Winters in Eastern Washington can be unpredictable and often times very cold. Could adding two follower boards at the end of each hive be an effective way of adding some insulation. I know the hive has a delicate balance and I wouldn’t want to disturb the ecosystem.
Adding two follower boards will theoretically add thickness to the wood, and so it will build up the insulation. However, there may be other more effective methods to protect your hive from chilly weather. First, if you have reduced the size of your hive by moving in the follower board, you will have some empty space in the hive box. Filling that empty space with insulating material (everyone’s favorite is an old wool blanket) will assist the hive greatly. Another consideration is adding insulation between the top bars and the roof. Again, old wool blankets are useful, although I’ve laid out burlap over the top bars and then stew wood shavings over the burlap. Some people have wrapped their hives in insulating material: Tar paper is sometimes a standard. But DON’T use plastic bags, or another item that will not allow for moisture evaporation. In the end, cold is rough on bees, but cold plus moisture is a real killer. Make sure their hive can breathe.