For home owners, landlords and occupiers of rental properties. Humidity and temperature is useful for identifying and monitoring ventilation and temperature improvements.

Tips for using dataloggers

  • Home owners should consider using ORIA Wireless Thermometer Hygrometer (20m (2 Packs)), currently costing about £10 each. (Multiple dataloggers helps track down the source of humidity). 
    Or for about £13 Brifit Wireless Thermometer Hygrometer, Bluetooth 5.0 with data display and 35M range.
  • Smart Hygrometers, with 50M blue tooth are useful for measuring sub-floor humidity that is under timber flooring.
  • Landlords should use Govee WIFI connected hygrometer, it costs about £42 through Amazon, this allows for remote monitoring. 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 and import it into Excel, or similar, you can apply the equation:
    • =243.04(LN(C2/100)+((17.625B2)/(243.04+B2)))/(17.625-LN(C2/100)-((17.625*B2)/(243.04+B2)))
    • Where cell b2 is temperature and c2 relative humidity.
    • For example 17.1°C and 50.82%RH established that dew point is 6.83°C.
    • Alternatively use
  • The data is useful for determining the root cause of excess humidity.
  • 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.

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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