Seasoned wood (moisture content)
Seasoned wood is firewood dried to below roughly 20% moisture content — the threshold at which a modern stove can burn it cleanly and efficiently. Fresh-cut (“green”) wood is 35–60% water, and burning it wastes heat boiling water while producing smoke and creosote.
Seasoning takes time: split and stacked off the ground with the top covered, most hardwoods need 6–12 months and dense species like oak often need two summers. Wood dries from the cut ends, so splitting is what actually exposes the interior.
A $20–40 pin moisture meter ends the guesswork: split a piece and measure the fresh inner face — the outside of a rained-on but seasoned split reads wet while the inside is fine, and vice versa for surface-dry green wood.
| Target | Below 20%; EPA cord-wood test fuel is specified around 19–25% (wet basis) |
|---|---|
| Green wood | 35–60%+ by weight when freshly felled |
| Energy in dry wood | About 8,600 BTU per pound, near-identical across species by weight |
| Cost of the water | Roughly 1,200 BTU per pound boiled off, before the extra flue loss |
| Drying time | 6–12 months for ash, birch, and maple; 12–24 months for oak |
| Basis conversion | 20% wet basis = 25% dry basis — check which one your meter reports |
| Creosote threshold | Flue gas condensing below about 250 °F is where creosote forms |
What the water actually costs you
Start with the direct arithmetic. A pound of wood at 50% moisture is half a pound of dry fuel — about 4,300 BTU of gross energy — and half a pound of water, which takes roughly 600 BTU to heat and evaporate. So around 14% of the gross energy in that pound never leaves the firebox as useful heat. That alone would be tolerable.
The second-order losses are the ones that hurt. All that steam keeps the firebox below the 1,000–1,100 °F that secondary combustion needs and below the temperature at which a catalyst stays lit, so a large share of the wood gas — well over half the energy in the log — leaves unburned as smoke instead of being converted to heat. The added water vapour also increases flue gas volume, carrying more heat out of the building for every pound of fuel.
And then the exhaust, cool and loaded with unburned tar, meets a chimney wall. Below about 250 °F those vapours condense as creosote. That is why wet wood and chimney fires are the same story told twice: the fuel that should have heated your house is instead lining your flue with something that burns at 2,000 °F.
How wood actually dries
Moisture leaves through the cut ends roughly ten to fifteen times faster than through the sides, and bark is close to waterproof. That single fact drives every piece of good practice: splitting is what does the work, because it opens end grain and exposes the wet interior; rounds left whole can sit for two years and still be soaked in the middle.
Stack in single rows off the ground, oriented to catch sun and prevailing wind, with the top covered and the sides left open. A tarp wrapped around a stack is worse than no tarp at all — it traps evaporated moisture and grows mould. Six to twelve months does it for ash, birch, poplar, and maple; oak, hickory, and elm are dense and commonly want two summers.
A few common shortcuts do not work. Standing dead timber is not seasoned — it is often waterlogged at the base and needs the same treatment as green wood. Wood stacked in a closed shed with no airflow dries very slowly. And a load bought in September labelled “seasoned” has usually been split in July. Kiln-dried firewood is the exception: it is genuinely under 20%, verifiably so, and priced accordingly.
Measuring it, and buying it
A $20–40 pin meter settles the question in seconds, but only if used correctly. Split a piece open, push the pins into the freshly exposed face across the grain, and read that. The outside of a split tells you about last week’s weather, not about the wood. Test several pieces from different places in the stack, and bring the wood indoors first: meters are calibrated near room temperature and read low on cold wood, so a 15% reading at 20 °F outside is not a 15% reading. Note also that meters differ on whether they report wet or dry basis — 20% wet basis is 25% dry basis, which matters when you are right at the threshold.
Without a meter, the indicators are cumulative rather than individually conclusive: the piece feels light for its size, the ends are checked with radial cracks, bark is loosening or fallen off, two splits knocked together ring rather than thud, and in the fire the ends do not hiss or bubble. Any hiss at the end grain is water boiling out, and it means the load is costing you heat right now.
When buying, know the units. A full cord is 128 cubic feet stacked — a 4 × 4 × 8 ft rick — of which roughly 85 cubic feet is solid wood. A “face cord” or “rick” is 4 ft high by 8 ft long by whatever the log length is, so at 16-inch splits it is a third of a cord; paying cord prices for a face cord is the oldest trick in the trade. Species matters for density, not for energy per pound: a cord of oak or hickory carries roughly 24 million BTU against 15–17 for pine, because there is simply more wood in the same stacked volume. Buy a season ahead and dry it yourself, and never burn painted, treated, glued, or salt-soaked driftwood — it destroys catalysts, corrodes flues, and puts genuinely toxic products into your house.
Frequently asked questions
What moisture content should firewood be?
Below 20%, measured on a freshly split face at room temperature. That is the level modern stoves are designed and certified around — EPA’s cord-wood test protocol specifies roughly 19–25% on a wet basis. Between 20% and 25% a stove will run but noticeably less well; above 30% you are boiling water instead of heating the house.
How long does firewood take to season?
Six to twelve months for ash, birch, poplar, and soft maple, split and stacked off the ground in sun and wind with the top covered. Oak, hickory, and elm are dense and usually need two summers. Rounds left unsplit can take twice as long or never get there — splitting is what opens the end grain that lets moisture out.
How can I tell if firewood is dry without a meter?
Look for radial cracks in the end grain, bark that has loosened or fallen off, and a piece that feels light for its size. Knock two splits together — dry wood rings, wet wood thuds. In the fire, dry wood catches quickly and burns with clear flame; wet wood hisses and bubbles at the ends. None of these is conclusive alone, which is why a $20 meter is worth buying.
Can I burn wood at 25% moisture?
It will burn, but poorly. At 25% the firebox struggles to hold the 1,000 °F secondary combustion needs, so more of the wood leaves as smoke, the glass blackens, you get less heat per split, and creosote accumulates faster. Mix a few damp pieces into a hot fire if you must; do not build a season around them.
Does covering firewood help it dry?
Covering the top does — it keeps rain and snow off the end grain. Covering the sides does not; a tarp wrapped around a stack traps evaporating moisture against the wood and encourages mould and rot. Cover the top only, leave the sides fully open to wind, and keep the bottom row off the ground on pallets or rails.