In case you intend to skim this, let me put the important information up front. Hygrometers measure relative humidity. They are easy to obtain and affordable (usually about $10-$15). Acoustics guitars are most comfortable between 45%-55% relative humidity. Some say 40%-50%, but I live in the swampy south, so I read it a little higher.
Now that that’s out of the way, let me share some of the anecdotal insights and tips that I’ve acquired throughout the years regarding ambient humidity, acoustic guitars, and how to make them get along. It all started with the Larrivee jumbo I picked out from Chuck Levin’s in 2003. It was my first high-end guitar. I was in high school and had squirreled away my busboy wages for a couple of years to afford it. I kept it exclusively in DADGAD and strung it up with the heaviest gauges I could find (much to the chagrin of my luthier).
One day in the midst of a northern Virginia winter, I noticed that the action had climbed up and it wasn’t intonating properly. Even more distressing, there was a hairline crack forming in what I hoped was just the finish. It was still under warranty, so I contacted Larrivee and arranged to have it inspected at their headquarters. A few weeks later I received a call from Jean Larrivee Jr. who graciously explained that wood, being an organic material, is susceptible to fluctuation in response to environmental factors such as temperature and humidity. This applies to musical instruments especially, which are typically constructed with pieces of wood that have been cut as thin as possible to allow for maximum vibration without caving in under the string tension. I’ve paraphrased him to make this sound as obvious as possible, but at the time, my seventeen-year-old mind was blown. After further discussion with him and my luthier, we concluded that the first step towards a “fix” would be to seal it up in its case with a soundhole humidifier for a couple of weeks, opening it periodically to re-moisten the sponge. It plumped back up beautifully within this time, and didn’t even require a setup. The finish crack, however, was there to stay; a subtle reminder of my previous ignorance.
A guitar humidifier, in the most basic sense, is a moist object kept in proximity to the instrument. They are most effective, however, when said object is suspended inside the soundhole between the 4th and 3rd strings. The old school method was an apple core wrapped in a punctured plastic bag, but nowadays, a small sponge in a hard, perforated plastic case is preferred. I’m being as specific as I can to avoid the following kind of mid-conception:
I once had a friend who would soak the sponges in his humidifiers to their maximum capacity so he wouldn’t have to “water” his guitars as frequently. As distasteful as the term may be, the imperative adjective here is “moist”. The sponge should be a little damp, pliable, softly wrung out. When it becomes dry and crispy, you re-wet it. It was difficult to convince my friend of this, as he was a scientist, and had conducted experiments that he maintained proved there was no difference in humidity within the case based on the wetness of the sponge. His error, it turns out, lay in the placement of the hygrometer. He was measuring the case humidity around the neck. Upon persuading him to place it near the humidifier, itself, the difference was clear. He was effectively hosing down the area around his soundhole, which could have resulted in structural issues down the line.
On the other hand, I once knew a guy who thought that having all his cased guitars stashed in a bathroom with an open toilet was sufficient humidification for the dry months of the year (usually in the winter, when you’re running heat). Granted, he and I both live in North Carolina where the winters are relatively mild, but it still gets cold enough for one bowl of toilet water to be dramatically ineffective for a whole stable of guitars. I never followed up with this guy after a number of awkward, plodding debates, but I sincerely hope that he has a giant, junglous John and all his instruments are okay.
Of course, guitars can definitely also get too wet. I learned this the hard way while homesteading in the rainforests of the big island of Hawaii. I was living in an open air shack near a gulch and I couldn’t get through a single song without my guitar going out of tune! I seriously considered getting a carbon fiber guitar at this point, but ended up buying a plane ticket home, instead.
Then there was the guy who took his brand new Collings with him to the tropics of Singapore. Upon noticing that it was becoming difficult to play, he took it to a local “luthier”, who rather than attempt to initiate the difficult conversation surrounding climate control, promptly cracked the truss rod in an effort to lower the action, and sent him on his way. Again, this was a highly educated man (a professor), but it took about three weeks and many phone calls for myself and my friends at Collings to convince him of the reality of what had occurred. Unfortunately, the neck had to be completely replaced and it was not covered under warranty.
Keeping a guitar from getting too wet is a bit trickier. Basically, you need something to absorb the ambient moisture around the instrument. An in-room dehumidifier or some silica packs will the trick. They’re are commercial accessories available as well, but silica packs (like you’d find in a new pair of shoes) can be reused if you bake them at a very low temperature.
Alas, the precise construction and delicate materials which make our guitars sound so good render them vulnerable to the elements. It’s not so bad, though. Just keep track of the humidity, take the necessary steps as needed, and you should be fine. If you’ve got a big stable of instruments, however, you may want to invest in a climate controlled music room. Lastly, they’re are some great carbon fiber guitars that are impervious to such things, but for those of us who prefer our wood with wire, it’s important to keep track of the air out there