Black Mould On Walls – Causes & Treatment

Black mould appearing on interior walls is a common issue for many UK homeowners. It’s more than an eyesore, it often signals a bigger problem at hand.

In this guide, we’ll explain:

• What black mould is
• What causes black mould
• How to remove black mould
• How to prevent it from returning long-term

What Is Black Mould?

Mould, or more specifically black mould, is a highly common problem experienced by homes, particularly in the UK. Black mould is directly a result of excess levels of moisture within a property, caused by any number of sources.

Black mould appears in the form of small black spots of mould that spread across walls, ceilings or window frames.

Common indoor mould species include Aspergillus and Cladosporium, which thrive in areas with poor ventilation such as behind furniture, in high-moisture rooms such as bathrooms or kitchens and on the walls of rooms that have cold spots, such as external facing walls.

What Causes Black Mould Growth?

Black mould forms when there is too much moisture in your home. Mould spores exist naturally in the air, but they can only develop when humidity or damp conditions allow them to.

Common causes of black mould include leaks, penetrating damp, rising damp and condensation.

Leaks

Leaks from household appliances, roofs, windows and pipes can all lead to water soaking into walls, ceilings and floors. Even a small leak that goes unnoticed for a while can create the conditions needed for mould to grow.

Penetrating damp

Contrary to popular belief, exterior walls are porous, meaning they will absorb moisture over time. This moisture is then transferred into the property causing high levels of damp.

This process is known as penetrating damp and is one of the most common causes of damp walls in the UK.

The UK’s climate adds to the challenge, with over 150 rainy days per year according to the Met Office. Combined with an ageing housing stock, this means many homes are vulnerable to water ingress from rainwater.

Rising damp

Water from the ground can be absorbed up through the walls of a property in a process known as rising damp.

A damp proof course (DPC) is designed to protect from this, however if this is failing then the moisture from the ground will simply soak into the bottom-level of a property and continue to rise up the external and internal walls.

Rising damp is generally rare and is often misdiagnosed instead of other, more common forms of damp.

Condensation

Condensation is created through the day-to-day living within our homes, from dryers, washing machines and showers, to name a few. Because of this, condensation can form in any house, making it a particularly common cause of mould.

When warm, moist air comes into contact with colder surface temperatures, the water is released causing condensation to form. This starts forming when the surface temperature drops below the dew point, usually 13°C–15°C.

This is most prominent in winter when indoor and outdoor temperatures vary widely. Cold walls combined with closed windows, heating and clothes drying indoors all raise humidity levels and make condensation more likely.

What Problems Does Black Mould Cause?

Black mould is more than just an eye-sore, it’s a sign of potential risk to the health of your home and those who live in it.

Health risks

Black mould can cause a number of health problems. As emphasised by the NHS, “if you have damp and mould in your home you’re more likely to have respiratory infections, allergies or asthma”.

Those most affected include:

• Young children
• The elderly
• People with existing respiratory or immune conditions

Even for healthy adults, breathing in mould spores can lead to coughing, skin irritation and other conditions if the issue persists.

Aesthetic damage

While not caused directly by black mould, high levels of moisture will lead to bubbling paint or wallpaper peeling. Over time this leaves walls discoloured and stained. Black mould itself leaves visible dark spots that can spread across plaster, making redecorating more difficult.

Odours

Black mould growth can leave a damp, musty smell in rooms which can be unpleasant. These odours come from microbial activity and are difficult to mask without addressing the underlying issue.

Structural damage

While mould growth itself isn’t a direct cause of damage, it often signals a deeper problem. Damp walls may point to cracked render, damaged mortar or porous bricks and render that are allowing water into exterior walls.

If left untreated, this moisture can further weaken exterior walls and lead to the need for more extensive repairs in the future.

How Do You Prevent Black Mould?

In order to prevent the growth of black mould on interior walls and ceilings, you must prevent the source of the moisture that is allowing it to grow.

Step 1: Identify the source of moisture

The first step to a home free of damp and mould is understanding where it’s forming as this often tells you what’s causing it.

• On external-facing walls – This could indicate penetrating damp, as rainwater may be soaking into the external wall and travelling through to the internal side. Any signs of water damage on exterior walls can also indicate this, such as peeling paint, cracks or damp patches.
• Near the bottom of walls – Mould close to the skirting boards could point to rising damp, which can only occur at the very bottom of the wall and gradually rise up through the floor-level. These walls must be external-facing walls.
• On cold walls – These areas are more prone to condensation because the lower temperature allows it to form, creating the perfect environment for mould to grow.
• Near windows – Mould here usually points to condensation as it builds up when warm air meets the cold glass, especially if the area lacks ventilation or is in a high-humidity room like a bathroom or kitchen.
• Behind furniture –  When air can’t circulate properly moisture gathers. This often happens behind wardrobes, beds or sofas placed against walls.

Step 2: Reduce the moisture

Once the cause of the damp and mould has been identified, the next step is to reduce this moisture within the home.

Condensation

If condensation is leading to black mould, improving airflow, reducing cold spots and lowering humidity is the key to success.

• Improve the ventilation in your home by opening windows regularly and using extractor fans in bathrooms and kitchens.
• Avoid drying clothes on radiators where possible.
• Leave a gap between furniture and the wall to allow for better airflow.
• Keep rooms evenly heated to prevent cold surfaces where moisture can form.
• Apply an anti-condensation paint to susceptible areas to reduce surface moisture, helping to prevent mould growth.
• Aim to keep indoor humidity below 50% to reduce the risk of condensation forming.

Good practice for any home involves keeping air moving. Even simple habits like opening a window make a real difference, something the Energy Saving Trust recommends for reducing mould.

Rising damp

Check whether your property has a visible DPC, which is usually a thin horizontal line between the first few rows of bricks. If it’s damaged or sitting below ground level, it may need to be addressed.

• Make sure soil, gravel or paving sits below the damp proof course. If not, this may bridge the barrier.
• Check paths slope away from the house, so water drains off instead of pooling against the wall.
• Keep air bricks clear by removing debris.
• Check gutters, downpipes and drains to make sure water is being directed away from your home.

If the damp persists, a professional can help address the DPC being bridged and ensure moisture doesn’t rise through the wall.

Penetrating damp

Penetrating damp happens when rainwater soaks through the external walls of your home, so to reduce the risk, focus on keeping the walls in good condition and protecting them from water being absorbed into the masonry.

• Inspect for damage – Look for cracks, missing mortar or damaged bricks that can let rainwater in. Also watch for peeling paint, discoloured patches or algae growth which are all signs that moisture is getting into the wall.
• Keep gutters clear – Blocked gutters can cause water to overflow on to exterior walls, so make sure they drain water freely.
• Repair defects – If render is cracked or brickwork is damaged, remove the loose areas and apply a suitable filler or mortar.

Learn more: How to repair exterior walls

Once the structure is sound, protecting the masonry from future water absorption is key to reduce the risk of more mould and damp.

Walls need to naturally breathe, so any treatment you use should keep rainwater out while still allowing water vapour to freely escape.

This is something sealers and standard masonry paint often fail to achieve. Sealers form a plastic-like film over the surface, which may block water in the short-term but seals the surface and traps moisture within the wall. Over time, that moisture forces its way out, causing the sealer to fail once damp problems have been made worse.

In this case of standard masonry paints, they reduce water absorption and allow some water vapour to escape, but to varying levels. Most still restrict the wall’s ability to breathe while still absorbing rainwater. This is why moisture is the most common cause of masonry paint failure, as it causes the paint film to bubble and blister often after 1-5 years.

To achieve genuine lasting protection, walls need a treatment that reduces water absorption while maintaining high breathability, something that sounds impossible.

That’s where nano-technology provides the answer. This creates a super hydrophobic surface that causes water to form beads that roll away, while not sealing the pores of exterior walls, maintaining high breathability.

This cutting-edge technology is built into Emperor Masonry Paint and Emperor Masonry Creme. Emperor Masonry Creme offers invisible protection for bare masonry, while Emperor Masonry Paint creates a coloured finish for brick, render and any exterior wall.

Both have been independently tested to reduce water absorption by up to 96%, while remaining highly breathable with SD values of 0.01 and 0.03.

Independent UKAS accredited testing has shown them to withstand a minimum of 25 years of harsh weathering without failing, which is why they come with a lifetime guarantee.

Learn more: How nano-technology works

How Do You Get Rid Of Black Mould?

Only once you have addressed the root cause of black mould should you start to remove it. Cleaning alone won’t stop mould long-term, but it’s an essential step.

• Make sure you are wearing appropriate PPE such as gloves and a mask.
• Always use a specialist mould remover to remove black mould. These biocidal cleaners kill mould spores at the root, helping prevent it from returning.
• Apply the cleaner with a cloth or sponge and leave for 10-15 minutes before wiping away.
• Dry the surface completely to avoid saturating the wall, before repainting.

Frequently Asked Questions

WILL BLACK MOULD REDUCE OVER TIME?

Black mould on walls will only begin to reduce when you prevent the source of the moisture that is allowing it to grow.

Whether this is from penetrating damp or from condensation, the only way to address black mould is to prevent the source of the moisture.

Where does black mould commonly appear?

Black mould grows where moisture builds up and airflow is limited, so bathrooms and kitchens are prime spots. Bedrooms are another common area, particularly on external-facing walls, behind wardrobes and near windows.

Can I paint over black mould?

No, you should not paint directly over black mould. This will lead to the mould simply growing underneath the paint and reappearing. Paint also cannot adhere to dirty, damp surfaces.

Before painting an area that has suffered from black mould, first remove it, address the source of the moisture and prepare the surface to be painted again.

Preventing black mould starts with understanding the cause and choosing the right long-term solution. Whether you’re tackling condensation, rising damp or penetrating damp, taking action now protects your home for years to come.

If you want to see how healthy your walls are, use our handy wall health check to spot issues early and keep your home stress free. Alternatively, chat to our in-house team of experts who are on-hand to give you tailored advice specific to your project. You can chat to them by emailing [email protected] or calling 01254 936121.

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This article was written by the Emperor Technical Team, a group of experts with an average of over 25 years experience in the building industry. They work daily with homeowners, tradespeople and specification professionals to deliver practical, expert guidance that promotes long-term results, supports wall health and helps create homes that are prepared for the future.