The Hidden Cause of Musty Smells in Your Home

Key Takeaways:

• Musty smells often come from dampness under the house, not visible mould or spills
  
• Poor air movement below the floor can lead to mould, warped timber, and stale indoor air
  
• Common warning signs include odours, peeling paint, uneven floors, and health symptoms
  
• Addressing the issue starts with identifying airflow problems and local drainage patterns
  

You’ve cleaned every surface, washed all the curtains, and even aired out the house for days — but that stale, damp smell just keeps coming back. It’s frustrating, especially when everything looks spotless. The scent seems to settle in, no matter how often you scrub or spray.

If you’ve already ruled out wet laundry, pets, or mould in the bathroom, it might be time to think deeper — quite literally. There’s a part of the home most people never check, yet it can influence the air you breathe every day. For many Australian homes, especially those built on piers or in humid climates, the source of that musty odour is often hiding underneath the floor.

Once you understand how under-house moisture builds up and why it lingers, it becomes easier to find the right way to deal with it — and stop the smell from creeping back.

Why moisture builds up in unseen spaces

Under your house, there’s a quiet zone most people forget about. It’s where pipes run, the air stays still, and dampness collects without much notice. In older homes, especially timber ones, this area is usually raised just enough to trap humidity after wet weather or when the ground underneath stays moist for long periods.

Australian soils, particularly clay-heavy ones, are known for retaining water. When it rains, the water doesn’t always drain away quickly — it lingers. Combine that with low airflow, and you’ve got a space that encourages condensation, stale air, and eventually, mould.

Homes along the coast or in leafy suburbs often deal with high ambient humidity, especially in spring and autumn. Without a way for this trapped air to move, the ground beneath your house stays damp, and that moisture slowly makes its way upward.

It’s not just weather or soil that contributes. Blocked vents, heavy garden beds up against the house, or even changes in landscaping can reduce the airflow that once kept things dry. Over time, that trapped moisture creates the perfect conditions for that musty smell to start seeping inside.

How damp underfloors affect indoor air

Once the underfloor space becomes damp, the impact isn’t just structural. It directly affects what you breathe. You may not see the mould forming down there, but the air doesn’t stay contained. It rises through gaps in the flooring, through wall cavities, or even through heating and cooling systems that run close to or through the underfloor zone.

That slight earthy odour? It’s a mix of mould spores, dust, and organic decay. And it doesn’t take much. Even a small patch of mould or a damp beam can produce enough airborne particles to create noticeable smells inside. Over time, these particles settle into carpets, curtains, and soft furnishings, making it even harder to pinpoint the source.

People with asthma, allergies, or chronic respiratory issues tend to notice it first. But even those without health concerns can feel the effects — stuffy sinuses, irritated eyes, and general discomfort, especially after long periods indoors. Pets can also be affected, particularly those that spend a lot of time close to the floor.

A few scented candles or a deep clean will likely fix it. And they might help for a few days. But unless the air under your floor can dry out and move freely, the same cycle of dampness and odour will keep returning.

The role of airflow in controlling mould and mildew

Good airflow is essential for keeping your home dry, and the process starts below your feet. The underfloor area of a house can easily become a trap for stale, humid air — especially during the cooler months or after rain. When that air doesn’t move, moisture builds up and settles into the timber and insulation. It creates exactly the kind of environment where mould spores thrive, even without any visible water leaks.

Under a home, conditions often change more slowly than they do inside. Warm days and cool nights can cause condensation to form on subfloor surfaces, and without constant circulation, that moisture has nowhere to go. This is why air needs to be able to flow freely through the space, not just occasionally but as a consistent feature of the house’s design.

Homes that have been fitted with subfloor ventilation systems tend to manage this balance much more effectively. These systems help maintain airflow beneath the structure by drawing in fresh air and allowing built-up moisture to escape. It’s not a quick fix or a gimmick — it’s a practical way to support the overall health of the building, especially in regions with high humidity or clay-heavy soil that holds water longer than expected.

Drying out the underfloor doesn’t just get rid of smells. It also stabilises temperature and humidity, both of which affect how your home feels on the inside. And once mould and mildew no longer have the right conditions to grow, the stale smell that’s been following you around often fades on its own.

Warning signs beyond just the smell

Smell might be the first sign something’s wrong, but by the time it’s noticeable, the issue is usually already affecting other parts of the home. You might walk into a room and catch a faint dampness in the air — then start spotting clues that confirm it’s more than just odour.

Timber flooring can start to lift or warp in subtle ways. Paint might bubble on skirting boards, or walls near the floor could feel cool and slightly soft to the touch. These are small but telling signs that moisture isn’t being properly managed. The damage often begins beneath the surface and slowly works its way up through the materials your house relies on.

You may also notice the air feels heavier than usual, especially during humid weather or after a stretch of rain. Carpets take longer to dry. Rooms with no open windows stay stuffy even with fans running. And in some cases, health symptoms become more noticeable — sinus irritation, itchy skin, or breathing discomfort that doesn’t line up with outdoor allergens.

Pets, especially those that spend time close to the floor, sometimes react to changes in air quality long before people do. A dog or cat avoiding their usual resting spot might not seem significant at first, but in a home with persistent underfloor dampness, it’s often part of a larger pattern.

While it’s easy to brush off these details as unrelated, they’re often connected. Odour, humidity, and structural changes don’t show up in isolation. They tend to signal an ongoing imbalance in how your home handles air and moisture. If these signs keep coming back, it’s time to look beneath the surface — literally — and assess what’s going on under the floor.

What to consider before tackling the problem

Once you’ve ruled out surface mould, leaky plumbing, and damp bathrooms, the next step is figuring out what’s happening beneath the floor. But solving the problem isn’t just about drying things out temporarily — it’s about identifying what’s causing the damp conditions in the first place.

In many homes, it comes down to airflow, but the reasons behind poor airflow vary. Some houses were never designed with underfloor circulation in mind. Others had vents that worked when they were first built, but over time, those openings became blocked by soil, leaf litter, or even changes to the surrounding landscaping. Retaining walls, garden beds, or renovations can all interfere with the way air moves under the house without anyone noticing.

It’s also common for drainage problems to contribute. If the ground around your home slopes the wrong way or stays wet after rain, water can seep underneath and linger for days. You might not see puddles, but the moisture is still there, gradually being absorbed by timber and insulation.

Addressing these issues properly usually requires a clear understanding of how your home is built and how it responds to the local climate. Every house is different. What works in a dry, inland suburb may not be enough for a coastal property that faces salt air and heavy rain. Rather than relying on surface-level solutions like sprays, dehumidifiers, or air purifiers, it’s worth having someone inspect the subfloor area directly.

A specialist can tell you whether the dampness is caused by poor drainage, a lack of airflow, or something else entirely. That kind of assessment helps prevent guesswork — and saves you from spending money on short-term fixes that don’t address the real issue. If the air under your house has been stale and damp for years, it won’t improve on its own. Taking the time to look beneath the floor can make a lasting difference to the air you live with every day.