Probably the number one cause of expensive damage to acoustic guitars is from lack of humidity. This is especially prevalent in the winter months when central heating can dry out the air inside buildings, but it can also be a problem in some areas such as the desert southwest in the summer during periods of lower humidity and high temperatures, with air conditioning removing even more moisture from the air. Lack of humidity in the air can dry out the wood in your guitar, causing it to shrink. Side effects and symptoms can include fret edges that stick out slightly from the sides of the fingerboard and feel sharp, a sunken top and bridge, which can lower the guitar's action and make it unnaturally easy to play, fret buzzes, and even cracks in the wood.
Fortunately, it's fairly easy to prevent this kind of damage. While commercial humidifiers are available, you can inexpensively make your own from parts you can get locally (or may even have laying around the house) and assembling them is easy.
The parts and tool list for this project are very basic. You probably already have all the tools you'll need sitting around the house, and the rest of the parts can be purchased at a local drug or dollar store.
I like 3M's O-Cel-O cellulose sponges in the 4.7" x 3" x 0.6" size. These are available in four and six sponge packs. These are treated with 3M's "StayFresh technology (™)", which locks hydrogen peroxide (a strong antimicrobial) into the fibers of the sponge and helps prevent the growth of bacteria, mold and mildew.
The first step is drilling the holes in the soap dishes. Remember - safety first! As always, you should wear eye protection and use care whenever operating any power tools. Peel off any stick-on product or price labels, then carefully drill about 15-20 1/4" holes on each of the soap dish's two main flat surfaces. You don't have to use any exact pattern, but spread them out so that there are holes across the entire surface area. Don't go too fast or press too hard or you might crack the plastic.
Once the holes are drilled, you can use a utility knife, jeweler's file or a bit of sandpaper to remove any remaining plastic flashing from the holes.
Next, use the smaller of the two soap dish halves as a pattern to trace the outline of the soap dish on to each of the sponges, and then use the scissors to cut the sponge so that it fits into the soap dish. Most soap dishes are thick enough that they will require two standard sized kitchen sponges, placed one on top of the other inside the dish to completely fill it.
Use distilled water to wet your sponges. Distilled water lacks the minerals in tap water, so over time you won't end up with mineral build-up in the sponge and on the humidifier case. If enough builds up, it can adversely affect the performance of the humidifier. A gallon of distilled water is inexpensive and will last you for a long time. Get the sponges nice and wet, but not so wet that they're dripping water out of them; gently squeeze out any excess so that it's not dripping wet. You don't want water coming out of the assembled humidifier and coming into direct contact with (and damaging) your guitar. Once I've assembled the humidifier, I usually give it a good shake or three to make sure no water is going to come out of it, then I dry the outer surface of the case.
Some soap case designs actually click or snap shut, so you may not need a rubber band or two to hold your case together, but even with those designs I prefer the added security of the added rubber bands to prevent the case accidentally opening and the wet sponge from coming into direct contact with the guitar. Obviously, that's something you'll want to avoid in order to prevent damaging your guitar, so be sure to check the condition of the rubber bands each time you re-moisten the sponge and replace them when they show signs of aging or wear.
Even with treated sponges and distilled water, one of the things you'll probably notice eventually is a bit of smell. That means bacteria is starting to build up, and it is time to clean your humidifier. This is relatively easy. Just take the case apart and wash it with soap and hot water. You can wipe it down with water with a little bleach added to it if you prefer. This will kill any mold or mildew that has started growing on it. As for the sponges, they'll need to be cleaned and sterilized too. There are three very effective ways to clean and disinfect your sponges. The first is to soak the sponge completely with distilled water and then microwave it on high for one to two minutes. Keep an eye on it, and make sure to use care when you remove it - it's going to be very hot! The second method is the bleach soak method. Prepare a half gallon of distilled water with half a cup of household bleach added to it. Use this solution to saturate the sponges. Soak the sponges in it for five to ten minutes, then rinse with distilled water. The third method is to boil the sponges for five to ten minutes. Each of these methods will kill over 99% of any bacteria, mold, and mildew that has accumulated on the humidifier, which are the main causes of the musty smell.
Monitoring the humidity levels inside your case is always a good idea. For that, you'll want a digital hygrometer. These are available online for about $30.
You'll see various different recommendations as far as humidity levels. If the manufacturer of your guitar has a recommended humidity level range listed on their website or in your manual, go with that. Otherwise, I'd recommend keeping the humidity level between 45% and 60%. Too much humidity can also be bad for your guitar, but you're far more likely to suffer from too little than too much. As long as you keep it within this range, you should be able to avoid the serious damage that can result from the guitar becoming too dry, or too moist. I live in Southern California where the summers are usually pretty dry and hot, so I keep two of these units in my acoustic guitar case - one near the guitar neck's heel, and one up near the headstock, and they keep the humidity levels inside the case right within that range.
Well there you go - these easy to make DIY humidifiers work as well or even better than the commercial units, cost next to nothing, and yet can protect your acoustic instrument from very serious and costly damage. Give your acoustic guitar the care it deserves, and treat it to a couple of these inside its case, and a hygrometer to monitor the humidity levels. Check it once a week or so, and refill the units as needed, and chances are excellent that your acoustic guitar will never suffer damage from drying out.