Wool works great, as long as you have enough absorbency underneath. However, it will NOT work over a disposable, as the disposable can't breathe, and if that gets sodden underneath, the wet will just flood out of the disposable, and wool can't work as a dam. the wool needs to constantly absorb and evaporate. I've just bumped the thread in the PURE WOOL forum,
which explains how it works. But it's fantastic at night, and I have found works best over 2 bamboo nappies, rather than trying to boost a single nappy.