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My cousin’s cabin had R-30 with tar paper backing stapled up between his floor joists. Squirrels got in and ripped off the tar paper and shredded the fiberglass. Mice and rats also got in and the whole thing was full of their excrement. He had to clean it all out through the access hole and didn’t bother re-insulating it.
Probably the best way is to use non tar paper lined batts smaller than the joist depth by 2″, push in after all plumbing and electric is done. Use 3″ nails on each side every couple feet to keep the insulation up. Drill 1 to 2″ holes at the rims and through any solid blocks and staple metal screening well, or install vent caps from the outside (in the area clear of insulation). Then use 3/8-1/2″ plywood or OSB nailed up with ring shank 8Ds or screwed with 1 5/8 drywall screws. At jigsaw cuts for plumbing, spray expanding urethane foam insulation in the small gaps. Then it will be vented for proper wood life, and be animal proof.
Most of the time there is a crawl space with vents and an access panel that is animal proof. Then there is no need for the rim vents or plywood to make the insulation animal proof. Often insulation is pushed up and in and then a mesh is stapled on the bottom to keep it vented with the crawl space vents installed every 12′ in the stem walls, and to keep the insulation from falling out (fiberglass or other batts).
Some crawl spaces require sump pumps if there is a water problem, and others require wrapped pipes, sometimes electrically heated.