Freshly chopped firewood has up to 50% water content and won’t burn in your fireplace. First, you must let the firewood season (dry), which allows the moisture to escape––the drier the wood, the cleaner the burn. When the wood gets down below 20% water content, it’s ready to burn. Burning unseasoned (green) or even partially seasoned wood in your stove or fireplace will cause creosote build-up in your chimney, which can lead to a chimney fire at the worst, and a lack of fire or a roomful of smoke at best. Every homeowner reliant on wood should know how to season wood.
Wood is essentially a mass of tiny long tubes, or cell cavities, that run the length of the tree. Moisture exists both as “free water” in these cavities and as molecular water that is locked in the cell walls. When a tree is felled, the slow process of drying begins, and the free water is the first to evaporate. Once the free water evaporates, the moisture content of the wood is around 30 percent. This is called the “fiber saturation point.” After this, water begins to leave the cell walls, and the wood starts to shrink and crack.
For optimal burning, firewood should be dried, or “seasoned,” until its moisture content is less than 20 percent. Firewood with a moisture content higher than that may eventually burn, but it is devilishly hard to light and just as hard to keep burning. Also, your new high-efficiency wood-burning stove or furnace is guaranteed to perform sluggishly as it struggles to burn freshly split, or “green,” firewood—much of the heat and energy content produced are wasted in drying the wood’s excess moisture. Just as important, the stove does not burn the tars and creosote in the smoke produced by the fire, and they end up lining the inside of your flue pipes and chimney. They also blacken the glass windows of your wood-burning appliances and produce a lot of blue-gray smoke, fouling your house and annoying your neighbors.
Seasoning wood has another important but less obvious benefit—when wood is properly cut and stacked right away, mold has less opportunity to establish itself. Throwing unseasoned firewood into a pile allows mold to spread throughout the logs, mold that you unwittingly release into your home’s environment when you bring the firewood inside throughout the heating season.
You can buy already seasoned firewood or buy it green and season it yourself. How can you tell whether wood is properly seasoned? It’s possible to test the wood with a moisture meter, which measures resistance to a small current and converts it into a moisture-content reading, yet this reading can vary widely from one area of the log to another. With a little practice, however, you can use the following tips to judge accurately for yourself whether your wood is dry. Use as many as you can for the best results.
There are advantages to buying firewood green, provided you have the room to store and season it for a year. For one, you’ll be absolutely sure the wood is seasoned, and it will cost a lot less, anywhere from $15 to $50 less per cord than seasoned wood. Also, seasoned wood may be in short supply in some areas, so you may not have a choice.
Firewood can take a very long time to properly season. Exactly how long is a matter of ongoing debate in wood-burning circles. The traditional rule of thumb is to season firewood for at least six months before the heating season; some hardwoods require at least one to two full years. The truth lies somewhere in the middle and depends on piece size, tree species and local climate.
The protective bark on a log helps prevent the interior moisture from evaporating, so firewood begins to dry significantly only after it is cut and split. By splitting the wood into smaller pieces, you create a greater surface area, and the greater the total surface area, the lower the overall density, which means the wood dries and seasons at a faster rate. Trees with a dense wood structure, such as oak and elm, season much more slowly than do ash and birch. “Diffuse porous” species, such as maple, birch and poplar, season more quickly than do “ring porous” species, such as oak and ash. Conifers have an entirely different cell structure than deciduous trees and take longer to dry, so they are best split into small pieces. Trees felled in spring when the sap is “up” also have a higher moisture content. Finally, if you live in a damp maritime climate, seasoning times may be longer.
As with a really good meal, seasoning makes all the difference in a quality fire. In the end, with the exception of truly dense hardwoods, such as oak, and large-split softwoods, most household firewood bought in the spring can be seasoned enough for burning by winter. It’s not the type of tree but, rather, the seasoning that makes or breaks your fuel supply.
If you plan to season firewood yourself, here are five simple guidelines to follow: