What is the dew point and why does it matter?

The dew point is the temperature at which air becomes saturated and water vapour condenses into liquid water. If any surface in your home (windows, cold walls, behind furniture) is at or below the dew point temperature, condensation will form. At 20°C and 70% RH, the dew point is about 14.4°C.

What relative humidity should I aim for?

Indoor RH should be maintained between 40-60% for comfort and to prevent condensation. Below 40% can cause dry skin and respiratory irritation. Above 65% increases condensation risk, and above 70% promotes mould growth. The ideal target is 55-60% RH.

How much moisture do people produce?

A typical household produces 10-15 litres of moisture per day: breathing (1.5L/person), cooking (3L), bathing/showering (1.5L), laundry drying (5L), and other sources. This moisture must be removed through ventilation or it accumulates as humidity and causes condensation.

What is the best ventilation for condensation?

A PIV (Positive Input Ventilation) unit is often the most effective whole-house solution. It introduces filtered, slightly warmed air from the loft, diluting moist air and maintaining positive pressure. Extract fans are essential in kitchens (30 L/s) and bathrooms (15 L/s) to remove moisture at source.

Will heating solve condensation?

Heating alone does not solve condensation — it raises air temperature which can hold more moisture, but without ventilation the moisture stays. The solution is adequate ventilation PLUS heating. Warm, well-ventilated rooms rarely suffer condensation. Cold spots (thermal bridges) should be insulated.

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Condensation Ventilation Calculator — Dew Point & Air Changes

Calculate dew point from temperature and relative humidity, determine air changes needed to reduce RH below 65%, and select ventilation type.

Room Temperature

Current indoor temperature

Relative Humidity

Current RH (use a hygrometer)

Room Volume

Length × Width × Height

Ventilation Type

How We Calculate This

This calculator determines the dew point and ventilation needed to reduce condensation risk.

The formula

Dew Point = (237.7 × alpha) / (17.27 - alpha)

Where alpha = (17.27 × T) / (237.7 + T) + ln(RH/100)

Air Changes = ((RH - Target) / 10) × 0.5 ACH

Risk levels

<65% RH: Low risk
65-80% RH: Medium — condensation on cold surfaces
>80% RH: High — widespread condensation likely

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.