Understanding Fire-Prevention Rating Systems

It’s important to remember that the top priority of fire requirements is the safety of occupants—to ensure that a fire, should one break out, is contained long enough to allow occupants to escape the building and to prevent structural collapse.

When evaluating building materials, such as insulation, and how they can contribute to the safety of an assembly in terms of fire, you will encounter specialized terminology and references to several standards and tests. Having a working knowledge of these will help you specify the best materials and systems for your project.

Combustible and noncombustible materials: The International Building Code (IBC) and other building codes distinguish between combustible and noncombustible building materials. By definition, a noncombustible material is one of which no part will burn or ignite when subjected to fire or heat. Materials that pass ASTM E 136 are considered noncombustible. In addition, materials consisting of a structural base of noncombustible material with a surfacing material no more than ¹/₈-inch (3.2-millimeter) thick, which have a flame-spread index (FSI) of 50 or less, are also considered noncombustible. An example of the latter is fiberglass batts with Class A rated facings, such as foil-scrim-kraft (FSK) and poly-scrim-kraft (PSK).

Image courtesy of CertainTeed Insulation

This graphic shows the ignition temperature of insulation materials.

Section 602 of the IBC defines five construction types, each with different levels of fire-protection requirements and allowable use of combustible materials. Building Types I and II require noncombustible materials, such as steel, concrete, and masonry. Building Types III and IV require noncombustible construction for exterior walls; code-compliant wood construction can be used for interior walls. Type V buildings allow any code-compliant construction materials.

Not all insulation materials are noncombustible; in fact, only unfaced fiberglass and mineral wool products are naturally noncombustible. Some insulation products, including cellulose and spray foam, are treated with flame retardants to improve their performance in the face of fire; however, these materials will eventually ignite.

Fire resistance: This is another important concept that can be applied to either individual materials or assemblies, both of which can be tested and rated. A fire-resistance rating refers to the period of time a building element, component, or assembly maintains the ability to confine a fire, continues to perform a given structural function, or both.

Building codes specify minimum fire-resistance ratings for various building components, such as interior and exterior walls, roofs, doors, and structural framing. To be recognized by code as providing a fire-resistance rating, a material or assembly of materials must be tested. The intent of the test is to determine the number of minutes or hours in which the material or assembly can contain or limit the spread of fire and/or restrict the thermal transfer of heat from a fire source to the protected component. The loss of this thermal protection can result in the spread of the fire and associated heat, smoke, and toxic gases throughout the building, and it can ultimately result in failure of the building itself.

Because fire resistance is often considered for assemblies rather than individual components, the rating considers the performance of the several materials that are incorporated into a wall, floor, or roof. Fire-resistance directories, such as the one provided by UL, include listings for products used to provide various hourly fire-resistance ratings for all types of building elements. The UL directory (http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/fireressrch.html) divides protection systems into different categories, including floor-ceilings, roof-ceilings, beams, columns, walls, and partitions. Fiberglass, mineral wool, cellulose, and spray foam insulation materials can all contribute to fire-rated assemblies when installed as specified in the designs.

Flame spread: While fire-resistance ratings can be used to judge the ability to resist flame penetration into the building, they do not necessarily provide information regarding flame spread or the tendency of a material to burn rapidly and/or spread flames. Both ASTM E 84 and UL 723 measure this quality to characterize materials into one of three classes. You have no doubt encountered products with “Class A” or “Class 1” fire rating. This means that laboratory testing has a flame-developed index (FDI) of 25 or less. The building code uses this measure to address interior finishes, including walls, paneling, acoustical materials, and insulation. Requirements for this characteristic vary depending on building type and application. For example, Types I and II buildings require all insulation products and facings to have flame-spread and smoke-developed indices that do not exceed 25 and 450, respectively. It’s important to note that exposed insulation is considered an interior finish and has a different requirement than insulation that is enclosed in a cavity.

Testing to the Standards

Now let’s take a closer look at the most common tests used to evaluate insulation materials and building assemblies.

ASTM E 136: Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 Degrees Celsius (Determines Noncombustibility)

Building codes specify ASTM E 136 as the test required to establish a material as a noncombustible material. This test does not apply to laminated or coated materials, such as faced insulation batts. For this test, a tube furnace is preheated to a temperature of 750 degrees Celsius. A preweighed material specimen is lowered into the furnace with one thermocouple attached to its surface and another located at its center. Testing continues until both thermocouples have stabilized at a maximum reading or until one of the acceptance criteria is violated. A material is rated as either pass or fail; no numeric value is given.

ASTM E 119: Standard Test Method for Fire Tests of Building Construction and Materials (Measures Fire Resistance)

ASTM E 119 evaluates how long building elements can contain a fire and/or retain their structural integrity. For the test, an assembly or structural member is placed in a flat furnace in either the horizontal or vertical position. If the specimen is a load-bearing element, a specific load is imposed on it. The specimen is then subjected to a controlled flame introduced from one side of the assembly, simulating exposure conditions in the field. The temperature of the controlled flame is increased to a maximum along a specific time-temperature relationship that simulates a flashover condition. The test continues until one of the following takes place: structural collapse occurs, the temperature of the unexposed surface of the assembly exceeds 250 degrees Fahrenheit, or cotton waste placed on the unexposed side of the assembly ignites. The assembly is classified based on time expired before failure. For example, a 1-hour fire-resistant assembly will withstand fire exposure for 1 hour before the structural integrity of the wall fails.

ASTM E 84: Standard Method for Surface Burning Characteristics of Building Materials

Building codes specify ASTM E 84, sometimes referred to as the “flame-spread test,” to assess the contribution of surface finishes on walls and ceilings to fire loading. It utilizes the Steiner Tunnel Test to measure the propagation of flame from an ignition source along a specified length of the material, comparing the distance of propagation to reference materials. The result is expressed as a FSI number between zero and 100, with zero representing asbestos cement board and 100 representing red oak. Materials are divided into classes based on the FSI as follows:

Class 1 or Class A: 0 – 25
Class 2 or Class B: 26 – 75
Class 3 or Class C: 76 – 100

The lower the number, the less the flame spread. A product that has a flame spread of 25 has a surface-burning spread that is 25 percent of red oak. A test report for a product may also include the smoke-developed index (SDI), which measures the concentration of smoke produced by a material as it burns. The SDI is required to be less than 450 for all classes of materials. ASTM E 84 is also known as UL 723. NFPA 255: Standard Method of Test of Surface Burning Characteristics of Building Materials utilizes ASTM E 84.

The NFPA Life Safety Code and Section 803.1 of the IBC limit finishes for interior walls and ceilings to materials in these three classes and give greater restrictions for certain rooms.

NFPA 286: Standard Methods of Fire Tests for Evaluating Contribution of Wall and Ceiling Interior Finish to Room Fire Growth

The flame spread test is not reflective of real-world conditions, so NFPA developed a second test to better simulate conditions in an actual fire. NFPA 286 measures flame spread in a room configuration, including fire spread along walls, ceilings, and combinations of both. Though this method is preferred over NFPA 255, it is more expensive. Test results for heat, smoke, and combustion product release from NFPA 286 can be used in fire models for performance-based design, whereas results from NFPA 255 cannot.