datalogger from a damp and mouldy property


Dataloggers are devices for monitoring recording data from an environment. The above graph comes from a property with extreme mould and condensation. The data we are most interested in is relative humidity (%RH) and temperature (°C), but other devices exist.

Example dataloggers include:

  • Humidity and temperature datalogger (known as hygrometers).
  • CO2 dataloggers (useful for determining sufficiency of ventilation), 
  • “rising damp” dataloggers for monitoring the groundwater (height of water table) and 
  • WME dataloggers, such as Protimeter for measuring dampness within a wall, as well as relative humidity and temperature – these are hard to set up and more useful for expert witness work, than standard damp surveys.

85% of damp in homes caused by excess humidity

85% of damp issues in the home involve some degree of excess humidity, be it condensation or insufficient evaporation. Data over time will help determine to what degree vapour is influencing the main area of concern.

Tips for using dataloggers

  • Consider using two or more ORIA Wireless Thermometer Hygrometer (20m (2 Packs)), currently costing about £10 each. (Multiple dataloggers help track down the source of vapour). Place one near, but not on the main damp patch. Place another in a central area, such as by the thermostat (if you have one). If you have more dataloggers, consider placing one in the bathroom (on an external wall), similarly the kitchen and bedroom.
    ORIA also sell dataloggers with displays (they are larger and heavier, so harder to place by damp patches or under floors) £13 Brifit Wireless Thermometer Hygrometer, Bluetooth 5.0 with data display and 35M blue tooth range.
  • Smart Hygrometers, with 50M blue tooth range are useful for measuring sub-floor humidity, that is under timber flooring.
  • Landlords should use Govee WIFI connected hygrometer. These costs about £42 through Amazon, the benefit is remote monitoring through WIFI (make sure you have WIFI). So the tenant can see the humidity and temperature and the landlord can monitor ambient conditions, and agree changes accordingly, for example proving improved ventilation or requesting the tenant increases nighttime temperature. After all it takes teamwork between tenant and landlord to maintain a property.
  • If you download the data from the datalogger and import it into Excel, or similar, you can apply the equation:
    • In eXcel apply this calculation to get the dew point =243.04(LN(C2/100)+((17.625B2)/(243.04+B2)))/(17.625-LN(C2/100)-((17.625*B2)/(243.04+B2))), where C2 in the relation humidity (0-100) and B2 the temperature (°C).
    • For example 17.1°C and 50.82%RH established that dew point is 6.83°C.
    • Alternatively use
    • I will support you with data in the survey report.
  • The data is useful for determining the root cause of excess humidity. Dew point is the temperature that condensation takes place. Dew point is a useful proxy for vapour pressure or quantity of vapour. If you compare dew points in one room to another, and or a standard property, you can determine if there is excess humidity and sufficient ventilation, there are many other ways to testing ventilation, so use of a datalogger is not critical, but useful.
  • Mould only grows where humidity exceeds 85%RH for 6 or more hours. There can be a difference between the wall temperature, and therefore the relative humidity and the datalogger. Sometimes you have to decrease the temperature by 1 – 2°C and therefore add about 5%RH to get the accurate %RH. Use a laser thermometer to determine the wall temperature.

Absolute humidity

Dew point is a proxy for the or the quantity of water in the air, known as absolute humidity which is measured as vapour pressure in kPa. In a hermetically sealed environment, the dew point will be constant, while temperature and relative humidity vary depending on surface temperature.

Datalogger from an uninhabited damp property, that was heated but sealed.

Armed with the dew point, we can work out the surface humidity of a cold wall using a laser thermometer to measure the surface temperature and comparing it to the dew point.

For example if the dew point is 6.8°C, the wall temperature is say 9°C, while ambient temperature is 17.1°C humidity is 50.8%RH.

A linear approximate is =100-(AD39-6.8)/(17.1-6.8)*(100-50.8), giving a surface relative humidity of 89.5%RH.

i.e. in this case, the wall temperature is low enough to push the surface relative humidity over 85%RH, the ideal conditions for mould to grow.

As surfaces become colder, so the relative humidity rises, until eventually the relative humidity is sufficiently high for mould to grow and condensation to form. See for daily forecasts.

Alternative dataloggers

  1. £5 with a hygrometer probe (also good for monitoring sub-floor humidity)
  2. £10 simple with 24 min/max.s;
  3. £30 datalogger with probe
  4. £10 datalogger
  5. £30 datalogger with probe

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