A ______ is not considered a plenum space.

A plenum space is defined as an area used for air circulation for heating, ventilation, and air conditioning systems, typically located above ceilings or below floors in buildings. When determining what constitutes a plenum, it's essential to consider functions and the design of the space.

Office spaces are not typically designed for air circulation purposes; they are primarily utilized for work environments, thus not meeting the criteria for plenum space. Mechanical rooms, while integral to the building's HVAC system, do not serve as paths for air distribution; they are designated areas where HVAC equipment is housed, and as such, they also do not qualify as plenum spaces.

In contrast, the space over a hung ceiling that is designated for environmental air would be considered a plenum since it is directly involved in air circulation and HVAC functionality. Therefore, the correct answer identifies the areas that do not serve a plenum role by identifying both office and mechanical rooms as exceptions.