The Musty Smell Is Telling You Something
Many Northern Rivers homeowners have learned to normalise the musty smell. “It’s an old house.” “It’s the wet season.” “It’s just the hinterland humidity.” After a while, you stop noticing it — and your visitors do notice it, every time they come in the front door.
Here’s the thing about the musty smell in your Northern Rivers home: it’s not a neutral atmospheric condition. It’s the chemical signature of active biological activity.
The musty smell that people associate with old homes, damp basements, and the Northern Rivers wet season is caused by MVOCs — microbial volatile organic compounds. These are chemical compounds released as metabolic by-products by actively growing mould colonies. Where there is a persistent musty smell, there is active mould producing it.
This guide explains where the smell comes from, what it means for your home and your health, and what to do about it.
What Causes the Musty Smell: The Science
Microbial Volatile Organic Compounds (MVOCs)
Mould and other microorganisms release hundreds of different volatile organic compounds as they metabolise organic matter. The specific compounds that produce the musty smell characteristic of indoor mould include:
- 1-Octen-3-ol (mushroom alcohol) — the compound most associated with the classic musty smell
- Geosmin — the earthy, wet soil smell
- 2-Isopropyl anisole — an earthy, anise-like compound
- Various terpenes and aldehydes — depending on the specific mould species
These compounds are released continuously by actively growing mould colonies. They’re produced in concealed spaces — inside wall cavities, in subfloor areas, in roof voids — and permeate through building materials into the living space.
Key point: MVOCs are detectable by human smell at very low concentrations — parts per billion. The fact that you can smell musty compounds doesn’t necessarily mean the concentration is at a level that poses direct health risk from the compounds themselves. But it does mean there is active mould growth producing them, which does mean there are mould spores and spore fragments in the air.
Why the Smell Is Strongest in the Wet Season
MVOCs are produced more actively when mould colonies are metabolically active — which means when moisture conditions are most favourable. During the Northern Rivers wet season (December to April), when ambient humidity exceeds 80%, mould colonies in walls, subfloors, and roof voids receive sustained moisture and are most active. The smell is strongest during these periods.
Conversely, during drier months, the same colonies may be relatively dormant — producing fewer MVOCs and generating less noticeable smell — even though the mould is still present.
This is why treating the musty smell as a seasonal phenomenon — “it just gets a bit musty in summer” — misses the point. The seasonal variation in smell intensity reflects the seasonal variation in mould activity, not the presence or absence of the underlying mould.
Where the Musty Smell Is Coming From: Common Sources in Northern Rivers
Subfloor Spaces in Elevated Homes
The most common source of musty smell in older Northern Rivers homes — particularly in Mullumbimby, Bangalow, Lismore, Casino, and Murwillumbah — is mould in the subfloor space. Mould growing on timber floor joists, bearers, or the undersides of floorboards in elevated homes produces MVOCs that are heavier than air, accumulate at floor height, and diffuse upward through the floor structure.
A musty smell that is stronger near the floor, or that is most noticeable when you first walk into a room, is a strong indicator of a subfloor source.
Roof Void
Mould in the roof void produces MVOCs that diffuse downward through ceiling materials — particularly plasterboard (which is semi-permeable) or older pressed metal and fibrous cement ceilings (which are perforated). A musty smell that is stronger in upper-floor rooms, or that you particularly notice in rooms directly under the roof (not on lower floors), suggests a roof void source.
Wall Cavities
Post-flood wall cavity mould is the defining source of musty smell for Lismore properties. Mould established inside wall cavities — behind the internal lining — releases MVOCs that diffuse through the plasterboard face. The smell is often strongest in the rooms that were most flooded.
Wall cavity mould produces a musty smell that tends to be evenly distributed throughout the affected room rather than concentrated at floor or ceiling height, and it is often accompanied by the specific “dirty flood water” smell that residents of flooded homes recognise.
Bathroom and Wet Area Structural Mould
Structural mould in bathroom ceiling plasterboard, behind tiles, or under bathroom floors produces MVOCs that permeate into the bathroom space and adjacent rooms. If the musty smell is most concentrated in or around the bathroom, the bathroom is the likely source — even if you can’t see obvious mould.
HVAC and Air Conditioning Systems
Air conditioning units that have mould growing in the evaporator coil area, in the condensate drain pan, or in the ductwork distribute MVOCs (and mould spores) throughout the home when they operate. A musty smell that is strongest when the air conditioning is running — and improves when it’s off — suggests the AC system is the source.
Is a Musty Smell Always Mould?
In the Northern Rivers context, almost always. There are a small number of other possible sources:
Damp soil (in subfloor spaces without mould): Wet soil can produce geosmin and similar compounds independent of mould, through soil bacteria. In a subfloor space with very wet soil but no mould on structural timbers, a mild musty smell is possible. However, in the Northern Rivers climate, wet subfloor soil almost always has associated mould on the timber above it.
Organic debris in gutters or stormwater pits: Decomposing organic matter (leaf litter in gutters, organic accumulation in stormwater pits) produces musty-adjacent smells. If the musty smell is strongest near external openings or after rain, check gutters and external drainage.
Aged or water-damaged building materials without active mould: Very old plasterboard that has been wet and partially dried multiple times develops a musty smell from the deteriorated paper facing, even without active mould. This is uncommon.
In the vast majority of cases in Northern Rivers homes, a persistent musty smell means mould.
What to Do About a Musty Smell
Step 1: Don’t Ignore It
The temptation is to normalise the musty smell — especially if it’s not getting dramatically worse. But musty smell means mould is active, and active mould means spores and MVOCs are in the air you’re breathing. Investigating and resolving it is the right response.
Step 2: Try to Locate the Source
Use the location clues described above to narrow down the likely source:
- Stronger at floor height → subfloor
- Stronger in rooms under the roof → roof void
- Strongest in a specific room, particularly one that flooded → wall cavity
- Stronger when air conditioning runs → HVAC system
- Strongest in or around the bathroom → bathroom structural mould
Step 3: Check Accessible Spaces
If the subfloor is accessible — lift an access cover and take a torch. Look for visible dark mould on joists and bearers. Smell test at the subfloor access point. If the smell is much stronger at the subfloor level, you’ve found your source.
Similarly, if roof void access is available, check for visible mould on battens, rafters, and insulation.
Step 4: Commission a Professional Assessment
If your location investigation doesn’t clearly identify the source, or if the source is in a space you can’t access, a professional assessment is the next step. Moisture mapping, air sampling, and where appropriate thermal imaging and borescope investigation will locate the source.
Read our guide to mould inspection and testing.
Step 5: Address the Mould, Not Just the Smell
Air fresheners, opening windows, and essential oil diffusers mask the smell temporarily. They do not address the mould colony producing it. The musty smell will return within hours to days if the mould is not treated.
Frequently Asked Questions
My home has smelled musty for years and nobody in the house has been sick. Does the mould still need to be fixed? Yes, for several reasons. First, the absence of obvious health symptoms doesn’t mean there are no health effects — mould exposure has chronic low-grade effects (increased respiratory sensitivity, worsening of subclinical conditions) that often don’t present as obvious illness. Second, the mould causing the smell is biological activity consuming your building fabric — it will cause or is causing damage to structural timbers, insulation, and building materials over time. Third, it affects the liveability and value of the property.
The musty smell is much worse in summer. Is the mould only active in summer? The mould is likely active year-round but most actively sporulating and producing MVOCs during the wet season when humidity is highest. The underlying colony is present and growing throughout the year — the seasonal variation in smell intensity reflects seasonal variation in activity, not presence and absence.
Can I neutralise the musty smell without removing the mould? Various products — ozone generators, activated charcoal, baking soda, essential oil diffusers — reduce musty smell temporarily. None of them treat the underlying mould. The smell will return. The only way to permanently eliminate a musty smell from a home is to eliminate the mould colony causing it.
My real estate agent says the musty smell is just “character.” Should I be concerned when buying a property that smells musty? Yes. A persistent musty smell in a property you’re considering purchasing is a material concern. It indicates active mould, which has remediation costs, potential health implications, and building damage consequences. A pre-purchase mould inspection will establish what’s causing the smell and what it will cost to resolve. Read our pre-purchase inspection guide.
After professional mould remediation, how long until the musty smell goes? The smell should improve significantly within days of remediation as the MVOC-producing colony is removed. In cases where MVOCs have been absorbed into soft furnishings, timber, or other permeable materials in the room, a residual smell can persist for weeks even after the mould itself is gone. If a musty smell persists more than a few weeks after professionally cleared remediation, additional investigation may be warranted — either for residual mould in a different location, or for persistent MVOC absorption in furnishings.
Get a Musty Smell Assessment
A musty smell in your Northern Rivers home is an investigable condition, not something to accept. Request a Free Quote — we’ll locate the source and tell you exactly what you’re dealing with. Assessment appointments available within 24 hours across Northern Rivers.