Having vents in your crawl space are major problems for numerous reasons. When you think about vents in a crawl space, it's easiest to look at it this way: on cold days, we are venting our crawl spaces with cold air. On hot days, we are venting our crawl spaces with hot air. On wet days, we are venting our crawl spaces with wet air. All of the air you bring into your crawl space then flows upward into your house.
A house with a dirt crawl space is dysfunctional, and a dysfunctional house is not worth building - at any price. Think about it this way, what you would and would not do in a basement, the same rules apply to a crawl space. We would never create a dirt floor basement - so why would we have dirt floor crawl spaces? Your house is one building. You can't have one part of the building that is sick and another part this is healthy. The crawl space air is bad. It's damp. It's cold if it's winter. It's full of mold spores. And it smells. Whatever is in your crawl space air is in your house and affecting you, whether you spend any time in the crawl space or not.
Organic simply means that it's a material that was once living, such as mold. When something is organic, dead and wet, mold grows on it and eats it.
Vented dirt crawlspaces attract mold, mildew, dust mites, and pests of all kinds. This is due to the Stack Effect. The Stack Effect occurs because hot air leaves the home through your attic and upper levels, as it does, a new air is pulled upwards from the crawlspace. If the crawlspace is vented, then that air is being drawn in from outside. That outside air carries moisture that condenses when it reaches the cooler crawlspace and then promotes mold and mildew growth. These mold spores are then drawn up into the rest of the home when warm air leaves the attic.
The best way to prevent this vicious cycle is to seal off the crawlspace from the outside air, allowing conditioned air from inside the home to cycle into a sealed crawlspace. This encapsulation increases your energy efficiency, prolongs the life of your air conditioning system, and saves your home from expensive repairs due to pest or mold damage.