Surface Mould Risk Calculator
INDOOR AIR PERFORMANCE — SURFACE MOISTURE RISK CHART
Psychrometric chart showing constant relative humidity curves. Coloured zones indicate surface moisture risk.
Your room's current air condition is shown as a ● dot.
The ✕ markers show where each VENTI Air Risk Zone threshold falls.
VENTI
v2.0
Why does this matter? Mould doesn't need visible water to grow. It starts on any surface
that stays cool enough for long enough — a corner behind a wardrobe, a north-facing windowsill, or an
uninsulated external wall. This tool shows you the surface temperatures at which mould can start
to take hold in your home, based on the temperature and humidity of the room.
How to use it: Enter your room's air temperature and humidity below. The calculator will show you the surface temperatures to watch out for — and what each risk zone means in plain terms.
How to use it: Enter your room's air temperature and humidity below. The calculator will show you the surface temperatures to watch out for — and what each risk zone means in plain terms.
Psychrometric chart — each curve is a constant humidity line. The coloured zones show VENTI's
Air Risk Zones. ● is your room's current condition. ✕ markers
show where each zone threshold falls.
Your room's air properties
Vapour pressure: – kPa · Moisture content: – g/kg
Vapour pressure: – kPa · Moisture content: – g/kg
The moisture content figure tells you how many grams of water vapour are held in every
kilogram of air. The higher this number, the more moisture is available to settle on cool surfaces.
| VENTI air risk zone | Surface temp |
|---|---|
| Zone 1 — Watch (ARZ-W · surface RH 66%+) The earliest mould species can begin to grow at this surface temperature. These are hardy moulds that don't need much moisture to get started — and they can be established within 12–24 hours. Check: corners, behind furniture pushed against outside walls. | – |
| Zone 2 — Caution (ARZ-C · surface RH 80%+) A wider range of mould species become active here, including those that spread quickly across soft furnishings and plaster. Risk develops within 48–72 hours. Check: north-facing rooms, unheated bedrooms, poorly insulated window reveals. | – |
| Zone 3 — Act Now (ARZ-A · surface RH 90%+) At this temperature, even moisture-loving moulds — including the type sometimes called black mould — can germinate within 7–14 days. Immediate action: improve heating, ventilation, or surface insulation. | – |
| Condensation threshold (Dew point · surface RH 100%) Below this temperature, water vapour turns to liquid on surfaces — the droplets you see on cold windows. Bacteria multiply rapidly here too. This is the point where damage to walls and structures can begin. | – |
Enter your room conditions above to see a summary.
Important: The VENTI Air Risk Zone (ARZ) notation is original to VENTI / The Coastal Group.
The surface humidity thresholds (66%, 80%, 90%) represent broadly accepted boundaries for mould germination
— there is considerable biological variation between species, and some can germinate at lower thresholds.
Required exposure times vary depending on conditions, surface porosity, and existing contamination.
This tool is for indicative guidance only and does not replace a professional mould survey or
qualified indoor air quality assessment. Calculations assume standard atmospheric pressure (101.325 kPa).
ARZ-W / ARZ-C / ARZ-A are proprietary VENTI notations.