Littlelamb nappies were designed to fit inside the Littlelamb wraps and they are made from 100% polyester with a Urethane film bonded to the inside. The Urethane film acts as a barrier to water molecules but smaller air molecules can pass through. The end result is it's waterproof but allows air to circulate reducing the temperature inside the nappy.
Most nappy sellers will say you need about four wraps for successful nappying one for daywear, one for night time and two in the wash.
Our own trials showed that one wrap change every two nappy changes was a more realistic figure and so each Littlelamb nappy kit comes with a nappy to wrap ratio of 2:1.