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I'm all set to start my first build but concerned about RH.

I live in San Jose, CA, where the annual average RH is around 60% according to web searches, and doesn't vary much from that. According to a calibrated RH gauge my shop and home are around 56% RH and pretty stable. All the info I read says you should build around 45% RH....so should I invest in a dehumidifier and try to lower the the humidity in my shop or just no worry about it?

Thanks,
Mark

yes if you can
keeping in the 45 to 55% range is a good place a little drier is better than too wet

I'm in the San Jose area as well. We are about to start the rainy season.

This worshipping of 45% humidity... If OP is building for himself, the guitar will likely stay near where it was built, what's wrong with accepting the ambient humidity (after the materials have stabilized) and build away? I can't see what's gained by drying the workshop to 45% and the guitar never seeing it that dry ever again. Vietnamese guitars don't come to grief at home, but in climates unlike where they were built.

I wrestle with understanding this too. My 30 year old L'arrivée has been through that many wood-heated winters on the humid west coast of Canada. My own first guitars are fine after several years of the same fluctuations. Maybe all of these are overbuilt? I think there is something to be said for building in the conditions the instrument will live in. But I would be very interested to learn from the experience of others. What exactly causes things to give? Is it temperature or humidity change, or the speed of the change, or the RH of the materials to begin with? And does the instrument become more forgiving as it ages? Too little time to learn all this stuff! Bruce W.

the biggest harm is from RH. If you can get the guitar to the lower side you will be better off.
If you build sat at 60% and you hit 35 % you will see a noticeable dip in the top for the shrinkage and
this may cause top cracks and for sure lower action to the point of making the guitar unplayable
if you are at 50% the dip is only a 15% change and would be within a safe range
so if you can plan to stay in that 15% range you will be fine , there is nothing more frustrating that seeing a guitar crack from humdity.

I don't have very good humidity control, and large swings in the humidity. It's caused all kinds of problems. On one occasion, I glued braces with a 15' radius on the back, and then the humidity changed. The back warped from the designed 15' convex radius to being a concave radius. Every winter the action on my guitars goes from being correct to being way low. I've now got a summer saddle and a winter saddle. I ought to pay more attention to humidifying my guitar cases.

Like MaineGeezer, I have a back (Guatemalan Rosewood) that went from a +15' curve to a -15' curve (slight exaggeration) due to a change in humidity. I like building a bit dry (~35-40% @STP) now that I can afford air conditioning in the summer. Drying out has often resulted in cracking for me, but modest humidifying effect of summer hasn't been much trouble.

Thanks everyone. I have ordered a dehumidifier for the garage and will see if that does the trick to keep it in the ~45% rh range. Still hovering in the mid to low 50's at the moment, but going to hold off gluing until I can get the humidity down and stable for a bit. Also ordered a wood moisture meter - was pretty inexpensive (about $20), so we will see if I learn anything meaningful from that.

MaineGeezer wrote:...On one occasion, I glued braces with a 15' radius on the back, and then the humidity changed. The back warped from the designed 15' convex radius to being a concave radius....

I had this happen twice with backs sitting on the shelf after bracing. They were bowing the wrong way one morning! I replaced the braces and keep my braces very stiff and tall now. I didn't even consider that the cause was probably humidity change and assumed they weren't good braces. I wasn't controlling RH% back then. They were the vintage style low, fat braces that bowed. I now use 5/16" wide but 3/4" tall for 3 and 4.