Condensation and black mould, fixed at the cause.

Why does black mould keep coming back?

Because the conditions that grew it are still there. Mould needs moisture, and in most homes I see, that moisture comes from condensation on a surface that is too cold.

It usually appears in the same places: behind a wardrobe on an outside wall, in the top corner of a bedroom, around a bay window reveal, behind a baby's cot. Those spots are the coldest surfaces in the room, and the air is often still behind furniture and curtains.

Many people who call me have already tried everything the shop sells. It looks fine for a few weeks, then the grey shadow and black spots return as soon as the weather turns. That is not a sign you are doing something wrong. It is a sign the cause has not been dealt with.

Why don't bleach, anti-mould paint or fogging work for long?

They treat the mould you can see, not the cold, wet surface it grows on. Once the surface gets damp again, new mould grows.

• Bleach and mould sprays: remove the staining, but the moisture returns every cold night, and new spores settle straight back on.
• Anti-mould paint and stain blockers: slow growth for a while, but the paint film still gets wet, and mould then grows on top of it or on the dust and grease that settle on it.
• Fogging and professional cleaning: can reduce spores in the air and on surfaces for a time, but they do nothing to the wall temperature or the ventilation.

Cleaning is still worth doing, carefully, while you sort out the cause. It just cannot be the whole answer.

What causes condensation? The dew point, simply explained

Warm air holds water vapour. When it touches a surface colder than a certain temperature, called the dew point, some of that vapour turns back into water on the surface.

A normal family adds a lot of moisture to the air every day from breathing, cooking, showers and drying washing. In a room at 20°C with fairly typical humidity, the dew point is around 12°C. So any surface colder than about 12°C will start to collect water.

Now picture an uninsulated solid brick wall on a freezing day. My calculation for a 1930s bedroom wall put its inside surface at about 14.5°C with 20°C in the room and 0°C outside. Corners, reveals and the wall behind a wardrobe are colder still, and those are the places that drop below the dew point. Mould does not even need visible water: a surface that stays very humid for long periods is enough.

After that wall was insulated, the calculated surface temperature rose to about 18.4°C, well clear of the dew point. That is the principle behind fixing mould for good.

Is it condensation, or damp getting in from outside?

That is the first thing I check, because the fix is completely different. Insulation helps with cold surfaces, but it must never be used to cover up water getting in.

During the free home visit I look for signs of penetrating damp and leaks: blocked or leaking gutters and downpipes, damaged pointing or cracked render, a leaking pipe or shower tray, soil or paths built up above the damp-proof course, and faulty flashing around bay roofs. Patterns tell a lot. Condensation mould tends to sit in cold corners and low-airflow spots; a tidemark or a patch that grows after rain points to water coming in.

If there is a leak or a defect, I tell you plainly and it gets repaired first. Only then does it make sense to talk about insulation.

Is black mould harmful to my family?

It is worth taking seriously, but it is not a reason to panic. Damp and mould can affect breathing and skin, and some people are more sensitive than others.

The main concerns are coughing, wheezing and breathing difficulties, asthma and allergies starting or getting worse, more chest infections, and irritated eyes and skin, including eczema flare-ups. Babies and children, older people, anyone with asthma or another lung condition, and people with weakened immune systems are the most vulnerable.

If someone in your home has symptoms, speak to your GP. On my side, the job is to remove the conditions mould needs, so your child's bedroom stays clean and dry through the winter, not just for a few weeks.

How do you stop mould coming back for good?

By dealing with the two causes together: cold surfaces and moisture that cannot escape. I start with a free home visit, find out exactly where and why it is happening, then recommend the simplest fix that will actually last.

• Warm the cold walls: internal wall insulation raises the surface temperature of cold solid walls. Cold bays are often the worst spot, and bay window insulation deals with them properly.
• Targeted treatments where suitable: for some smaller problem areas, a thin internal cork coating can warm the surface without losing room space. I will tell you honestly if it suits your wall.
• Moisture control: ventilation that actually removes wet air from kitchens, bathrooms and bedrooms.
• Everyday habits: simple changes to heating and airing that make a real difference once the walls are warmer.

Every system I use is chosen after moisture modelling, so the wall stays able to dry and the problem is not simply moved somewhere else.

What ventilation does my home need?

Enough to remove the moisture you make, without making the house cold. Insulation and ventilation work together: one keeps the heat in, the other lets the damp, stale air out.

Typical improvements include trickle vents in windows, extractor fans in kitchens and bathrooms that are powerful enough and actually used, gaps under internal doors (around 10mm) so air can move between rooms, and in some homes a heat recovery system. For context, current guidance for new ventilation design in England gives a whole-house rate of at least 31 litres per second for a three-bedroom home. An existing house needs its own assessment, which is why ventilation is more than just fitting a fan.

I recommend what your home needs and organise the work with trusted electricians and joiners, so it fits in with the rest of the job.

I'm a landlord. What do I need to know about damp and mould?

You need a clear diagnosis, a lasting fix and a record of what was done. Repainting and cleaning between tenancies becomes an expensive cycle, and tenants rightly lose patience.

In England, Awaab's Law already sets strict deadlines for social landlords to investigate and fix damp and mould hazards. The government intends to extend it to private rented homes, but that extension is not yet in force, so the same deadlines do not yet apply to every private landlord. Private landlords still have existing legal duties on damp and mould, including under the Housing Health and Safety Rating System and the Homes (Fitness for Human Habitation) Act 2018. A minimum EPC rating of C for private rentals has also been proposed.

I can survey the property, explain the cause in writing, carry out the insulation and plastering, and organise ventilation work with trusted trades. That gives you something solid to show your tenant and a property that stays in better condition.

What can I do right now while the cause is sorted?

A few simple habits will reduce condensation straight away. They will not fix a cold wall, but they make the room healthier while you plan the proper work.

• Put lids on pans and use the cooker hood every time you cook.
• Run the bathroom fan during and for about 20 minutes after a shower, with the door closed.
• Dry washing outside or in a room with the window open or an extractor running, not on radiators in bedrooms.
• Leave a gap of 5–10cm between furniture and outside walls so air can move behind it.
• Keep trickle vents open and heat the room steadily rather than in short, hot bursts.
• Wipe condensation off windows and sills each morning.

Cleaning mould safely needs the right protection: a proper mask, gloves and eye protection, a fungicidal wash rather than bleach, and the window open. Never brush or scrape dry mould, as that spreads spores into the air. If the patch is large, keeps coming back, or anyone in the home has asthma or a weak immune system, leave it to a professional.

How much does it cost to fix condensation and black mould?

It depends entirely on the cause, so I price it after a free home visit. Some homes need a ventilation upgrade and a change of habits; others need a cold wall or bay insulating.

Where insulation is the answer, internal wall insulation starts from about £2,000 per wall and bay window insulation from about £1,700 per bay. You get a written quote that explains the cause, what I recommend and why, so you can decide calmly. My guide on how to stop mould is a good place to start if you want to read more first.