Crawl Space Insulation: the pros and cons of different types of insulation in a crawl space
Although some crawl spaces have fiberglass insulation in the floor above them, others do not. One reason for insulation above a crawl space is to help keep the cold air during the winter months from effecting the house above. The crawl space is cold in the winter because of open vents, which should be closed permanently to help regulate the temperature of the floor above.
A second fact that should be known is that fiberglass insulation only works in a closed cavity. This type of insulation is loose and allows air to pass right through it. Fiberglass insulation is most often installed between the joists in a vented crawl space, with the paper or foil side up and the unfaced side down. Not only does this paper-faced insulation fail to insulate, but it is also extremely prone to attracting mold. Mold loves paper more than wood. Foil-faced insulation is also supposed to be a radiant barrier because it is installed with the foil face against the heated side of the crawl space. This seems like it would insulate extremely well except for the fact that a half inch air space is left in front of the foil radiant in order for it to reflect heat. Because of this gap, an air space is created producing a thermal bypass where air flows on both sides of the insulation; this makes the insulation pointless.
One last important thing to remember about insulation, if just a little bit of it is damp, it loses a lot of its insulation value.