Indoor climate, relative humidity: Measured by wooden probes (NT BUILD 432)

  • Report #: NT BUILD 432
  • Approved: May 1995
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The test method specifies equipment and a procedure to measure the average relative humidity of air over a long period of time. The average relative humidity of air in an environment is determined by first deploying a number of wooden probes and after a period of time there will be a balance between the humidity in the probe and the surrounding air. The probes are then brought back to the laboratory where the relative humidity measurements on the probes are performed.


The method can be used to determine the average relative humidity level of indoor air, i.e. rooms heated to a common temperature. One example of an application is to investigate if a certain level of relative humidity has been exceeded in order to assess the risk of condensation and mould growth. In such cases an accuracy of ±5 %RH will normally be acceptable. The method provides an average of the relative humidity over a period of time e.g. two weeks. Fast fluctuations such as daily variations will be averaged while a general increase or decrease during the exposure period will influence the test result. Measurements by this method are not recommended at temperatures other than common room air temperatures. For example, measurements in cavities in building constructions will require correction of the results due to differences in temperature. If the fluctuations in the relative humidity are needed other measuring methods must be used. The method is especially suitable for measuring in a large number of environments simultaneously. Other advantages are that there is no need for apparatus on the site where the relative humidity is to be determined and no need for skilled personnel for the deployment or collection of the probes, which can be performed by the inhabitants or users. The method is suitable for measuring relative humidity in the range of approximately 20 to 95 %RH. The minimum time of exposure is dependent on the type of probes used, but will typically be two weeks or more.

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