Vermiculite is good for where you need stable high humidity for long periods of time. With some South American and Asian species we mix in 20-30% vermiculite with coco fibre to help keep the humidity high. Though bear in mind no spiders like stagnant conditions so make sure you also have a good airflow. We use the same mix for spiderlings as it make burrowing easier. Also great for using as a substrate when incubating reptile eggs.