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Relative humidity doesn’t measure humidity

April 28, 2013

thermo-hygrometerIt is common for people to use the term relative humidity to describe how humid an environment feels.

They may describe a dry environment as having a low relative humidity (RH 40%); while a damp environment as having a high relative humidity (RH 70%). I hope to prove to you that this is not the case at all.

This afternoon I placed one thermo-hygrometer outside and another inside and left them there for an hour to acclimatise. I recorded the air temperature and relative humidity from each instrument and recorded my findings. You can use 2 low cost device as shown above. I bought this one from ETI Ltd.

Readings taken internally

Air temperature = 18.2oC

Relative humidity = 44%

Readings taken externally

Air temperature = 10oC

Relative humidity = 51%

Now the question is, which air is more humid, the inside air at 44% RH or the outside air at 51% RH?

Some people may say that the outside air has more water because it has a higher relative humidity. Let’s work out the actual quantity of water per kg of dry air, for each air sample just to see if there is a link between RH and the actual quantity of water.

Now there are three different ways of finding out the mass of water vapour in an air sample.

1. Using the Psychrometric chart

2. Using the Mixing Ratio formula to calculate the moisture content (g/kg of dry air)

3. Using a web-based calculating tool. (e.g. Vaisala humidity calculator)

I tend to use the Vaisala calculation tool because it’s quick and very accurate. You can check it out yourself.


Based on the recorded thermo-hygrometer readings above for internal and external air, I obtained the following…

Mass of water internally at 44% RH and 18.2oC = 5.72 g/kg of dry air

Mass of water externally at 51% RH and 10oC = 3.88 g/kg of dry air

This is proof that the relative humidity doesn’t tell you anything about the actual humidity of the air. Relative humidity actually changes with a change in the air temperature as well as the mass of water vapour in the air.

For a great tutorial on relative humidity, check out Richard Clements on U-Tube.


From → 3. Dampness

  1. Hi,

    The RH piece was very informative and short. I feel that I have leant something. I suppose the rule here is, the warmer the air, the more moisture it can hold. The lower the temperature the lower the dew point.

  2. This is really interesting from the perspective of a wannabe surveyor, and something I’ll have to remember.

  3. Hi, very useful as always !
    at what level does actual humidity become a problem?
    is 5.72 g/kg of dry air a lot of water?
    or does condensation/mould/rot depend more on Relative Humidity?


    • Thanks for the comment.

      Internal humidity becomes a problem when it causes either damage to internal finishings or allows organisms to colonise and breed. This depends on air temperature, ventilation rates and how absorbant and viable the materials are within the room.

      For the control of house dust mites the mass of moisture in the air must be kept below 7g/kg of dry air. 7 grams seems to be enough water to sustain dust mites.

      Condensation is directly linked to vapour saturation of the air. VS point changes with air temperature.

      Moulds start to colonise a viable surface when the RH of adjacent air typcially reaches 70%

      Timber decay (rot) could occur if its moisture content passes above 20%. In Spruce this would occur at around 90% RH over a prolonged period of time. Hence the need for sub-floor ventilation.

      However, the point in the post was merely to caution people against using RH as a way of quantifying the air’s moisture content.

      Does this help?

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