Dan wrote:
Bought a mid 1970's vintage house in the Seattle area last April. The
house has had ongoing problems with rats in the crawl space. If
the assortment of insulation between the joists is any indication,
some has been replaced at least twice, likely from this problem, and
a bunch of it is torn/pulled down/soiled now from the animals as
well. It goes without saying that the goal is to keep rodents out of
the space, and I am working on that (just found one primary point of
entry that was missed by TWO exterminators that I know of, and
probably more). However I plan to have ALL the fiberglass removed &
replaced with new insulation. I am wondering if there is any type of
insulation, maybe a rigid foam board material, or something sprayed
on, which is resistant if not impervious to rodents getting into it,
on top of it, pulling it down & nesting in it. Any helpful insights
into this appreciated.

Get a device called "Barn Cat." Maybe two. It's ecologically sound.