· by Michael Blair, Nashville's Chimney Sweep
Every problem a wood fire produces, smoke in the room, glass that blackens overnight, a chimney that needs sweeping twice a season, a fire that will not throw heat, comes back to one number: the moisture content of the wood. Freshly cut hardwood is around half water by weight. Wood ready to burn is below twenty percent. Everything between is burning water first and heating the house second.
What seasoned means
Seasoned wood has dried to a moisture content below about twenty percent, which for split hardwood stacked off the ground and under cover takes somewhere between six months and two years depending on species and split size. Oak, which is most of what gets sold here, is at the slow end. The word on a delivery ticket means nothing on its own; the number does.
A pin moisture meter, the kind sold for woodworking, reads it directly. Split a piece and test the fresh face in the middle, not the end grain and not the outside, which dries first. Under twenty is ready. Over twenty-five is firewood for next year, whatever the seller called it.
What wet wood does to the chimney
Burning water cools the fire, so combustion is incomplete and the smoke carries unburned gases and tar up the flue. The cooled flue gases condense on the flue walls as creosote. Wet wood produces more creosote, of a stickier and more flammable kind, than dry wood does, and it does it faster. A household that burns a cord of wet oak in a winter can build a hazardous layer in one season.
It also stains the glass on a stove or insert black overnight, smokes into the room when the door is opened, and delivers a fraction of the heat the same wood would when dry, because the energy went into boiling off the water. Dry wood fixes all four at once.
- Signs of wet wood: hissing, bubbling at the ends, black glass, heavy smoke
- Signs of dry wood: cracked ends, gray color, lighter weight, a hollow sound when knocked together
- The only reliable test is a meter on a fresh split face
Species that burn well here
Middle Tennessee firewood is mostly oak, hickory, ash, maple and hackberry, with some cherry and locust. Oak and hickory are dense, burn long and hot and are worth their price when dry, and they are the slowest to dry. Ash and hackberry dry faster and burn cleanly. Maple is in between. Softwoods, pine and cedar, burn fast and resinous and are for kindling rather than the main fire.
A mixed load is practical: faster-drying species for this winter and oak stacked for the next. The one rule is that whatever the species, it has to be dry, and a dry hackberry beats a wet oak every time.
Storing wood in Middle Tennessee humidity
Wood dries by air movement, and our summers are humid enough that wood stacked in the wrong place gains moisture rather than losing it. Stack off the ground on pallets or rails, in a single row where possible so air passes through, with the top covered and the sides open. A tarp thrown over the whole pile traps moisture and grows mold; a roof over the top with open sides does the job.
Bring a few days of wood indoors or onto a covered porch ahead of use so the surface dries after any rain. Keep the main pile away from the house, because a woodpile against the siding is a termite and rodent bridge as well as a moisture problem for the wall. Buy next winter’s wood this spring and it will be ready; buy it in October and test it before it goes in the stove.