Cased with a inside the case humidifier is an option if you maintain it.

For me, I leave them out and use a whole house humidifier. I can only get about 35 % as the temps outside approach zero but 32 and above it stays at 45%.

quote:

Originally posted by P220 Smudge:

In point of fairness, every Martin 12 string, if it lives long enough to tell the tale, the action is going to creep ridiculously high, and for several reasons that a Taylor won't.

Warpage associated with long-term string tension:

This device really does work and has an almost imperceptible effect on tone. I would argue that it improves the tone of a lot of guitars.
It is not necessary to wait to "repair" a guitar. I had one built into a custom and the top has remained dead nuts flat for about 15 years now, in Utah heat/no humidity.

http://www.stewmac.com/Luthier...D_Bridge_Doctor.html

I take Bob Taylor's advice to heart and make an effort to keep my guitars humidified in their cases. He recommends 47%, builds and sets up his guitars in that environment. My house gets very dry in the winter even with a whole house humidifier in the HVAC system. It will drop and stay the mid 20's in the winter, not good for anything wood inside the house except firewood. Today the RH in my house is 17% due to the cold snap.

I like and use the Oasis humidifier as recommended earlier in this thread. Oasis uses crystals inside a Gore-tex tube that suspend distilled water in a gel. This prevents water from leaking out of the device, but allows moisture to migrate through the membrane. I have seen damage to guitar backs from other sound hole humidifiers that leaked or were over filled. The Oasis helps prevent that, but does not last forever either, the glued seams can fail after many years, so I replace them as necessary. In the winter the small Oasis OH-1 will keep my guitars in the low 40s RH. I'm thinking about going to the larger OH-5 next time for winter use.

For convenience I also use Oregon Scientific wireless hygrometers in my guitar cases that broadcast the humidity to a base station display. I can check humidity in every guitar without opening the case by simply pushing a button. I find I need to refill the OH-1 about every 2 weeks in the winter.