Wood racks and humidity

I have a Timbernation rack that is over 2 years old. Live in a very high humidity (swampy) location and humidity inside the house can get fairly high since my wife is cold blooded and keeps the AC very warm. I've never noticed the slightest issue with the rack. Sample size of one doesn't tell you much but better than nothing! 

Sounds like actual experience is you will be fine. Manufacturers often exaggerate to cover themselves against having to replace stuff customers screwed up. If you want a great DIY rack rock solid yet dirt cheap you are welcome to copy mine. https://systems.audiogon.com/systems/8367

I made my own rack using two inch think hard maple with a live edge. I bought an eight foot length and halved it. Sanded it down nice and stained it and lots of coats of poly. For legs I used 3/4" threaded pipe and flanges and I placed a thick rubber washer between the metal flange and the wood surfaces. I bought shorter pipes for the legs from floor to bottom shelf and longer for between shelf to shelf to give some clearance above the gear for heat dissipation. Spray painted the legs gloss black and with the shiny maple it looks great. After looking at the cost of the Butcher Block Accoustics I'm glad I did! What makes them accoustic?!

I am a carpenter/woodworker w/ 45-50 yrs experience. MC has given good info. However, the shrinkage is calculated for RAW wood. Sealing the wood is necessary. IMO, its NOT an option. It seals the wood against shrinkage. Does that mean there will be NO shrinkage? No. Most woodworkers shoot for 7-8 %. This is the percentage that is usually achieved in a kiln. Air dry is 10-15%, depending on how long it is left to dry. I only use kiln dried which is safer. However I have a friend who has used a lot of air dried with good results.  I won't take the chance on that. But many do. That said, I have an all wooden turntable including the platter. The platter is made with a resin and covered with a veneer (cocobolo). There is no cracks whatsoever in it. There are a few minor surface cracks in the veneer on the plinth. But they are small and have no effect on the sound. I also have built a stand with sand box on top using wood. Again, I'm sure the wood moves some but what is the detrimental effect? I think none

Another problem encountered is the use of thick boards. They retain more moisture than  the thinner stock. And most racks use 2 inches thick or more. IOW, the moisture content which is sealed in can cause a problem & often can cause problems. It can be remedied so as not to crack but that adds cost. Often thicker stock is air dried. And air dried material is less expensive. 

So the question is not whether the wood moves? Most everything does expand & contract with weather change. Steel does , although at a much lower rate. The question to me is this. Does the movement cause troubles. I don't think it does. But in the end, everyone must choose for themselves. Considering the low humidity levels that  Timber Nation specifies makes me believe they probably use air dried material. My own house fluctuates between 30 & 55% humidity. According to their specs, I doubt any house in the nation would be an appropriate candidate for their racks unless they use humidifier in winter and dehumidifier in summer, which is more exception than rule.