Where it usually turns out to be
- Behind or under fitted furniture on an external wall — the wall behind a wardrobe runs several degrees colder and never gets air.
- Under carpet or laminate on a ground floor, especially where a slab has no or a failed damp-proof membrane.
- Inside a wall cavity or under a floor void where a slow leak has been running.
- In a roof space where a small leak keeps insulation damp.
- Under a shower or bath where a failed membrane has soaked the screed — the classic one, because nothing is visible in the bathroom itself.
- In a cupboard against an external wall, closed up and unventilated.
- Kitchen kickboards and under-sink units.
Finding it
- Follow your nose room by room with the doors closed, in the morning before the house has been aired. That is when it is strongest.
- Move furniture off external walls and check what is behind it.
- Open every closed cupboard and smell inside.
- Lift a corner of carpet against an external wall if you can do so without damage.
- Check the room below any bathroom, and the ceiling in it.
- If it is strongest when the heating comes on, suspect a roof space or a void.
Why air fresheners are the wrong move
They mask the gas without touching the growth, and they remove the one early-warning signal you had. Meanwhile the growth continues on whatever it is feeding on — plaster, timber, adhesive, dust — and by the time it is visible it is a bigger job.
The smell is doing you a favour. Use it to find the source.
Once you find it, what you do depends on which fault it is: mould on walls covers the visible growth, and the four types of damp covers what is keeping it wet.
The smell stops when the water stops, not when the room is aired. Finding which of the four damp mechanisms is feeding it is the first step of damp proofing in Cape Town.
Related reading
Start with The Four Types of Damp for the overview, or:
Common questions
Why does my house smell musty but I cannot see any mould?
Because the growth is behind, under or inside something — a wall cavity, under flooring, behind a wardrobe, in a floor void. Smell reliably precedes sight, which is why it is worth acting on rather than masking.
Is a musty smell harmful?
The compounds producing the smell are not usually harmful at household concentrations, but they indicate active mould growth, and mould spores can affect people with asthma, allergies or compromised immunity. Treat it as a signal worth following.
Will a dehumidifier fix a musty smell?
It helps if the cause is general humidity and condensation, and it is a reasonable short-term measure. It will not fix a leak, a failed shower membrane or rising damp — it just removes some of the water those faults keep supplying.