Let me give some insight here into building in tropical humidity. First, this is not meant to contradict Michael (what a font of knowledge!) or any other posters here who look down on this sort of thing. But their analysis is a bit incomplete. They assume the basic techniques of building are universal. Unfortunately so do some new builders in these tropical environments. But building techniques that work when an instrument doesn't travel are problematic when shipping instruments out of high humidity areas. That’s what isn’t understood; that’s what causes the problems.
So first, look at where the typical construction falls short when building in high humidity and shipping to a lower environment. You have to look at the instrument piece by piece. The most problematic area by far is the hardwood back. A hardwood top is not much better. Sides can split but their movement is not as restricted, so damage is relatively rare. Necks just need to be made of slow dried stable timber. The most straightforward examples of how to build are in places like Veracruz and Northern South America. Various instruments there have bodies that are simply made out of a hollowed solid block. The soundboards aren’t hardwood but softwood, allowing much greater flexibility. Not only are the thicker body elements more resistant to warping, but glue joints are cut down as well (another topic).
We have always built in Central America. Our first shop, one we built in for almost a decade, was not only not controlled for humidity, the roof leaked as well. During the rainy season, we had to often be at 100% humidity. Having designed our way around that sort of environment with solid wood furniture exports, we felt that with the narrow stock needed for Ukuleles there was no cause for concern. The first prototypes showed us our mistake as a couple of Soprano prototype backs cracked during their first winter. We had overlooked how the thinner material of an Ukulele back would be more susceptible to splitting than what we had been using for furniture. Okay, it was 20% with the central heat, but if we could do that, so could someone else.
We didn’t want to try to market the hollowed out solid wood body; there’s nothing wrong with it, but in spite of being a very refined technique in good hands, it has the flavor of something crude up north. So we wanted to build in a more traditional “European” style and hit on a way to do that.
Rene LaCote was perhaps the premier builder of Romantic Era Guitars, and favoured a Cypress lining on his backs. He did it for a “sweeter sound”, but of course it solves the problem of the weakest point of tropical construction in European style. It does affect both the depth of resonance and the sound, but the sound quality was not inferior, just different. Some Spanish guitars are still built this way; our builder was very familiar with the construction and enthusiastic about it. It took no time at all to adjust the other parts of the body to the parameters of the stiffer, almost Australian style back.
We ended up using different wood for side material: solid, stiff and slightly thicker than normal. Of course good “bendability” was extremely important as well once you go that route. We used carbon fiber reinforced necks. Soundboards needed a curve to allow for movement (a bit on the backs as well). We used German Spruce soundboards in the beginning, but in the end, just to be on the safe side, went to an extremely stable soundboard: Alaskan Yellow Cedar. It not only moved less (better intonation) but we ended up with a sound that paired better with our stiff bodies than we had had with German Spruce.
That construction served us for years: instruments went to all sorts of climates and never gave us a problem. So the point in all this is that once, instruments were built in their environment for players in that environment. Now that builders are shipping all over the world, it doesn’t rule out certain building conditions; it just means you need to do a bit of engineering. That’s all the “golden 45%” rule tries to do. It just tries to hit a “median” humidity so an instrument can travel through a certain (though still limited) range of environments. A 45% building environment, for example, won't cause immediate problems for a customer in a place like here in Louisiana, but with our 70-80% typical humidity range, it's less than ideal.
I’ve always thought that one great feature of Ukulele design is portability. And that means it should be able to withstand an even greater range of humidity than other stringed instruments. But you don’t necessarily have to control your building environment to do that. Our construction, for example, was one solution that allowed for even greater portability than standard design. Just think about your instrument; think about your environment; think about your woods; don’t rely on traditional formulae; find your own.
P.S: We are working with a new shop now as our previous builder has retired. It is causing some adjustments and taking time to reset some parameters. This fellow has taken the modern approach and has a humidity controlled shop. Re-adjusting some of the design technique to the new environment (the kind y’all are used to) has been a surprisingly long process. Almost done now, but sometimes I kind of wish for the old days of the wild tropics.