Smoke control and exhaust duct is a very important part of the smoke exhaust system in the building. When a fire occurs, its fire insulation performance can determine the safety of personnel and buildings to a certain extent. Therefore, China has also issued relevant regulations - "Technical Standard for Building Smoke Control and exhaust System" clearly stipulates the fire resistance limit of smoke control and exhaust ducts set at different locations.

Fireproof wrapping of air duct is a common scheme to improve the fire resistance of smoke exhaust duct. Use Class A fire-resistant material to cover the surface of the smoke exhaust duct. As a kind of insulation material with excellent fire protection and heat insulation performance, rock wool is usually used as one of the fire-proof wrapping materials for smoke exhaust duct.

However, it is not the use of rock wool to fireproof the smoke exhaust duct to achieve the corresponding fire resistance limit requirements, but also the specification parameters of rock wool, such as volume weight.

Bulk weight refers to the mass of a material per unit volume, usually expressed in kilograms per cubic meter (kg/m³). For smoke control and exhaust systems, selecting the appropriate volume weight can ensure that the rock wool material has sufficient density and thermal insulation properties to meet the corresponding fire resistance requirements. Too low rock wool bulk weight may cause the fire resistance limit to meet the requirements, while too high insulation bulk weight may increase the load of the smoke exhaust system and affect its normal operation.

Taking the above factors into account, the bulk weight of rock wool insulation used for fireproof wrapping of smoke exhaust duct is generally 120kg/m³, and under this bulk weight, rock wool with a thickness of 50mm can ensure the fire resistance limit of smoke exhaust duct for 2 hours.

It should be noted that the fire resistance limit of the smoke exhaust duct is for the whole duct, not for a single rock wool. Therefore, in order to reduce the bulk weight of rock wool and save costs, some manufacturers may use fireproof paint with low bulk weight rock wool, or use low bulk weight rock wool with fire retardant board, which can also reach the fire resistance limit of 2 hours (requiring professional testing institutions to test).

The bulk weight of rock wool insulation for fireproof wrapping of smoke and exhaust duct is generally 120kg/m³. It is also possible to use fire retardant coatings or fire boards with rock wool, but this may reduce the volume weight of rock wool.