If the logs are wet, they can encourage mold growth in the woodpile. This could make the wood unusable, and burning it can be harmful to your health. In any case, damp wood is much less flammable, so it's important to monitor the wood's moisture content.
How do you know if firewood is dry?
Start with a visual inspection. With the naked eye, you can see that the wood is clean, hard, odorless, cracked at the ends, and transparent (most of the time), but it also has clearly visible growth rings. This assessment can also help you eliminate logs with fungus or mold. If it's not too cold, you can also tap the logs against each other to gauge their moisture by the sound they make. A resonant sound indicates fairly dry wood, while a barely audible knock is more likely to indicate wood that's soaked with moisture.
If you don't have a moisture meter to measure the moisture content of wood (a device you can buy at any hardware store), you can test the wood with... detergent! Pour a few drops on one end of the wood and blow on the other end. If bubbles form, the wood is completely dry. This indicates that the wood grain is dry enough to allow the air circulation needed to create bubbles.
Efficient firewood storage: the special case of green wood!
