Interior separations of zones, such as ceilings and partitions (interior walls), are handled in a much simpler way than exterior constructions. A U-value is all that is required (along with the temperature differences found in the set points of the zones that hold the adjacent spaces) to determine the heat transfer across these surfaces. Ceilings are also used for plenum heat transfer, and partitions are normally only used when different zones contain spaces which are adjacent to each other. Interior windows (but not interior doors) are also used in determining the heat transfer across a partition. The only construction never used in the load calculation engine is the slab.