As an owner of many guitars :roll: :lol: I've obsessed about it for ages.
It's sudden changes in temperature or humidity that do the real damage more than any particular level. In other words, if you make a long journey on a hot day with the guitar in your car boot, don't run into a cold basement & whip it straight out (the guitar Afghan, this isn't a Koch thread) but leave it in the case for the temperature to even up a bit. Likewise, don't walk 20 miles in driving snow to get to that gig at the local sauna & open the case as soon as you're through the door.
PhillyQ sort of has a point - this country rarely sees the really high levels of summer humidity that, say, the southern US sees. My hygrometer rarely registers >60% (& almost never >65%). However, it's not high humidity that does the real damage but the low humidity you typically get in cold winters. Anything under 35% & I don't get my acoustics out. Really low humitity can lead to cracking in untreated wood (i.e. fretboards & the inside of acoustics). In my experience a house kept at reasonably comfortably temperatures rarely sees humidity outside the 35-55% range which is OK for most guitars.
