Musty smell at home: where damp smells come from and how to get rid of them

Why does my home smell damp?

A musty smell — that smell of wet earth, cellars, clothes stored too long — comes from compounds released by moulds and bacteria growing on damp materials. You can often smell it weeks before any visible stain appears.

Mould doesn't need dripping walls. It only needs the humidity right at a surface to stay above ~80% for a few days. That happens behind a wardrobe pushed against a cold wall, inside a closed cupboard or under a sink with a small leak.

In winter it gets worse: windows shut, laundry drying indoors and a family of four putting about 10 litres of water into the air every day. If your home smells musty mainly in the morning or when you get back from work, moisture is building up — and somewhere a surface is growing mould.

Mould, drains or something else? How to identify the smell

Telling the smells apart saves time. A trap with no water lets sewer gases up and is fixed in seconds; hidden mould means finding the source and cutting off the moisture.

Traps dry out by evaporation in drains left unused for weeks: the bidet, the floor drain in a guest bathroom, the kitchen sink of a home left closed. If the sewage smell returns within a few hours of running water, the trap or seal is damaged and it's a job for a plumber.

Where hidden mould lurks

• Behind wardrobes and beds pushed against outside walls — air doesn't circulate and the wall is colder.
• Inside wardrobes: the back panel, shoes, suitcases and cardboard boxes stored on the floor.
• Under the kitchen sink and the washbasin: dripping traps, badly sealed waste pipes, swollen chipboard at the base of the unit.
• Behind the fridge and the washing machine, where there may be condensation or small hose leaks.
• In the washing machine door seal and filter, where water sits between washes.
• Under rugs and floating floors on ground floors and in basements.
• By the windows: curtains touching the glass, roller-shutter boxes, tracks of aluminium frames.
• In false ceilings and behind tiles, when there's slow ingress or a leak in the flat above.

Air conditioning can also smell musty when it switches on: the condensate tray and filters stay damp between uses. Clean the filters following the manufacturer's instructions and, if the smell persists, have the indoor unit cleaned.

How to find the source of a musty smell

1. Close the internal doors at night. In the morning, walk into each room coming from outside: the room where the smell is strongest is your starting point.
2. Open wardrobes and cupboards and move furniture away from outside walls. Shine a torch on the backs of furniture and along the skirting.
3. Open the cupboards under sinks and basins. Put dry kitchen paper under the traps, run the water for a minute and see whether it gets wet.
4. Measure the humidity at 7am in each room with a ~€10 hygrometer. Above 70% for several days in a row, condensation is likely.
5. Measure suspect walls with an infrared thermometer (€15–20) and compare with the dew point calculator.
6. Turn off every tap and check whether the water meter moves. If it does, there's a leak — and the smell may come from it.

If you then find stains, the diagnosis helps you work out whether the cause is condensation, ingress, rising damp or a leak. Choose «smell» for the appearance and say where you notice it.

Quick fixes and fixes for the cause

Quick fixes remove the smell for a few days or weeks. They're worth doing, but they only solve the problem when paired with fixing the cause.

To clean mould, use diluted bleach — 1 part to 4 of water — wearing gloves and a mask with the window open, and never mix it with other products. Use biocides safely. Always read the label and product information before use.

Absorbers work well in small, closed spaces such as wardrobes and cupboards; in a whole room they make no difference. The guide do damp absorbers work? explains the limits. Air fresheners and scented candles just cover the smell: the mould keeps growing underneath.

Basements and ground floors: when the smell comes from below

Basements almost always smell damp. They're in contact with the ground, cold and poorly ventilated. Water can rise by capillary action, come through buried walls or condense on cold surfaces — often all three at once.

One detail surprises people: in summer, opening the windows of a cold basement in the heat of the day can increase the humidity. Warm, humid outdoor air cools as it comes in and the water condenses on the walls. Air the basement early in the morning or at night, when the outside air is cooler.

In basements and rooms below ~15 °C, a desiccant dehumidifier usually works better than a compressor one. There's more in the guide damp in basements and ground floors and on the basement page.

In basements, store things in closed plastic boxes rather than cardboard, away from the walls and raised off the floor on shelves or pallets. Cardboard and fabrics absorb moisture and are among the first materials to grow mould.

When to ask for help

Ask for a professional opinion if the smell persists after 2–3 weeks of airing, cleaning and measuring; if you can't find any visible source; if the water meter moves with everything off; or if the smell gets worse after rain. In these cases water is getting in from somewhere and the problem isn't just habits.

• Traps, hoses and plumbing leaks: a plumber (canalizador).
• Stains after rain, on the ceiling or by the windows: a builder or waterproofing company — or the condomínio (owners' association), if the source is a common part.
• Mould spread across several rooms that returns after cleaning: measure the humidity and consider mechanical ventilation.

Professional mould cleaning costs from €48, but it treats the symptom. It makes sense once the cause is fixed, or when the area is large and nobody at home should be cleaning it.

If you rent, tell the senhorio (landlord) about the problem in writing, with photos and readings. The guide for tenants explains, in general terms, what the law provides.