Inner Cover and Cover Foil
Lexikon
Between the topmost box and the actual outer cover lies the inner cover.
Between the topmost box and the actual outer cover lies the inner cover. It is a solid component, often with an insulating panel fitted inside and a stainless steel ventilation screen. It keeps the warmth in the colony, provides air exchange, and serves as a travel screen during migration.
To be distinguished from this is the cover foil, a thin, usually transparent sheet of plastic that lies directly on the frame top bars. It allows a first look into the colony without disturbing the combs, helps regulate moisture, and can be changed easily. A cover cloth is also used instead of a foil.
The practical benefit of this layer is additionally that it protects the inner cover from being propolized. Anyone who fits the insulating panel into the inner cover without a foil underneath it will find it firmly propolized after one season.
In hives without an inner cover, another component takes over its task. In the Warré hive this is a cushion filled with loose material, over a cloth.