It is generally agreed that a moisture content of less than 20% is best.

There are three factors which determine how much heat you can get from a log when you burn it. The variety of the wood (and how 'dense' it is) and the efficiency of the stove are two, but these are fairly insignificant when compared to the importance of the third: moisture content. Depending on the variety, a log from a recently felled tree can have as much moisture as 60% – something like a pint of water for every kilo of log. A seasoned log will produce twice as much heat as a freshly cut (aka 'wet' or 'green') log because this water must be boiled away first before the log gives up any real heat. This makes unseasoned wet logs an incredibly expensive heat source, not to mention the damage that burning them does to our air quality and your chimney system, so it is vital that your logs are always 'dry' and ready to burn.

Virtually every wood burning stove manufacturer recommends only burning wood logs with a moisture content of less than 20%. Somewhere between 10% and 20% is ideal. Once the moisture content gets less than 10% then the wood quickly burns away, sucking in too much air and cooling the flue gases as it does this, which in turn increases unwanted emissions. A little moisture is good for moderating the burn rate.