Granted I live in Gainesville now, which is place where humidity is born, but I started this hobby when I was stationed in Virgina. I've had no problems holding humidity in either location with my racks. I use a paper based substrate, so all my humidity comes from a combinaiton of existing ambient and the water bowl.

When making your holes, I suggest you start out with about half as many as you think you'll need, and adjust from there. You can always add more if you need to.