I am suspicious of those companies claiming to make a ventilated crawlspace into an unventilated sterile clean space. There are 150+ year old houses using crawl spaces over bare dirt with no problem. This hermetically sealed crawlspace idea is untested in the long term IMO.
Are you skeptical of the companies that propose to condition crawl spaces, or the actual theory behind doing this? Because the research I've done makes it seems like the way to go, at least for my case. And the evidence that this works for 150 year old houses on the east coast doesn't necessarily mean that it'll work well for "newer" west coast houses.
Moisture is the enemy. Lay down a vapor barrier but leave the crawlspace vented. My exterminator quoted me $350 to do the job, which is less than I could buy the plastic for!
Again, this may be a regional difference, but I don't think moisture is the issue here in California. We have some humidity in the summer, but not like the east coast. Here, however, the temperature can swing be 40+ degrees between the day and night. But, the humidity could be an issue in that we have a conditioned house and an unconditioned crawl space, separated by wood floors. These floors are getting warped within one year of having them redone. So, whatever the cause (temp or humidity) the solution is the decrease that temp/humidity differential. You're suggesting insulating the floors but keeping the crawlspace vented. Can you provide a reference as to why this would work better than conditioning the crawlspace?
If cold floors are bothering you, either hire a contractor to spray foam or DIY with *enclosed* fiberglass batts and supporting rods. Also check local codes to see if your crawlspace has too much ventilation. If you can, closing some of the vents in winter (only after vapor barrier installation) might make the crawl space and your floors warmer. Just be sure to reopen the vents in the spring as humidity creeps up.
It's the combination of cold floors, high heating/cooling bills, and warping of hardwood that has use looking to improve the situation.
And to be clear, I'm thankful for your input. Your suggestions seem to contradict the research that I've done, so I'd be more likely to go your route if there were some references or data points to review. Thanks!