It's not much to look at but this is an absolutely essential piece of equipment for overwintering hives in cold climates. The dado on your inner cover should be facing up, the dado on this board faces down to line up with the other. This provides excellent top ventilation and an upper bee entrance.
Eliminating moisture in winter is critical to hive health. The absorbent homasote (a natural, recycled-paper product) prevents condensation from the cold outer cover from dripping on the broodnest. We recommend open screened bottom boards and slatted bottom boards for the winter. If you take care of ventilation, and leave your bees adequate honey, they will take care of the temperature inside the hive.
Once nights get start to get cold, put a mouse guard on the entrance and be sure to have an insulating cover on before Thanksgiving.