Below is information regarding the adverse effects of too-low relative humidity on wooden instruments (guitars in particular). This partially modified info was originally obtained from Taylor Guitars.
No environmental condition can do more damage to your instrument than low relative humidity. Many areas of the U.S. experience naturally low humidity all year long. The Rocky Mountain region, for example, has notoriously low RH levels, as do Phoenix, Reno, Las Vegas, Bend, Oregon, and a number of other cities. In most regions of the country, however, low RH levels accompany the onslaught of winter, mostly due to the effects of artificially heating our homes and buildings. If you’ve read the various references to humidity-related problems published in our quarterly news-letter, Wood&Steel, you know that the colder months wreak havoc on acoustic guitars made of solid wood. As the weather turns wet and temperatures free fall, there is a natural tendency to crank up indoor heating units. Unfortunately, what many don’t realize is that heating a room forces the relative humidity down to a level that poses a real threat to quality acoustic guitars by literally drying them out. The bottom line regarding dryness is that if your guitar is not kept within a comfortable range of 40-55% relative humidity, it will be in danger of cracking. We strongly recommend the use of a soundhole humidifier, along with a digital hygrometer to accurately measure the humidity level where you store the guitar.
Symptoms of a dry guitar: The arch in the top sinks, taking the bridge with it; the neck bows forward, necessitating a truss rod adjustment; and the fingerboard shrinks in width, causing the sharp ends of the frets to extend beyond the edge of the neck. Combined, these events can make a guitar look like it has a badly made neck. Actually, the neck is fine — the problem is with the body, and that can be corrected with a truss rod adjustment and the use of a soundhole humidifier. However, many customers still find it hard to believe that so simple a solution as humidifying the body will fix what appears to be a grave problem with the neck. Let us convince you. When we make a guitar, the wood first is dried, “seasoned,” and acclimated to a certain moisture content. As a result, all guitars leave the factory inthe same condition, and all will react more or less the same when exposed to changes in humidity. Our factory is climate-controlled to maintain a temperature of 75 degrees and a relative humidity of 47 percent. This consistency causes the wood to equalize at a specified moisture content. As the wood’s moisture content changes, so does the size of the wood. Spruce, in particular, shrinks and expands a tremendous amount as it gains and loses moisture. For example, let’s say we condition a spruce top in a room that is 47 percent RH, and then cut that spruce to a width of 16 inches. If we then were to lower the room’s RH to 30 percent, that same piece of spruce would shrink to 15.9 inches in width — shrinkage of almost 1/8 of an inch! If, instead, we were to raise the room’s RH to 60 percent, the spruce would swell to 16.06 inches, an expansion of almost 1/16 of an inch. Relative humidity is not an arbitrary matter. We prefer 47 percent RH because it is a very “normal” or “median” humidity. When built at that RH level, a guitar can be exposed to more or less humidity and still perform well. The more extreme the temperature and/or humidity fluctuations, the sooner the guitar will be adversely affected. The good news is, your guitar can be protected from many changes simply by storing it in its case. If it does become too dry, both its moisture content and its shape can be restored by exposing it to humidity. Room humidifiers, soundhole humidifiers and gel-type humidifiers accomplishes this very well.
If using sponge-type humidifiers:
BE SURE TO SQUEEZE EXCESS WATER FROM THE DAMPITS AND WIPE THE OUTSIDE DRY. THEY SHOULD NOT DRIP WHEN HELD VERTICALLY. IF DRIPS OCCUR, SQUEEZE MORE WATER OUT UNTIL THEY STOP DRIPPING.
Always store your instrument in a hardshell case? Try an Arion humidifier! The Arion design is unique and has none of the shortcomings of traditional humidifiers. It relies on a proprietary special solution combined with hydrogels enclosed in a space-age plastic that allows moisture to consistently escape through its walls. The Arion Humidifier will not leak in your case, requires only plain tap water every 1-3 weeks and will last for many years.