How humidity affects mould growth

Why moisture matters most

Mould spores are everywhere. They only grow when there's enough water, which can come from leaks, condensation or simply humid air. When indoor relative humidity stays above about 60%, mould can establish and spread. Condensation on windows or bathroom walls is a sign the humidity is too high.

Moulds have moisture preferences

Not all moulds need the same amount of water. Scientists group them by how damp the surface they grow on needs to be:

• Primary colonisers can grow in relatively dry conditions, at as little as 65–70% relative humidity at the surface. These include some Aspergillus and Penicillium species.
• Secondary colonisers prefer 80–90%. Cladosporium is one example.
• Tertiary colonisers need very wet conditions, above 90%. They include Stachybotrys, Chaetomium and Fusarium.

When high humidity or leaks persist, the mix of moulds in a home shifts towards the water-loving species, which usually means materials have been wet for some time.

Where moisture collects

• Bathrooms and kitchens, especially under sinks and around tiles
• Roof spaces and wall cavities, where leaks can go unnoticed
• Subfloors, where damp air can collect
• Air conditioning units and ducts, where condensation builds up

Timber, plasterboard and wallpaper become vulnerable once their moisture content reaches about 16%.

How to keep humidity under control

• Keep indoor relative humidity between 30% and 50%, and below 60%.
• Dry wet areas within 24 to 48 hours after a leak or spill.
• Use a dehumidifier or air conditioning to manage humidity.
• Run exhaust fans that vent outside, not into the roof space.
• In humid coastal areas, ongoing ventilation or dehumidification may be needed.

Uncontrolled airflow can work against you. In warm, humid weather, drawing outside air in can add moisture rather than remove it.