Mould in New & Airtight Homes
If your new Adelaide home has a mould problem, the building itself isn't broken — but the ventilation probably is.
The counterintuitive truth: new homes in Adelaide develop mould more readily than older, draughtier homes — precisely because they are better built. Airtight construction is a feature, not the problem. The problem is airtight construction without adequate ventilation.
Why new homes are more vulnerable to mould
Australian homes built over the last decade are significantly more airtight than older construction. Improved building techniques, better window seals, vapour control membranes and more careful detailing all reduce uncontrolled air leakage — which is exactly what modern energy efficiency standards require.
The problem is that many of these homes were built without any corresponding increase in controlled ventilation. The old draughts and gaps that used to inadvertently dilute indoor humidity are gone — but there's nothing to replace them.
Four reasons moisture builds up in new homes
Airtight construction traps moisture
Modern homes are deliberately built with lower air leakage to reduce energy use. This is a good thing — but it means moisture generated inside the home (from showers, cooking, breathing, even houseplants) can no longer escape through random gaps and cracks. Without a controlled outlet, relative humidity rises.
Construction moisture takes months to dry out
A new home contains a significant amount of moisture locked into the concrete slab, mortar, plaster and timber framing. As the building dries out over the first 6–18 months, it releases that moisture into the indoor air. In an airtight home without adequate ventilation, this is a significant and often overlooked source of humidity.
Cold surfaces create condensation
Mould doesn't grow in mid-air — it grows on surfaces where moisture condenses. Windows, external wall corners, the backs of wardrobes against external walls, and bathroom ceilings are all common condensation points. In an airtight home with elevated indoor humidity, any surface below dew point temperature will see condensation and, eventually, mould.
Extraction fans that vent into the roof cavity
A very common installation error: bathroom and kitchen exhaust fans ducted into the roof cavity rather than directly to the outside. This is non-compliant under NCC 2022 (Part 10.8.2) and pumps warm, humid air into the roof space where it condenses on the underside of the roof and in insulation — leading to mould, timber decay and insulation damage that homeowners often don't discover for years.
What doesn’t work — and why
The usual advice — open your windows, run your exhaust fans, use a dehumidifier — addresses the symptom rather than the cause. These approaches place a burden on occupant behaviour and don’t reliably solve the problem:
Opening windows: Intermittent and weather-dependent. In Adelaide winters, cold air coming in can actually increase relative humidity in warm rooms. Occupants stop doing it.
Bathroom exhaust fans: Only operate when the room is in use. They don't address background moisture load from breathing, cooking and drying. Many are also installed incorrectly — ducted into the roof, not outside.
Dehumidifiers: Energy-intensive, require emptying or draining, and treat symptom not cause. They don't supply fresh air — they just dry the existing stale air.
Anti-mould paint: A cosmetic treatment. It may slow visible mould regrowth on a specific surface but does nothing to address the moisture load driving it.
What actually works: controlled mechanical ventilation
The only reliable solution is a ventilation system that continuously dilutes and removes indoor moisture regardless of occupant behaviour or weather conditions. For airtight homes in Adelaide, that means MVHR — Mechanical Ventilation with Heat Recovery.
An MVHR system runs 24 hours a day, drawing stale, moist air out of bathrooms, kitchens and utility areas while simultaneously supplying fresh, filtered outdoor air to bedrooms and living spaces. A heat exchanger recovers up to 90% of the energy from the outgoing air, so there’s minimal heating or cooling penalty.
When properly designed and commissioned, an MVHR system keeps indoor relative humidity in the 40–60% range where mould cannot establish — not occasionally, but continuously, year-round.
NCC 2022 and the ventilation requirement
The National Construction Code 2022 now requires homes tested at under 5 air changes per hour (ACH50) to have mechanical ventilation meeting a minimum calculated airflow rate:
Airflow (L/s) = 0.05 × Floor Area (m²) + 3.5 × (Bedrooms + 1)
This change recognises that modern building practice — insulation, draught-sealing and energy-efficient construction — naturally produces homes with lower air leakage, and that these homes require designed ventilation to maintain healthy indoor air quality. If your new home was built without compliant mechanical ventilation, you may have grounds to raise it as a building defect.
Mould in Adelaide’s climate
Adelaide’s climate creates particular ventilation challenges. Hot, dry summers followed by cool, wet winters mean the outdoor humidity profile changes dramatically across the year. In winter, homes are closed up and heating cycles drive indoor moisture onto cold surfaces — particularly windows, external corners and north-facing wall cavities.
In the Adelaide Hills, where overnight temperatures are lower and winter humidity is higher, mould problems in new homes are especially common. The combination of airtight construction, thermal mass floors, and closed winter living creates ideal conditions for condensation without adequate ventilation.
Frequently asked questions
Why does my new home have mould?
New homes are built more airtight than older homes, which means moisture generated inside — from cooking, showers, breathing and drying clothes — has nowhere to escape. Without a mechanical ventilation system, that moisture accumulates on cold surfaces and creates ideal conditions for mould growth, even in a brand-new house.
Does opening windows fix mould in a new home?
Only partially, and not reliably. Opening windows reduces humidity temporarily but doesn't address the root cause — continuous moisture generation in an airtight building. In Adelaide winters, opening windows also loses heat and can introduce cold, humid air from outside. A mechanical ventilation system with heat recovery (MVHR) provides continuous, controlled fresh air without the energy penalty.
Is mould in a new home a building defect?
Under NCC 2022, homes tested at under 5 ACH50 are required to have mechanical ventilation capable of providing a minimum calculated airflow rate. If your new home lacks compliant mechanical ventilation and develops mould as a result, this may constitute a building defect. The NCC 2022 changes specifically address this issue by requiring builders to account for the relationship between airtightness and ventilation.
What is the best ventilation to prevent mould in a new home in Adelaide?
For high-performance and airtight homes in Adelaide, MVHR (Mechanical Ventilation with Heat Recovery) is the gold standard. It provides continuous, balanced ventilation — supplying fresh filtered air to living spaces and extracting humid, stale air from bathrooms and kitchens — while recovering up to 90% of the heat from the extracted air. This keeps the home fresh and dry without wasting energy.
Dealing with mould in a new or airtight home?
HiPer Haus specialises in MVHR design and installation for high-performance homes across Adelaide and South Australia. We can assess your home and specify the right system.