Almost every crawl space in Huntsville was insulated the same way: fiberglass batts pushed up between the floor joists, paper facing toward the house. Walk under most homes built before about 2010 and you will find that insulation lying on the ground.
It did not fall because it was installed badly. It fell because in this climate it was always going to.
Why Batts Between the Joists Fail
Fiberglass does not stop air, it slows it. In a vented crawl space, humid summer air passes straight through the batt and meets the cool subfloor behind it. Moisture condenses there, out of sight, where nothing dries it.
The batt absorbs that damp and gets heavy. Wet fiberglass loses most of its insulating value, so the floor above gets colder, which makes the subfloor cooler still, which increases condensation. Eventually the weight overcomes the friction fit or the wire supports rust through, and it drops.
The paper facing makes it worse. Facing the warm side is correct in a heating climate. In a cooling climate for much of the year, it becomes a vapor barrier on the wrong side, trapping moisture in the assembly.
What to Do Instead
Once the crawl space is sealed and dehumidified, the insulation belongs on the perimeter walls, not between the joists. The crawl space is then inside the building envelope, and the floor above no longer needs to be a thermal boundary at all.
Rigid foam board against the foundation walls, sealed at the joints and carried up to the sill, does the job. It does not absorb water, it does not sag, and it does not care about humidity. Closed-cell spray foam is the alternative and performs well, at a higher price.
Building codes require a thermal barrier over foam in some situations, and there are fire-rating considerations, so this is worth having done properly rather than as a weekend job.
The Order, Again
Insulating a wet crawl space is money set on fire. Water out, humidity controlled, liner down, then insulation. Any contractor willing to insulate before the moisture is handled is either not paying attention or does not expect to be called back.
What You Notice Afterward
Floors that are not cold in February. Rooms over the crawl space that stop being the coldest in the house. Air conditioning that keeps up better in August, because the ducts are no longer running through a hot damp cavity and losing their cooling on the way.