What Is Mould?
Mould is a type of fungus found naturally throughout the environment. It helps break down organic materials, including leaves, wood and other biological matter. As a result, mould is present in many outdoor and indoor environments.
Inside buildings, mould can become a problem when moisture allows it to grow on surfaces and building materials. Unlike outdoor areas, where sunlight, airflow and natural drying can help limit growth, indoor spaces may retain moisture and provide conditions that encourage mould to develop.
Mould spreads by producing microscopic spores that move through the air. These spores are commonly present in both indoor and outdoor environments. When they land on damp surfaces containing suitable organic material, they can begin to grow and form colonies.
Indoor mould is frequently associated with water leaks, condensation, flooding, inadequate ventilation and high humidity. If these moisture conditions continue, mould may spread across visible surfaces or penetrate porous materials. It can also develop within concealed building spaces before any obvious signs appear.
How Does Mould Grow?
Mould requires particular environmental conditions to develop and spread. Moisture is the most important factor because mould cannot grow without sufficient available water.
The conditions that commonly support mould growth include:
- An ongoing source of moisture or elevated humidity
- Organic material that provides nutrients
- Temperatures suitable for fungal growth
- Areas with restricted airflow or limited drying
Many materials used in buildings can support mould growth when damp. These include plasterboard, timber framing, insulation, carpet underlay, ceiling tiles, fabrics and household dust. Even surfaces that are not themselves organic may support growth when moisture and accumulated dust are present.
Mould does not always develop where it can be easily seen. Water may enter wall cavities, roof spaces or subfloor areas and remain trapped for extended periods. As a result, contamination can develop within the building structure before becoming visible on internal surfaces.
When conditions are favourable, mould may begin growing within 24 to 48 hours of water exposure. Once established, it can spread through the movement of airborne spores, ventilation systems or the disturbance of contaminated materials.
Common Causes of Mould in Homes and Buildings
Mould is often a sign of an underlying moisture problem rather than an isolated issue. Identifying the source of that moisture is essential for determining why mould has developed and reducing the likelihood of it returning.
Water Leaks
Leaking roofs, damaged plumbing, faulty windows and defects in the building envelope are common sources of moisture that can lead to mould growth. These problems may remain undetected when water enters concealed areas of a property.
Over time, moisture can accumulate behind walls, beneath flooring or inside ceiling spaces. If affected materials remain damp, mould may develop and spread without immediately visible signs of contamination.
Condensation
Condensation forms when warm, moisture-laden air comes into contact with a cooler surface. As the air cools, water droplets can collect on surfaces such as windows, walls and ceilings. This commonly occurs in bathrooms, kitchens, laundries and other areas where ventilation is limited.
Repeated condensation can maintain the moisture needed for mould to grow. If the problem continues, growth may develop on building surfaces and within insulation or other materials.
Condensation can also indicate broader building performance issues. Inadequate ventilation, insufficient insulation and thermal bridging may contribute to recurring moisture accumulation.
Flooding and Water Damage
Flooding and substantial water ingress can saturate building materials and create conditions that encourage rapid mould development. Prompt and thorough drying is important to reduce the risk of contamination following a water damage event.
Removing visible water does not necessarily mean that the affected area is dry. Porous materials such as plasterboard, timber and insulation may retain moisture internally, allowing mould to develop later.
When drying is delayed or incomplete, the likelihood of more extensive mould contamination increases.
Poor Ventilation
Inadequate ventilation can allow moisture and indoor pollutants to build up within a building. Without sufficient air exchange, humidity may remain elevated, particularly in enclosed areas with limited airflow.
Wardrobes, cupboards, roof cavities and subfloor spaces can be especially susceptible to these conditions.
Modern, energy-efficient buildings may also experience moisture problems when reduced natural airflow is not supported by appropriately designed and maintained mechanical ventilation. Managing ventilation is therefore an important part of controlling indoor moisture.
High Indoor Humidity
High indoor humidity can contribute to mould growth even when there are no obvious leaks or visible water damage. Everyday activities, including cooking, showering and drying clothes indoors, release moisture into the air.
When humidity remains high over time, building materials and other surfaces may absorb moisture and provide conditions suitable for fungal growth.
This type of problem can develop gradually and affect multiple areas of a property, particularly when seasonal humidity changes or limited ventilation make moisture difficult to control.
Where Does Mould Commonly Grow Indoors?
Mould can develop on a wide range of indoor surfaces when sufficient moisture is available. It is more likely to occur in locations where water accumulates, airflow is restricted or materials remain damp for prolonged periods.
Common areas affected by mould include bathrooms, ceilings, internal walls, window frames, carpets, roof cavities, subfloor spaces and air conditioning systems. Areas that have experienced previous leaks or water damage may also be at increased risk.
Visible mould does not always show the full extent of a problem. Growth may also occur behind wall linings, above ceilings or within other concealed building cavities where moisture can remain undetected.
A persistent musty smell may provide an early indication of hidden mould. These odours can be associated with microbial volatile organic compounds (MVOCs), which are released by microorganisms during growth.
Types of Mould Found Indoors
Thousands of mould species occur naturally, although only a smaller number are commonly encountered in indoor environments.
Common indoor mould genera include Aspergillus, Penicillium, Cladosporium, Alternaria and Stachybotrys.
Different mould groups favour different environmental conditions. Some are associated with materials that remain wet for extended periods, while others may develop in areas affected by intermittent moisture or elevated humidity.
The colour or appearance of mould cannot reliably establish its species or determine the level of risk. Different types may look similar, and their appearance can vary depending on environmental conditions and the surface on which they grow.
Where accurate identification is required, laboratory analysis can help determine the types of fungi present in a sample. This information may assist with a building assessment or the planning of appropriate remediation measures.
How Mould Can Affect Indoor Air Quality and Health
Mould can affect indoor air quality by releasing spores, fragments and other microbial by-products into the surrounding environment. These materials may become airborne through normal air movement, ventilation systems or the disturbance of affected surfaces and building materials.
The potential impact on indoor air quality depends on several factors, including the amount and location of mould, the effectiveness of ventilation and the sensitivity of people occupying the building.
People in buildings with significant mould growth may experience irritation affecting the eyes, throat or respiratory system. Persistent musty odours and general discomfort may also be reported.
People with asthma, allergies or existing respiratory conditions may be more sensitive to indoor mould and the conditions associated with its growth.
Assessing mould contamination and the contributing moisture conditions can help establish whether further investigation or remediation is warranted.
Signs of a Possible Mould Problem
Mould is not always visible, particularly when it develops behind building materials or inside concealed areas. Recognising early warning signs can help identify a potential problem before it becomes more extensive.
Common indicators include:
- Persistent musty or damp odours
- Staining or discolouration on walls and ceilings
- Paint that bubbles, peels or lifts from surfaces
- Warped or moisture-damaged building materials
- Recurring condensation or damp patches
- Mould that repeatedly returns after cleaning
Recurring mould is an important sign that the underlying conditions may not have been resolved. If moisture continues to enter a building or remains trapped within materials, cleaning visible growth alone may not prevent it from returning.
Where signs persist or the source of moisture is unclear, a more detailed assessment may be needed to determine the extent of the problem and identify contributing building conditions.
Why Might Professional Mould Testing Be Required?
A professional mould assessment can help establish the location, extent and significance of suspected contamination within a building. It can also help investigate the moisture conditions contributing to mould growth.
Depending on the circumstances, an assessment may include a visual inspection, moisture measurements, air sampling, surface sampling and laboratory analysis. Post-remediation verification may also be appropriate following mould removal to assess the condition of the affected area.
Mould testing is not limited to confirming whether fungi are present. It may also help clarify the nature of the contamination, investigate suspected hidden growth and provide information relevant to remediation planning.
Professional investigation may be particularly useful following significant water damage, when mould repeatedly returns after cleaning, or when there are concerns about contamination within walls, ceilings or other concealed building spaces.
The appropriate assessment method depends on the building, the observed conditions and the purpose of the investigation. Testing results should be considered alongside the inspection findings and relevant moisture information.
Preventing Indoor Mould Growth
Preventing mould requires effective moisture management throughout a building. Because mould spores are naturally present in the environment, prevention focuses on controlling the conditions that allow them to grow rather than attempting to eliminate every spore.
Practical measures include keeping building materials dry, maintaining suitable ventilation, managing indoor humidity and investigating water leaks promptly. Areas affected by water ingress should be dried thoroughly, with particular attention given to materials that may retain moisture.
Ventilation systems should also be appropriately designed and maintained to help manage indoor moisture and airflow.
Where a building has an unresolved leak, persistent condensation or inadequate ventilation, mould may continue to return even when visible growth is repeatedly cleaned away. Addressing the source of moisture is therefore essential for achieving more lasting results.
Conclusion
Mould is a naturally occurring fungus that can become a building concern when excess moisture supports growth on internal surfaces and materials. Although spores are commonly found in indoor and outdoor air, mould develops when environmental conditions are suitable.
Understanding the causes of mould, the locations where it commonly develops and the warning signs of moisture problems can help property owners identify issues and take appropriate action.
Long-term mould management involves more than removing visible growth. Identifying and correcting the underlying moisture conditions is essential to reducing the risk of recurring contamination and supporting better indoor environmental conditions.
Frequently Asked Questions
Yes. New and recently constructed homes can develop mould when moisture is present. Building materials such as timber framing, plasterboard and insulation may absorb moisture during construction or following leaks after completion. Even well-sealed modern properties can experience mould growth if humidity remains high or ventilation is inadequate.
Not necessarily. Cleaning may remove visible mould from a surface, but it does not automatically resolve the moisture conditions that caused the growth. Moisture may remain within walls, ceilings or concealed cavities, allowing mould to return. A lasting solution may require identifying and repairing the moisture source, drying affected materials and addressing any remaining contamination.
Significant mould contamination may affect a property’s condition and how it is perceived by prospective buyers or tenants. Persistent moisture can indicate problems such as hidden water damage, inadequate ventilation or deterioration of building materials. Where contamination is extensive or concealed, further investigation and remediation may be necessary.
The term “black mould” describes mould that appears dark in colour and does not identify one specific species. Some moulds, including Stachybotrys, are associated with materials exposed to prolonged moisture. However, colour alone cannot determine the species present or establish the level of risk. The significance of mould depends on factors such as the type of contamination, its extent and the conditions within the affected environment.
Possible signs include persistent musty odours, mould that repeatedly returns, unexplained dampness, bubbling paint, warped surfaces and moisture stains that appear or disappear. These indicators do not confirm hidden mould on their own, as other building issues can produce similar signs. A professional inspection and appropriate moisture assessment may be needed to determine whether contamination is present inside wall cavities or other concealed spaces.
About the Author
This article was reviewed and contributed to by Ibrahim Ech, Certified Occupational Hygienist (COH®) and an occupational hygiene consultant.
Ibrahim has more than 20 years of experience in occupational hygiene, environmental health, indoor air quality and mould investigations. His experience includes assessing more than 1,000 residential, commercial, government and industrial properties across Australia.
His areas of expertise include mould contamination assessments, moisture investigations, post-remediation verification (PRV), asbestos assessment, indoor air quality testing and environmental laboratory analysis.
Ibrahim is a Full Member of the Australian Institute of Occupational Hygienists (AIOH) and has held Certified Occupational Hygienist (COH®) status since 2012. He is also associated with SESALAB, a NATA-accredited laboratory providing airborne and surface fungal analysis using direct microscopy.