Talking to a friend this weekend, she has had animals getting into her crawl space. To fix this you have to first get the animals out. Then to keep them out, you should encapsulate the crawl space closing off all the vents, holes, gaps, and cracks. This will keep it critter free, improve indoor air-quality and comfort, and reduce energy usage. An encapsulated crawl space is a sealed and conditioned space like the rest of your home. This approach lines the floor and walls with a sealed 12 mil poly and air tight insulation along the top of the foundation wall to the underside of the floor sheathing above. The walls should have a rigid foam material behind the poly to keep a constant temperature. You could also foam the floor or pour a “mud” slab in the crawl space, but this is often considered a luxury step. The final piece of the puzzle is to take the space to positive pressure to further push the gas and moisture out of the space.