Many facilities have walls that contribute to the HVAC load but which are not exterior walls (or floors or ceilings). These are normally called partitions, and they add to the building load an amount proportional to the U-value of the wall and the design temperature difference.
For those elements with a 17°F recommended temperature difference, other than exterior walls, an approximation in computer simulation programs is to designate the partition as a north-facing exterior wall. (This is the usual way to handle fully shaded walls on any exposure.) This emphasizes the need to insulate partitions just as thoroughly as other walls that are in direct contact with the outside.