I have never used inner covers. None of the beekeepers in Alabama (that I knew) used them. Now that I am in Kentucky, with much colder winters, should I use one? What is the main benefit of them?
BTW.. I will be using a top entrance by opening the front of the top cover (Michael Bush style)
I was told (about 45 years ago) that the main purpose is the keep the moisture off the outer (telescoping) cover so it would last longer. I have come to like it so I can remove the outer cover without exposing the top frames to the elements and I can get a quick read on the bees mood. -Mike
We make our inner covers with a round hole cut out in the middle big enough for a narrow mouth jar feeder to go in. We put a screen over that hole on the same side as the notch in the edge for the top entrance. Very handy. I can put an empty medium super over the inner cover, to protect the jar feeder and then put the telescoping cover over all of it. In theory, you could put 4 of those holes in the cover to feed a gallon at a time.
The inner covers also help keep the yellow jackets and bumblebees out.
In the winter, I use 2 inch shims full of sugar or candy on top of the brood box and then put the inner cover over that. You can insulate or put a quilt box on over that, under the telescoping cover.
The inner cover keeps them from locking the tele cover down with propolis. If it has the 3/8 inch deep side up, it gives the bees access to the top to keep the ants out. Without access, the ants build a nest up there and raid the honey supers, killing the hive.
If you use a telescopic cover, you will need the inner cover. If you use a migratory cover you will not need the inner cover.
http://www.bushfarms.com/beesfaqs.htm#innercover