Yeah, it all depends on how your floor structure works if using the foam board will work. On my house, for instance, the floor beams are perpendicular to the floor joists and would stop me from getting a good seal. So it wouldn't work for me.
Amazingly, they don't recommend insulating the ground with conditioned air crawlspaces. If the crawl space floor is at or above terrain level they just recommend insulating 2 ft into the dirt space (over the poly). The reason it is done that way is so cold earth temperatures outside the warm envelope can't migrate to the crawlspace earth. So that is the reason it makes sense to just insulate the perimeter of the crawl space and vent it to the house interior.
Apparently the warm air is enough to warm the surface ground temperature over time. I think it must mean that dirt is a pretty good temperature insulator and cold temperatures from farther below the ground surface don't pull the warmth away. Yeah, it is hard to believe. But that is apparently what is behind the whole idea making sense. I eventually plan to do it as one of my (many) long term projects.
BTW, I'm only about 70 miles from where you live. Lakeport, Ca.
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