CBP Magazine: Articles

Fire safety as a critical part of building design contributes to more sustainable buildings and greater occupant safety.

One of the most important factors in building design is using materials that will make the structure as fire-resistive as possible. Throughout our series on building science, we have covered a full range of topics, from heat transfer to acoustic-control techniques. In our seventh installment of the series, we will show how to design buildings for optimum fire safety. We'll cover building codes; industry fire resistance testing for materials; fire-resistive assemblies; and the evaluation of joints, penetrations, and wraps.

Fire safety begins with a well thought out building design, and for good reason. According to the National Fire Protection Association (NFPA), Quincy, MA, there are structural fires every 61 sec. throughout the year in the U.S., on average. Smoke from building fires accounts for 73% of fire-related deaths. Building codes require that many building products and assemblies resist flame spread and smoke development for extended periods of time. We begin with a closer look at some of these building codes.

Life safety is the principle reason for fire codes. It all goes back to the great Chicago fire of 1871, which left hundreds dead and a large part of the city destroyed. Over the years, fire-safety concerns have evolved, and today all building codes require that building materials and systems be evaluated for fire performance. Two national fire codes are the Int'l Fire Code (IFC) and the NFPA's National Fire Codes. An historical example of a regional code is the Uniform Building Code (UBC). In addition, there are local codes issued by municipalities all across the U.S. and Canada.

In general, codes attempt to establish minimum safety requirements. Building codes are based on at least three criteria: type of occupancy, intended use, and type of construction. Building materials are evaluated and rated for their flammability, especially interior finish materials. Construction and components are rated for combustibility, and assemblies are rated for fire resistance.

The Int'l Building Code (IBC) classifies construction into five different building types. Types 1 and 2 require only non-combustible building materials in their assemblies, including iron, steel, concrete, and masonry. Types 3 and 4 require non-combustible exterior walls and code-permitted interior walls. Type 5 allows exterior walls and interior walls to be made of any code-permitted materials. Various industry tests are used to determine the fire-resistance levels of building materials.

Objective fire testing includes temperature, ignition times, rate of temperature increase, flame propagation, rate of burning, and smoke generation and properties. Subjective observations by experienced testers also play a part in some of the tests.

Materials don't just burn-they burn at different rates and produce different kinds of smoke, so there are several test methods that measure these properties, such as flash- and self-ignition times and temperatures. Testers evaluate smoke for density, and how much it obscures the view. The heat- and smoke-release rates are an indication of the fuel content of the test samples. Combustion gas properties are also tested. Test equipment to evaluate combustion properties includes tube furnaces and oxygen-consumption calorimeters, which can determine the chemical composition of smoke.

Various fire test standards for all types of building materials and assemblies include:

The chart in Figure 1 shows how building materials are categorized by class, and how test results are shown for each material tested by flame spread index and smoke developed index, by class. Return air plenums are their own class for testing purposes, as these assemblies can quickly carry superheated air, flames, or smoke from room to room.

Now that we have an understanding of the tests used to measure the fire safety of building materials, we can show how they fit into building code requirements. For example, here's a rundown of building code requirements as they relate to fiberglass insulation. Building Types 1 and 2 require a flame spread index of 25 or less, and a smoke developed index of 450 or less. Types 3 through 5 have similar requirements, but allow combustible insulation facings, which are then covered by code-approved finishing materials. Return air plenums require a flame spread index of 25 or less, and a smoke developed index of 50 or less.

There are other test methods to evaluate surfaces for flammability. All surfaces in interior spaces must be tested. Materials in this category include suspended and finished ceiling systems, wall coverings and coatings, exposed attic insulation (generally found in residential or light-commercial construction), roof coverings, and exposed foam plastics. Surface flammability tests measure and rate materials for flame spread, rate of burning, ignition point, and smoke generation. Some tests use a radiant heat source instead of a flame to ignite and burn the test samples.

Exposed finished surfaces on materials, such as ceiling tile, gypsum board, and wall coverings and coatings, are tested using ASTM E 162. In this test, materials are exposed to a radiant heat source to measure ignition time, burning rate, and smoke generation. This test also determines whether or not materials emit flammable gasses when exposed to high temperatures.

Roof coverings have their own testing methods, such as ASTM E 108 to measure surface-burning characteristics. This test simulates a fire originating outside the building, such as a fire at an adjacent building. The roof surface material is subjected to flames and wind to spread the flames, and burning embers are dropped onto the surface to mimic real fire conditions. This is predominantly a test conducted by trained observers who watch the test and evaluate videotapes of the testing. It is used to measure surface burning, flame spread, and the material's cohesiveness under fire conditions.

ASTM E 603 mimics a fire in a wastebasket in the corner of a room, and is used to test materials, products, or complete assemblies. This test observes the fire over time, from ignition to flashover, into the next room or space. This measures temperatures over time, smoke generation, time to flashover, combustion gas, and total energy-release rates. Referred to as the "room corner test," it involves a corner of a room constructed in an accredited testing facility, using standard building materials.

Materials to be tested are mounted on the walls and ceiling of the room corner. A fire source-either a wood crib or a diffusion burner-is placed in the corner and ignited. The test mimics a wastebasket-sized fire that spreads to something nearby. To perform satisfactorily, the tested material must not spread the fire or generate excessive amounts of smoke.

ASTM E 119 establishes the effectiveness of a building assembly to act as a fire barrier over time. These are large components, including walls, floor-ceiling assemblies, roofs, and windows and doors. In this case, a wall assembly is tested for structural integrity and the ability to contain a fire. The assembly is mounted to a specially constructed furnace. Gas burners are lit as thermocouples record temperatures, and the flames mimic heat from an adjacent fire. Observations are made through viewing windows in the furnace and with instrumentation. Temperatures and time to system failure are recorded. A hose stream test follows to measure the assembly's resistance to water pressure.

ASTM E 119 uses a furnace-heating schedule (see Figure 2), a timed increase of temperature, which brings the furnace up to 1000 deg. in 5 min., up to 1700 deg. in 1 hr., and to 1850 deg. in 2 hr. Assemblies must survive these temperatures to be successfully fire rated.

Other standards cover the evaluation of joint systems and penetrations. ASTM E 1966 is one standard for testing fire-resistive joint systems and includes sealants, coatings, and materials used in joints. ASTM E 814 includes testing wall and floor penetrations, which are sealed with fire-stop materials. Typical materials used as fire stops through penetrations are stone wool and specialized caulk. Finally, there are test methods for wrapping systems. ASTM E 1725 measures how well electrical systems and components resist fire. ASTM E 2336 evaluates the fire safety of fire-resistive grease duct enclosure systems.

Fire safety is a very important part of design, as it contributes to more sustainable buildings and greater occupant safety. Though it is often difficult to prevent fires at their sources, ensuring that building materials have good fire ratings and meet building code requirements is the best way to hinder the spread of fires and minimize losses.

Stanley D. Gatland II is the manager of building science technology for CertainTeed Corp.'s Valley Forge, PA, Insulation Group. He is responsible for generating and providing technical information to architects, engineers, builders, trade contractors, building-envelope consultants, building scientists, and building-code officials on the system performance of new and existing building-envelope materials, as well as building-science educational training. He has expertise in the areas of building science and architectural acoustics. He is a graduate of the Univ. of Massachusetts, Amherst, with BS and MS degrees in mechanical engineering. He is a member of ASHRAE, ASTM, ASME, and BETEC.

Editor's note: This print series is based on the CertainTeed video series "Commercial Building Science: Concepts and Practices."

Building Design for Fire Safety