You are not making it up. Plenum boxes are not typically required in a plenum ceiling. Plenum ceilings are designed to allow for the movement of air from one space to another, and this air movement can help to cool or heat the space. Plenum boxes are typically used to control the airflow in a plenum ceiling, but they are not always necessary. In some cases, it may be possible to design a plenum ceiling without using plenum boxes. Flag for review