The Forestry Commission report that moisture content has a greater effect on calorific value of firewood than any other variable. To burn the log must dry before it combusts. This uses up valuable energy that could go into heating your house.
Wet logs don’t burn cleanly. Wet wood contains moisture which creates smoke and particulates when burned, which reduces air quality. It will also create soot and tar which will cause damage to the chimney liner and stove.
Winstree logs have been seasoned for two years. This means that their moisture content is below 25% which reduces particulates when burnt and gives you more heat.