I know this is probably a dumb question, Karen, but how much air-movement do you have in you greenhouse?
I don't have rodents (is chipmunk a rodent?) but if I had the problem you have, I would cover all the vents with heavy-duty screening/mesh. I know I once saw a copper mesh specifically indicted for just this use. There is also "Excluder", a steel mesh, but that is perhaps too dense for your needs. I don't know how much air can move through this particularly mesh.
