Freshly cut wood may contain as much as 100 percent moisture, which means the water in the wood weighs more than the wood. Building a fire from wet wood, also called green wood, is less efficient and less safe than using properly dried or "seasoned" firewood. Firewood should have 20 percent moisture or less, according to the Washington State Department of Ecology.
Burning wet wood is less efficient than burning dry wood because so much energy is lost in warming water to steam. Vaporizing a pound of water wastes about 1,200 British thermal unit, or BTU, a measure of heat. In contrast, a pound of firewood that contains 20 percent moisture provides about 7,000 BTU of heat, according to University of Missouri Extension.
Burning wet wood in a fireplace may also be dangerous to your health. Wet wood produces more smoke than dry wood, which releases more pollutants and small particles into the air. Burning wet wood in a fireplace can also cause creosote buildup in your chimney, which may create a fire hazard, according to University of Illinois Extension. This occurs because wet wood produces less heat, which allows creosote to build up on the fireplace flue.
If you're not sure whether your firewood is dry, check the ends of the pieces of wood. Small splits in the wood mean the wood is probably dry, while a smooth texture indicates a higher moisture content. You can also knock two pieces of wood together. Wet wood creates a quiet noise, while dry wood makes a louder clinking sound. In addition, you can use a wood moisture meter to check the moisture content of firewood.
Season freshly cut firewood by storing it in a covered outside location off the ground for at least six months. Dense hardwoods may require a year or more to season effectively. Place the wettest wood in the back of your storage area or on the bottom of the firewood stack to ensure you use seasoned wood first. If you purchase firewood, verify that it has been dried in a covered location for at least six months.