What Does Relative Humidity Mean?

Relative humidity is a tricky concept to explain to a tenant or homeowner during a damp survey. Yet, I feel it is so key to helping people understand the causes of condensation and mould in a property.

Years ago while on a field trip to the Malaysian rainforest I simply could not understand how the air could be 98% humidity. If it was so wet in the air then why wasn’t I swimming through the forest?

The picture below shows the key understanding that relative humidity only refers to how much water the air can hold at a particular temperature. Warmer air can hold more water in it than colder air. If you have ever been skiing you will know that your lips can dry out really easily – that’s because there is hardly any moisture in the air. Antarctica has a relative humidity of 0.03% – the polar opposite of the rainforest (hilarious pun intended).

If you imagine each box below as a room then you can see that as the temperature reduces the size of the imaginary space holding the water also shrinks until water literally spills onto the walls. The space available for water has shrunk to a point where no more water can be held.  No water is added to the room yet the relative humidity increases.

what is relative humidity simple

How is Dew Point Related To Relative Humidity?

The dew point is a term which is sometimes included in Landlord Damp Surveys as it has an important function which can explain why condensation problems are occurring.

The dew point is simply the point at which air can no longer hold anymore water and so the water will start to fall out of the air.  In the diagram above you can see that when the temperature drops to 10degreesC the water spills out. This point of spillage is the dew point and is a variable point determined by temperature and the amount of water in the air – i.e. the relative humidity. 

The dew point can be calculated by using a psychrometric chart, however, for survey purposes equipment is used which will automatically calculate it. 

Knowing the dew point helps in determining the cause of moisture in a property since if it is found that the walls are often at or below the dew point then we can know that the water seen is likely coming from the air as condensation. 

What Does 100% Relative Humidity Mean?

If you ever find yourself in an environment that is 100% relative humidity, as I did in Malaysia, you will notice that you are not actually swimming.  Chances are you will be sweating like crazy and quite uncomfortable but nevertheless, swimwear will not be required. 

What the 100%rh actually means is that the air cannot hold anymore water within it. Therefore any more moisture produced, by trees or in buildings cooking/showers etc, will immediately fall out of suspension in the air as condensation. In a property this will generally happen on wall or window surfaces. 

You can help reduce condensation on windows through some fairly simple lifestyle changes as detailed in this post. 

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