Insulating a crawlspace the Norwegian way

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Q: I'm originally from Norway and I've seen several Norwegian Web sites that say you should cover the underside of the insulation in your crawlspace with exterior-grade plywood. However, the American sites say to leave the bottom of the insulation open or to cover it with a non-tight covering. I tend to trust the Norwegian way of doing things because they have to deal with such crazy weather, but I want to make sure I'm not missing something here if I don't do it the "American" way. --Rune I. A: Air will naturally move from a warm area to a cold area. The warm air in your house wants to move toward the colder crawlspace, and as it does it will take any moisture vapor it contains along with it. For that reason, when installing floor insulation, the paper or foil face of the insulation is installed facing up, against the underside of the subfloor, to create a vapor barrier that prevents most of that moisture from ever reaching the insulation. However, that floor insulation vapor barrier ...