Either way, I don't think the cats will be pleased ......!

Does it ever freeze where you are? This would dictate what you did with your water and soil pipes. They definitely need to be on the "warm" side of any insulation.

If you did the plastic-encased fiberglass, would that surface be permanently exposed? And to what? If contained in a closed crawl space, it would be fine, I think, but if the space is not enclosed (and it sounds like it might not be since the cats can get in) I would be thinking about what could be mounted on the bottom side (towards earth) of the fiberglass to protect it. It would be a shame to have it vulnerable to varmints, inlcuding mice.

Can you concoct a hybrid: fiberglass between joists and something solid (reflictix or something else) attached the bottom of the joists?

Foam (I think you mean spray foam) is excellent around wire entry points. That alone can make a big difference. Wires in a 70 yo house should be fine just buried in the insulation, but for peace of mind and elimination of any future problems, I'd do a careful inspection of each one before encompassing it in insulation. Easier, and cheaper, to replace or repair now, than later.

Glad your house withstood all the bad weather you had earlier.

Molly~

Thanks Molly,
The underneath of my home is exposed to the elements, all of them. Rain water will pool under my house in some areas. I have dug a trench to help it escape. I had extra roofing felt, so I stapled it around the inseide of the most exposed sides (2). Helped a little, and since it is black, it didn't take away from the house. Cats being chased by dogs and an inconsiderate plumber have torn down parts of it.
Yes, we freeze in my area. Low 20s to mid teens, however we warm up to the 40s - 50s the same day. Last night was in the low 50s and my house is 68 in the middle of the day. Brrrrr. I get cold easily. As for my pipes, I have a variety: iron, PVC, and some kind of clay. Eventually, I will try to replace all the plumbing. Mice are not a problem beacause of the cats. But, I do have raccoons and opossums that frequent my home. How does styrofoam work as an insulator? Cost is a concern.
Your advice is greatly appreciated.

Cost is always a consideration with older homes! LOL

And in LA, things may be even dicier this year.

I'd start at one of the big-box stores and price everything as a baseline. Comparing R-value for the $$ works, but you also have to factor in where and how you're going to use it. Combinations of materials work, too. Later this fall in a portion of my attic I plan to install a thinnish layer of unfaced fiberglass laid directly on the fragile, uneven keys of my old plaster. Above that I'll have insulating foam (what I think of as "rigid pink", but it may be another brand/product) sheets which give more R per inch than fiberglass and are easier to install, and to mouse-proof, in this particular situation. These two materials will completely fill the usable cavity space I have, without crushing, which would degrade the possible R-value.

You may also want to consider one of the infiltration barrier fabrics if your floors are leaky.

With your reported low winter temps (those temps would lead the local news with stories of a "warm spell" up here in January!), you will have to make sure your water pipes, and to some extent your waste pipes, are kept on the warm side of any insulation you put up, which would block heat from radiating downward and saving them from freezing up.

French drains will help re-direct surface water, and your totally open undercarriage is the only situation where I'd put a vapor barrier on the bottom side of the insulation/joist (as opposed to directly on the ground). It would be good to keep moisture from entering the interior of an insulated floor assembly. And add chicken wire if that's what it takes to excluded coons and possums. What about snakes, they always got in my Virginia house!

Good luck!

Molly~

I have a raised home ,in south la. ..it is 8 feet high, my floors are plywood floors underneath.....also concrete paved under the house ...no standing water issues...what would be the best way to insulate under floors and materials...

I am in Gulfport same problem. My house is a 946 Sq ft 1949 bungalow. I was all set to call an insulatiom company and then read an article suggest than many companies would use the fiberglass which could cause moisture=mold, mildew and thereby rot. So I think I just by a lot of throw rugs. Good luck.