Thanks for the replies. The house is in a suburban area, but heavily
wooded. I don't think it's an issue of garbage or dog food. The footprint
of the crawlspace is probably 1300 sq'. If you're as small and clever as a
rat, and basically have nothing else to do when not looking for food &
screwing, the odds of you finding a gap for entry are pretty favorable. I'm
thinking given the history, and the complexity of finding & irreversibly
sealing off every conceivable entry point, if there were an insulation
alternative that wasn't quite as accomodating as fiberglass batts, it might
be a good choice.

Dan