It sounds like you may have a bit too much moisture in the tub.
My first attempt at mixing vermiculite had just a bit of moisture collecting on the top. I would just wipe it off with a paper towel when I went to air out eggs. Last year, I never did have a lot of condensation on the top, and I didn't worry about the small bit on the sides. I just tried to keep the water from dripping on the eggs.