What is relative humidity?
Wood swells and shrinks in response to changes in the relative humidity of the air around it. Relative humidity (RH) is the amount of moisture contained in the air, compared to the maximum amount of moisture that the air is capable of holding. The moisture content of air is affected by weather as well as conditions and activities within the home, while the moisture- holding capacity of air varies with temperature. One way of thinking about RH is that it is a measure of air’s tendency to absorb or release moisture to its surroundings. Thus when the RH of air in a room increases, moisture will tend to transfer from the air to wood and other absorbent materials in the room. When the RH of air decreases, moisture will transfer from other materials back into the air. The RH of the atmosphere is always changing by the hour, and more dramatically, with the seasons. Consequently, the wood and felt parts in your piano are constantly changing dimension as they absorb and release moisture.
Since RH depends upon the temperature and moisture content of the air, it is not possible to maintain a constant RH by controlling room temperature alone. In fact, maintaining an even temperature while moisture content varies will cause RH to change.
The preceding article is a reprint of a Technical Bulletin published by the Piano Technicians Guild, Inc.It is provided on the Internet as a service to piano owners.