10.3.1　Fire Resistance Rating of Buildings

China's national standards for building fire protection design classify the fire resistance ratings of buildings into four grades—Grade I, Grade II, Grade III, and Grade IV—based on the importance of the building, fire hazard, building height, fire load density, characteristics of the occupants, etc. Grade I has the highest fire resistance performance, and Grade IV the lowest.

The fire resistance rating of a building is jointly determined by the fire resistance performance of all the components constituting the entire building, but the fire resistance performance of the main load-bearing components such as walls, supporting columns, beams, floor slabs, and the roof load-bearing system plays a decisive role. The fire resistance performance requirements for building components in buildings with Grade I and Grade II fire resistance ratings are shown in Table 10.5.

The functional requirements of the hospital clean operating department require that the building housing it should have a relatively high fire resistance rating, adopting a building with a Grade I or Grade II fire resistance rating or being located within a building with a Grade I or Grade II fire resistance rating. In design, a fire resistance rating as high as possible should be adopted; the ceiling materials should preferably be non-combustible materials, and when flame-retardant materials are used, their smoke generation index, flame spread index, and heat release should be reduced as much as possible.

[Reference] Fire resistance performance of building components. The fire resistance performance of a building component is determined by its fire resistance limit and its combustion performance.

(1) The fire resistance limit of a building component refers to the duration (h) from the moment the component is subjected to the action of fire until it loses its load-bearing capacity, or its integrity is destroyed, or it loses its thermal insulation function, when any building component is subjected to a fire resistance test according to the standard time-temperature curve. In China, the fire resistance limit test of building components is conducted in accordance with Test Methods for Fire Resistance of Building Components (GB/T9978). The judgment criteria are:

  1. When the integrity test is performed using the cotton pad specified in the standard, if the cotton pad is ignited, it indicates that the specimen has lost its integrity.

  2. If the average temperature rise on the unexposed face of the specimen exceeds the initial temperature of the specimen surface by 140℃ or the maximum single-point temperature rise exceeds the initial temperature by 180℃, it indicates that the specimen has lost its thermal insulation property. 3) If the specimen collapses during the test or its deformation exceeds the specified value, it indicates that the load-bearing capacity has been lost.

When any one of the above three conditions occurs, it indicates that the specimen has reached its fire resistance limit.

(2) The combustion performance of building components also depends on the combustion performance of the materials that make up the components.

According to the Classification Method for Burning Behavior of Building Materials (GB8624), the combustion performance of building materials is classified into 4 grades: Class A₁ (non-combustible), Class B₁ (flame-retardant), Class B₂ (combustible), and Class B₃ (easily flammable) materials. The relevant test methods can be found respectively in the Test Method for Non-combustibility of Building Materials (GB5456-85), the Test Method for Flame-retardancy of Building Materials (GB8625-88), and the Test Method for Combustibility of Building Materials (GB8626-88).

Table 10.5　Fire Resistance Limits and Combustion Performance of Corresponding Building Components for Buildings of Different Fire Resistance Ratings

Building Category Building Category Single-story or Multi-story Single-story or Multi-story High-rise High-rise
Component Name＼Combustion Performance and Fire Resistance Limit/h Grade 1 Grade 2 Grade 1 Grade 2
Walls Fire wall Non-combustible construction 4.00 Non-combustible construction 4.00 Non-combustible construction 3.00 Non-combustible construction 3.00
Walls Load-bearing walls, walls of stairwells and elevator shafts Non-combustible construction 3.00 Non-combustible construction 2.50 Non-combustible construction 2.00 Non-combustible construction 2.00
Walls Non-load-bearing exterior walls, partition walls on both sides of evacuation walkways Non-combustible construction 1.00 Non-combustible construction 1.00 Non-combustible construction 1.00 Non-combustible construction 1.00
Walls Partition walls between rooms Non-combustible construction 0.75 Non-combustible construction 0.50 Non-combustible construction 0.75 Non-combustible construction 0.50
Columns Columns supporting multi-story buildings Non-combustible construction 3.00 Non-combustible construction 2.50 Non-combustible construction 3.00 Non-combustible construction 2.50
Columns Columns supporting single-story buildings Non-combustible construction 2.50 Non-combustible construction 2.00 — —
Beams Non-combustible construction 2.00 Non-combustible construction 1.50 Non-combustible construction 2.00 Non-combustible construction 1.50
Floor slabs Non-combustible construction 1.50 Non-combustible construction 1.00 Non-combustible construction 1.50 Non-combustible construction 1.00
Roof load-bearing components Non-combustible construction 1.50 Non-combustible construction 0.50 Non-combustible construction 1.50 Non-combustible construction 1.00
Evacuation stairs Non-combustible construction 1.50 Non-combustible construction 1.00 Non-combustible construction 1.50 Non-combustible construction 1.00
Suspended ceilings (including ceiling joists) Non-combustible construction 0.25 Flame-retardant construction 0.25 Non-combustible construction 0.25 Flame-retardant construction 0.25

10.3.2　Fire Separation within Buildings

(1) The importance of fire separation. As far as building fires are concerned, there are mainly two possible modes: being ignited by fires in adjacent external buildings, and the spread and expansion of combustion within the interior. In fact, the vast majority of building fires are caused by fires breaking out inside the building. When a fire occurs in a certain space within a building, the fire spreads to other spaces through heat radiation, convection, and conduction during the combustion process, or through the floor slabs of the fire room, damaged spots inside the walls, or door and window openings. If there is no effective fire suppression or fire control intervention and the fuel is sufficient, the fire may eventually develop into a fire involving the entire building. Therefore, certain measures are taken to separate certain spaces within the building that have greater fire hazards, contain important equipment, or cannot be Separating the premises where work cannot be interrupted from other parts, and confining the fire within a designated area for a certain period of time, is very important.

Providing fire compartmentation zones between the relevant usable spaces inside a building can effectively prevent fire from spreading horizontally and vertically within the building, confining it to a specific local area. This is especially true for places such as the clean operating department of a high-rise building: because there are many vertical shafts such as stairwells, elevator shafts, pipe shafts, and exhaust ducts, and because the building has many stories with relatively long horizontal and vertical evacuation distances, if the fire compartmentation measures are not properly handled, the consequences of a fire are often relatively serious.

In order to enable people in the fire area to evacuate quickly to a safe place, effectively control the extent of fire spread, give full play to the capability of the fire protection facilities and the fire brigades to control the fire, and ensure convenience of use, fire compartmentation shall be rationally provided and the floor area of horizontal fire compartments shall be strictly limited. With regard to fire compartments, by referring to foreign practice and in light of domestic conditions, buildings of different natures and uses, and of different fire resistance ratings and fire protection categories, have different principles for the division of fire compartments. Generally, in addition to satisfying the area requirements for fire compartments and/or the construction requirements for fire separation and fire separation elements specified in the Code for Fire Protection Design of Buildings, the Code for Fire Protection Design of High-rise Civil Buildings, and the Code for Design of General Hospital Buildings, it is also necessary, in light of the building's plan shape, functional use, spatial form, and the flow of people and goods, to properly arrange the locations of the fire separation elements; fire-fighting passageways required for fire suppression should be provided inside the building; fire separation should be provided between the various hazardous areas and between areas of different functions; and it should be ensured that the stairwells, vestibules, and corridors used for refuge are free from the threat of smoke and fire and remain unobstructed at all times. For pipe shafts and the like in high-rise hospital buildings, fire compartmentation measures should preferably be provided on a story-by-story basis.

[Reference] Fire compartment. A fire compartment is an enclosed space formed within a building, in accordance with the characteristics of the building, by means of fire walls, floor slabs, and other fire separation measures as required by the regulations, for the purpose of controlling and preventing the spread of fire to its adjacent areas. It is the basic spatial unit for controlling fires in fire-resistive buildings.

The division of fire compartments should be considered not only from the perspective of restricting fire spread and reducing losses, but also with regard to day-to-day use and management, so as to save investment. The design generally comprises two parts: horizontal fire compartments and vertical fire compartments. Horizontal fire compartments are used to prevent fire from spreading and expanding in the horizontal direction; vertical fire compartments are used to prevent vertical fire spread from story to story in multi-story or high-rise buildings—the compartmentation of vertical shafts such as elevator shafts and stairwells falls into this category. The fire compartments specified in the Code for Fire Protection Design of Buildings are shown in Table 10.6.

Table 10.6 Requirements of relevant codes for the floor area of fire compartments

Fire resistance rating Maximum permitted number of stories Maximum permitted length between fire compartments/m Maximum permitted floor area per story of fire compartment/m² Remarks
Single-story and multi-story, Grade 1 and Grade 2 As specified in Article 1.0.3 of the Code for Fire Protection Design of Buildings 150 2 500
Grade 3 5 stories 100 1 200 The inpatient portions of hospitals and sanatoriums shall not be located on the third story or above, or in underground or semi-underground buildings
Grade 4 2 stories 60 600 Hospitals, etc. shall not exceed 1 story
Category 1 high-rise buildings 1000
Category 2 high-rise buildings 1500

Note: For a fire compartment equipped with an automatic fire extinguishing system, its maximum permitted floor area may be increased by 1 time (i.e., doubled) in accordance with the provisions of this table; where an automatic fire extinguishing system is provided only in part of the compartment, the increased area may be calculated as 1 time the area of that part.

(2) Fire compartments. A clean operating department is a relatively independent space or building. When it is an independent building, the division of its fire compartments shall be carried out in accordance with the provisions of the current national standards Code for Fire Protection Design of Buildings and Code for Fire Protection Design of High-rise Civil Buildings. If it is built together with buildings for other medical purposes, it should preferably be divided into a separate fire compartment. In addition to satisfying the corresponding building fire protection provisions according to its building height, the area of its fire compartment should also be completely separated from other areas. If it is located in the same ... as rooms of other uses Even when located within a single fire compartment, the clean operating department shall also adopt effective fire and smoke separation measures, and shall be separated from other areas by partition walls with a fire resistance rating of not less than 2h, ensuring that it forms a zone of its own.

At the connections with non-clean operating department areas, both fire/smoke protection and the need for continuous operation of the operating rooms within the clean area shall be considered; fire doors shall be installed where conditions permit. The doors shall be fire doors with a fire resistance rating of not less than Class B. Where the needs and convenience of use—such as the passage of surgical carts—must be taken into account, measures such as providing a vestibule or fire shutters may be adopted for fire and smoke protection, and the door leading into the vestibule shall still be a fire door with a fire resistance rating of not less than Class B. The design of fire doors shall pay attention to the following:

  1. Fire doors shall be side-hinged doors opening in the direction of evacuation, and after closing shall be capable of being opened manually from either side.
  2. Fire doors used in evacuation corridors, stairwells, and vestibules shall be capable of closing by themselves.
  3. Double-leaf and multi-leaf fire doors shall be fitted with door coordinators (sequential closing devices).
  4. Fire doors normally held open shall, in the event of a fire, be capable of self-closing and of providing signal feedback.
  5. Fire doors located near movement joints shall be installed on the side with the greater number of floors, and when opened the door shall not span the movement joint, so as to prevent smoke and fire from spreading and expanding through the joint.

When fire shutters are used, their fire resistance performance and smoke control performance, as well as smoke/fire sealing measures, shall be considered. When fire shutters that have passed testing in accordance with the national standard Fire-resistance Tests—Elements of Building Construction (GB/T9978) are used, a fire cooling water curtain need not be provided; when fire shutters that have passed testing in accordance with the national standard Fire Resistance Tests for Door and Shutter Assemblies (GB7633) are used, a fire cooling water curtain shall be provided. Judging from the actual use of fire shutters, the gaps between the shutter guide rails and roller shafts and the building components (such as beams and slabs) are the main passages for smoke spread in a fire; they shall be tightly sealed with fire-stopping materials, or sealed with special smoke seals (such as the smoke/fire sealing components for fire shutters and fire doors from Tianjin Shengda Safety Science & Technology Industrial Company, see the notes below).

Where the connection with the non-clean operating department area leads directly to an open exterior corridor, or where a door opening directly to the outside is provided, fire doors need not be installed.

[Reference]

① Fire doors. Fire doors (Figure 10.2) are classified into three grades by fire resistance rating: Class A, Class B, and Class C. The fire resistance rating of Class A fire doors is not less than 1.2h, that of Class B fire doors is not less than 0.9h, and that of Class C fire doors is not less than 0.6h. By combustion performance, they can be divided into non-combustible fire doors and difficultly-combustible fire doors; there are usually two types, wooden fire doors and steel fire doors, and the door leaves are usually single-leaf, double-leaf, or multi-leaf.

Non-combustible fire doors are generally constructed with a skeleton of thin-walled section steel, with thin steel sheets 1–1.2mm thick nailed to both sides of the skeleton and the interior filled with a wood core, mineral wool, or glass wool. Domestic products are mainly steel fire doors. A steel fire door uses steel plates for the door frame, door panel, and skeleton, with non-combustible material filled inside the door leaf.

Difficultly-combustible fire doors are generally made by nailing one or two layers of wooden boards alternately arranged together, using 5–7mm thick asbestos board or slurry-soaked felt at least 15mm thick as interlayers, wrapping the wooden boards tightly from one or both sides, and nailing sheet iron on the surface; alternatively, they are constructed with fire-retardant timber for the interior and outer frame, covered with decorative materials on the outside. Domestic products are mainly wooden fire doors. A wooden fire door is a door composed of a door frame, door panel, and skeleton made of timber or timber products, fitted with hardware, which can meet the requirements for fire resistance stability, integrity, and thermal insulation.

Steel fire doors, fire windows, and fire roller shutter doors have become one of the important measures for fire compartments. They are usually used at fire walls, stairwells, entrances and exits, and the openings of pipe shafts, playing an important role in preventing the spread of smoke and fire and reducing losses. They are now widely used in civil buildings such as high-rise buildings, hospitals, commercial buildings, exhibition buildings, power dispatching centers, high-grade office buildings, scientific research facilities, and archive buildings, as well as in industrial plants, warehouses, and other buildings.

② Fire windows. Fire windows are divided into two types: fixed type and self-closing sliding type; among them, self-closing sliding fire windows designed with temperature-sensing devices should be given priority. Figure 10.2　Appearance of Fire Door

③ Fire roller shutters. A fire roller shutter consists of curtain slats, brackets, a winding shaft, guide rails, a counterweight, a protective cover, an electrical control box, control buttons, etc. (Figure 10.3). The curtain slats are generally made of metal sheets such as steel plates or aluminum alloys, formed into a rollable chain-like plane by buckle rings or hinged connections, or made of non-metallic composite fire-resistant materials. Steel fire roller shutters are usually of two types: light-duty and heavy-duty. The steel plate of a light-duty shutter is 0.5–0.6 mm thick; that of a heavy-duty shutter is 1.5–1.6 mm thick.

Fire roller shutters are divided into several types, such as single-panel, composite, and inorganic shutters. Normally they are retracted at the top; when a fire occurs, the shutter descends automatically to confine the fire within a smaller area. The bottom rail of the shutter and the guide rails are fitted with sealing rubber strips, which can greatly improve their sealing performance.

Fire roller shutters can be rolled up and down in two ways: electric and manual. For the electric type, an electric motor controlled by buttons or linkage control equipment is installed at the rotating shaft to roll the shutter; for the manual type, the shutter is rolled directly by hand using a pull chain or the like.

(3) Fire protection of technical mezzanines. Many pipes and lines, such as filtration devices, electric wires, gas pipes, and ventilation ducts, are arranged in the technical mezzanine of the clean operating department. Moreover, the mezzanine is enclosed with respect to the space below, so once a fire breaks out, it is not easily discovered and the location of the fire is difficult to determine. By the time it is discovered, the fire is often already rather difficult to control. The combustion state of such a fire is usually ventilation-controlled, and if improper measures are taken, it may even trigger a flashover, causing greater harm. In addition, in the operating room there may be situations in which people are still undergoing surgery when a fire occurs; they are highly sensitive to smoke and fire and difficult to evacuate. Therefore, in the design, careful consideration should be given to the treatment of the parts connecting the technical mezzanine of the clean operating department with the operating rooms, auxiliary rooms, etc., and necessary fire and smoke separation measures should be adopted, such as a suspended ceiling made of fire-resistant boards with a fire resistance rating of not less than 1 h (including the ceiling panels and the fixing support system). When ventilation ducts pass through the separating elements, fire dampers and the like should be installed at the separation points.

[Reference]

① Ventilation-controlled combustion. Ventilation-controlled combustion is a phenomenon that occurs when the opening size of a confined space is within a certain range and the fire area has developed to a certain scale. During the combustion process, the burning rate of the combustibles is controlled by the rate of air flowing into the space. Its hazard lies in the fact that when the opening size of the space is suddenly increased (e.g., the integrity of a weak spot in the suspended ceiling is destroyed) or the ventilation conditions are changed, it can easily cause the entire confined space to catch fire over its full extent or the burning rate to accelerate suddenly.

Flashover is a transient phenomenon in the development of an indoor fire in which, owing to the massive accumulation of heat released by combustion, under certain conditions almost all combustibles in the space ignite and burn over their full extent. In building fire protection, efforts should be made to avoid the occurrence of this phenomenon. Figure 10.3　Schematic Diagram of the Fire Shutter Structure

  1. Curtain slats (curtain surface) 2. Smoke prevention device 3. Guide rails 4. Lintel

② Fire damper. A fire damper is a valve installed on the supply and return air ducts of ventilation and air-conditioning systems. It remains open under normal conditions and closes when the gas temperature inside the duct reaches 70℃ during a fire. Within a certain period of time, it can meet the requirements of fire resistance stability and fire integrity, and serves to block smoke and resist fire. It is generally composed of a metal outer frame, metal valve blades, a rotating shaft, fusible alloy pieces, an electric control mechanism, and springs.

Fire dampers are usually of two types according to their mode of action: the gravity flap type and the spring type; according to their shape, they are usually divided into circular and rectangular types; according to their blade configuration, they can be classified into two categories: single-blade and multi-blade.

The operating temperature of a fire damper is generally 70℃. It can be closed manually, manually reset, and have its valve angle manually adjusted; it can also be linked with the fire alarm system to close automatically. Whether closed automatically or manually, it can send out an electrical feedback signal. Fire dampers are installed on ventilation ducts and remain open under normal conditions.

Fires have shown that when a fire breaks out in a building equipped with ventilation and air-conditioning systems, the vertical and horizontal air ducts passing through floors and walls are the main routes of fire spread. Installing fire dampers at appropriate positions on the ducts of ventilation and air-conditioning systems can effectively prevent this from happening, stop the fire from spreading and expanding through the ducts, and ensure the reliability of horizontal and vertical fire compartments.

In the fire protection design of buildings equipped with ventilation and air-conditioning systems, the positions and construction of fire dampers shall meet the following requirements:

(i) A fire damper shall be installed where a ventilation duct passes through a floor of non-combustible construction. A smoke-preventing fire damper shall be installed where a ventilation duct passes through a fire wall, or double fire dampers shall be installed on both sides of the fire wall. (ii) Fire dampers shall be installed at the locations where the main supply and return air ducts pass through the partition walls and floors of ventilation and air-conditioning machine rooms, so as to prevent a fire in the machine room from spreading through the ducts to other rooms in the building.

(iii) Fire dampers shall be installed at the locations where supply and return air ducts pass through the partition walls and floors of rooms of significant importance or with high fire hazards, such as rest rooms, multi-function halls, conference rooms, flammable substances testing laboratories, storehouses with large quantities of combustible goods, and rooms for valuable items.

(iv) In multi-story and high-rise buildings, fire dampers shall be installed on the horizontal duct sections at the junctions where the horizontal supply and return air ducts of each floor connect with the vertical ducts.

(v) Fire dampers shall be installed on both sides of the locations where ducts pass through the deformation joints of a building.

(vi) The closing direction of a fire damper should be consistent with the airflow direction inside the ventilation duct, and the ventilation duct in which the fire damper is installed should have a certain strength; separate supports and hangers should be provided at the duct section where the fire damper is installed to prevent deformation of the duct.

③ Requirements concerning ventilation duct materials in foreign codes and standards for heating, ventilation, air conditioning and smoke exhaust ducts.

(i) United Kingdom.

The United Kingdom has the "DW/151 Specification for Plastics Ducting", the "DW/191 Practical Guide: Glass Fibre Ductwork", the "DW/144 Specification for Sheet Metal Ductwork", etc.

Section 10 of the "Building Regulations 1991 / Approved Document B" stipulates the following regarding the combustion performance of materials and assemblies:

In general, it is necessary to insulate sheet metal ducts for the purposes of sound insulation and thermal insulation. The duct is usually lined internally with a sound-absorbing layer and, at the same time, covered externally with an insulation layer. There are two commonly applied methods of external duct insulation: one is to apply the insulation layer around the outer wall of the duct, and the other is to fill the space between the duct and its outer casing with insulating material. Whichever method is adopted, when the insulation material is not non-combustible, a large amount of heat and smoke will be generated in a fire.

Although rigid sheet metal ducts themselves can resist the penetration of fire and smoke, flexible joints or connectors made of materials that are not non-combustible must generally be provided as part of the system. These locations are the weak links of the system, and therefore their use must be limited.

Duct components must be carefully selected to ensure that materials connected to, or in the vicinity of, ducts that are not adequately insulated will not be ignited by the heat radiation or heat conduction from a fire within the duct.

Clause 18.14 stipulates that where a duct or duct shaft is constructed for the ventilation and smoke exhaust of a basement, a fire-resistant structure of non-combustible materials must be adopted

(ii) Australia.

The Australian Standard "Ductwork for air-handling systems in buildings" (AS4254:1995) stipulates:

Fire dampers, branch ducts, ducts and related equipment used in smoke exhaust systems, as well as kitchen ventilation smoke exhaust systems, must possess the following fire-resistant performance:

Non-combustible material: The materials used should be regarded as non-combustible materials.

Melting point: The melting point of the materials shall not be lower than 1000℃.

Insulation: The melting point of the insulation material should not be lower than 500℃ and, if it is applied inside the duct, the melting point of its perforated metal surface shall not be lower than 1000℃. Joints (silencers, sealing components or similar accessories) connecting the duct outlet to the fan, with a total length not exceeding 2m, are not required to comply with this provision. Internal insulation systems of the glass fiber and rock wool types whose perforated metal surfaces have a melting point not lower than 1000℃ are deemed to comply with this provision.

Flexible joints: The melting point of flexible joints shall not be lower than 500℃.

Branch ducts must be made of sheet metal with a thickness of not less than 2mm, or their fire resistance rating must not be lower than the requirements for duct shafts.

For kitchen smoke exhaust systems, where the smoke exhaust duct is not installed in a dedicated independent duct shaft, it must be made of tinplate with a thickness of not less than 1.2mm, stainless steel with a thickness of not less than 0.9mm, or other suitable materials.

(iii) United States.

Auxiliary materials such as duct exterior finishes, duct liners, vapor barriers, adhesives, fasteners, gaskets, and the core materials of ducts and the flame spread index of structural spaces above ceilings, cover panels, silencer materials of duct systems, etc. shall not exceed 25, with no sustained burning, and the smoke developed index shall not exceed 50. If the exterior covering and lining of the duct need to be attached with adhesive, the same adhesive must be used for testing, or, with the adhesive in its final cured state, the flame spread index of the silencers of the duct system, etc. shall not exceed 25, with no sustained burning, and the smoke developed index shall not exceed 50.

The exterior surfaces and linings of duct cover panels and of structural spaces above ceilings used as air ducts, when tested in accordance with a duct exterior covering standard similar to the Standard Test Method for Hot-Surface Performance of High-Temperature Thermal Insulation (ASTMC411), shall not ignite, generate heat, or produce smoke. The test temperatures are all above 250℉ (121℃).

(iv) Singapore.

According to the requirements of its Code of Practice for Fire Precautions in Buildings, the construction of ducts for air-conditioning and mechanical ventilation systems shall meet the following requirements: all air-conditioning or other ventilation ducts, including their frameworks, must be constructed of steel, aluminum, fiberglass panels, rock wool panels, or other approved materials; all air-conditioning or other ventilation ducts must be adequately supported.

The exterior surfaces and linings of ducts must be non-combustible materials. However, when non-non-combustible materials have to be used: if tested by the methods required by the Code of Practice for Fire Precautions in Buildings, the surface flame spread index of the material shall be no lower than Class 1; but in certain areas of a building where the ceiling is required to be a Class 0 surface, the exterior surfaces and linings of ducts are required to be Class 0 materials; when they produce the least amount of smoke and toxic gases in a fire, they shall be at least 1m away from fire dampers.

Thermal insulation materials for ducts used in air-conditioning and mechanical ventilation systems shall meet the following requirements: the thermal insulation materials of ducts, together with their vapor barrier linings and adhesives, must be tested by the methods specified in this code, and their surface flame spread index shall be no lower than Class 1; however, in certain areas of a building where this code requires the ceiling to be a Class 0 surface, the exterior surfaces and linings of ducts are required to be Class 0 materials.