Perlite

When growing mushrooms, perlite is used mainly to maintain a very high level of humidity within a terrarium/grow chamber. It provides aeration and optimum moisture retention for superior mushroom growth. Other benefits of horticultural perlite is its neutral pH and the fact that it is sterile and weed-free.

If you’re making a DIY kit with a five gallon bucket and a humidity dome.  Perlite is the easiest and most effective way to achieve high amounts of humidity.