Maintaining proper humidity levels is crucial for indoor comfort and various applications. During the winter months, heating air with low moisture content can cause dryness, making it essential to regulate humidity levels. In most applications, a relative humidity (RH) range of 40% to 50% is suitable. However, for critical environments like animal rooms, humidity levels must be carefully controlled between 50% and 60% RH. In such cases, seeking advice from experienced humidifier manufacturers, such as J.S. Humidifiers, is imperative. Let’s explore the calculation of humidity to understand how to determine the necessary moisture levels accurately.

Calculation of Humidity

To calculate the required moisture content, we can utilise a psychometric chart and follow a few simple steps. Let’s consider an example where the outside ambient temperature is -3°C, and the design requirement is 50% RH at 21°C.

  1. Plot the two points on the psychometric chart:
    • Outside ambient: -3°C
    • Design requirement: 21°C dry bulb / 50% RH
  2. Draw a horizontal line from each point across to the right-hand side of the chart.
    • Read the values indicating the kilograms of moisture per kilogram of dry air required at each point.

For example:

  • At -3°C: 0.0029 kg/kg
  • At 21°C dry bulb / 50% RH: 0.0079 kg/kg
  1. Calculate the difference between these two values:
    • Difference: 0.0079 kg/kg – 0.0029 kg/kg = 0.005 kg/kg
  2. Multiply this figure by the air volume, let’s say 1.5 m³/s.
  3. Multiply the result by 3600 to convert it into the requirement per hour (60 seconds × 60 minutes = 3600).
  4. Finally, divide the answer by the specific volume (m³/kg) at the entering air point.

For example:

Kilograms per hour of humidity required = (0.0079 kg/kg – 0.0029 kg/kg) × 3600 × 1.5 m³/s / 0.769 m³/kg Therefore, the humidity requirement amounts to 35.11 kg/hour.


Accurately calculating humidity is essential for creating optimal indoor conditions, ensuring comfort and meeting the specific requirements of critical applications. By following the steps outlined above and utilising a psychometric chart, HVAC professionals can determine the necessary moisture content for a given space. It’s crucial to remember that maintaining proper humidity levels is vital for occupant well-being, preventing dryness or excessive moisture-related issues. For critical environments, seeking guidance from experienced humidifier manufacturers, such as J.S. Humidifiers, is highly recommended. By incorporating precise humidity control, we can enhance the overall comfort and functionality of indoor spaces.