The ideal scenario would be no water ever entering the basement or crawl space at all. However, the nature of the basement and crawl space makes that difficult. The basement and crawl space are below ground and made of concrete, a naturally porous material. Being below grade you have to deal with the natural elements, water in the soil and science. Water from the soil is pulled toward the foundation through capillary action. There has to be a waterproofing system to prevent the water from entering the foundation wall even if there aren’t any foundation cracks. That’s why French drains must be installed too.
Even if you choose an exterior waterproofing system, the water that may get into your basement needs to have a way to get out. That is why you need French drains in your basement or crawl space. The interior French drains line the perimeter of the basement or crawl space and filter any water out that may come in. Interior French drains are designed as a trench with a perforated pipe embedded into the floor. The perforated pipes are usually lined with a filter fabric or something to prevent sediment build-up. Then rock or pea gravel is set around the pipe. This gives the water somewhere to go and this in combination with a sump pump system keeps the basement dry.
Dealing with water problems is the number one step if you are finishing your basement or lining your crawl space. If you don’t deal with the water problems first and make sure it will stay dry, you will end up with mold, mildew, rot, and pest problems. Not to mention indoor air quality problems just from having high humidity in your basement or crawl space. French drains help keep the space dry.