Why is there a damp smell in my house?
A method for tracing the source rather than covering it — where to look, in what order, and what each finding means.
- Difficulty
- beginner
- Time
- 45 min
- Read
- 3 min
- Safety
- caution
Short answer
The smell is mould growing on something. Work from the coldest, least ventilated places outwards: behind furniture on external walls, under sinks, inside cupboards, and under the floor. Air freshener treats the symptom and lets the cause continue.
That musty smell is a by-product of mould, so it always has a physical source. It is usually somewhere with no airflow and a cold surface, which is why it is so often behind something rather than in plain view.
Safety
What you'll need
- Torch
- Hygrometer (optional)
- Particle mask and gloves (optional)
Step by step
- Follow your nose room by room, with the doors shut.Close the internal doors for an hour, then go in one at a time. It isolates which room it is coming from, which is otherwise surprisingly hard.
- Pull furniture away from external walls.The most common answer. A wardrobe or sofa against a cold external wall stops air moving, the wall stays cold, and mould grows on the back of the furniture or the wall behind.
- Check under every sink and behind appliances.Slow leaks from waste traps and washing machine hoses are the second most common cause. Feel the pipework and the cupboard base, and look for staining.
- Open the cupboards you rarely open.Under the stairs, the airing cupboard, a bathroom cabinet on an external wall. Enclosed and unventilated is exactly the condition.
- Look at the lowest and coldest places.Behind skirting, in corners of ground-floor rooms, and under a suspended floor if you have access. Rising and penetrating damp both show low down.
- Check the loft.Blocked eaves ventilation, a small roof leak or a bathroom extractor venting into the loft rather than outside all produce a musty smell that drifts down.
- Measure the humidity.A hygrometer costs very little. Consistently above about 60% means the whole house is too humid, and the answer is ventilation rather than a single source.
- Deal with the cause, then the smell.Fix the leak or the ventilation, dry the material out completely, then clean or replace it. Porous things that have gone mouldy — plasterboard, insulation, some fabric — often cannot be cleaned and are better replaced.
Tips
- If it is worse after rain, suspect a leak or penetrating damp. If it is worse in cold weather, suspect condensation. That one distinction narrows it enormously.
- Move furniture five centimetres off external walls permanently. It costs nothing and it prevents the most common cause outright.
- Air freshener and dehumidifier crystals both mask this. Neither stops the mould growing.
If it doesn't work
Worse in winter, better in summer
Cause: Condensation on cold surfaces — Fix: More ventilation and background heat; move furniture off external walls.
Worse after heavy rain
Cause: Penetrating damp or a leak — Fix: Check gutters, roof, external walls and window seals.
Confined to one cupboard
Cause: No airflow, or a leak behind it — Fix: Empty it, check the pipework, and leave it open to dry before treating.
Everywhere, all year
Cause: Whole-house humidity — Fix: Measure it. Sustained above 60% needs ventilation and possibly a dehumidifier.
Questions people ask
What causes a musty smell in a house?
Mould growing somewhere on a damp surface. The commonest sources are behind furniture on external walls, under sinks, in unventilated cupboards and in the loft.
Will a dehumidifier get rid of a damp smell?
It helps if the cause is condensation, and does nothing if the cause is a leak or rising damp. Find the source before buying one.