A crawl space window is a small window installed in the foundation of a building to provide ventilation and natural light to a crawl space or basement. Crawl space windows are typically made of glass or acrylic and are set in a frame that is installed in the foundation wall. The frame is typically made of wood, metal, or vinyl. Crawl space windows are an important part of a building’s ventilation system and can help to prevent moisture and mold problems in the crawl space.