A great deal of valuable advice can be found on the web about maintaining ideal relative humidity for all-wood acoustic guitars. Martin Guitars, and in particular Bob Taylor have informed and advocated on the subject. Your music store will no doubt also provide excellent advice. I cannot improve on these expert resources, but I will add my voice to this message. The winter months here in the north-eastern USA can be brutal on all-wood guitars. Relative humidity drops with the lower temperatures, and heating and ventilation systems further dry the air. Maintaining a humidity level between about 45% to 55% is a proactive step that protects the instrument from damage, and can prevent expensive repairs due to the wood shrinking as it dries. In the best cases, dryness affects playability. The top will sink, and the guitar will come out of adjustment with the frets beginning to extend beyond the width of the fingerboard. You may get some buzzing during play, with uncomfortable fret feel along the fingerboard and neck. In the worst cases, the guitar will crack, the bridge or neck might separate at the glue joints and the neck will warp.
An acoustic guitar is under constant stress due to the tension on the strings. It is constructed primarily out of different types of wood, but also contains other materials such as steel, nickel, plastic, bone, etc. along with various parts, like the neck and bridge, which are typically glued together. These different materials expand and contract differently with changes in temperature and humidity. Don’t become paranoid, guitars are very strong in their construction, with a basic design that has matured over centuries. As long as environmental changes are gradual, and the guitar is not exposed to extremes, or adverse conditions over an extended period of time, there is nothing to be concerned about. The key concept is to manage conditions so that the guitar can moisturize when needed, and any changes are gradual so that the instrument is able to adjust to the ambient environment naturally. For example, traveling with your guitar in the trunk/boot of your car during winter, or leaving your guitar inside a parked car in the hot summer sun will certainly damage it, even if it is in the hardshell case. However, a guitar that becomes dry after a few hours of play, can be quickly brought back to normal condition with a simple, no effort, inexpensive care regimen, that anyone can do.
Guitars are beautiful and many people like to keep them out on display. In this case, it is important to manage the relative humidity of the room where you keep your guitar. This is easily accomplished with a low cost digital hygrometer. Add a warm mist room humidifier, and you can keep an eye on the hygrometer to maintain humidity levels in the range of 45% to 50%. Various wall hanging solutions exist for display and if you utilize these, then do not mount your guitar against an exterior wall, due to the differences in external and internal temperatures.
During winter months, I prefer (and recommend) to keep the guitar in it’s hardshell case, with a sound-hole humidifier. (See illustration).
This type of humidifier is elegantly simple, cheap, easy to use, with few opportunities for user error. Just moisten the sponge, suspend the humidifier off the strings into the soundhole, then enclose in the hardshell case. The sponge naturally evaporates into the wood on the inside of the guitar body. The wood inside the body is not treated with lacquer, or other finishes, and therefore absorbs the moisture. The closed case maintains a climate controlled environment, and insulates the guitar from sudden and drastic changes in external temperature and humidity. And that is all you need to do in order to maintain playability during winter, protect your instrument from humidity related damage, and your wallet from potential repairs or replacements.