Airtightness and Mould
If your new Adelaide home has a mould problem, the building itself isn't broken — but the ventilation probably is.
New homes in Adelaide shouldn’t have mould. Yet it’s an increasingly common complaint in homes that are only a year or two old — and, counterintuitively, it’s often the better-built, more airtight homes where it shows up first. This article explains the building science behind why that happens, and why the fix is almost never “make the house less airtight.”
16 min read
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.
Key takeaways
- Mould needs moisture and a surface to grow on. Airtight homes don't create moisture — they simply stop it escaping the way it used to in leakier, older construction.
- The real cause of mould in new airtight homes is almost always inadequate or absent mechanical ventilation, not the airtightness itself.
- Relative humidity, not just "dampness", is the number that actually predicts mould risk.
- Thermal bridges create cold surfaces where condensation forms even when average indoor humidity looks fine.
- MVHR directly addresses the root cause by continuously removing moisture-laden air and supplying fresh air.
Jump to a section
1. Why mould grows — the building science
Mould is a fungus, and like any fungus, it needs three things to establish and grow: a food source (timber, plasterboard paper facing, dust and even paint all qualify), a suitable temperature, and moisture. Of these three, moisture is the only one that’s practically controllable in a finished home — moisture control is therefore the entire game when it comes to preventing mould.
Mould doesn’t need standing water or an obvious leak to grow — it can establish at surprisingly modest, sustained levels of surface moisture or elevated relative humidity, particularly on organic surfaces like timber framing or the paper facing on plasterboard.
2. Relative humidity — the number that actually matters
Relative humidity (RH) — the amount of moisture in the air relative to the maximum it could hold at that temperature — is the key metric for mould risk. See our full explainer on what is relative humidity for the underlying physics.
3. Surface condensation and thermal bridges
Condensation happens when air meets a surface colder than its dew point. Windows, external wall corners, wardrobes against external walls, and bathroom ceilings are all common condensation points, precisely because they tend to be the coldest surfaces in a room.
Thermal bridges — localised points where insulation is interrupted or bypassed — create surfaces colder than the surrounding wall or ceiling. Even in a home with generally good average humidity, a thermal bridge can create a small, persistently cold, persistently damp spot — exactly where mould commonly first appears, often in a corner or along a single stud line rather than across an entire wall.
4. Why new homes are more vulnerable to mould
Australian homes built over the last decade are significantly more airtight than older construction. The problem is that many of these homes were built without any corresponding increase in controlled ventilation — see why airtight homes need ventilation and what makes a house airtight.
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 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.
Cold surfaces create condensation
Mould doesn't grow in mid-air — it grows on surfaces where moisture condenses. Windows, external wall corners, wardrobes against external walls, and bathroom ceilings are all common condensation points.
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 outside. This is non-compliant under NCC 2022 (Part 10.8.2) and pumps warm, humid air into the roof space.
HiPer Haus field note
At an existing limestone home in Mount Gambier (built by Scanlon), solid masonry walls were lined with a false wall and building membrane specifically to manage moisture, and a Zehnder ComfoSpot 50 decentralised heat-recovery unit was core-drilled through — delivering continuous filtered fresh air without central ductwork. It shows the same moisture-control principle applying to existing solid-masonry retrofits, not just new airtight builds.
5. 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:
Opening windows: Intermittent and weather-dependent. In Adelaide winters, cold air coming in can actually increase relative humidity in warm rooms.
Bathroom exhaust fans: Only operate when the room is in use. Many are also installed incorrectly — ducted into the roof, not outside.
Dehumidifiers: Energy-intensive 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 does nothing to address the moisture load driving the problem.
6. Airtightness myths, corrected
“Airtight homes cause mould.”
Airtight construction doesn't generate moisture — people, showers, cooking and construction drying do. Airtightness simply stops that moisture escaping accidentally.
“A house needs to breathe to avoid mould.”
Uncontrolled leakage is an unreliable, energy-wasteful way to manage moisture. A designed mechanical ventilation system manages moisture far more effectively.
“If I just open windows more, I'll be fine.”
This depends entirely on consistent occupant behaviour and works against you in cold weather.
“Older, leakier homes don't get mould, so leaky is safer.”
Older homes get different moisture problems — often rising damp or poor original vapour control — rather than none at all.
“New home mould means the builder used bad materials.”
In most cases the materials are fine — the issue is a ventilation system that wasn't specified, sized, or commissioned correctly.
7. How MVHR reduces mould risk
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 — see what is MVHR.
An MVHR system runs continuously, 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 around 90% of the energy from the outgoing air. 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.
8. NCC 2022 and the ventilation requirement
The National Construction Code 2022 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)
See our full NCC 2022 ventilation guide for SA-specific commencement detail. If your new home was built without compliant mechanical ventilation, you may have grounds to raise it as a building defect.
9. 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.
Try the tool
Want to see how occupancy, airtightness and climate drive moisture, humidity and mould risk in a particular home? Try the Home Comfort Explorer.
Open the Home Comfort Explorer →Related calculators
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.
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 while recovering up to 90% of the heat from the extracted air.
Does making a house more airtight increase mould risk?
Only if ventilation isn't upgraded alongside it. Airtightness itself doesn't generate moisture; it just stops moisture escaping accidentally. Pairing airtightness improvements with adequate mechanical ventilation avoids the problem entirely.
What's the difference between mould from airtightness and mould from rising damp?
Airtightness-related mould in new homes is driven by trapped indoor moisture and inadequate ventilation. Rising damp is a different mechanism, common in older homes, where groundwater moisture moves up through masonry due to an absent or failed damp-proof course.
What relative humidity level should I aim for indoors?
Generally 40–60% is considered a comfortable, low-mould-risk range. Sustained levels above roughly 60–70%, particularly at cold surfaces, increase mould risk.
Can a thermal bridge cause mould even if my house isn't humid overall?
Yes — a thermal bridge creates a localised cold surface that can reach dew point and condense even when average humidity in the room seems reasonable, producing isolated mould spots rather than widespread growth.
How quickly can mould appear in a new home?
It varies, but mould can appear within the first year, particularly during the first winter, when construction moisture is still drying out and heating cycles create the largest indoor-outdoor temperature differences.
Will a dehumidifier fix my mould problem?
It can provide temporary relief in a specific room but doesn't address the underlying cause and doesn't supply fresh air. It's a symptom-management tool, not a substitute for proper ventilation.
Does my exhaust fan need to vent outside, not into the roof?
Yes — venting into the roof cavity is non-compliant under NCC 2022 (Part 10.8.2) and a very common cause of hidden roof space mould and timber decay.
Who can help assess and fix mould caused by inadequate ventilation in Adelaide?
HiPer Haus specialises in MVHR design and installation for high-performance and airtight homes across Adelaide and South Australia, and can assess your home and specify the right system.
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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.
Written by
Jonathen HindryFounder of HiPer Haus. 25+ year plumber turned Certified Passive House Tradesperson — blower door testing, MVHR design and heat pump hot water across Adelaide and South Australia.
