What additional safety feature is required when a type "B" vent passes through a ceiling or a floor?

When a type "B" vent passes through a ceiling or a floor, it is necessary to have a firestop as an additional safety feature. The firestop serves to prevent the spread of fire and smoke between the different floors of a building, maintaining the integrity of fire-rated assemblies. This is especially important in areas where combustible materials may be present, as it helps to ensure that any potential fire does not escape into other areas of the structure, which could compromise safety.

Firestops are designed to seal any openings that might exist around the vent, ensuring that there is no pathway for flames or heat to travel through. This functionality is crucial in preventing a small fire from becoming a larger, more destructive one, giving occupants more time to evacuate safely and reducing damage to the building.

While options like a heat shield, an expansion joint, and a moisture barrier may have their respective roles in other scenarios, they do not specifically address the critical need for fire safety when a vent penetrates a barrier like a ceiling or floor. Therefore, the firestop is the required safety feature in this context.