When storing guitars and other instruments made of wood, one of the most important consideration is the environmental conditions that the guitar will be subject to during storage. This would include extremes of temperature and most importantly, relative humidity.
Wood is very susceptible to relative humidity levels (basically the amount of moisture in the air). Too little humidity and the wood drys out and your guitar can develop any number of problems. Conversely, if your guitar is exposed to conditions where there is too much moisture in the air, the wood can swell and cause different problems. Here is a link Humidity and Guitars that tells you more about how humidity can affect an acoustic guitar and the optimal humidity range.
Many people choose to store their guitars and other wood instruments in a hard-shell case with either a sound hole or an inside the case humidity control system.
There are several sound hole humidifiers that work great for acoustic guitars stored in their case. Here are some of the popular varieties.
This gadget drops in the sound hole and is suspended by the strings. You wet the small sponge inside whenever it dries out.
This is a Dampit Soundhole instrument humidifier. It can be used in instruments with F holes as well as larger soundholes so it will work with violas or cellos for example. Again it uses a sponge to hold moisture.
This is another Planet Waves Product called Humidipack and is a two way humidifier. According to the description on
Planet Waves dot com this is a
maintenance free, two-way humidity control system for guitar. It automatically maintains the optimal 45-50% relative humidity level within your instrument case, eliminating the guesswork and potential mess related to refilling a humidifier. Unlike refillable humidifiers, this system provides true “two-way” purified humidity control by adding and removing moisture on demand.
There are also many YouTube videos describing how to make a simple homemade humidifier using a sponge and something like a plastic sandwich bag with small holes punched in it. By making your own, you can size it to conform to any instrument or to whatever air space (however tight) might exist in your instrument's case. I have found that denser sponges tend to hold moister longer than the cheaper light weight ones. You can cut any sponge to fit the size you need.
If not all of your wood instruments are stored in a hard shell case or if you want to leave one on display or handy so you can play it daily without removing it from the case - you might consider a room humidifier to maintain the optimal humidity level in the room where your instruments will reside. Many models of humidifiers will automatically maintain a pre-set humidity level. You set the level and then leave it on. When the humidity drops below the pre-set level, it will automatically come on and run until the desired humidity level is restored.
A room humidifier will not work well in a wide open space. Ideally it should be used in a room with a door that can be kept closed during the dry heating season. You might also add some foam weather stripping to seal any gap below the door to make it even more efficient.
Regardless of which method you choose, it is important to maintain an adequate humidity level for the health of your instruments. With 20% or below, I am afraid your instruments may be susceptible to deleterious effects of drying.