September 16, 2015
By Paul A. Hough
A suspended acoustical ceiling represents a significant percentage of a room’s surface. As such, these ceilings are critical to controlling the noise level in a space by providing a means of absorbing unwanted sound generated inside and blocking unwanted sound generated outside.
Suspended acoustical ceilings are also important in controlling the growth of a fire by providing a barrier between the room and the space above the ceiling. The separation a suspended ceiling offers can delay a blaze starting in a room from reaching potentially combustible materials in the space above the ceiling. It can also slow the spread of flames across the surface of the ceiling material, therefore delaying the spread to other spaces.
Most building codes rely on flame spread/smoke developed of the material ratings, and fire resistance of the assembly ratings to evaluate suspended ceilings, and roof/ceiling or floor/ceiling assemblies. These ratings are the result of testing based on CAN/ULC standards, with compliance determined by tests conducted at non-governmental, independent labs.
It is important to note flame spread, smoke developed, and fire resistance ratings are separate issues and must be addressed independently in selection and specification.
Flame spread and smoke developed ratings
Flame spread classification (FSC) and smoke developed (SD) ratings are the result of testing according to CAN/ULC-S102, Test for Surface Burning Characteristics of Building Materials and Assemblies. FSC is the relative rate at which a flame will spread over the surface of a material. This index is compared against a value of ‘zero’ for inorganic reinforced cement board and of ‘100’ for red oak. It is the rate at which the flame actually spreads along the surface and not an indication of the fire resistance of the material.
SD is a measure of the amount of smoke a material emits as it burns. Like FSC, it is based on an arbitrary scale in which asbestos-cement board has a value of ‘zero’ and red oak has a value of ‘100.’
Acoustical ceiling panels that are part of a plenum require an FSC of ‘25’ or less and an SD of ‘50’ or less. These ratings pertain to the face, back, and any surface exposed by cutting or drilling through the panel. Plenum-rated panels are designed to limit the spread of flames and production of smoke across the ceiling panels into a space, rather than to contain or compartmentalize a fire.
Fire resistance assembly ratings
The fire resistance assembly rating indicates the degree, measured in minutes or hours, to which an entire floor/ceiling or roof/ceiling assembly can withstand fire exposure and high temperatures. Essentially, it defines an assembly’s ability to prevent the spread of fire between spaces and offer additional protection to the structure.
The primary purpose of acoustical ceilings in a fire-resistance-rated assembly is to limit the spread of fire across the ceiling panel either below or above the occupied space, and to limit the movement of heat and smoke through the panel or assembly. By doing so, they provide protection to the rest of the building elements comprising the fire-resistance-rated roof/ceiling or floor/ceiling assembly.
Roof/ceiling assemblies are one of the types of fire-resistance-rated construction assemblies having acoustical ceiling systems as a component. These assemblies include:
Floor/ceiling assemblies are the second type of fire-resistance rated construction assembly to have an acoustical ceiling. They include:
While the tests to determine FSC and SD ratings only measure an individual product (e.g. a ceiling panel), procedures to determine fire resistance ratings are true ‘system tests’ that include all components in a roof/ceiling and floor/ceiling assembly. This means every material used in the construction, such as bar joists, acoustical panels, and steel fireproofing are included. A malfunction of any components in the assembly can result in failure of the fire resistance test because all components are tested as a complete system.
Fire resistance tests—such as CAN/ULC-S101, Fire Endurance Tests of Building Construction and Materials—measure the time an assembly contains a fire and is able to slow the temperature rise of building elements in the space above the ceiling so as not to ignite combustible material on the unexposed side of the assembly. The resultant fire-resistance-rated assembly and its time rating are then published in the UL Fire Resistance Directory or other such resources.
The UL Fire Resistance Directory provides criteria for the specific products installed in each component of the assembly. It also defines the specific manufacturer, type, size, and thickness of the ceiling panel and suspension system that must be used, as well as all other assembly elements. Any deviation results in a violation of the assembly’s design, and then in a loss of its fire resistance rating.
Fire resistance assembly requirements
Only acoustical ceiling panels specifically designed and tested as part of a fire-resistance-rated assembly can be used in a fire-resistance-rated floor/ceiling or roof/ceiling assembly. They are always identified as such, usually on the back of the panel.
Similarly, only suspension systems specified in the UL or other testing laboratory designs can be employed. Helping maintain the structural integrity of the ceiling, these systems are usually identified by the presence of expansion-relief notches in the main beams. If the notches are absent, the grid will expand and distort as it becomes hotter, eventually allowing the ceiling panel to drop into the space below.
Another key component of a fire-resistive assembly is the weight of the ceiling panel. Fire-rated panels must weigh a minimum of 4.5 kg/m2 (1 lb/sf). This will offset the effect of hot fire gases, lifting a lighter ceiling panel and pushing it out of place, resulting in lost protection.
In many building codes and UL-tested and listed system designs, hold-down clips are required on each panel—they must be installed if panels weigh less than the specified amount. However, installation of hold-down clips requires additional time, labour, and costs, and there is no quick assurance the clips are always installed in the proper position.
Smoke barriers, smoke partitions
The fire safety requirements for an acoustical ceiling can vary depending on the specific building’s use, occupancy, and construction details.
For example, smoke barriers and smoke partitions, frequently equated with corridor walls and ceilings, are two types of construction often found in hospitals. Smoke barriers are designed to completely stop the movement of smoke from one space to another, while smoke partitions are designed to limit the passage of smoke from one space to another, but not stop it completely. As a result, smoke partitions are not intended to provide a safe haven that is completely free of smoke. (A suspended ceiling can be part of a smoke partition, but not part of a smoke barrier.)
The function of each requires smoke barriers to have much more stringent requirements than smoke partitions. For example, smoke barriers must extend from the floor to the underside of the above floor or roof deck assembly. The wall must extend through any concealed spaces such as those above suspended ceilings, and through any interstitial structural and mechanical spaces. Additionally, any penetrations and joints must be fire stopped.
Smoke partitions can extend from the floor to the underside of a monolithic or suspended ceiling system as long as:
The space above a ceiling is often used as a return air plenum in many types of commercial buildings, meaning exhaust air from the room below is moved from the space, through the plenum, and then to ductwork leading to the central HVAC system.
In hospitals, however, this is rarely the case—the sheer number of service lines running through the space, and the need to minimize the chance of any airborne infection being spread through the plenum to other parts of the facility, prevent it.
Smoke partition considerations
Suspended ceilings used in smoke partitions are not required to have a fire-resistive rating. However, the panels must weigh at least 4.5 kg/m2 (1 lb/sf). If they are fire-resistive, they automatically meet the weight requirement. If not, verification is needed.
While not necessarily required by code, there are numerous other considerations to keep in mind regarding acoustical ceilings in smoke partitions.
Most acoustical ceiling panels are available as either square lay-in or tegular panels. Tegular panels have a reveal edge and are preferred because they fit into the grid system tighter, reducing the possibility of leaks. They also provide a better fit at the wall moulding when cut in properly.
Ceiling panels are usually offered in 610 x 610 mm (2 x 2 ft) or 610 mm x 1220 mm (2 x 4 ft) sizes. The larger-size panels are preferred because they require fewer grid components, once again resulting in less possibility for leaks.
To minimize air leakage, which may allow smoke transfer between spaces, it is important to seal the moulding on the wall with a firestop sealant that has an air leakage rating. Other penetrations through the ceiling—such as light fixtures and air diffusers—may also require treatment.
Regardless of the building, suspended ceiling systems play an important role in controlling the growth of a fire within a room. Both their flame spread/smoke developed rating and their fire resistance assembly rating allow fire protection engineers to address potential problems involving the possibility of occupants being trapped in a building before orderly evacuation can be accomplished, and the rapid spread of fire through a building or area before fire protection measures can be used to control or extinguish the fire.
Paul A. Hough is manager of product fire and seismic performance for Armstrong World Industries, and is responsible for all fire compliance testing, seismic testing, and code-related activities. He is a graduate of Indiana University of Pennsylvania, and is an active participant in a variety of ASTM committees, including E05 (Fire Standards), E06 (Building Performance), and E33 (Environmental Acoustics). He serves on the International Organization for Standardization (ISO) TC92 on Fire Safety and ISO TC92, SC3 on Fire Threat to People and the Environment. Hough can be reached at email@example.com.
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