The ideal humidity for acoustic guitars is between 40% to 50% RH. If your home is normally in this range you’re OK. A little higher or lower should not be a problem. I would be concerned if the RH got down close to 30% or lower. Low RH can cause the wood to shrink resulting in cracks. Too high RH can result in the wood swelling and causing other problems including loosening of joints. Sudden changes in temp or humidity can also cause cracking or swelling of the wood depending on which way the RH swings. For example you don’t want to take your guitar (acoustic or electric) from a climate controlled environment such as your house and leave it in the car over night if it’s a cold winter or during the day in the heat of summer. If you ever have your guitar in a situation where it had gotten very cold or hot don’t open the case immediately. Give it time to adjust before opening. This will allow the guitar acclimate gradually rather than shocking it. If you find the RH is too low in your house you can use a product like Alfonso suggested or another alternative is a room humidifier for the room you store your guitars in, keep the door closed, monitor the RH and adjust accordingly.
Hope this helps.
Ditto. When I first got my Taylor (410 custom, used), I was clueless to this particular fact. The very first winter, my top split! TRAGEDY!! I took it into the local dealer and he said that it wasn't really fixable, but that the crack didn't go through the wood itself (just the gloss and a tiny bit of wood on the top) so it shouldn't affect the sound much. I got lucky, really.
I got a few sponges (mentioned above) that day and it's been fine since. But now I have a full-house humidifier that keeps things above 50%, so I don't even bother wetting the sponges when it's in the house during the winter. Haven't had any problems.
To me, the guitar doesn't sound quite as "warm" when it's too dry...maybe Rick can comment on that...When it's in the right range, your guitar will sound correct (which is relative to that specific instrument, of course, as all acoustic guitars sound different). When it's too humid it starts to sound too warm, i.e. "muddy," that happens a lot when playing outdoors in the summer (and guitars have a tendency to de-tune a bit then too). Apparently maple wood guitars are better for outdoor play for that reason--and they're darn pretty too.