humidity in house

I'm of the mind that the exact % of humidity is not the determining factor. It's more the fluctuation that becomes a problem.

here in the Mojave 16% is somewhat high for summertime. We are often at 10% or even lower. Right now, on a rather common winter day we are at 38%

My numbers above are regarding ambient (outdoor) levels. 

Yeah, my Sire H7 Semi Hollow 335 style guitar surprisingly had the wood with plastic binding on the neck apparently shrink due to low humidity and even though the frets were curved downward on the ends the binding not covering the end (the binding is just notched at the fret ends) and the sharp ends were slightly extending past the binding on both sides of the neck. While it was much easier to fix than my instruments with necks with no binging,  I was surprised as my other instruments with binding covered the ends.

Low humidity can lead to static charge developing, potentially zapping poorly designed solid state gear, in areas of the circuit where it interacts with the outside world, eg inputs, outputs.

I’ve fixed a few things where this seemed the most likely cause of their failure.

Exactly. Your gear is most comfortable in an environment that you're comfortable in.
I don't own any solid bodies, but fret sprout on any guitar is enough for me to keep all of them properly humidified.
I don't subscribe to the 50% humidity rule many recommend. I find 35-40% just fine.

Those numbers come from the factories.  For the wood itself, there really is no magic number.  The wood doesn't care.

Wood expands with more moisture, and contracts with less.  A cabinet back will often have an unglued tongue and groove joint, so the tongue can move in and out of the groove as the humidity changes.  If it was glued up as a solid panel, it would crack in the winter and bow in the summer.  A guitar top does not have a tongue and groove joint, and would be like the cabinet back if it was made wrong and glued up solid.  It is glued to the sides, so if it shrinks, it has nowhere to pull away from.  Those sides are incredibly rigid.  Most acoustics are made with some arch to them, which can act like a strut and give some movement (part of why the action drops when it dries out), and different pieces of wood have different amounts of elasticity, and a different modulus of rupture.  Eventually there is so much tension that it pulls apart, usually in the widest part of the body.  Too much humidity, it pushes out and has to bubble up.  

So, the top is going to fit in the sides where it was glued best at the humidity at which it was when it was glued there.  Run a factory at 10% humidity, and it will be happiest there.  It doesn't matter that 10% is crusty booger dry, that is where it'll fit best.  Run one at 75% humidity, and you'll struggle to get enough moisture in the thing to not crack in any other non-tropical environment.  

All that said... 45-50% is what most factories say they keep the shop at.  In my experience, it is usually on the lower end of that, and 50% tends to be a bit too high. I think people go straight for 50% because of the belief that humidity is like food for your guitar, and it needs to stay alive.  But, the guitar doesn't care.  It just wants its pieces to be the same size as they were when it was glued together at the factory.  These are sometimes the people (usually GC employees) who think you can't over humidify a guitar.

How far you can deviate is not easily predicted.  Some tolerate it better than others.  Bracing doesn't matter.  When wood wants to move with humidity, it ABSOLUTELY will move.  That is a VERY strong force.  In the New Hampshire granite quarries, they used to drill holes in the rock, put in pieces of wood and then pour water on the wood so it would expand and crack the rock.  If the giant rock wasn't enough to "brace" the wood to keep it from moving, a couple spruce bars won't do it, either.

This is why electrics really aren't affected.  A tele body isn't glued out to a rim anywhere, it if needs to shrink a little bit, it will, and immeasurably.  If you wanted to special order a giant set of calipers to measure the width at the lower bout in different humidity levels on a solid body you probably could.  But, since a solid body is 1 3/4" thick and fully finished (usually), I wouldn't expect quick changes.  Laminate woods tend to be stable, but once in a blue moon I'll see some that will move with humidity.  Most often it is a veneer acting up, like the flame maple tops on an ES-335.  

Yeah, most of my manufacturing/repair experience has been in drier climates, so I know less about dehumidifiers... except that when Benedetto set up in Savannah, they had a bit of a struggle. But, that was mostly with lacquer blushing. For others, my guess is that they say they keep it at 45-50%, but are struggling to keep it at 45, so 40-45% might be a more reasonable number and there will still be seasonal fluctuation. At one place I worked this is where it was... then if we got a bunch of rain, we had to open the whole shop because it would become a steam room inside.

Damage from over humidification isn't as common, usually they just balloon up, sound like crap and are hard to play. Stuff can pop loose though. Back braces and the ends of the bridge are the most common. Usually when I see something over humidified to the point of damage it was because someone was putting in a case humidifier when the outside humidity was either adequate or too high to begin with. I've never lived in the southeast, though. I expect that stuff balloons a bit and that the damage mostly occurs if/when instruments are stored in extra dewey places, and being kept inside in a livable space is enough to keep it at normal range. So, inside in the AC and you're fine, outside in the storage shed is going to give damage. Just my guessing, though.

Yeah, newer home constructions are super tight and hold onto a lot of moisture in the winter, and if you don't have wood heat or forced air, you have that helping you, too.

On the other hand, old farm houses sometimes get super humid in the winter, because moisture comes up from the basement, which is often just dirt or slate.

I only started using a humidifier a couple of years ago and so, I have two running between the kitchen and the living room. That being said, I have guitars hanging on my living room walls that have been there, when not being played ( ), for more than twenty years. Haven't even seen the inside of their cases in all that time (except for transport). Not one of them has an issue and they keep their tune. Solids and semis alike. I do keep my acoustics in their case though and as far as the humidifiers go, they are more suited to keeping the static low. No fret sprout, surface cracks or any other anomaly related to temp, humidity or age. I saw the lowest humidity I've seen in my house during our recent severe storm @ 8%. On average, it's around 35%. Why it's this way maybe do to the mostly wet climate I live in I guess. 

Me no likee neck heel adjustment.

Yep, not such a big deal for solid bodies but for the neck. It's obviously not a big deal to adjust the truss rod but keeping them in a humidified environment will lessen the yearly variance.

I have two acoustics that I play regularly; one doesn't care where it lives, sounds great in general (though really, it sounds it's best in the summer and gets a bit cranky in the winter), the other will balloon in the summer and sound like a wet sponge and contract in the winter to the point that I was worried that It'd crack apart such that I ultimately created a humidified space. Now all my guitars, acoustic AND electric are happy, stable, and consistent.

All that said, my '84 Les Paul that I've had since '89 and never gave any special treatment (lived in the open air for most of the ensuing years, mostly in non-insulated houses here in Michigan with absolutely no ill effects) has, in the last year, developed a hump in the neck near where it joins the body. Go figure...

I tend to think that any guitar is gonna reveal it's quirks (with regard to humidity) in its early years. The wood available now is much younger and consequently much squirrelier. How many of those gazillion dollar pre-war Martins do ya think were humidified at any time in their life?

Amazed you didn’t have more issues there. You probably didn’t have insulation in the walls and possibly the attic did you?

What was the basement/foundation on that house?  I knew a family in a farm house from 1866, and it had a dirt/slate basement, and in the winter the heaters would draw up moisture from the basement (below the frost line) and it was basically a big humidifier and left condensation on all of the windows.  I've similarly seen over humidification on other old farm house instruments.