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I am more concerned about the amount of water in solution (Absolute Humidity) because RH alone is not the whole picture.
Examples:
40% RH at 25/77 = .009 kg H2O/M3 of air
40% RH at 23/73.5 = .008 kg H2O/M3 of air
40% RH at 18/65 = .006 kg H2O/M3 of air
50% RH at 18/65 = .008 kg H2O/M3 of air
55% RH at 18/65 = .009 kg H2O/M3 of air
30% RH at 25/77 = .007 kg H2O/M3 of air
30% RH at 23/73.5 = .006 kg H2O/M3 of air
30% RH at 18/65 =.005 kg H2O/M3 of air

So you can work at an RH of 55% at 18c/65f and it will be the same Absolute Humidity as 40% RH at 23c/73.5f. The easiest way to decrease RH is to increase temp.

With that said, a piece of wood is stable at the same relative humidity no matter the temp. Wood stability doesn't correspond to the amount of moisture in the air. Yes, air at higher temps can hold more moisture so it requires more moisture to be at the same RH, but not sure how that relates to wood movement?

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Formerly known as Adaboy.......

The room got very hot, easily into the 90s. Too hot to work in. I remember the humidity gauge reading just over 80%. The small windows were opened but there was very little air exchange. It was pretty much an attic environment. The glue was Titebond. The only body I regretted losing was a wenge reso body.

I do appreciate all the discussion of RH, but seems to me we have not really addressed the full intent of the original question, and I would like to know what others think of using highly figured wood, etc. I for one like them aesthetically for sides and backs just because you can't readily buy guitar with them off the rack, and there are some many options out there for the hand builder (if I can use the work "hand" without diverting this thread).
Thanks,
Marty

Pretty, unusual varieties of wood are probably half the reason most folks get into lutherie. After you find out that no wood is the holy grail there's nothing left but to push on to find more pretty stuff. Or not. If you really believe that guitars are just musical tools then you've reached the end of the line and it doesn't matter what you make them out of. "Pretty" becomes irrelevant. Glorious wood is just a trophy. But its a nice trophy.