Black mould on bedroom walls: why does it keep coming back after cleaning?

If you have scrubbed black mould off a bedroom wall only to watch it creep back, you are not doing anything wrong with the cloth. Mould grows where a surface stays damp. Cleaning removes the mould you can see, but not the moisture that fed it. Until that moisture is found and dealt with, the mould will usually return. Government guidance for landlords puts it plainly: simply removing surface mould will not stop damp and mould reappearing.

Most people searching for black mould want three answers. Is it harmful? How do I get it off? How do I stop it coming back? This article takes them in turn. The third gets the most space, because it is the one that actually solves the problem.

Why black mould keeps coming back

Mould is a type of fungus that grows in moist environments. Wipe it away and the patch looks clean, but the wall underneath is still cold and damp. Within a few weeks the conditions that grew the first crop will grow another.

GOV.UK guidance says condensation is the most common form of damp. Moisture made inside the home cools and settles on colder parts of the building. Examples include window frames, corners and low points on walls behind sofas or wardrobes. That moisture comes from cooking, showering, drying washing and simply from people being in a room.

Bedrooms are a classic spot for it. The RICS consumer guide says condensation mould typically forms around windows, on external walls and on ceilings close to external walls. It also notes that moisture can travel from kitchens and bathrooms and condense on cold external walls elsewhere in the house. Put a wardrobe against an outside wall, close the door and window overnight, and you have a cold, still corner where moisture can settle.

Condensation is not the only possibility. Water can get in from outside through defects in walls, roofs, windows or floors. It can also come from leaking pipes inside. There may be more than one cause at once. GOV.UK guidance notes that water getting in or leaking can make condensation worse. The RICS guide gives an example of water coming up through a solid floor, raising humidity and causing condensation as a second problem.

Is black mould on bedroom walls harmful?

GOV.UK guidance says damp and mould primarily affect the airways and lungs, but they can also affect the eyes and skin.

It also says everyone is vulnerable to the health impacts of damp and mould, but people with certain health conditions, children and older adults are at greater risk of more severe health impacts. That is a good reason not to leave mould where someone sleeps.

The same guidance says anyone concerned about symptoms they are experiencing should consult a healthcare professional.

How to clean it off safely

Before you clean, take a photo of the patch and, if you can, draw around its edge. GOV.UK guidance suggests this because a record of where the mould grew helps identify the source. Then:

• Use a mould and mildew cleaning product. GOV.UK guidance prefers these to bleach, for health and safety reasons.
• Wear a mask, gloves and goggles, however small the patch.
• Keep the bedroom door shut so spores do not spread, but open the window during and after cleaning.
• Let the surface dry completely afterwards.
• Wash or throw away the cloth or sponge, and wash any protective clothing.

GOV.UK guidance says most tenants could reasonably be expected to remove condensation and very small amounts of mould. Larger areas should only be tackled by qualified professionals. The RICS guide gives an example of a larger area: a substantive part of a whole wall or ceiling. Mouldy carpets, soft furnishings and other absorbent items may be difficult or impossible to clean completely.

A musty smell with no visible mould can mean mould is hiding behind something, such as wallpaper, panelling or furniture. Take care here. Pulling off wallpaper or lifting carpet can release a lot of spores, so wear protective equipment and get advice if you are unsure.

How to stop it coming back

The first step is to find where the moisture is coming from and deal with that. If it is condensation, the aim is to make less moisture, let more of it out and keep walls warm enough that it does not settle.

Make less moisture

• Cover pans when cooking.
• Keep kitchen and bathroom doors closed while in use, and use extractor fans when cooking, bathing or showering.
• Dry clothes outdoors where you can, rather than over radiators.
• Make sure any tumble dryer is vented outside or is a condenser type.

Let moisture out

Windows do not need to be open all the time. The RICS guide says about five minutes for every hour you are in a room is enough. Trickle vents or a secure window latch also let in a small, steady amount of air. If your bedroom has an air vent, do not block it. Leave a gap between furniture and external walls so air can reach the wall behind.

Keep surfaces warmer

The RICS guide suggests keeping internal heat levels ideally between 18C and 21C. The SPAB, a building conservation charity, adds that heating run constantly at a low level, rather than in short bursts, can keep surfaces above the point where water condenses. We know heating costs are a real worry. The RICS guide makes the point that even if you can only heat yourself, your home still needs ventilation.

Wipe up condensation daily

Wipe condensation off windows and other cold surfaces each day with a dry cloth. The RICS guide says this helps prevent mould growth. A dehumidifier can help too, but the SPAB notes it works well when rooms are warm and damp and is of limited benefit when they are cold and humid.

These habits help, but they are not the whole story. GOV.UK guidance is clear that it is totally unreasonable to blame damp and mould on lifestyle choices. Everyday tasks like cooking and washing will always produce moisture. The fundamental cause is usually the building itself: defects, poor ventilation, inadequate heating or poor energy efficiency.

Why black mould loves a cold gable wall in Yorkshire and Lancashire

Many older stone terraces and back-to-backs across Yorkshire and Lancashire have solid walls and face plenty of wet Pennine weather. A bedroom on a cold, exposed gable or a north-facing wall is exactly the kind of cold surface where condensation settles. The SPAB notes that diffuse damp on north-facing walls is commonly mistaken for rising damp and used to justify unnecessary damp-course work.

That matters before anyone sells you damp-proofing. The RICS guide says that in some homes, such as older buildings with solid walls, damp-proofing measures may not be appropriate and may make damp worse. It also warns that modern cement in mortar or render on older buildings can trap moisture in walls. The SPAB adds that double glazing, heavy draughtproofing and blocked flues or air bricks can reduce the ventilation old buildings relied on.

So if bedroom mould appeared after new windows went in, a fireplace was blocked up or a wall was rendered, that is a useful clue. It is also worth stepping outside on a wet day. Look for overflowing gutters, leaking downpipes, worn pointing, cracked render and soil or paving built up against the walls. Exposed gables take the brunt of the weather, and a small defect there can keep an inside wall damp.

When to ask a surveyor to find the cause

If mould keeps returning to the same patch despite cleaning and changes to your routine, covers a large area, or comes with a musty smell you cannot trace, it is sensible to get an independent opinion. The RICS guide suggests a building surveyor with experience in building pathology, which is the study of why buildings fail. It advises asking for an inspection that complies with the Joint Position Statement 2022 (JPS22). Such a report should come from someone with no commercial interest in selling you a product.

Be wary of anyone who relies only on a damp meter, especially on solid walls. The RICS guide explains that these meters are accurate in timber, but in other materials can give high readings from things other than moisture.

If you rent, or you are a landlord

If you rent, report damp and mould to your landlord or managing agent promptly, and check your tenancy agreement for what you are responsible for.

If you are a landlord in England, GOV.UK guidance says you are legally responsible for addressing damp and mould, whatever the type of damp. It says you should find and tackle the underlying causes, not just remove visible mould. It also says you should revisit at least six weeks after the work to check the mould has not reappeared. If it has, the guidance says you should investigate further.

This page gives general information, not legal or medical advice. For your own situation, use the organisations listed under "Where this comes from" at the foot of the page.