Have you discovered black patches of mould creeping up on the walls? Well, there are many curious questions that may arise, including what causes mould on walls?. Most people blame humidity problems, general poor ventilation, and damp conditions in and around the house and that’s not far from the truth. The problem with mould is that it causes serious health risks to your loved ones like allergies and respiratory diseases.
This guide will take you through the main causes of mould on walls: condensation, rising damp, and penetrating damp
- Causes of mould in your home
- Effective treatments and prevention tips.
By the end, you should be in a position to know just about everything required to deal with mould at its source and keep the walls dry and potentially mould-free.
Let’s begin..
What Causes of Mould on Walls?
Mould develops in damp and unventilated surroundings, and mainly three primary causes of moisture to build up on walls include:
1️- Condensation is the number one reason black mould forms. These are the coldest surfaces to which the warm, moist air from cooking and bathing is directed. This happens mainly in the winter when, due to heating, windows are tightly closed and moisture is trapped inside.
What Condensation Causes?
🔹 Poor ventilation (keep windows closed, no extractor fans)
🔹 High humidity from cooking, drying clothes, and showering
🔹 Blocked air vents, stopping airflow
Signs of Condensation Mould:
Black mould patches on walls, mainly on corners and behind the furniture
Excess moisture on windows and walls
Musty smell in the affected rooms
How to Prevent Condensation Mould:
✔ Ventilate – Open windows, start extractor fans, and allow the air to circulate.
✔ Get a dehumidifier – This will help to lower moisture levels indoors.
✔ Don’t dry clothes indoors. If you must, keep the window open.
✔ Paint over third-party anti-condensation paint & mould killers to prevent future growth.
💡 Recommended Resource: Read the UK Government’s guide to damp and mould prevention.
2- Penetrating Damp: Mould Formation Due to External Moisture
Is the water seepage in walls from outside due to structure defects or poor maintenance?
Major Reasons Behind Penetrating Dampness:
🔹 Leaks in gutters, roofs, or downpipes
🔹 Damage to brickwork, cracked render, or bad seals around windows and door frames
🔹 Rain on poorly maintained external walls
Signs of Penetrating Damp:
✅ Damp patches on walls after rain
✅ Peeling wallpaper and bubbled paint
✅ Musty smell in affected areas
What to Do About Penetrating Damp:
✔ Repair roof leaks –inspect damaged tiles, gutters, and chimneys.
✔ Seal cracks in walls – Apply waterproof treatments such as Stormdry Masonry Protection Cream to prevent any water entering.
✔ Inspect seals around windows and doors – Replace any defective or broken seals.
💡 Pro tip: If the damp is persistent, then should consider a professional damp survey for structural problems.
Read more about penetrating damp
- Rising Damp: Moving Moisture Up the Walls.
Rising damp is when ground water rises through porous masonry due to a defective or absent damp-proof course (DPC).
What Factors Contribute to Rising Damp?
🔹 Failures of the DPC
🔹 Elevated levels of ground water
🔹 External works (like steps, flowerbeds, etc.) bridging the DPC
Rising Damp Symptoms:
❑ Tide marks or damp patches extending to about 1m.
❑ Peeling plaster, salt stains, or crumbling skirting boards
❑ Old, stale suffocating air and visible mould growth.
Treatment of Rising Damp:
✔ Installing chemical DPC – creates a moisture barrier.
✔ Remove and replace damaged plaster – waterproofing with membrane/tanking slurry before replastering.
✔ Improvement or ensuring drainage – as much as possible avoid having ground levels higher than the internal floors.
How To Remove Black Mould from Walls
Once you know what causes that moisture on walls, it is very important to remove it so that it does not grow again.
Step-by-step guide to the Removal of Mould:
1️⃣ Wear protective gear-head, hand, and eye – gloves, mask, and safety goggles.
2️⃣ Clean the surface-as for example biocidal mould remover and do not simply use bleach.
3️⃣ Apply a fungicide- that is at root in killing the mould.
4️⃣ Let the walls dry completely- use a heater or dehumidifier.
5️⃣ Apply fresh paint of mould-resistant type- for long-term effect.
💡 Pro-tip: It might just be washing with bleach to get rid of stains but will not prevent growth of mould.
Final Thoughts:
To sum up, what causes mould on walls? condensation, penetrating dampness, and rising damp are main factors and all three require different treatments.
Ways to avoid mould in the house:
✔ Increase ventilation and the use of dehumidifiers to reduce moisture inside.
✔ Attention to leaks and external damage so as to prevent water from getting in.
✔ Attend to rising dampness by installing or repairing an adequate DPC.
✔ Use mould-resistant paint and treatment for long-term protection.