Combustion air ducts that traverse rated construction without protective dampers must be?

Combustion air ducts that traverse rated construction must be enclosed in a shaft to maintain the integrity of the fire-rated assembly. When ducts pass through walls or ceilings that have fire-resistance ratings, it is crucial to prevent the spread of fire and smoke between compartments. Enclosing these ducts within a shaft provides a protective barrier that helps to contain any potential fire within that space and ensures that the rated assembly's performance is not compromised.

This approach is particularly important in building design, as it adheres to safety regulations and standards aimed at protecting occupants and property. The use of shafts for combustion air ducts allows for proper air intake while still maintaining the fire rating necessary for the structure's safety. Other options such as being completely open, protected by insulation, or made of non-combustible materials do not fulfill the requirement to ensure the fire-rated assembly’s performance is maintained when ducts are installed through those types of construction.