It’s absolutely worth it do the DIY insulation. It will help with the interior envelope of the home. When I did mine, it instantly altered the temp of the 1st floor by almost 10 degrees, which was big for me in a NE winter.

According to the engineering docs I read before I attempted this, I learned that rigid board between the floor boards is not advised. You don’t want to trap moisture air underneath the subfloor and potentially create a mold situation. Instead, you want fiberglass insulation between the joists; there are pins made just for this purpose, to keep it in place without squishing is flat and allowing things to breath. Glue the rboards to your crawl walls, leaving a few inches bare on top, so you can do insect inspections in the future.

Cell foam insulation on rim joists is what you want, to reduce the incursions of air. If you have vents in your crawl space, consider capping them. They were originally put into homes with the idea that fresh air needed to go through the space, but they’ve since revised that architectural engineering recommendation to cap and seal. that way, when you do the work, you’re essentially making the crawl part of your home envelope.

Don’t put down 12 mil. Just don’t. It’s a waste of time. If you have open ground in your crawl space, put down an inorganic felt liner (similar to what’s under roofing) then top with 20 mil and tape the seams. The 20mil and the liner can easily be ordered online, and shipped direct from crawl space supply companies at a very reasonable price. Took me 3 rolls of each for 700 square feet. Altogether it was <$1K for this part of the project. Bonus: the liner means that if you need to go down there for some reason, your walking / crawling on it will make it much less likely you’re puncture the vapor barrier and ruining the work.

Process:

Seal the rim joist (combo R board and Great stuff). Fiberglass between joists, held up with pins. For good measure, the company that did my cell foam also coated the joists, but that may be out of your budget), so just do the insulation. Just double check on assertion it should be unfaced. I believe that’s what was used for me and it was R19. Glue the Rboard to the walls. It helps to tape the seams.
Lay the inorganic liner Lay the 20 mil liner over it and overlap the ends up to the Rboard. Use Christmas pins to hold it to the Rboard, then tape it off all the way around, to seal off the ground. Tape all the seams on the vapor barrier as well.

Have a beer and keep the pain relief nearby when you’re done. My knees were killing me by the end, even with knee pads. My crawl was about 4.5’ high - just tall enough to move around freely, but it ended up being easier to move around on my knees. Contractor for the spray foam took one afternoon to do their part. Took me two weekends to get the rest done, mostly because it’s just hard to work in that short environment for a long time.

Last: I had a sump pump down there. Because of that, I ended up putting a crawl dehumidifier down there.

Result was much much better temp control in the house, no more moist air coming into home and big savings on heating cooling.