I haven't tried to measure humidity in mine at all. We're pretty humid here in Central Texas most days. I usually have a watering can full sitting in there. I've some white mildew/mold issues on top the dirt in the plants, but once I started running fans longer that went away - for the most part. When I occasionally get it, I use the peroxide mix and it works like a charm. Got some of the stuff
@Drdawg recommended too, but haven't had to use it just yet. I run a small fan 24/7 and a bigger one 8 hours a day. My GH is small, so probably not a gauge of much of anything