As the nights draw in and cool down, many people's minds turn to the prospect of keeping warm during the long winter months. Increasingly in the UK, wood is being used either as the primary source of domestic heat or as an effective (and aesthetic) top-up to central heating. Sustainably harvested, locally sourced wood can, when burned in an efficient stove, be a low-carbon alternative to fossil fuels. However, managing firewood supply for a home requires more planning than gas or electricity, and for some, the pleasurable work of gathering, cutting, splitting and seasoning.
Why season?
Freshly cut 'green' wood has a moisture content of between about 35 and 60 per cent. When burned, much of the energy released is devoted to evaporating this moisture. Seasoning gives this moisture an opportunity to leave the wood before it is burned.
As a rule of thumb, for every percentage point of moisture lost, you gain 1 per cent in useful energy from the wood. So reducing the moisture content of wood by 20 per cent means you get 20 per cent more useful heat - that’s a fifth less wood to purchase or gather, chop and store, for the same amount of heat.
The benefits of seasoning do not end there. Because so much heat is devoted to evaporating moisture in green wood, combustion is inhibited, resulting in the release of more ‘products of incomplete combustion’, including many climate-warming gases and particles. These condense into smoke and form deposits in flues and chimneys, and can lead to chimney fires.
Burning damp logs to ‘keep the fire in’ is done at the expense of heat, the flue and the environment.
Doesn’t seasoning take years?
Seasoning can take a long time, particularly if you store your wood in long logs (‘cords’) with the waterproof bark on. Guidance on seasoning wood more quickly is pretty consistent: cut and split the wood to increase surface area; allow good airflow; protect from rain and leave for a year or more.This translates into some serious forward-planning and a substantial amount of work, however.
Is all this work - and time - really necessary? Is it essential to protect wood from the rain? Is there really a significant advantage to splitting it all before stacking, increasing its volume and the labour of stacking? And need it really take a full year?
To answer these questions, earlier in 2010 my company, Wild Stoves, designed a simple experiment to quantify the impacts of cutting and splitting firewood ahead of seasoning, and the impacts of exposure to wind, sun and rain. Branches 15-20cm in diameter were removed from a single pine tree felled in January 2010 and either: left as ‘cord wood’, approximately 1m in length; cut into shorter logs, 30cm in length; cut and chopped into ‘split wood’, approximately 10cm x 10cm x 30cm.
The cord wood was then weighed individually (total three cords), while logs and split wood were placed into net bags for weighing and storage, each containing approximately 22kg (total six bags).
Some cord wood, logs and split wood were placed in an open-sided wood store, protected from the rain, with reasonable exposure to wind and sun. Some logs and split wood were placed outside (exposed to rain, wind and sun) and other logs and split wood under cover (protected from rain, wind and sun). In each case the wood was elevated on pallets. According to the results of the study, a more detailed version of which will appear on our website this autumn:
Wind and sun work best
During spring, protection from rain did slightly speed up seasoning, but come summer the wood left outdoors and exposed to rain, wind and sun seasoned fastest. Therefore if it's a choice between a shady, protected, dry area and an exposed sunny windy area, results indicate that the latter will season wood more quickly, at least during summer.
Playing catch-up with wet wood
Wood left outside dries less consistently than that protected from rain, as during rainy periods it takes on some water. However, the speed with which the rained-on wood catches up with that in the wood store suggests that it can lose this water quickly. If you do leave your wood without cover, make sure you bring it in on a dry day.
Split for speed
During spring, cord wood did season, but logs lost moisture almost twice as fast. Split wood was almost completely seasoned (down to approximately 20 per cent moisture content) by the middle of May - seasoning three times faster than cord wood. So if speed is of the essence, split wood down and expose the grain.
Choose cords if you have the time
In the longer term, the benefit of splitting wood became largely irrelevant, as by September logs ended up with moisture content just a few per cent higher than split wood. If you have space and time there is little advantage to splitting. If you have a year or two to wait, leave wood in cords for even easier and more space-efficient stacking, and it will season.
By the end of the study, the logs and split wood were fully seasoned, having lost 54 per cent of their weight. If we assume that the pine started off at 60 per cent moisture in January, after losing 54 per cent of its total mass it would be down to approximately 15 per cent moisture content by now. This is confirmed by the use of a digital moisture meter.
What does this mean for this winter?
Received wisdom largely agreed with the findings of this study. However, it’s comforting to know that when wood is rained on it has surprisingly little impact on seasoning; that splitting becomes less critical with just a few more months of drying; and that it’s possible to fully season some wood in just a few months. It was remarkable to see how reluctant cord wood is to relinquish its moisture.
The split wood used in this study was almost completely seasoned after three cold and wet months. That means if you act now, you could still be using seasoned wood by Christmas.
Jonathan Rouse is an energy specialist and owner of Wild Stoves, which specialises in efficient use of wood for outdoor cooking.
Editor's note: seasoning wood will also reduce the emissions of smoke, an important health consideration. Use of a modern wood stove is recommended both for efficient warming and for low emissions.
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