Cool dry air is necessary for human comfort. Maintaining relative humidity below 50% prevents dust mite infestations, inhibits mold and mildew growth, and inhibits bacteria. Lower humidity also reduces the off-gassing of VOCs. In colder climates, wintertime humidity levels must be even lower - generally 30-40% - to prevent condensation on windows and other surfaces.
So part of what we have to plan for is dehumidifying wet air, and/or cooling down and humidifying hot/dry air. This process can be easily accomplished using a mechanical system, but when we do not use a system to manage it, we need to get creative. Some people warm up a crock of water on their wood stove to increase humidity. Others use misters in their greenhouse (though that can wreak havoc on your structural system if that system is wood-based). Some use fountains and waterfalls to serve the dual purpose of cycling the water in the cistern to keep it fresh while creating humidity within the livable areas. You might even use water walls with plants to increase humidification and oxygenation. Whatever you do, if you work with water, you need to make a plan for maintenance too… interior ponds and waterfalls require filtration and algae control to work correctly.
Alternately, people in humid climates may need to utilize salt de-humidifiers, using calcium chloride in most cases, which is readily available at your local agricultural store. This technique has been used by farmers for generations to keep hay from molding. You can also use air-based heating and cooling like stoves and forced-air systems to remove humidity from the air.
Note: interior ponds and waterfalls require filtration and algae control.
