Wet wood is a bad fuel. Water in the log has to be boiled off before the wood itself can burn, and that energy comes out of the heat you were trying to produce. Both seasoning and kiln drying solve that problem. They differ in how long they take, what they cost, and what else they do to the wood.
The number that matters
Firewood should be under 20 percent moisture content when you burn it. Freshly felled hardwood can be 50 percent or more, which means half its weight is water.
At 50 percent moisture, a large share of the fire’s energy goes into evaporating water rather than heating the room. The fire also burns cooler, which means the volatile gases driven out of the wood do not fully combust. They travel up the flue, cool, and condense as creosote, which is what chimney fires are made of.
So this is not a question of preference. Burning wet wood gives you less heat and a dirtier chimney at the same time.
Seasoned wood
Seasoning is air drying. Wood is split, stacked with airflow, and left for six months to two years depending on species and climate.
Split matters. Wood dries from the exposed face, not through the bark, so a round left whole for a year is still green inside. When a seller says the wood was cut a year ago, the question to ask is when it was split.
Well seasoned wood typically settles at 15 to 20 percent moisture, in equilibrium with the ambient humidity. It will not go lower outdoors, and it will go back up if it is rained on or stored on soil.
Time by species, split and stacked properly:
| Species | Time to season |
|---|---|
| Ash | 6 to 9 months |
| Pine and softwoods | 6 to 9 months |
| Birch, cherry, maple | 9 to 12 months |
| Red oak | 12 to 18 months |
| White oak, hickory | 18 to 24 months |
Climate shifts all of these. The same stack dries faster in Denver than in Houston, because humidity sets the floor that air drying can reach.
Kiln dried wood
Kiln dried wood is heated in a chamber, typically to 160 degrees Fahrenheit or higher for a set period, which drives the moisture out in days rather than months. It comes out at 10 to 15 percent, drier than air drying can achieve.
You get three things for the premium:
Immediacy. It burns properly the day it arrives.
Consistency. Every piece in the load is at the same moisture, which air dried stacks rarely manage.
Pest treatment. The heat kills insects, larvae and fungus in the wood. This is the reason kiln dried firewood can legally cross quarantine lines that untreated wood cannot, which matters in states with emerald ash borer or oak wilt restrictions. Look for a USDA certified heat treatment stamp if you need to transport it.
The premium is typically 30 to 60 percent over seasoned wood.
Which to buy
Buy kiln dried if you need to burn it this month, you are transporting wood across a quarantine line, you have nowhere to store a stack for a year, or you are burning indoors in a fireplace where you want a clean, predictable fire and the quantities are small.
Buy seasoned if you burn regularly, have somewhere to stack it, and want the best heat per dollar. Well seasoned wood at 18 percent burns nearly as well as kiln dried at 12, for meaningfully less money.
Buy green if you are buying in spring for next winter, have a proper stacking spot, and want the cheapest fuel available. This is the best value in the market and the only cost is patience.
Testing what arrived
A moisture meter costs less than a face cord and settles every argument. Split a piece and press the pins into the freshly exposed face, not the outside. The outside of any stack reads dry.
Without a meter:
- Checking. Cracks radiating from the centre of the end grain indicate drying. Green ends are smooth.
- Colour. Seasoned wood greys and dulls. Fresh cut is bright.
- Bark. On many species it loosens and falls off as the wood dries.
- Weight. Seasoned wood is noticeably lighter than green wood of the same size, which becomes obvious once you have handled both.
- Sound. Two seasoned pieces struck together make a sharp knock. Green wood makes a dull thud.
- On the fire. Wood that hisses, bubbles at the ends, or refuses to catch without constant attention is wet.
Storing it so it stays dry
Whatever you buy, poor storage undoes it. Kiln dried wood left uncovered on the ground will reabsorb moisture and end up no better than green.
- Off the ground, on pallets or rails.
- Single row where possible, sides open to prevailing wind.
- Cover the top only. A tarp to the ground traps ground moisture and rots the bottom of the stack.
- Bring in a few days’ worth at a time, not the whole load.
To compare what you are paying for the heat you get, including the moisture penalty, run your quotes through the firewood cost calculator.