So many people ask this, so I thought I'd post this explanation on here as well. I know most people don't even consider wool covers as an option, because they don't understand or believe that they can possible work, but they really do. They are particularly good for addressing the overnight problem, as they allow the nappy underneath to dry a bit between wees, thus increasing total overnight capacity considerably. What's more, you don't get those horrid red marks which ealsticated PUL can leave on your baby's skin. Wool is just gorgeously soft, gentle and also cooler for your baby, so really comfy for him.
Here's how it works.
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.
You do need a good absorbent nappy underneath - wool isn't a dam, so the nappy needs to absorb initially, but once that has happened, the wool will start to do its work.
You need to keep the cover lanolised, as this will keep it waterproof, and also clean the wool without the need of washing very frequently, but lanolising is now a very simple procedure, if you use the new soluble lanolin. Wool covers only need washing if they start to smell a bit after they have aired, or when they start to get grubby looking. (You can spot wash small stains) Lanolin is anti-bacterial, so just hanging to air will sanitise your cover. Re-treating with lanolin, which probably needs doing every couple of months or so, is no more difficult than adding fabric softener to the final rinse, if you use soluble lanolin. Just add some soluble lanolin to warm water, and leave the cover to soak overnight. Then lay flat to dry, and you cover is all ready and re-conditioned to do another couple of months.
And they are so cute looking too!