The stack effect is an effect that occurs whenever you have an opening in the attic and in the basement or crawl space. Most homes have some sort of opening in the attic or roof, and homes with open crawl space vents or basement waterproofing problems will also have an opening in the basement or crawl space.
The base premise of the stack effect is that warm air rises. When air enters your crawl space, it will either be warm or cool. If the air is warmer than the air inside, it will start rising immediately. If the air is cooler than the air inside, it will take some time to warm up, then start to rise. It will continue to rise all the way through your home, no matter how many stories it is, until it exits through the roof.
The thing is, the air isn’t the only thing that rises. This air also carries with it airborne particles. This can be anything from crawl space condensation to mold spores to even small insect droppings. The stack effect inevitably causes health and cleanliness concerns throughout your home.