Humidity

Because of the cold winter(s) here and the furnace running constantly, I run a room humidifier 24/7 and keep the guitar area between 40% and 50%. (Monitored by a StewMac hygrometer) This range works.

Living in the Mojave Desert leaves me with very low natural ambient humidity

I don’t do anything to mess with it.

Maybe I should, I guess I am reliant on the idea that consistency is my friend.

Around here summer will bring single digit numbers and winter a little higher.

Probably doesn’t vary more than about 25 points generally speaking.

The guitars seem to be holding up just fine.

45-55% is probably ideal with temps 65-75.

I have hygrometers in the studio and a couple downstairs rooms. You can get three for like $20 on amazon.

My nicer instruments get played a lot so stay out of their cases. I notice the grain beneath the french polish of my classical becomes a bit more noticeable around 40%. In the winter if the humidity dips below 35% In a certain space I’ll move it to a room with better numbers.

Used to be more careful and try to control humidity inside the case etc but at this point just want to play as much as possible and the guitars don’t seem to mind.

Our recent cold snap has the furnace running more than ever, and a few of my wall hangers have shown it. I was playing my recently cobbled Tejas Thinline the other day and I swear the trussrod went slack under my fingers. It's an Allparts 22 fret/12" radius neck. Yanked it and checked, the heel adjustment wasn't loose but it took some significant tightening to get the relief back in the ballpark. I usually expect this with new builds but this was by far the most out of whack.

I need a hygrometer for my studio.

45-50%.

Why those numbers? Because that is where most guitars are built. The numbers aren't magic - it will just be closest to its intended dimensions when it is at the same humidity it was when all of the pieces were sized out.  Wood moves with humidity, and guitars aren't designed for movement... at least, not much. Think of old style cabinet backs with tongue and groove joints that aren't glued - they're there to allow movement. Your guitar back and top don't have tongue and grooves. They do have a bit of an arch which can rise and fall, but that is it.

How much variance it can take is more or less random. Every wood has a different modulus of rupture. In my experience, anywhere in the 40s is safe, and anywhere in the 30s will drop the action and make the setup go crummy, but actual structural damage doesn't happen until you go lower than 30%. Many will survive the 30s, but teens and 20s are pretty bad. Again, it isn't guaranteed.... some pieces of wood are more elastic than others... and everyone has a story about "well, my brand X never cracked, but my brand Y did! While the next person will insist "My brand Y never cracked, but my brand X did!".

If you get a humidifier, don't trust what the machine tells you. Those are never accurate. Get a hygrometer and put it somewhere away from the humidifier so it isn't just getting the moisture as it comes straight out of the machine. The ThermoPro ones are consistently good for the money, and good enough for guitar work.

Love, repair guy in the desert.

It's usually optimal here in San Diego, but the worst happens when we have Santa Ana conditions where dry mountain air blows our way. We've been going through it this week with temps in the 80s! and RH in the 15-30% range. It seems to be easing up though. My weather app shows a high of 57F and a 49% chance of rain on Tuesday. Can't wait! I also tend to drop things when it's dry - no finger grip.

My house stays fairly well regulated but it will get affected when it's too dry outside. Some guitars need truss rod tweaks and my Taylor is all buzzy. It's not even all solid wood. I've had it in a 45% room all week and it's evening out. It was buzzing only on the G and now it buzzes less, but is more consistent across all strings. I think I can loosen the truss rod and it'll be good. Will try that tonight.

Wow - 4 %. Now that’s dry! Right now I’m going through 1.5 gallons of water every 12 hours trying to maintain things at 45%. 
Another trick I use is to keep a laundry rack in the music room throughout the winter and hang a few things on it after a load of laundry.  It helps.