When foundation vents were installed in crawlspaces they allowed cold air to enter. In the winter months the cold air had the potential of freezing water pipes and allowing the floors to remain cold. This challenged the heating system to try to warm the home. Fiberglass insulation had to be added to the floor joists in an attempt to prevent the cold crawl space air from making the floors cold. The same foundation vents that allow the cold air to enter in the winter also allowed humid air to enter in the summer.
The humidity would condense in the cool crawl space and collect on the fiberglass insulation. Between the humidity and occasionally rodents, the fiberglass insulation often would fall to the ground. Styrofoam insulation is also often used to insulate the exterior crawl space walls. This is a preferred method. If foundation vents are installed the insulation value will be decreased by the foundation vent openings allowing cold air to bypass the insulation.
Creating a "Conditioned Crawlspace" utilizing Styrofoam insulation and eliminating foundation vents is the preferred method for energy efficiency and humidity control. Crawlspace Doctor has developed systems for all crawl space insulation needs.