Which construction is NOT required for the space between the corridor ceiling and the floor above when used as a return air plenum?

When considering the requirements for the space between the corridor ceiling and the floor above when used as a return air plenum, it is crucial to understand the purpose of these spaces in terms of air distribution and fire safety.

Fire-resistance-rated construction is typically necessary for areas involved in return air systems to help prevent the spread of fire and smoke. However, certain designs allow for more flexibility in plenum spaces, particularly regarding their fire-resistance ratings, provided that other safety measures are in place.

Non-combustible materials, flexible ducting systems, and sheet metal encasements are indeed essential components for ensuring safe and efficient airflow in a return air plenum. Non-combustible materials help prevent fire from spreading through the air handling space, while flexible ducting systems provide necessary adaptability for routing air. Sheet metal encasements serve as a durable option for protecting ductwork and also play a role in maintaining the integrity of the airflow pathways.

Understanding this, we can see that while fire-resistance is important, the specific requirement for fire-resistance-rated construction can vary based on the design and materials used in the building, which is why it is not mandated in every circumstance.