What Causes Mould?

Mould is one of the most common and frustrating, problems found in homes. It appears as dark, fuzzy patches on walls, ceilings, windowsills, and even furniture, often accompanied by a musty smell. But while people often blame “old houses” or “bad weather”, mould has a few very specific causes. Understanding them is the first step to preventing outbreaks and keeping indoor spaces healthy.

1. Excess Moisture: The Root of the Problem

Mould cannot grow without moisture. If the air or a surface stays damp for long enough, mould spores (which are always present in the environment) will settle and multiply.

Common sources of excess moisture include:

• Condensation on cold surfaces, especially windows, external walls, and corners
• Leaks in roofs, pipes, or appliances
• Flooding or water ingress
• Drying clothes indoors without adequate ventilation

Even small, repeated moisture events, like steam from a shower or a kettle, can accumulate over time.

2. Poor Ventilation

Stale, unmoving air traps moisture and creates ideal conditions for mould to thrive. Modern homes, especially those with energy-efficient insulation, often retain moisture if ventilation is inadequate.

Problem areas often include:

• Bathrooms without extractor fans
• Kitchens with ineffective ventilation
• Bedrooms where windows remain shut overnight
• Cupboards or wardrobes placed against cold external walls

Without consistent airflow, humidity builds up, and surfaces stay damp for longer.

3. Cold Surfaces and Thermal Bridging

When warm, moist air meets a cold surface, it forms condensation. This is why mould frequently appears in:

• Corners of rooms
• External walls
• Behind furniture
• Around window frames
• On ceilings, particularly near the edges

Poor insulation or “cold spots” (thermal bridges) make these areas vulnerable.

4. Everyday Activities That Increase Humidity

A typical household generates litres of moisture every day simply through daily living. Activities that raise humidity levels include:

• Cooking without lids or ventilation
• Long, hot showers
• Drying laundry indoors
• Using unvented tumble dryers
• Keeping doors closed while creating steam

Individually these seem harmless, but combined they can significantly raise indoor humidity.

5. Building Issues and Structural Problems

While lifestyle factors play a role, sometimes the building itself is the culprit.

Structural causes of mould include:

• Rising or penetrating damp
• Damaged pointing or roof tiles
• Blocked gutters causing water to seep into walls
• Poorly installed insulation trapping moisture
• Inadequate damp-proof courses

These issues usually require professional assessment.

6. High Humidity Climates

In damp or coastal regions, the surrounding environment contributes to mould growth. Homes may need extra precautions such as dehumidifiers, improved ventilation, or enhanced insulation to keep indoor humidity under control.

How to Reduce the Risk

• Keep rooms well-ventilated, especially after cooking or showering
• Use extractor fans and keep them running for 10–20 minutes after use
• Avoid drying clothes indoors where possible
• Leave space between furniture and external walls
• Insulate cold spots and repair any leaks promptly
• Monitor indoor humidity – ideally around 40–60%

Final Thoughts

Mould thrives wherever moisture, cold, and stagnant air combine. By understanding these triggers, you can take steps to control the environment and stop mould before it starts. With the right balance of ventilation, insulation, and moisture management, your home can stay fresher, healthier, and mould-free.