This topic has been on my radar for quite some time now and I have been doing a lot of research on it. The OP indicates they are in the Ozarks. I am in central Missouri, on the northern edge of the Ozarks so basically the same climate. While I don't have any personal
experience with it I can share what I have learned so far on my journey.
I was fortunate
enough to visit
Wheaton labs in 2015 and took a tour of both Cooper Cabin and Allerton Abbey. The day we went up there it was 104 degrees outside and I was very impressed with the "air conditioning" of those two buildings. I thought to myself that even if these buildings still required supplemental heating in winter, this "air conditioning" feature was worth the effort to bury the building in earth. However the climate in the mountains of western Montana is much drier than that of Missouri. We have very humid, muggy summers here and if I built a wofati just like one of those at the lab, I can only imagine the amount of condensation dripping from the walls and floors inside. So I too began looking for a way to passively dehumidify the indoor air so that I could have effective off grid air conditioning for my climate.
Earth Tubes: I read about earth tubes in John Hait's book on PAHS. Then I went and visited
Abundance Ecovillage in Fairfield, IA just 3 hours drive from my home. The homes built at Abundance are very modern with high end features including hardwood and tile floors, granite counter tops, stainless steel appliances and all the other finishes you would expect in a modern home. But many of the homes here feature earth tubes for cooling purposes. They buried corrugated drain tile, the perforated kind, in the earth around the homes and used a fan to draw air through the tubes and push it into the house. The condensation would drain out of the tubes into the surrounding soil through the perforations in the tubes. I arranged a tour of the village with the developer and asked a lot of questions on how this was working. He said that it worked very well until about the end of August/early September. By that time the soil around the tubes had warmed up to a point that it was no longer effectively cooling. In addition to that issue, this area experienced a drought in 2012 which opened up deep cracks in the heavy clay soil. This allowed radon to enter the earth tubes and be pumped directly into the house. They were able to work around this by pressurizing the tubes and thus preventing the radon from entering but that just added another level of mechanization. The developer told me that
solar panels had gotten so cheap and mini split air conditioners had gotten so efficient that they were headed away from the earth tube idea and moving towards a 4000 watt solar array and a mini split AC. Hardly a passive solution, but it does work.
That said, there are some key differences between the earth tubes described in Hait's book and those used at Abundance Ecovillage. First of all, if you look at the picture of earth tubes on the Abundance website link above, they are spaced very close together. Hait describes having these tubes further apart from each other, at least 4 feet from the next tube if I remember correctly. If the tubes were spaced further apart I believe that this would eliminate the issue of the earth around the tubes getting too warm by the end of summer and they should be able to provide cooling and dehumidification for the entire cooling season. In other words we need to optimize, not abandon, this system. But that doesn't take care of the radon issue. Hait's earth tubes are made from solid, smooth wall, pipe. No perforations means that radon, or any other sub soil gasses, can't enter the tubes or your home. So, in order to deal with the condensation that occurs in the pipes Hait's earth tubes feature a constant slope away from the building all the way to daylight, much like a waste water drain. I haven't had the opportunity to meet someone who lives in a house with this type of earth tube but poking around here on permies.com seems to confirm my suspicions. I believe that even with a constantly sloping pipe there will still be enough moisture in those tubes to create mildew issues, so any earth tubes of this style should be built with a way to clean them. I imagine using a rope to pull a large rag soaked with some mildew killing solution through the tube every once in a while would do the trick. I would also be tempted to use potable water pipe, instead of drain pipe, since the potable water pipe is resistant to mildew growth. That type of pipe is more expensive, but still cheaper than a solar array and an air conditioner.
I also visited Dancing Rabbit Ecovillage in northern Missouri which has a very high concentration of owner built,
straw bale, homes. And I asked them what they were doing for dehumidification of indoor air. It turns out that many of the residents there run a dehumidifier for about an hour a day and some of them have air conditioners. The houses who don't dehumidify have a musty smell. DR has been around a while and as experienced as they are at living off grid and sustainably, they haven't figured out this one yet. It's a tough issue for those in the midwest and deep south.
I have some different ideas for an "almost passive" dehumidifier but I'll save that for another post. I would love to see more discussion on this topic because I am desperate to build a wofati stye building here in Missouri but really want a workable solution to the moisture issue before building.