Often the first thing you notice is not a stain or a drip. It is the smell. A musty smell in basement rooms, in a downstairs room built in under the house, in a garage cut into a slope or in a cupboard under the stairs is common.
The smell is a sign that something is staying damp. It is not the problem itself. In dark, closed or below-ground spaces, damp can come from several places. The patterns you notice can point towards some causes more than others. The cause still has to be found before anyone spends money on a repair.
Why do basements and dark rooms smell musty?
A musty smell usually comes from mould or mildew (a kind of mould) growing on materials that stay damp.
Dark, closed rooms give damp more chances to linger. A room that is shut up for weeks gets little fresh air. Poor ventilation in a basement or a below-ground room can add condensation on top of any water that is coming through the walls or the floor.
Where does a musty smell usually show up?
- Basements and rooms that sit partly below the ground.
- Downstairs rooms built in under the house.
- Garages and rooms cut into a slope, and the downhill side of a split-level home.
- Cupboards under the stairs.
- A musty wardrobe on an outside wall.
- Built-in wardrobes that back onto a bathroom.
A musty wardrobe that backs onto a bathroom can be a sign of a leaking shower. That case has its own guide, Shower leaking into the next room? What to check before anyone retiles it.
What can a musty smell point to?
In a basement or under-floor space, moisture can come from water in the ground, surface water running towards the building, a plumbing leak or rising damp. Each source needs a different repair. The main ones are listed below, in short.
- Condensation. This happens when warm, humid air meets a cold surface. The wall is simply the coldest thing in a humid room. It is not a leak. In Queensland it is a genuine problem in air-conditioned rooms, poorly ventilated bathrooms, built-in wardrobes against outside walls and rooms that are closed up for weeks.
- Lateral damp (water in the soil pressing sideways through a wall). Basements, garages cut into a slope and the downhill side of split-level homes are the classic places for it.
- Rising damp (ground water climbing up through the base of a wall). It is common across the Gold Coast and coastal Brisbane, often in older homes on sandy ground with a high water table (ground water close to the surface). It often happens where the damp-proof course (a barrier near the base of a wall that stops ground water climbing up) has failed or been bridged by later paving or garden beds.
- Damp coming up through a concrete slab. This is often only seen once the floor coverings are lifted.
- A plumbing or wet-area leak. Water often travels a long way inside a wall or a floor before it shows, so the place where you smell it is not always where the fault is.
- Water from above. In a basement car park under balconies or a podium (a raised outdoor slab), water can come through the ceiling.
A room can have more than one of these at once. For the full patterns of the three most common causes, see our guide Rising damp, lateral damp or condensation? How to tell them apart.
What clues are worth noting?
You cannot name the cause from these clues alone. They are still very useful, both to you and to whoever investigates. Here is what to look for and what each clue can point towards.
- A musty smell after rain. Damp that gets worse after rain points towards water coming in from outside, such as water in the soil pressing on a below-ground wall.
- Where the damp sits. Whether it sits low along one wall, in a level band near the floor, or high in corners helps tell the main causes apart. Our guide Rising damp, lateral damp or condensation? How to tell them apart sets out the full patterns.
- White salt deposits, crumbly mortar (the material in the joints between bricks or blocks) and blistered paint along the lower rows of blocks or bricks. These are often seen in below-ground rooms with damp walls.
- White, crusty salts and dark spotting along the edges of the floor and at door thresholds when floor coverings are lifted. This can point towards damp coming up through a slab.
- Dark staining, damp patches or white salt deposits along cracks and joints in a basement ceiling, or rust-coloured staining around cracks and pipe penetrations (places where a pipe passes through a slab or a wall). These point towards water getting through the concrete from above.
- Garden beds, paving or decking built up against the house above the level of the inside floor, and downpipes that empty onto the ground beside the house. Damp is often worst where garden beds, downpipes or paving send water at a wall.
A note on the white deposits. Efflorescence (a white, powdery or crusty salt deposit) is left behind when water passes through concrete or masonry (brick, block or stone) and then dries. It shows that water has been moving through that surface. Water can travel a long way before it shows, so where the deposit sits is not always where the water gets in.
Will airing out, a dehumidifier or fresh paint fix it?
These steps can make a room more pleasant. They do not tell you where the damp is coming from.
- A dehumidifier (a machine that takes water out of the air) will reduce condensation and make a room more comfortable. It does nothing about water coming through a wall from the ground.
- Airing the room out does nothing about water arriving through the walls, the floor or the ceiling.
- Fresh paint covers the marks for a while. Repainting over the wrong cause is the most common way owners end up paying twice.
Waiting has a cost too. Long-term wetting breaks down mortar, render (a cement coat over a wall) and timber, and salts attack masonry. A damp problem tends to get more expensive the longer it is left.
Can a moisture meter tell you the cause?
Not on its own: a cheap moisture meter will read wet whatever the cause, and walls with salt in them can read wet even when they are dry. Readings help to map a problem, but they do not name it.
How to find the source of a musty smell
Finding the source means reading the pattern of the damp against the building. A HydroIQ investigation follows these broad steps. Our guide How a waterproofing investigation actually works explains each one in more detail.
- The history. When did the smell start? Does it follow rain, showers or neither? What has been repaired, repainted or renovated?
- Thermal imaging. A thermal imaging camera (a camera that shows surface temperature) can show damp patterns the eye cannot see. It does not see water, so a thermal image is a lead to check, never a finding on its own.
- Moisture mapping. Readings are taken on a grid across a wall or a floor and compared with a known dry area of the same material. The pattern of where it is wettest points towards the source.
- Reading the pattern. The investigation maps where the moisture is, at what heights, in what pattern and how it behaves with the weather, then reads that against the building.
- Testing where a leak is suspected. Leak source identification combines thermal imaging, moisture detection, pressure testing (checking whether a pipe holds its pressure) and dye testing (adding a coloured dye to water so its path can be traced). Water or dye testing is used where needed to confirm a source. HydroIQ can also look for leaks in concrete slabs, including plumbing under the slab.
The methods are non-destructive. If a small inspection opening is ever the only way to confirm a finding, it is discussed with the owner first and kept to the minimum.
What should a report tell you?
A useful report sets out what was seen, what was measured and what was concluded, with photos and readings. It says where the water gets in and how far the damp has spread. It describes what needs fixing in terms a contractor can price and plan from.
Who fixes the problem once the cause is known?
HydroIQ investigates and reports. It does not carry out repairs, and that keeps its findings independent. An investigation is a paid service.
The repair is work for a licensed contractor, such as our sister company S&S Remedial. A leaking water pipe is work for a licensed plumber. Each cause needs a different fix:
- Rising damp needs the path from the ground cut off.
- Water pressing through a below-ground wall needs to be stopped outside, or the wall waterproofed from the inside as a designed system.
- Condensation needs better ventilation, insulation or a change in how the room is heated and cooled.
Work applied to the inside face of a basement wall or slab deals with moisture coming through it. It does not fix the drainage or the water pressure on the outside. That is why the source matters before any work starts.
For background, the National Construction Code (the national set of building rules) requires wet areas and external areas to be built so that water does not damage the building or make it unhealthy to live in. The code changes over time, so check the current edition.
Getting the source found
If your downstairs room smells musty, or you are living with a damp basement on the Gold Coast or a damp smell in an under-house room, the patterns above will help you describe it. They will not give you the answer for your building. Get in touch with HydroIQ to arrange an investigation and find out where the damp is really coming from.
Common questions
General information for South East Queensland home owners, not advice about your building or a legal matter. Standards and time limits change; check the current edition or rule before relying on a figure here.