Mould Risk Factors and Early Indicators

Mould growth in buildings is never random. It requires specific conditions: moisture, a suitable temperature, nutrients (found in most building materials), and time. Understanding these factors allows housing providers to identify at-risk properties before mould becomes visible—and before tenant health is affected.

The Critical Role of Humidity

Relative humidity (RH) is the primary controllable factor in mould prevention. Research consistently shows that maintaining RH below 70% significantly reduces mould risk. Above 80% RH, many common mould species can germinate and grow within 24-48 hours given suitable temperatures.

Key thresholds to monitor:

• Below 60% RH: Low mould risk, ideal conditions
• 60-70% RH: Moderate risk, acceptable for short periods
• 70-80% RH: Elevated risk, action recommended
• Above 80% RH: High risk, urgent intervention needed

Temperature and Dew Point

Temperature affects mould risk in two ways. First, most mould species grow optimally between 15-30°C—typical indoor temperatures. Second, and more importantly, temperature determines where condensation occurs.

When warm, humid air contacts a surface below its dew point temperature, water vapour condenses into liquid water. This surface moisture is what enables mould growth. Cold spots in buildings—thermal bridges, uninsulated corners, areas behind furniture against external walls—are high-risk locations.

Early Warning Signs

Before visible mould appears, several indicators suggest elevated risk:

• Persistent condensation on windows: Indicates high indoor humidity
• Musty odour: Often detectable before mould is visible
• Humidity readings consistently above 70%: Environmental data doesn't lie
• Cold spots on walls: Detectable with thermal imaging or surface temperature sensors
• Tenant reports of allergic symptoms: Some occupants are sensitive to mould before it's visible

High-Risk Property Types

Certain building characteristics increase baseline mould risk:

• Solid wall construction without insulation
• Single-glazed windows
• Properties without mechanical ventilation in wet rooms
• Flats above unheated spaces (garages, commercial units)
• North-facing properties with limited solar gain
• Properties that have had previous flooding or water damage

The 72-Hour Warning Window

Continuous monitoring can provide 24-72 hours of advance warning before conditions become critical. This window allows for intervention—adjusting heating, improving ventilation, contacting tenants—before mould growth begins.

This proactive approach transforms mould management from reactive remediation to preventative maintenance.