General information only. If anyone in your home has asthma, allergies or a weakened immune system, or the problem is large, get professional advice.
This guide is for someone who has noticed black spots creeping along a bathroom ceiling, a musty smell in a bedroom cupboard, or windows streaming with water every winter morning: a renter in an older flat, a family in a newly insulated house, a homeowner with a damp basement. Mould is common, and small patches are usually manageable. The key is that cleaning alone never solves it. Mould grows where there is moisture, so the real job is finding and fixing the source of the water.
Why it matters
The US Environmental Protection Agency’s brief guide to mould, moisture and your home explains that moulds produce allergens, irritants and in some cases potentially toxic substances. Inhaling or touching mould or its spores may cause allergic reactions in sensitive people, and mould can trigger asthma attacks in people with asthma who are allergic to it. The EPA adds that mould exposure can irritate the eyes, skin, nose, throat and lungs of people with or without mould allergies.
Mould also damages what it grows on: paint, plaster, wood, fabrics and stored belongings.
The rule that matters most
The EPA states that indoor mould growth can and should be prevented or controlled by controlling moisture. It also notes that if wet or damp materials are dried within 24 to 48 hours after a leak or spill, in most cases mould will not grow.
That gives you two priorities: stop water getting where it should not be, and dry anything wet quickly.
Find the cause
Damp has three main sources, and each needs a different fix.
| Type | Typical signs | Common causes |
|---|---|---|
| Condensation | Water on windows and cold walls; mould in corners, behind furniture and on window frames; worse in winter | Cooking, showers, drying clothes indoors, poor ventilation |
| Leaks | A patch that grows after rain or in one spot; staining on ceilings below bathrooms | Roof, gutters, plumbing, failed seals around baths and showers |
| Rising or penetrating damp | Tide marks low on walls; damp patches on outside walls; flaking plaster | Damaged brickwork, blocked air bricks, ground levels too high outside |
Condensation is the most common
Everyday activities release a lot of moisture into the air. When warm, moist air meets a cold surface, it condenses. The EPA notes that condensation can be a sign of high humidity.
Look for patterns
Mould spread evenly across cold corners and behind wardrobes on outside walls usually points to condensation. A single patch that grows after rain points to a leak.
Cleaning removes the mould. Only fixing the moisture stops it coming back.
Measure the humidity
The EPA recommends keeping indoor relative humidity below 60 percent, and ideally between 30 and 50 percent. It notes that humidity can be measured with an inexpensive moisture or humidity meter, available at many hardware stores.
Place a meter in the problem room for a week, checking it in the morning and after showers or cooking. If readings regularly sit above 60 percent, condensation control is your first job.
Reduce condensation
Ventilate at the source
- Run the bathroom extractor fan during showers and for a while afterwards
- Use the cooker hood when cooking, vented outside if possible
- Keep lids on pans
- Open a window a little in the room you are using, if the weather allows
Dry clothes carefully
Drying laundry on radiators releases a large amount of water into the air. Dry clothes outside when possible, or in a room with the door closed and a window open or a dehumidifier running. If you use a tumble dryer, make sure it is vented outside or is a condenser model that is emptied regularly.
Keep air moving
Leave a small gap between furniture and outside walls. Avoid overfilling wardrobes and cupboards on cold walls.
Keep rooms reasonably warm
Cold surfaces attract condensation. Keeping rooms at a steady, moderate temperature is usually better than letting some rooms get very cold.
Use a dehumidifier
In persistently humid rooms, a dehumidifier can bring humidity into the recommended range. Empty it regularly and keep the filter clean.
Fix leaks and structural damp
Leaks
Fix plumbing leaks and other water problems as soon as possible, as the EPA advises. Check gutters and downpipes for blockages, look for missing roof tiles, and reseal around baths, showers and sinks.
Penetrating and rising damp
These need a closer look at the building: damaged pointing, cracked render, blocked air bricks, or soil and paving piled above the damp-proof course. A surveyor or reputable damp specialist can diagnose the cause. Be wary of anyone who recommends expensive treatment without first identifying the source of moisture.
Cleaning small areas of mould
The EPA’s guidance suggests that if the mouldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, you can usually handle the job yourself. For larger areas, or after significant water damage, it points to more detailed professional guidance.
Protect yourself
The EPA recommends limiting exposure to mould and spores. Its suggestions include:
- An N-95 respirator, fitted properly according to the instructions
- Long gloves extending to the middle of the forearm; ordinary household rubber gloves are fine for water and mild detergent
- Goggles without ventilation holes, to keep spores out of your eyes
Clean
- Scrub mould off hard surfaces with detergent and water, then dry completely, as the EPA advises
- Absorbent materials such as ceiling tiles and carpet may need to be thrown away if they become mouldy, because mould can be difficult or impossible to remove completely from porous materials
- Do not paint or caulk over mouldy surfaces; the EPA notes that paint applied over mould is likely to peel. Clean and dry first
Bathrooms
The EPA’s bathroom tip is practical: in places that are often damp, mould that keeps reappearing can usually be prevented or kept to a minimum by increasing ventilation, running a fan or opening a window, and cleaning more often.
When to call a professional
- The mouldy area is larger than about 10 square feet
- There has been significant water damage, such as a flood or burst pipe
- Mould keeps returning despite good ventilation
- You suspect mould inside walls, under floors or in the heating or ventilation system
- Anyone in the home has health problems that may be linked to mould
- The cause appears structural
Hidden mould
A persistent musty smell with no visible mould can mean growth behind panelling, under carpets or inside walls. Investigating can release spores, so if you suspect hidden mould over a large area, it is worth getting a professional to open things up and assess it safely.
If you rent
Tell your landlord promptly and in writing, with photos and dates. Many tenancy rules require landlords to fix leaks and structural problems, while tenants are usually expected to ventilate and heat reasonably. Keep a record of humidity readings and the steps you have taken. Local housing authorities or tenant advice services can explain your rights where you live.
Damp and mould checklist
- Type of damp identified: condensation, leak or structural
- Humidity meter placed; target below 60 percent, ideally 30 to 50
- Extractor fans used during and after showers and cooking
- Laundry dried outside or with ventilation
- Furniture moved slightly away from outside walls
- Leaks fixed and seals replaced
- Wet materials dried within 24 to 48 hours
- Small mould patches cleaned with detergent and water, wearing protection
- No painting over mould
- Professional help sought for large or recurring problems
- Landlord notified in writing, if renting
Takeaway
Mould is a moisture problem. Work out whether you are dealing with condensation, a leak or structural damp, keep humidity below 60 percent as the EPA recommends, dry any spills within 24 to 48 hours, and clean small patches with detergent and water while wearing protection. Call in professionals for anything larger than about 10 square feet or anything that keeps coming back.
