I know wool is difficult to believe but it really does work.
So many people ask this, so I thought I'd post this explanation. Wool is absorbent, but absorbs very slowly. Because of the body heat of your baby, and the warmth of the wool, it also evaporates, and this happens at the same rate as it absorbs.
When evaporation takes place, the wet is turned into gasses. Most people think this will be steam, but in fact it's not. Steam is created when evaporated water condenses again, and this happens when the air containing the evaporated gasses, hits something cold, and there is enough wet in it to saturate the air. (This is called the dew point)
If you have a PUL wrap, the inside of the PUL will not let the wet get any further, so the amount of evaporated gasses builds up, and also because PUL has a cold feel, the saturated air will cause wet to condense again, and get absorbed back into the nappy. This doesn't happen with wool, because it is warm, and can breath, so "dew point" is never reached, and the gasses continue one their journey, unrestricted by the wool , baby clothes, bedding etc. It will finally condense possiby when it hits the window of your baby's room in winter, but basically the amount of wet in the air never gets high enough to cause a dew point.