I had a damp crawl space. Plastic sheeting and a fan solved the
problem.
Lay plastic sheeting on the ground. Overlap as necessary. A small
hole here and there when crawling or putting things on it is not a
problem.
My crawl space was in the shape of an L, with the small end not having
a vent. After the sheeting was laid down, the humidity was still too
high. They make vent-shaped fans that automatically turn on and off
with the humidity. That solved the problem.