What is dew point and why does it matter indoors?

The dew point is the temperature at which air becomes saturated and water vapour condenses to liquid. Indoors, when a surface drops below the dew point, condensation forms on it. This moisture feeds mould growth and causes structural damage over time.

At what humidity level does mould start to grow?

Mould can begin growing when relative humidity exceeds 70% for sustained periods. The ideal indoor humidity is 40-60%. Above 60% is a concern; above 70% is a mould risk zone.

How do I stop condensation on windows?

Condensation forms when the glass surface is below the indoor air's dew point. Solutions: increase ventilation to lower humidity, install double-glazing, use a dehumidifier, or increase heating to raise surface temperatures above the dew point.

Dew Point & Condensation Risk Calculator

Enter air temperature and humidity to find the dew point and whether condensation will form on your windows, walls, or floor. Includes mould risk level and a dehumidifier sizing tool. Nothing uploaded.

Dew point ? Absolute humidity ? Wet-bulb temp ? Surface condensation risk Mould risk indicator Dehumidifier sizing °C / °F toggle

Results

Dew point

°C

Absolute humidity ?

Wet-bulb temp ?

°C

Heat index ?

Mould Risk

Low mould risk

Mould can grow when relative humidity stays above 70% for extended periods, especially in warm rooms. Reduce humidity or improve ventilation.

Dehumidifier Capacity Needed

Estimated for the room volume and conditions above. Actual results vary by insulation, occupancy, and air change rate.

Learn more: Dew point and indoor condensation control

What dew point means and why it matters indoors

The dew point is the temperature at which air becomes saturated and water vapour condenses to liquid. Indoors, when a surface (window, wall, cold pipe) drops below the dew point of the surrounding air, condensation forms on it. This moisture feeds mould growth, damages plaster, and causes structural rot over time.

Humidity levels and mould risk

Mould can begin growing when relative humidity exceeds 70% for sustained periods, even without visible condensation. The ideal indoor humidity for health and building integrity is 40-60%. Above 60% is a concern; above 70% is a mould risk zone.

Condensation on windows and other surfaces

Condensation on windows means the glass surface is below the indoor air's dew point. Solutions: increase ventilation to lower indoor humidity, install secondary glazing or double-glazing to raise glass surface temperature, use a dehumidifier to reduce absolute moisture, or increase background heating.

Dehumidifier sizing and selection

The calculator estimates dehumidifier capacity needed based on room volume, current humidity, and target humidity. This helps you choose an appropriately sized unit that will handle condensation without over-running.

Frequently asked questions

What is dew point?

The temperature at which air becomes saturated and water condenses to liquid. When a surface drops below the indoor dew point, condensation forms on it.

At what humidity does mould grow?

Mould begins above 70% RH sustained. Ideal indoor humidity is 40-60%. Above 60% is a concern; above 70% is a mould risk zone.

How do I stop window condensation?

Increase ventilation, install double-glazing, use a dehumidifier, or increase heating to raise glass surface temperature above the dew point.

Last reviewed: June 3, 2026