Basic moisture / temperature physics

Moisture & temperature physics basics no.1


All examples and data provided here are for normal environments indoor or outdoor at normal altitude and pressure on planet Earth. Numbers are roughly.

Elements: Breathable Air, fresh Water and common building material.


Water has three states, 1. Liquid, 2. Solid (ice), 3. Gas

1. Liquid water occurs at temperatures between 0 - 100 degrees Celsius.

2. Solid water (as ice) occurs below 0 degrees Celsius.

3. Gas of water, all water becomes gas above 100 degrees Celsius. Below 100  degrees Celsius we can still have water in gas form in the air. Even at freezing temperatures (below 0 degrees Celsius) there can be water in gas form in the air.


Invisible water

The most interesting state of water in building technology is the water in gas form (vapor where the water is hovering around as gas-molecules in the air), the one you can't see. No you can not see the the gas of water in the air. If you can see it it's not in gas form. This state is perhaps also the most sneaky form of water in buildings, since you can't see it you don't know where it is.


Where is the temperature?

And what has the temperature to do with the water? Well my friend the temperature is the "Pedro el negro", as they say in spanish, of this physics when it comes to moisture in building technology. The temperature is everything, almost.

First the temperature "controls" which state water is in, obvious, as stated above. Second it also "controls" the relative and absolute content of water vapor in the air, se below.


Relative and Absolute Humidity

But the most interesting thing is something called "Relative Humidity" (RH) and Absolute Water Content in air (also called Absolute Humidity). This numbers differ depending on the temperature.

Absolute humidity: The warmer the air, the more moisture as water vapor (water gas) it can hold and the opposite when it gets colder. Think of the warm air as a large drinking glas and the cold air as a smaller drinking glas. The warmer the air the larger glas and the colder the air the smaller glas.

Absolute humidity is expressed as grams of moisture per cubic meter of air (g/m3) or grams of moisture per grams of air (g/g).

Relative humidity: Is the quota (%) of moisture as water vapor (water gas) in the air relative to the maximum level (absolute humidity) at a given temperature. Now is probably the time to pick those drinking glases again... At, for example, +10 degrees Celsius our drinking glas can hold a maximum of 7 cl and at +25 degrees Celsius our drinking glas can hold a maximum of 20 cl of water. Okey, so one small glas and one larger glas. If we put 3,5 cl water in the small glas and 10 cl in the large glas they are both half full so both is 50% filled (of what they can hold). If you are thirsty which glas would you want? Both are 50% full but the absolute water content is different. So there we have it the temperature rules both the relative- and the absolute humidity.


For further education you can look up a diagram called ix-diagram also called hx-diagram or Mollier-diagram. They have curves of relative humidity dependig on the temperature and the absolute humidity. If you have two of the three values you get the third. In the future you will most probably find an ix-diagram here.


End of Basic Moisture & temperature physics no.1.


This article was updated on 19 February 2024