I have always used shredded aspen and have never had any issues, other than dust giving me allergies. As long as there is enough circulation in the tub & it's cleaned quick enough, I get no mold. Humidity is fine for me as well but then again I live in a humid city. I am currently using aspen sani-chips, which isn't as effective as the shredded aspen I think, but it has way less dust and looks nice lol.