Mould is rarely “just a cleaning problem”. In most buildings, it’s a sign that moisture is building up faster than it can escape, whether through leaks, humidity, cold surfaces or poor ventilation.
In England alone, 2 million people live with significant damp and/or mould in their homes, and the health risks are now being recognised. The death of Awaab Ishak brought renewed attention to the need for faster, better responses to mould, reflected in the introduction of Awaab’s Law.
In this guide, we’ll share the key principles for investigating and preventing mould growth in buildings.
What Is Mould?
Mould is a type of fungus that grows on damp surfaces and spreads by releasing spores. You may recognise it as a dusty bloom on fruit, or as black patches on walls in a poorly ventilated bathroom.
Mould spores can affect health, especially when they build up in indoor air in damp spaces. That’s why preventing mould is less about removing the mould you see, and more about controlling the conditions that allow it to grow.
Types of Harmful Mould
There are over 100,000 types of mould. We can identify many of these by their appearance and location, but laboratory testing is needed to confirm the specific type.
Harmful moulds can be classified as:
- Allergenic: Triggering allergic reactions or asthma symptoms
- Pathogenic: Affecting people who are unwell or immunocompromised
- Toxigenic: Causing serious health issues
The colour of the mould alone isn’t a reliable indicator of risk, so the safest approach is to focus on prevention: reduce moisture and persistent high humidity.
What Causes Mould to Grow Indoors?
Mould is most likely to grow when moisture remains in a building and the relative humidity (RH) stays high. This happens especially on cold surfaces, such as external corners, windowsills, and areas behind furniture.
In general, the risk of mould increases when humidity is persistently high. Mould can begin to establish when conditions are very damp (often discussed as around 90% RH for germination), and can continue to grow when humidity remains high over time (around 70% RH).
Mould also needs something to feed on. Dust and debris in a home can provide enough nutrients for growth, which is why mould often appears in poorly ventilated and hard-to-clean areas.
When investigating the root cause of mould, it’s important to consider:
- How the building is used: Daily moisture from cooking, bathing and drying clothes
- The building fabric: Leaks and dampness
- How well the home can remove moisture: Such as through heating or ventilation
Mould prevention works by removing the conditions that allowed the mould to grow in the first place, rather than focusing solely on the mould you can see.
If you want to build the skills to investigate mould with confidence, our Level 3 Award in Damp, Mould & Condensation in Buildings is a practical two-day course that will equip you with the knowledge you need.
How to Investigate Mould
Investigating the cause of mould in a building is the most effective and economic way to prevent it in future. These are the key steps to follow:
1. Desktop Assessment
Note the building’s age and construction type, regulatory/alteration and repair history, and its location (including exposure to wind-driven rain and nearby sources of moisture).
2. Speak to Occupants
Understand day-to-day use, including moisture-generating activities (cooking, bathing, clothes drying), heating patterns, and how ventilation is used in practice.
3. External Inspection
Confirm whether the building is sheltered or exposed. Inspect the envelope for defects and changes that could affect moisture performance (e.g., rainwater goods, openings/junctions, and later interventions). Record external conditions (temperature, RH, dew point) for comparison.
4. Internal Inspection
Work through the building methodically. Record where mould, damp and condensation appear (and when), identify cold/stagnant areas, and assess ventilation effectiveness (not just whether fans exist, but whether they work and are used).
Your investigation should end with a final analysis and a clear conclusion that explains why mould is occurring and what needs to change to resolve it.
How to Stop Mould Growth
We stop mould growth in a building by tackling both the visible mould and the conditions that allowed it to develop. If moisture and high humidity aren’t addressed, mould is likely to return, especially on cold surfaces with poor air movement.
Your investigation should point to the most appropriate actions, which may include:
1. Deal With Mould Safely
Small areas of mould, such as around a bath or a patch on a wall, can usually be handled through routine housekeeping. If mould is widespread, recurring, or covers a large area, specialist support may be needed to remove it properly.
2. Improve and Check Ventilation Performance
Effective air movement is critical because it reduces how long RH stays high. If the building has extractor fans, confirm they are working, being used correctly, and are delivering adequate extraction. A vane anemometer with a hood can help you assess performance.
In traditional buildings, check background ventilation routes that may have been reduced over time. These may include sealed draughts, blocked vents or redundant flues. These changes may have unintentionally trapped moisture indoors.
3. Address Any Moisture Ingress and Building Defects
If the building is in poor condition, fix leaking plumbing and moisture ingress through the building fabric. Damp materials and defects can also reduce surface temperatures internally, making them more vulnerable to mould.
4. Address Heating Patterns and Cold Spots
Mould often develops on cold surfaces, so make sure the building is sufficiently heated. Where possible, aim for steadier indoor temperatures and reduce cold or stagnant zones. These may be areas behind furniture or rooms that are rarely heated.
How We Can Help
Preventing mould growth isn’t about repeatedly treating what you can see. It’s about understanding why and where moisture is building up, and what needs to change to prevent high humidity indoors.
When surveyors and contractors have the competence to investigate mould and apply the right principles, problems can be resolved more effectively, with fewer repeat visits, less disruption, and better outcomes for the building and its occupants’ health.
If you want to develop these skills, our Level 3 Award in Damp, Mould & Condensation in Buildings is a government-approved, Ofqual-regulated qualification delivered through a two-day course at the Environment Study Centre.
It covers the common causes of damp and mould, including the risks of ill-informed retrofit, and supports a holistic approach across all building types, including traditional construction. Learn more about our Level 3 Award.
We also offer a 2-day, on-site training course on investigating damp, mould, and condensation in buildings, available only to groups.
