Guitars are made of wood that has been cured and dried to ensure that the individual parts remain stable – they exhibit minimal expansion and contraction. However, the relative humidity of the guitar’s operating environment can have a profound effect on a guitar’s sound, playability, and finish.
The ideal environment for a guitar has a relative humidity of about 45%. If your room is much more humid (65% or greater), the top can rise, making the action high or even unplayable. High humidity can also cause lacquer to check, and impressions of the bracing may appear on the top. Puckers may appear where the top is glued to the internal structure of the guitar, such as at bridges, braces, head blocks, and tail blocks. It also restricts movement of the top resulting in a guitar that may sound “tight.”
High humidity is difficult to deal with, but it can be conquered. Air conditioning will really help you keep the humidity at a reasonable level. As the temperature lowers, so does the saturation point of the air. Water will condense, and the relative humidity will decrease. Do not use a swamp cooler in an already hot, humid area, as it will almost certainly spell out problems for your guitar by causing high humidity conditions.