There is always air movement occurring between your crawl space and house. Even if your floor is insulated, air is still traveling throughout your house. This process is called the stack effect or convection. Warm air in the upper level of your home rises and is expelled through windows, vents, cracks, and gaps. As it rises, air from lower levels, including the crawl space, of your home rushes in to fill the vacuum.
Air from your crawlspace enters your home along with everything in it. Odors, mold spores, and dust mite feces enter your home. This can cause a variety of respiratory problems, including allergies and asthma. Air, often filled with more moisture, gets sucked into the crawl space from the outdoors. In this sense, a poorly designed crawl space acts like a bad air machine.