JSW Twin Pack of drainage membrane which consists of 2 x sheets of drainage membrane measuring 12 inches square each sheet, plenty enough for 2 good sized larger enclosures or many smaller ones. It should be  cut to the exact internal size of the bottom of your Terrarium/Enclosure, not the outside, then placed over the top of the Bio-Active balls to create a layer between the Bio balls and the substrate. (See our shop for Bio-Active balls and Spider substrate ) Once completed you should soak it all down until the Bio balls contain a reservoir of water at the bottom which can be kept wet using a turkey baister which is an ideal tool for this. As Jumpers do not require large quantities This is a great way to keep the cost of your set up down by only getting what you need