Relative humidity (% RH) is an expression that tells us how far the air around us is towards becoming water-vapour saturated (100% RH).

For example, at 50% RH, the air is halfway to saturation. The higher the air temperature, the greater the capacity for air to hold water-vapour.

Therefore, relative humidity falls with rises in air temperature and increases as air is cooled.

Why is it important to control humidity in the museums industry?

Most organic materials react with their surrounding environment and will absorb or desorb water accordingly from the air to become acclimatised or at ‘equilibrium’.

Consequently, many works of art within museums and heritage could potentially be damaged if relative humidity is too high or too low.

For example, very low humidity may cause shrinkage and cracking of panel paintings, or very high humidity could lead to distortion of paintings or corrosion of metals.

For most general collections, the acceptable band of humidity is between 40% and 60% RH at normal room temperature (17-25°C), with no more than 10% fluctuation over 24 hrs.

Most heritage collection environments are now monitored for relative humidity and temperature surroundings with the plotting of analytical graphs, using electronic sensors coupled with software-based data logging systems.

To correct high humidity, dehumidifiers are used and for low humidity problems, humidifiers are used.

Optimal humidity

Consider how a biscuit, being crisp-baked, will go soft if left open in your kitchen environment. That is because it will equilibrate or balance with the higher room relative humidity of your kitchen.

The same too will happen with absorbent materials like wood, leather, canvas & textiles in collections.

Humidity instability may give rise to fatigue damage, caused by constant absorption/desorption resulting in repetitive expansion/contraction.

Relative humidity (% RH):

An expression that tells us how far the air around us is towards becoming water-vapour saturated (100% RH).

Absolute humidity (or mixing ratio):

The amount of water in the air by weight g/kg. Typically 1kg air =1.3 cubic metres.

There are around 8g of water in 1kg of dry air at ambient room conditions of 50% at 20°C. That is about a tablespoonful of water per cubic metre.

Dew point temperature:

The temperature at which the air reaches 100% RH as it cools °C (td). Often cold surfaces attract condensation because they are below the dew point temperature of the air.