So many people ask this, so I thought I'd post this explanation.
I have also published a more detailed BLOG site about the my nappies and about wool.
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 with a well lanolised cover, 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 possibly 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.
Lanolin in wool nappy covers has 2 purposes. Firstly it is anti bacterial, so it basically reacts with the damp from urine, and sanitises the wool as it dries out. Wool absorbs up to 30% of it's own weight of wet before it actually feels wet. The second purpose is to slow down the rate at which the wool absorbs wet. If there is enough lanolin in the wool it will absorb at the same rate as it evaporates the wet.
Over time the lanolin content will rub off and then there may not be enough to slow down the rate of absorption sufficiently to allow evaporation to take place at the same rate. So you need to re-lanolise if either the soaker isn't as waterproof as it was, or it begins to smell AFTER it has aired and dried. (or both of these) Wool does need relanolising now and again, but this is a very simple process if you use our ready emulsified lanolin