The threshold that matters
Mould spores germinate when the surface they land on has a water-activity value above roughly 0.75 — which corresponds to a relative humidity of about 75–80% sustained against that surface. The air in the room can be lower; what matters is the local humidity against the cold corner of the wall, behind the wardrobe or under the window sill.
Because cold surfaces locally raise relative humidity (cooler air holds less moisture for the same absolute water content), the relevant metric is not just room humidity but also the dew-point — the temperature at which the air would saturate.
Because cold surfaces locally raise relative humidity (cooler air holds less moisture for the same absolute water content), the relevant metric is not just room humidity but also the dew-point — the temperature at which the air would saturate.
What good and bad humidity look like
30–50% RH: healthy range. Comfortable for occupants, low mould risk, acceptable for wooden floors and furniture.
50–60% RH: elevated. Common in poorly ventilated bathrooms after showers, kitchens after cooking, and bedrooms in winter. Not yet a mould problem but a clear ventilation signal.
60–70% RH (sustained): mould-prone. If a cold wall is involved, expect mould within weeks unless you change something.
Above 70% RH (sustained): active mould risk. Open windows, run extractors, dehumidify.
50–60% RH: elevated. Common in poorly ventilated bathrooms after showers, kitchens after cooking, and bedrooms in winter. Not yet a mould problem but a clear ventilation signal.
60–70% RH (sustained): mould-prone. If a cold wall is involved, expect mould within weeks unless you change something.
Above 70% RH (sustained): active mould risk. Open windows, run extractors, dehumidify.
Catching mould early with continuous data
A weekly humidity-and-temperature curve tells a clearer story than any one-off reading. Watch for: bedroom RH that climbs steadily overnight; bathroom RH that does not recover within 30 minutes after a shower; kitchen RH that stays elevated long after cooking. Each is a signal to fix something — usually a fan, an open window or an air-brick — before mould appears.
For landlords, a continuous humidity record across the tenancy is also the most useful single piece of evidence in a damp dispute. See our guide for landlords.
For landlords, a continuous humidity record across the tenancy is also the most useful single piece of evidence in a damp dispute. See our guide for landlords.
How Envora One helps
Envora One logs relative humidity and temperature minute-by-minute alongside CO₂ (the ventilation gauge) and PM2.5 (the cooking gauge). The dashboard surfaces mould-risk warnings when sustained RH and dew-point thresholds are hit, and the same data flows out via API, MQTT and CSV.
For landlords: pair with Envora Fleet for a multi-property humidity dashboard, tenant-safe access links and monthly property health reports.
For landlords: pair with Envora Fleet for a multi-property humidity dashboard, tenant-safe access links and monthly property health reports.