You notice a musty smell in the home, but you cannot see mould anywhere. This guide explains where that smell usually comes from, how to look for it without breaking anything, what not to do and when it makes sense to call a professional. Cleaning visible mould on walls is covered in the guide mould on walls, which we do not repeat here.
Updated on 4 November 2026. General information; it does not replace a technician’s assessment or your doctor’s advice.
The essentials
- According to the FOPH (Switzerland), a clear mould smell with no visible mould is a symptom of hidden mould. The US EPA and the 2026 English guidance say the same.
- Hidden mould can sit behind wallpaper and panels, behind furniture, around leaking or condensing pipes, and in ventilation equipment.
- The smell shows there is a moisture problem. Hiding the smell fixes nothing: you have to find the water and remove it.
- Do not paint over it. The EPA warns that paint applied over mouldy surfaces is likely to peel.
- Disturbing hidden mould (pulling off wallpaper, carpet or panels) can release large numbers of spores. If in doubt, ask for help.
Where the smell comes from
The FOPH writes that damp and mould go together: materials kept wet for a few days are very likely to become mouldy. A musty smell is the sign that somewhere there is wet material, even if you cannot see it. The sources we read point to these places:
| Place | What the sources say |
|---|---|
| Behind wallpaper, panels, suspended ceilings | EPA and English guidance: mould grows on the back and stays out of sight. |
| Behind furniture against outside walls | EPA: walls behind furniture, where moisture condenses. See mould in wardrobes and cupboards. |
| Around leaking or condensing pipes | EPA: wall cavities next to pipes that leak or sweat. |
| Ducts and ventilation units | EPA and English guidance: mould can be inside ducts and ventilation units. |
| Cold corners and outside walls | FOPH: outside walls cool in cold weather and become damp; a frequent cause. |
The DGS (Portugal’s health directorate) notes that excess humidity can condense on cold surfaces and lead to mould. For the mechanism, see bedroom condensation and ventilation.
What about drains? A sewer smell is not a mould smell, but people confuse them. We found several home-tips pages (mostly Brazilian) naming a dry trap as a common cause, but no official Portuguese source. So we give no rules: if the smell comes from a drain, a plumber can confirm it quickly.
How to look for the source
Go in this order, without opening walls or stripping coverings.
- Confirm the smell. The FOPH says people perceive smells very differently. Ask someone else to walk into the room without telling them what you are looking for.
- Find where it is strongest. Close doors and windows for a while, then sniff room by room, near the floor, the corners, the wardrobes and behind furniture.
- Pull furniture away from outside walls and open the wardrobes. Look for dark spots, discolouration, or a wall that feels cold and damp.
- Check the wet areas: under the kitchen sink and the basin, behind the toilet, around the shower tray, the ceiling below the bathroom above. See bathroom and extractor fan.
- Photograph everything before cleaning or touching: the English guidance recommends photographing mould before removing it, to help identify the source.
- Think about what changed: a leak, building work, drying laundry indoors. For the type of damp, see damp walls: the four types.
The FOPH adds that, with large mouldy areas, building materials should not be removed by the occupant: professionals have non-destructive methods to locate moisture. Measuring volatile organic compounds in the air is generally neither reliable nor conclusive.
What not to do
- Do not paint over it or mask the smell. The EPA advises cleaning the mould and drying the surface before painting, because paint over mould tends to peel. See also anti-damp, anti-mould paint: does it work?.
- Do not run a dehumidifier before the visible mould is removed. The FOPH repeats this warning in every damage category. See dehumidifier: when it helps.
- Do not tear off wallpaper, carpet or panels just to peek, without protection. The EPA and the English guidance warn this can release large numbers of spores.
- Do not call the job done while it still smells musty. The EPA says a cleanup is not finished while a mouldy odour remains. And dead mould can still cause allergic reactions, so it has to be removed.
- Do not ignore the cause. The English guidance says cleaning surface mould will not stop it returning if the moisture, ventilation or building defect stays unresolved.
Health: what the sources say
According to the FOPH, mould can irritate the eyes, skin and airways, and sometimes cause allergies. The 2026 English guidance mentions cough, wheeze and shortness of breath, and lists among those most at risk people with asthma, allergies, lung disease or weakened immune systems, pregnant women, children and older people. The EPA says mould can irritate even people who are not allergic.
People with severe allergic asthma, cystic fibrosis or a weakened immune system should talk to their doctor, and the FOPH says that in such cases detection and removal should not be done by the occupant. The DGS says poor indoor air quality can worsen chronic respiratory disease and allergies.
If you have symptoms that worry you, speak to your family doctor or call SNS 24. We found no DGS or SNS 24 page linking a particular smell to a diagnosis.
When to call a professional
- The smell persists after moving furniture, ventilating and checking the wet areas. The FOPH says that where someone feels unwell or vulnerable people live there, building-damage and building-physics specialists should be hired without delay.
- Suspected pipe leak or infiltration: call a plumber or check your home insurance.
- A large area of mould. The EPA suggests professional help for areas above about 10 square feet (roughly 1 m2) or after heavy water damage. The English guidance also refers larger areas to professionals.
- Rented home: the FOPH advises telling the landlord or building manager by registered letter, describing the place and the type of smell. Otherwise you can be blamed for secondary damage.
What we could not confirm
- An official Portuguese rule on musty smells. We found none from the DGS, SNS 24, LNEC or ADENE.
- The origin of sewer and drain smells from an official Portuguese source. We found only unofficial tips pages.
- Prices for an inspection or leak detection. We found none in a public source.
- Which air freshener or spray works against the smell: no official source confirms it, and they treat the smell, not the cause.
- How many Portuguese homes have the problem. We found no statistic.
Sources (read on 4 November 2026)
- Swiss Federal Office of Public Health (FOPH), Bolor em espaços habitáveis: informações para locatários, inquilinos e senhorios, Portuguese version, February 2021 (section “Cheiro a bolor sem bolor à vista”, damage-category tables, causes, note on air measurements).
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home (hidden mould, where it hides, painting, when to hire a professional, health effects).
- U.S. EPA, Mold Cleanup in Your Home (cleaning with detergent and water, complete drying; does not cover odour).
- Department of Health and Social Care (UK), Understanding and addressing the health risks of damp and mould in the home, updated 1 April 2026 (smell and hidden mould, symptoms, risk groups, cleaning). The first 100,000 characters were read; the annexes were not.
- Direção-Geral da Saúde, Qualidade do Ar Interior (condensation, mould, vulnerable groups). Undated page.