The moisture content of wood has the greatest effect on its calorific value or CV – the heat energy it releases when it burns fully.
Any water in the timber has to evaporate before the wood will combust and this will reduce the net energy released as useful heat – as opposed to steam going up the chimney. Logs that aren’t dry will result in a fire that smoulders and creates lots of tars and smoke. These tars can be corrosive, potentially damaging the lining of your flue and increasing the danger of a chimney fire. Wet logs will tend to blacken glass in stoves even if it’s been designed to keep clean. Dry logs can have approximately twice the CV of green logs, so always buy it dry – preferably kiln-dry – or buy green logs and take the time to season them yourself. Radial cracks and bark that comes off easily are both good signs of dry, well-seasoned wood.