Relative Humidity

Relative Humidity (RH) is a crucial concept in understanding moisture dynamics within indoor environments. It measures the current amount of moisture in the air relative to the maximum amount the air could hold at the same temperature, expressed as a percentage. This parameter is critical for maintaining comfort and ensuring a healthy indoor climate, especially in preventing condensation, damp, and mould issues.

Significance of Relative Humidity in Buildings

The indoor climate significantly affects the comfort, health, and well-being of the occupants. High levels of relative humidity contribute to:

  • Condensation: When warm, moist air comes in contact with colder surfaces, it cools down, and its relative humidity increases, leading to condensation once it reaches the dew point.
  • Dampness: Persistent high humidity can cause dampness in walls, floors, and other building materials.
  • Mould Growth: High relative humidity provides a conducive environment for mould growth, especially when it rises above 85%RH for six or more hours at a time.
  • Allergenic dust mites and pathogens thrive in similar humid environments, posing health risks to the occupants.

Surface Relative Humidity and Measurement

Surface relative humidity refers to the moisture level at the surface of materials. It is a vital measure as it directly impacts condensation and mould growth on those surfaces.

Rule of Thumb: Temperature Drop and RH Increase

A useful rule of thumb is that for every 1°C drop in temperature, the relative humidity will increase by roughly 5%RH until it reaches the dew point, where condensation occurs. For instance, if it’s 20°C and 65%RH in the centre of a room, it could be 14°C and 95%RH in a cold corner of the wall, especially behind furniture, near poorly insulated areas, or reduced airflow zones from radiators.

Effective Ventilation and Humidity Control

Managing indoor relative humidity is crucial for maintaining a healthy and comfortable living environment. Some effective strategies include:

  • Targeted Extraction: Utilise extractor fans in high-humidity areas like bathrooms and kitchens to remove moist air.
  • Heating and Insulation: Proper heating and insulation help maintain a consistent indoor temperature, reducing the likelihood of condensation.
  • Dehumidifiers: These devices can help reduce indoor humidity levels, especially during colder months when windows are kept closed, and natural ventilation is reduced.


Understanding and managing relative humidity is pivotal in maintaining a healthy indoor environment and preserving the structural integrity of buildings. By monitoring and controlling indoor humidity, employing effective ventilation, and ensuring proper insulation, homeowners and property managers can significantly mitigate the risks associated with high relative humidity, including condensation, damp, and mould growth.

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