All wood contains moisture, and for good combustion, firewood will need to be kept for at least six to nine months outdoors and have a moisture content of 20 to 25 percent to burn well and not generate a lot of smoke. Of course, the drier the wood, the more heat you will get back, so the longer you dry it and keep it dry, the more you benefit. When comparing the kinds of firewood, one thing to remember is the denser (or heavier) the wood, the more heat output, given the same moisture content of similar sized pieces.
Firewood less than six inches in diameter can dry as is, over that size and the wood will need to be split in order to "season." Seasoned firewood is easy to spot. The ends will be checked and have gray color. This is important especially if you are buying your firewood. If you are splitting your own wood to dry, stack it in a crisscross fashion to allow the air to circulate through the stack. If possible, do not stack directly on the ground or know that the bottom layer will attract decay fungi and a number of insects, as well as absorb soil moisture. Because of this, do not stack the wood next to the house so it touches the siding either. Stacking it away from the house also will increase airflow.
Firewood will need to be covered to keep rain and snow from being reabsorbed after the wood is dry. Use simple materials like a tarp or a waste piece of plywood. Whatever you use, be sure to secure it so it will not blow off, but not so much that it takes too much time to remove and put back on when you are stacking or removing wood to burn this winter. The covering should extend down the pile at least a foot or extend past the edges protecting much of the wood.
We have all thrown a piece of unseasoned wood on a fire and then felt a real drop in heat output. Dry, seasoned firewood will provide 20 percent or more heat than an equal amount of green or unseasoned firewood. It takes energy to burn off the moisture before the firewood burns. If you are going to burn green wood, add it in small amounts to an already well-established fire. It is not recommended that you burn green wood indoors in the fireplace. Green wood generates a lot of smoke and more importantly, creosote can build up in the home's chimney. The key to a successful fire that provides the most heat is going to be that properly dried wood.