The Problem With Wet or “Green” Firewood
Freshly cut wood has a very high moisture content. As much as 60% (or more) of the weight of a tree is water. At least some of this water must be removed before trying to use it as a fuel wood (See below for a moisture content table). Several bad results can occur from burning wood that is not fully dried to below 25% moisture content. (Such wood is referred to as “green” wood).
In green wood, the effective available heat is MUCH less, not just because there is less wood fibers in each pound of wood put in the wood burner, but because a good percentage of that heat must be used to evaporate all that water before those wood fibers can burn. Another VERY important consequence of burning green wood is that the presence of all that moisture tends to keep “putting out” the fire, therefore making it burn very poorly, which tends to produce a lot of creosote and pollution.
What is Firewood Seasoning?
Generally, the way firewood drying is accomplished is by “seasoning” the wood. Firewood is cut to length, split, and then seasoned (dried) in a stack, with air being able to get to it, for at least 9 months before burning (the DEP states it should be dried for 6 months minimum). The natural 60%-70% moisture content must be reduced to about 20% to burn well. The wood cells don’t lose much moisture through the bark; the moisture is most effectively removed through the cut cells at the ends of each piece.
Indicators of Properly Seasoned Firewood
- Color – Wood should have a dull gray color.
- Bark – A sure sign that firewood is dry is bark that is falling off the wood.
- Weight – Dry wood weighs much less than wet wood (be sure you are comparing the same wood species).
- Checks – As wood dries it will begin to split on the ends and display cracks.
- Touch – Split a piece of wood. If the exposed surface feels damp, the wood is not seasoned.
- Sound – Two dry pieces banged together sound hollow; wet pieces sound solid and dull.
Why Firewood Species is Important
Something to consider when shopping for firewood is whether or not the cord wood is mixed species or all one type.
If it’s all one type, you can use the below chart to get an idea of which wood species need longer drying times. For example, if you are looking at a supplier who only sells ash then you can have a slightly higher confidence level it’s all seasoned since it’s at a much lower moisture content to begin with. Looking at the below chart will show that seasoning a cord of ash will take far less time then a cord of red oak. I have stacked split ash in the sun for 4 months and it was perfectly ready to burn (although if I stacked it in the shady woods I am sure it would have required another month or so to properly season).
Considerations When Burning Green Wood
Sometimes you can’t help but burn unseasoned wood (though it’s not recommended). How do you know which species would be best under those conditions? It turns out that the desirability is NOT the same as for seasoned wood! While they are living, various species of trees have different moisture contents. If you suitably dry them all, that difference disappears. But, while still green, it becomes significant.
It’s possible to correlate both the heat-content of the wood fibers and the green moisture content to form a table of desirability for those situations when green wood must be burned.
| Species | Excess Moisture to dry weight |
GREEN ranking |
SEASONED ranking |
|---|---|---|---|
| Ash | 15% | 1 | 8 |
| Beech | 17% | 2 | 4 |
| Black Locust | 17% | 3 | 1 |
| Red Spruce | 18% | 4 | 16 |
| Shagbark Hickory | 19% | 5 | 2 |
| Sugar Maple | 21% | 6 | 5 |
| Norway Pine | 19% | 7 | 14 |
| Tamarack | 21% | 8 | 10 |
| Black Cherry | 22% | 9 | 11 |
| Yellow Birch | 23% | 10 | 7 |
| White Birch | 24% | 11 | 12 |
| Red Maple | 24% | 12 | 9 |
| White Oak | 25% | 13 | 3 |
| Silver Maple | 27% | 14 | 13 |
| Red Oak | 31% | 15 | 6 |
| White Pine | 31% | 16 | 21 |
| White Elm | 35% | 17 | 15 |
| Basswood | 38% | 18 | 22 |
| Aspen | 40% | 19 | 19 |
| Butternut | 41% | 20 | 18 |
| Balsam Fir | 44% | 21 | 20 |
| Hemlock | 44% | 22 | 17 |
Excess moisture is that percentage above the desirable 20% seasoned moisture content. So if you need to burn green wood, look for ash or beech, rather than fir or hemlock.
Tips on Buying Good Firewood
- Ask friends and neighbors who burn wood for recommendations on reliable suppliers.
- Shop around and select the dealer who seems most reliable and comes with the best recommendations.
- Do not order wood by phone. Go to the storage area to inspect the wood and take a tape measure to check piece length and pile size.
- Look for wood that is clean. Sand and mud on firewood makes it less desirable.
- Either measure the piled wood before delivery or stack it (or have it stacked) at home before paying so you can measure it and confirm that you get the volume you pay for.
- If possible, get the wood in spring and stack it in your own yard so you can control the seasoning process.
For further information on firewood or to buy firewood from Barts Tree Service located in the Danbury CT area click here.
