45%-55% relative humidity is optimum. If this is obtained, you minimize the risk of damage.
Temperature and humidity can have a large impact on a guitar’s tuning. Too much or too little moisture will warp the wood of your guitar, making it difficult to tune. Innapropriate levels of humidity may eventually lead to cracks or may even cause the instrument to break or snap.
The optimum level of humidity for a guitar is between 45% and 55%. Generally, dry air poses a more frequent and damaging risk to the instrument. When the humidity drops below the optimum range, the wood of your guitar beings to bleed moisture, leading to shrinkage and cracking. Fortunately, there are some easy steps and tools you can use to protect your instrument from damage.