Instead of letting moist air out, vents brought humid air in during the summer. During the winter, crawlspaces got colder and so did the floor above. Builders responded with insulation.
Insulating the floor above a crawlspace is not a bad idea, but not for most crawlspaces. Joists are filled with obstructions that make installation tough, including ducts, pipes, cables, and light fixtures. Gaps, of course, degrade the performance of insulation. In addition, there are numerous holes for plumbing, cables, and registers, all of which allow air to pass from the crawlspace to the living space above.
To make matters worse, fiberglass insulation itself is not an effective air barrier. Air can pass through it (unlike some other forms of insulation). You can see this for yourself by examining fiberglass insulation that has been in place for several years. The fibers are often discolored by dust and dirt. In effect, the batt has functioned more like a filter than a barrier.
Even if the insulation was effective, it often meant that air ducts would have to travel through a cold crawlspace, loosing some the heat they were supposed to deliver. Plumbing pipes became more susceptible to freezing.
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