For a duct that connects not more than two stories, what is required to protect annular space around the duct?

To protect the annular space around a duct that connects not more than two stories, the requirement for using an approved noncombustible material aligns with safety regulations aimed at preventing the spread of fire and smoke through these spaces. Noncombustible materials are essential in maintaining fire resistance and ensuring that if a fire were to occur, it would not easily transfer through the ducting and into other spaces of the building.

The selection of approved materials is crucial as they must meet specific fire safety standards determined by codes and regulations. These materials help seal the annular space effectively without compromising the duct's structural integrity or the performance of the HVAC system. Using noncombustible materials also assists in upholding building safety measures and achieving compliance with local fire safety regulations.

Other options, while they may signify some form of protection or insulation, do not meet the necessary criteria for fire safety and might not provide the same level of protection against fire risks as approved noncombustible materials.