Surface Relative Humidity

Surface Relative Humidity refers to the relative humidity level at the coldest surface point within a given space. It is an invaluable metric for assessing the risk of condensation and mould growth within properties. The concept is rooted in the principles of thermodynamics and psychrometry, where it’s calculated using room temperature and relative humidity, in conjunction with the coldest surface temperature.


Understanding point surface relative humidity allows for a more nuanced interpretation of the environmental factors contributing to dampness and mould growth. This can lead to more targeted solutions for damp remediation.

Measurement Tools:

  1. Hygrometer: To measure the ambient relative humidity and temperature in the room.
  2. Laser Thermometer or Thermal Camera: To measure the temperature of the coldest surface in the room.
  3. Infrared Lens: For a detailed thermal map of the surfaces.

Mathematical Formulae:

  1. Calculating Dew Point (TD):TD=243.04×(ln⁡(RH/100)+(17.625×T)243.04+T)17.625−(ln⁡(RH/100)+(17.625×T)243.04+T)TD=17.625−(ln(RH/100)+243.04+T(17.625×T)​)243.04×(ln(RH/100)+243.04+T(17.625×T)​)​
  2. Calculating Vapour Pressure (e):e=RH×0.01×ese=RH×0.01×es​where eses​ is the saturation vapour pressure, which can be calculated using various methods, including the August-Roche-Magnus approximation.
  3. Calculating Point Surface Relative Humidity (RH_s):RHs=100×e×exp⁡(17.625×Tsurface243.04+Tsurface)e×exp⁡(17.625×TD243.04+TD)RHs​=100×e×exp(243.04+TD17.625×TD​)e×exp(243.04+Tsurface​17.625×Tsurface​​)​

Most Complex Calculation:

The formula for point surface relative humidity (RH_s) is the most intricate, especially when factoring in the various variables needed to accurately assess the condition. It incorporates dew point, vapour pressure, and surface temperature, all of which can be influenced by a multitude of factors, such as ventilation, insulation, and building material properties.

Recommendations for Use:

Regular measurement of point surface relative humidity is advisable for those interested in reducing the risk of condensation and mould growth. Implementing this practice can significantly improve the accuracy of damp surveys and the efficacy of subsequent remedial actions.

By understanding and regularly checking this metric, you can make your living or working environment healthier, more comfortable, and less susceptible to damp-related problems. Regular checks can be performed using simple tools and straightforward calculations.

Final Thoughts:

Being able to calculate and understand surface relative humidity is a cornerstone of effective damp surveying. It provides a comprehensive view of the moisture levels within a property, aiding in accurate diagnosis and solution implementation. We recommend using a data logger for all surveys.

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