What describes ducts that penetrate non-fire-resistance-rated floor assemblies?

Ducts that penetrate non-fire-resistance-rated floor assemblies are best described as combustible penetrations because they involve elements that can contribute to fire spread if not properly protected. When ducts penetrate these types of floor assemblies, they need to be evaluated in terms of their material composition and how they interact with potential fire hazards.

In many cases, ducts made of combustible materials can allow for the passage of smoke and flames from one area to another, potentially compromising the safety of the building and its occupants. This relationship between combustible materials and non-fire-rated structures necessitates adherence to specific building codes and standards to ensure adequate firestopping measures are implemented.

The other choices do not accurately capture the nature of the ducts that penetrate non-fire-resistance-rated floor assemblies. While non-combustible ducts might be used in fire-rated applications, they do not apply here since the question focuses on non-rated assemblies. Floor vent ducts and air transfer ducts describe specific functions but do not address the fire safety considerations of penetrating a non-rated assembly. Thus, the correct classification of these ducts in such situations is as combustible penetrations, highlighting the importance of fire safety in building design.