Because air flows upward into the upper levels of your home from the crawl space, it brings the humidity from the crawl space with it. The effects it can have on your home can include: buckling hardwood floors, increased heating and cooling bills, mold, and aggravated asthma and allergies.
At Healthy Spaces we suggest 4 steps to totally eliminating the crawl space from having any negative effects on the rest of the house. Fix the water leakage (if there is any). Isolate the house from the earth. Seal the vents and other outside air leaks. And lastly, 'condition' or dehumidify your crawl space air.