But there are diminishing returns and he's in SF, which has a moderate climate. Between unfinished basements and the living space typically isn't insulated. My basement is probably ~55F all year long, in SF I wouldn't be surprised if a crawlspace, with some limited vents, is not too much different.
Someone else pointed out that some more info on what the real objective is would be helpful. He said that after replacing carpet with wood flooring, it takes a lot longer to heat the house. I don't think that makes much sense. I can see carpet feeling warmer, but from a heat transfer perspective, I doubt carpet vs wood is going to make much difference. Also, he says he has a 20 year old furnace. If heating bills are an issue, replacing that might make more difference in how much energy it takes to heat the house than insulating the crawlspace.