The humidity level in a home that houses acoustic instruments should be slightly different than homes without. Pianos, guitars, mandolins, and other types of string instruments need to be housed with a certain amount of humidity to preserve the instrument. Since many of these instruments are quite expensive, it's important to understand what levels you should keep in your home.
Guitars and Fretted Instruments
For guitars and other instruments with frets, stores will keep the humidity set at around 50 percent. In a home, this can be uncomfortable. You should keep the humidity level in the room at between 35 to 50 percent. Even at 35 percent, you may experience some shrinkage in the tops and the frets can start to protrude. The closer to 50 percent relative humidity you can get, the better your instrument will fair.
A piano can be easily damaged by improper humidity levels. Extreme levels on either side can create trouble. It's best to keep the room right around 40 percent for optimal conditions. Direct sunlight and heat sources can also create problems for a piano, so it's best to keep them away from these.
A hygrometer should be placed in any room where acoustic instruments are stored. This will allow you to monitor the relative humidity and make adjustments as necessary.