Condensation and Relative Humidity

We all know that warm air can hold more moisture right? Well this is true, but there are different ways in which we can consider HOW MUCH moisture there is within the air, this being: relative humidity (RH) and vapour pressure.

RH does not actually tell us how much physical moisture there is in the air. What it tells us is how much moisture is in the air RELATIVE to the total quantity of moisture that the air can hold at a given temperature.

It is expressed as a proportion, i.e. a percentage.

If we have 50% RH, that air is holding half of the total moisture that it could hold, at a particular temperature.

Relative Humidity is a proportion, not a physical quantity.

If we don’t change the amount of moisture in the air, but heat it up or cool it down, what we’re doing is influencing what the air can hold, nothing else, but as a result of this the RH proportion would change.

As simplistically as I can make it and using the example above, i.e. air with 50% RH, if we heat up the air the RH might drop to 25% (warmer air can hold more). If we cool down that air, the RH might increase to 75% (cooler air can hold less).

Remember, we’ve not changed how much moisture is in the air, we’ve just influenced how much moisture the air can hold.

The same volume of moisture in a smaller container (the air) means it is more 'full', equating to a higher % of 'fill'.

So… condensation occurs when you reach 100% RH, i.e. the air is cooled to a point whereby it cannot hold the moisture, and this condenses, i.e. changes from vapour (a gas) into its liquid state.

Why does condensation happen in certain areas?

This is why water condenses on cold surfaces, such as windows, window reveals, poorly insulated substrates etc. etc., and it is why homes need to be heated/insulated in order to prevent issues of condensation.

If you are experiencing problems with condensation speak to a waterproofing specialist who will investigate the problem.