Crawlspaces
Some crawlspaces have insulation in the floor above and some do not. One reason for insulation in the floor above a crawl space is that the crawl space is cold in the winter due to the open vents.
But, because, fiberglass insulation is loose and air passes right through it, it only works in a closed cavity, the fiberglass must contact the floor or its ability to reduce the transfer of heat will be greatly reduced. When it is between the joists in a vented crawl space, with the paper or foil side up, and the unfaced side down, it does not function. If your crwal spaceeven has insulation it's probably installed incorrectly.
Our preferred method is to apply an inch or two of closed cell spray foam to the underside of the floor. This gives the home both a much needed vapor barrier as well as some insulation.
This web site, crawlspaces.org has a lot of really good information on vented and closed crawlspaces. It's really informative and it answeres every possable question or concern about crawlspaces.