I'm pretty obsessed with this too since I'm in Piedmont NC and conventionally-built houses regularly have to be evacuated and torn down because of mold - black mold is no joke, and it's everywhere here. Air conditioning stops seeming like a luxury product when your books mold through and your shoes start to decompose.
There are some documents about passive dehumidification modeling in Florida using dessicant panels that are dried via solar heat - I'm set up to try it on a pretty small scale this summer, via:
- digital humidity gauge
- activated charcoal pellets (the kind prepared for fish tank filtration)
- kitchen scale
- glass-front box (basically a large picture frame with some air movement holes)
Design:
- Separate charcoal pellets into 2 lots of equal volume
- Take a completely dry starting weight per volume (dry charcoal pellets in the oven at 200 for multiple hours)
- Take a completely saturated starting weight per volume (soak charcoal pellets in water and then drain in a colander)
- Load oven-dried charcoal pellets into the glass-front box, weigh the whole shebang, then weigh several times over the next days/weeks until the weight is stable (keeping a log of day weight + house humidity)
- Take the point at which the weight stabilizes as the charcoal's humidity absorbtion capacity in my house
- Partially vent the box and put it on top of my car in direct sun, see if I can evaporate humidity out of it back down to its starting dry weight.
Happy to report back, as I think this is a pretty big sustainable building hurdle in general - as more of the country gets hot and humid, energy use for AC is only gonna rise.
I will say that even here, a wood stove can dry a house out pretty thoroughly in the winter - but that's depending on the temperature getting low enough to run the stove continuously. Right now the biggest mold events tend to be in the hinge seasons, where it's rainy and 70 degrees with 99% humidity for weeks.
Edited to add: the modeling paper I read for Florida also just straight-up suggested a UV coil to keep the air return from growing mold. I'm not sure how efficient those are, and they're definitely not low-tech/off-grid. The high-labor low-input solution might have to involve cleaning or solarizing your system regularly - I'm sure Earthship people can weigh in there.
