Hey Tone. Most of the year our humidity level runs right around 46% which is great. During the winter months when we run our heater a lot (gas), the humidity drops to 30-35%. That's the time we usually start having problems. I start watching the guitars closer when the humidity drops below 40% for signs of humidity problems. Things like changes in the top of the guitar, the dreaded 12th fret hump, or unidentified buzzes and such. If I see any signs I go ahead and case the guitar with a humidifier.

Some of the guitars are more susceptible to humidity problems than others. I'm not sure why.