How to Make a Homemade Humidifier

It's important to maintain correct humidity, especially during the winter when heated air in your home causes the relative humidity to drop.  It's also important to keep an eye on it during the summer, though, because air conditioners will lower RH.  Ideally, you want to maintain an RH of 40-50%, with a max range of 35-65%.  When your instrument was made, it was put together under certain conditions.  Suppose your builder had an RH of 30% in his shop.  When wood takes on moisture, it swells, thus if you took an instrument from 30% to 60% RH, it will swell.  Since the instrument is assembled and since pieces are glued to one another, this swelling must go somewhere and you'll see the top bulge, the neck back bow, and the back bow out.  It's possible to cause damage by over-humidifying!  Conversely, if the instrument is built at, say, 65% RH and you live in AZ with 20% RH, the wood is going to shrink.  Again, it's held together at the sides and this shrinkage must come from somewhere- eventually the wood cracks along a grain line.  As a compromise, most builders assemble their instruments at right around 50% RH, give or take.  This means that, the majority of the time, the instrument will maintain the shape under which it was built.  But, in the winter, or in particularly dry climates, or if the wood wasn't totally dry, or for a wide range of reasons, it may be necessary to humidify (or de-humidify).

Check out this page on Common Problems for some ways to check your guitar for dryness.

The first thing you need to do is find out the RH.  As we've seen, high humidity can cause just as many problems as low humidity.  There's a wide range of affordable digital hygrometers on the market.  Their accuracy will vary, but you can calibrate them easily.  Simply fill a bottle cap with salt and then fill that with water.  Put the cap and the hygrometer in a ziploc bag and seal it.  Leave it for a couple hours- it should read 75% RH.  The Planet Wave hygrometer in this shot reads about 5% too high, the Radio Shack unit is dead-on, and the other one needs a new battery! I'm not that worried about 2% here or there.  As long as they're around 45-55%, I'm happy.  Keep a hygrometer wherever you keep your instrument.
You can buy fancy humidifiers, but I typically have a lot of guitars around between mine and my customers, so I just make my own humidifiers. I've been using these for 15-20 years with no problems. Start with a high quality cellulose kitchen sponge. Don't use a cheap plastic sponge- get the good stuff.
Cut a piece of the sponge approx 2" wide.  I like to use the "SnakPak" size Ziplocs.
Fold the Ziploc over several times and punch a row of holes in the folded-over section.  Don't worry if the punch doesn't "clean" the hole- moisture will still escape just fine.
Get the sponge wet and then squeeze it out.  A good sponge will absorb water and shouldn't drip, but you can flick the it a few times to get rid of excess water, if you're worried.
And here's the completed humidifier.  If it's really dry or I have the instrument hanging on the wall, I drop this right into the guitar and just leave it there.  People will often ask "Doesn't it drip?".  Again, a good kitchen sponge isn't going to drip!  If it does, you have WAY too much water in it.  Also, if it did drip, the water should collect in the corners of the Ziploc and re-absorb as soon as the humidifier is moved.  But, once again!!!, it shouldn't drip.  If the instrument doesn't need full humidity, I'll stash the bag under the headstock or in-between the neck heel and case (if there's room).  And, of course, I have a small hygrometer in the case to tell me what's going.

One CAUTION: This humidifier can put out some serious humidity.  Until you get used to it, check your guitar frequently (like every day) to be sure you're not over-humidifying.  Use common sense.