Moisture Theory Crash Course

  • An introduction to the term Relative Humidity

    Most people are aware that air contains moisture but what does relative humidity really mean?
    We start by stating what we know about water:

    • Water has its’ highest density at 4 °C
    • There are three forms of water; solid, liquid and gas
    • Water is mostly in the form of gas in the air despite temperature – this is called water vapour

    What does relative humidity mean and what importance does it have on different climates and material? The term relative humidity (RH) is defined as:

    The amount of water vapour in the air
    in relation to maximum amount of water vapour
    at a certain temperature.

    At 100 %RH the air contains maximum amount of water vapour. The pictures below show two different environments. The temperature can be approximately the same but the humidity in the air differs significantly. In the rainforest, the air is so humid that water is precipitated in the form of fog. It is close to 100 %RH in the rain forest while only 5-10 %RH in the desert.

    olika klimat regnskog och öken

    The example below show two very different climates with the same level of RH but different temperatures. The first picture shows a day with some snow, the temperature might be around 1 °C with 90 %RH while the other picture shows an early morning with dew in the grass where the temperature can be around 10 °C but still 90 %RH.

    olika klimat snö och dagg

     klimat kondensIn sum, relative humidity (RH) is a measure of how much water the air contains at a certain temperature. RH changes as the temperature changes. A good example of this is when condensation forms on the window; warm indoor air is being cooled down by the window – the temperature in the air decreases, RH increases and reaches saturation vapour content which results in condensation being formed.

     

     

     

     

     

    Why is it important to understand RH?

    When warm air gets cooler, RH increases. Too high RH can cause moisture damage.

    The graph below shows various examples of at what level of %RH that reduces quality in certain materials. Certain hygroscopic products such as certain medicines and electric equipment start to gel already at 5-10 %RH. At 50-60 %RH some material starts to rust and at 70 %RH, you can expect mold infestation. Therefore, it is important to know what material you are handling. Rule of thumb is that 50 %RH is good for most materials. It is also convenient for humans and animals, but it is difficult to reach 50 %RH, at most locations, the annual value is around 80 %RH. The high relative humidity is even in colder climates. The relative humidity can be as high in the Winter as in the Summer.

    graph showing relative humidity and moisture damage

     

     

    Do you want to learn more about drying techniques? Attend one of our seminars within Corroventa Academy.