This thread merits constant bumping - important stuff
Thanks, Light - Humidity control (especially here in the Northeast) is critical.
There were some posts about too much humidity and Taylor's approach. This might clear things up a bit.
Here in Vermont, we have wide variations in temperature from day to day. The temp can fluctuate from 20 degrees F. above zero to 20 below overnight. When it gets really cold, or pretty cold for a long time, the relative humidity drops and drops - sometimes to 5% or less.
When wood takes on moisture, it tends to swell a bit. You can find the belly of an
acoustic guitar might rise a little and your action might feel a bit high. But it rarely causes damage.
Dryness is a whole different deal. When wood gives off moisture (as it will this time of year in the Northeast) the wood cells shrink. The trouble is that the wood, say, in the top, is glued to the sides, glued to the bridge plate, glued to the braces, glued to the pickguard... and the glue will not yield its place. At some point the stress from shrinking will cause the wood to snap, and you have a crack.
You can begin to see damage, for instance, when fret ends start moving neck binding around - the steel shrinks at a different rate than the neck wood. My Taylor was refretted and the frets always cut my left hand in February, even with some humidification. That's reversable, as the frets settle down with more moisture. But it's a warning.
With a little too much moisture the guitar might not perform to its capability.
But let it get too dry and it will break into pieces. The best target Relative Humidity is around 40 % - 50 %. Try not to let it get below 30%. And without humidification, a steel string guitar under tension is very likely to develop a top split when the RH drops to 10% or below.
Tree
doesn't everyone have a guitar/cigar humidor room? right next to the wine vault... 
