facing walls. Black spotting shows up in wardrobes and behind furniture against external walls. Musty smells settle in rooms that do not get cross-ventilation. In a poorly ventilated subfloor or roof space, the damage happens to the timber before anyone smells it, which links this straight to subfloor ventilation and to timber decay, since sustained damp is what rot needs.
Renovations that seal a house up for energy efficiency without adding ventilation are a growing cause. The house gets tighter, the moisture stays in, and the mould follows.
What an inspection tells you
We report visible mould, the moisture readings and staining that explain it, whether wet-area and roof-space exhaust actually vents outside, and whether the roof space and subfloor are ventilated. Where mould is a surface symptom of a hidden moisture problem, we say the source needs to be traced, because painting over mould without fixing the ventilation simply delays its return. A pre-purchase inspection reads these signs together rather than treating a mouldy corner as a cosmetic note, because the corner is usually telling you something about the whole house.
Why modern houses trap moisture
There is a mild paradox at the heart of the condensation and mould problem: as houses have become better sealed and more energy efficient, they have in some respects become more prone to internal moisture problems, not less. An older, draughty house leaked air constantly, and with the air went a great deal of the moisture that daily living generates, cooking, showering, drying clothes, breathing. A modern, well-sealed house holds that air, and its moisture, inside far more effectively, which is excellent for keeping conditioned air in and energy bills down, but means that without deliberate ventilation the moisture has nowhere to go. It accumulates, and when moist air meets a cold surface, it condenses, and persistent condensation is what feeds mould.
This is why ventilation has become a more prominent concern in building requirements as energy efficiency has tightened. The two have to be balanced: a house needs to be sealed enough to be efficient but ventilated enough to manage the moisture that sealing traps. Get the balance wrong, seal a house tightly without providing adequate ventilation, and you create the conditions for condensation and mould, which are not just unsightly but a genuine problem for the building fabric and for the health of the people living in it. The requirements around ventilation exist precisely to stop energy efficiency from being achieved at the cost of a damp, mould-prone interior.
Where condensation shows up in a Perth house
Condensation concentrates where moist internal air meets cold surfaces and where the moisture-generating activities happen, and in a Perth house that is a fairly predictable set of locations. Bathrooms and laundries, where showering and clothes drying pump moisture into the air, are the first, particularly where the exhaust ventilation is inadequate or vents into the roof space rather than outside. Bedrooms, where closed doors and windows overnight trap the moisture of breathing against cold glass and walls, are another, often showing as mould in the corners of ceilings and behind furniture on external walls. Kitchens, where cooking generates steam. And the roof space itself, where moist air that has risen from the living areas can condense on the underside of a cold roof if the space is not ventilated.
Perth's climate gives the problem its own character. The winters are cool and wet enough to produce the cold surfaces and the closed-up houses that condensation needs, while the lifestyle, plenty of showering, indoor drying of clothes in winter, generates the moisture. Mould in the corners of bedroom ceilings, on the walls behind wardrobes, around window reveals, and in poorly ventilated bathrooms is a common finding, and it is usually a symptom of a ventilation and moisture-management problem rather than a one-off. Reading it as a symptom, and looking for the moisture source and the ventilation gap behind it, is more useful than treating the visible mould as the whole problem.
What an inspection reads, and what an owner can do
An inspection assessing a house for condensation and mould reads the visible evidence and the conditions that produce it. Visible mould, staining, and the musty smell of persistent damp are the symptoms; behind them the inspection looks at the ventilation, whether the wet areas have adequate exhaust and whether it vents outside, whether the roof space is ventilated, whether the house has the means to manage the moisture its use generates. Where mould is present, the useful question is always what is feeding it, a specific moisture source like a leak, or a general ventilation inadequacy, because the answer determines the fix.
For an owner, much of the defence against condensation and mould is manageable through use and maintenance: ventilating wet areas properly and running the exhaust fans, drying clothes outside or in a ventilated space rather than in a closed room, opening up the house to let moisture-laden air out, and dealing with any actual leaks or damp sources promptly. Where the problem is built in, inadequate exhaust ventilation, an unventilated roof space, a house sealed without the ventilation to match, the fix is a building one, improving the ventilation to match the sealing. Either way, an inspection that identifies mould and traces it to its cause gives an owner or buyer something they can act on, rather than a cosmetic observation that leaves the underlying moisture problem to keep producing more.
Common questions
Why does my house get mould?
A house constantly generates moisture from washing, cooking and drying. If that vapour cannot escape it condenses on cool surfaces and mould grows. The NCC treats it as a ventilation and condensation-management problem.
What is interstitial condensation?
Moisture condensing inside a wall or roof cavity, out of sight. It can affect a building's structural integrity and often goes undetected until the cost to remedy is significant.
Does sealing my house cause condensation?
It can. The NCC notes a well-sealed house with poor ventilation is at higher condensation risk, while one that is both well-sealed and well-ventilated manages it well. Sealing without ventilating makes it worse.
This comes up on every pre-purchase building inspection and building & timber pest inspection we carry out.
Read next: the little grille in the brickwork is doing a job and structural timber decay: a moisture problem in a timber costume.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.