Relative Humidity and guitars

Seasoned wood is hygroscopic and reaches a moisure content that is largely dependant on the Relative Humidity (RH) of the surrounding air. When it no longer gains or looses moisture it is at an Equilibrium Moisture Content (EMC). Many substances including guitar case linings and silica gel display similar behaviour.

When the EMC of wood changes, there will be corresponding shrinkage or expansion trying to take place. This can cause changes in the geometry of a guitar affecting playability and can lead to permenant damage, such as cracks or failed joints. Acceptable RH may vary depending on the guitar; 40% to 55% would normally be considered desirable.

It may be necessary to humidify or dehumidify air that surrounds guitars to keep the moisture content of the wood in an acceptable range, but significant benefit is derived by just changing the temperature and therefore RH where the guitar is stored.

The following calculator shows the expected RH if temperature were to change without any change in absolute humidity levels. This can occur between different parts of a building as well as between external and internal air.

An example would be a guitar stored at 21oC (70oF) at an RH of 33% being placed in a cooler room in the house at 17oC (63oF) The RH would then be 42%, and no other remedial action would be required.

Select input temperature scale: Celsius    Fahrenheit  

Current Temperature:   o

Current Relative Humidity:   %  

Revised Temperature :   o

The new Relative Humidity would be:   %  

Dewpoint constant at:  o