Air pressure also comes into play with air movement. Air flows from areas of high pressure to areas of low pressure. When there is a greater difference between indoor and outdoor temperatures, the more significant this airflow becomes. (Source: Fine Homebuilding)
Even though you can’t actually see airflow, it might help to see this in action to get a better understanding of the stack effect. The Cold Climate Housing Research Center has several informational YouTube videos, including “Your Nothern Home: Stack Effect” that illustrates the stack effect.
So, if there’s humid air or mold in your crawl space, it’s also in the air that is circulating throughout your home.