Is denim insulation more inviting to packrats?
To add to the other answers; Mice seem to like fiberglass.  I have found nests in my grill, and under a rain barrel that were made from fiberglass they've managed to collect out of my house.  Mice have managed to get into my basement and nest in the fiberglass batts on the basement ceiling many times over the years.

I try to air seal the foundation sill area when I can.  I feel that if they can sense the warm air leaking they will try harder to find a way in. PestBlock (mentioned by KCasper) is a new product.  I've bought a can.

I've been told that regular expanding foam is not something that rodents have trouble chewing through. Another technique is to stuff steel wool in the crack and foam that in.  Rodents don't like chewing on steel wool.