Firewood moisture content is such an important issue, I wanted to write this post exclusively to discuss and explain this important & topical issue.

Trees obviously contain sap when they are growing, in fact when harvested in the summer they can contain up to 50% or half their weight in water.

The water is required to carry nutrients up from the roots of the trees through the xylem and the sugars from photosynthesis back down through the phloem.

Often we will try to procure wood harvested in the winter when a lot of the sap migrates to the bottom of the tree.

So we have fresh cut timber at say 45% moisture. The problem with that for burning is that the fire must evaporate off most of that moisture before it can burn. Evaporating water takes a huge amount of energy (2,584,841J = 617 Kcal per Kg water)

So you can see wet firewood can only give out a fraction of its energy to heat your room

There are additional issues with burning wet wood related to the amount of steam in the smoke, and the cool flue temperatures. These are the ideal conditions to create creosote, especially with softwoods, that will coke your flue, and can ultimately cause chimney fires.

Incomplete burning of the wood gasses also creates more particulates in the smoke which increases pollution.

OK, so we appreciate we must remove this moisture from the wood, before we burn it, so what are the methods to do this?

Primarily, the process is referred to as “seasoning” which is the traditional term for leaving logs to dry naturally over time.

Nature and time will dry firewood logs, but the law of diminishing returns cuts in. It takes proportionally longer to get each percentage point of moisture out.

The law of diminishing returns affect firewood seasoning

Wood gives up the first 15% quite quickly and easily, but it then starts to get much harder (except for larch, which I will talk about in a bit).

So how dry should logs be before burning?

This is a judgment call, the scientific data does suggest that the benefits of drying keep going right down to bone dry or 0% (Click to access full DEFRA report careful 327 pages!).

However, that just increases the costs of firewood production. Tests have shown that the flue gases become “clean enough” at around 20%. This moisture level has been accepted by the regulating authorities and is the level set by Woodsure as being suitable to be “Ready to Burn”.

With all the recent press interest in the pollution aspects of wood burning, I tried to bring it all together in this blog post:

Wood Burning Stoves and Pollution Summary

So if it takes too much time to get under 20% naturally, how do firewood companies achieve this?

The answer is by using firewood drying kilns.

These machines burn a fuel, ranging from natural gas, through oil to woodchip, to heat the firewood up in dry air to force out the moisture.

This process obviously increases the costs and environmental cost of your wood fuel.

Here at Firewood Express, we have the space to use a hybrid system. We air dry our logs for as long as possible, achieving mid 20% moisture content. Only then do we dry the logs in our kiln.

We find that given the pre-drying, it is quite straight forward to get the last 5% out. We find that to dry from “Fresh” or “Green” takes too much energy and time.

We also would like to point out 2 things about our kiln:

1. It is fired by woodchip, a renewable fuel, including wood waste from our production system.  

2. We use low temperatures, around 30-40 degC, we believe some of the high temperature kilns “cook” out some of the energy from the wood, creating a product that appears to “flare” away to fast.

Larch Firewood and Moisture Content.

Larch is a unique and fantastic wood fuel for many reasons, but the moisture topic is specifically interesting.

Larch appears to expel its moisture. It can be dried naturally to below 17% moisture content within 4-6 summer months. Achieving a “ready to burn” status very quickly.

So, this means that larch can help create drier firewood with a lower environmental cost.

Because it is so quick, during the summer months, we offer a semi-seasoned larch product for your to finish in your own log store.

So in summary, this is really the most important feature of firewood, with greatest impact on burning qualities. So look out for declared moisture content below 20%, but consider buying larch in the summer months for DIY drying/seasoning.

Click to view all Ready To Burn Firewood Options