What is the two inch bar going around the top of your hive? Is that a custom made piece? Is it for insulation? Is it lined with somthing metallic? Do you leave it on all year? Do you use an inner cover with it? Sorry for the noob questions, but as I said I'm just starting out.
It is a insulation shim made out of 2" foil backed foam board. Foil prevents the bees from chewing through it. I use the same material for the cover. Without the shim, there is not enough room to feed sugar or pollen patties. By putting it on in the fall, the bees have time to seal up the cracks and get the air tight nest they desire.
It is put on in the fall, and taken off in the spring before any flow starts to prevent them from filling it with comb. If the timing is right, I get none to very little comb built in the space. Occasionally, I do get a hive that fills it with comb, but if caught early enough in the spring, it can be removed before the queen lays eggs in it.
No inner cover is used with it, the 2" cover foam board is places right on top. Then I either put a piece of coroplast election sign or a telescopic wood cover as shown in the picture. This just gives the insulation board a little protection and stores the cover until spring.
In spring, the shim and insulation cover are removed and standard wooded honey supers are put on top with standard inner cover and telescopic cover.
Purpose being you don't want anything with a lower r-value than the hive bodies otherwise it can be a place of condensation. By keeping the insulation values high, my hives stay bone dry. Not to mention, the heat retention allows them to raise brood faster and in larger quantities as shown in the picture. Brood needs heat, so any heat you allow to escape reduces the amount of brood they can raise.
Just FYI, I had some non-foil backed insulation board given to me this year and I made some shims out of it and lined it with a layer of clear shipping tape and it worked fine.