Fire-Protective vs. Fire-Resistive Glazing: Radiant Heat, Tests and Ratings  

In designing safe building structures, architects have three methods for protecting against the loss of life and property due to fire. One is detection, which includes smoke and heat detectors as well as alarms. A second, suppression, encompasses sprinklers and fire extinguishers. The third, compartmentation, is uniquely architectural in nature. This method describes the use of material barriers and design layout to separate zones within a building from each other, preventing the migration of smoke and fire from one area to the next.

While it may seem counterintuitive, glass can be an effective material and system component for compartmental barriers. Properly specified and designed, glazing can successfully protect building occupants from fire, smoke and radiant-heat hazards.

Due to the increasing interest in preventive fire protection and the simultaneous growth in the use of architectural glazing, this subject is especially timely. Architects increasingly seek open, transparent, and communicating buildings, allowing more views and daylight for occupants. To serve this need, fire-resistive glazed partitions, doors and windows have become widely available over the last two decades.

Just as prevalent, however, have been misconceptions and misinformation about the proper specification and application of fire-rated glazing. For that reason, architects and their project teams seek better understanding of the types of fire-protective and fire-resistive glass; their system requirements, standards and codes; and their performance in actual fires. Also important is a good working knowledge of effective uses of fire-rated glazing-and the misuses, too.

Fire-Protective vs. Fire-Resistive Glazing

To help compartmentalize a building design, fire-rated glazing are available. There are a few general types of fire-rated glass, and each product in these categories has its own fire rating, measured by time. The time describes how long the product can meet the fire endurance test for either-fire protective or fire-resistive standards.

One important difference between "protective" and "resistive" glazing concerns radiant heat, a mechanism of fire and heat propagation. It is also a serious life-safety risk, and so is covered in detail below.

Fire-protective performance can be described by means of a Fire-Protection Rating. According to the Glass Association of North America (GANA), this is "the period of time that an opening protective assembly will maintain the ability to confine a fire as determined by tests." The tests include NFPA 252, NFPA 257, UL 9, UL 10c, ASTM E 2010, and ASTM E 2074.

This narrow ability to "confine" a fire is different from Fire-Resistance, which GANA defines as "that property of materials or their assemblies that prevents or retards the passage of excessive heat, hot gases or flames under conditions of use." It is described by means of a Fire-Resistance Rating: "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, as determined by tests," including NFPA 251, ASTM E 119, and UL 263 (wall assemblies).

In this context, fire protection means confining fire (smoke, flame and, to some degree, heat) for a period of time. Fire resistance means the qualities of a material that similarly confine fire, smoke and radiant heat, and at the same time maintain that material's structural characteristics.

The critical distinction in the performance of fire-rated glazing is the ability of glass to limit or control radiant heat-the often dangerous heat transmitted from the fire side of a building separation to the non-fire side. At critical intensities, this heat transferred through a construction assembly can cause harm to building occupants and the spontaneous ignition of building materials and contents-again, on the non-fire side.

To compare fire-protective and fire resistive products, the categories-which are significantly different from each other-are described in Section 8 of the Glass Association of North America (GANA) Glazing Manual, titled "Fire Rated Glazing Products."

Fire-Protective Glazing

Fire-protective glazing offer fire ratings of 20 minutes and up to 3 hours. The glass types are typically in thicknesses of 3/16" inch to 3/4" inch, and include such types as polished wired glass, specialty tempered glass, ceramic glass, and specialty laminated or filmed glass, either with wire or without and insulated glass units.

Traditional wired glass (non-safety) is limited in terms of its use to fire-rated and non-hazardous locations not requiring human impact safety. Wired glass fire ratings range from 20 to 90 minutes. Filmed or laminated ceramics offer fire ratings from 20 minutes to 3 hours, and when filmed or laminated, can meet the safety requirements for use in hazardous locations. Both unfilmed or unlaminated wired glass and ceramics have important limitations in terms of their use in locations that require protection for hazards associated with human impact. Also, wired glass and ceramics rated 60 minutes and over are restricted to 100 sq. inches in size due to radiant heat concerns.

Other fire protective glazing that carry a 20-minute rating include monolithic tempered glass impact safety rated to Cat. I and II and laminated glass impact safety rated to Cat. I with added radiant-heat protection. These 20-minute rated products may be used as vision panels in doors up to 9 square feet. Another type of fire protective glazing are radiant heat reducing specialty tempered monolithics fire rated from 45 to 60 minutes and impact safety rated to Cat. I and II. This product is used in doors, transoms, sidelites and openings under 60 minutes. Currently, this product requires the approval of authorities having jurisdiction (AHJ) because it is fire-tested without hose stream.

In general, fire-protective glazing is limited by code to 25 percent of a wall area due to radiant heat concerns. Other restrictions and exemptions on their use also apply.

Fire-Resistant Glazing

Glazing in the category fire-resistant or fire-resistive-the terms are used interchangeably -includes intumescent multiple-laminate and fire retardant-filled transparent units. The listings for the products classify them as "transparent walls" that are not restricted to 25 percent of a wall area when the framing system has a rating equal to the glazing. The fire-resistive class includes products also rated for acoustical performance, blast and bullet resistance, and hurricane protection.

Intumescent products are sandwiches of annealed glass containing special interlayers that expand and turn dark when heated. This quality helps contain heat and smoke from fire. Fire retardant-filled units have a clear semi-solid material in their cavities that crystallizes during a fire, helping to contain smoke and heat.

Protective vs. Resistive: The Big Picture

To summarize, fire-protective glass is not designed to block or stop radiant heat; fire-resistive glazing does. Fire-resistive glass is considered as a "wall," tested just like a brick, masonry, or gypsum board assembly. All glazing rated at 1 hour and over is fire-resistive, except wired glass and ceramic, which have ratings of up to 90 minutes and 3 hours, respectively. But because wired glass and ceramic products do not protect against radiant heat, they are defined as "fire protective."

The ratings required by the U.S. building codes are determined based on applications for fire-protective and fire-resistive constructions. The application requirements are based on time-the time to allow for safe egress and to maintain the structural integrity of the building.

So why does the distinction between fire-protective and fire-resistive matter? Because it's an important factor in ensuring occupant life safety.

Standards, Testing and Ratings

Glazing products receive fire ratings and listings from groups like Underwriters Laboratories and Intertek/Warnock-Hersey. The ratings are measured and awarded in terms of time, based on the how long an installed glass panel can reliably perform in an actual fire. Glass ratings may range from 20 minutes to 3 hours. Directly heating glass panel specimens in frames is one of the main tests. A second protocol called the "hose-stream test" (discussed in detail below) evaluates structural behavior at high temperature by means of pressurized water.

However clear it may seem, the distinction between fire-protective and fire-resistive glazing is not described in such a straightforward way by the subsequent ratings the products earn. And fire ratings do not always and fully describe the fire performance of a glass product.

Fire Resistance Testing

Heat testing is one of the main tests for fire-rated glazing and fire-resistive assemblies. For glazing, glass panels in frames are subject to heat that gradually approaches approximately 1,600 degrees F. For its rating, the glass must stay fixed in its frame for the length of time: 20 minutes, 45 minutes, and so on.

A second test, the "Hose-Stream Test" (see section below) is used to evaluate the material's structural behavior at high temperature. In this test, the glass panel is subjected to a high-pressure stream of water from a fixed hose. (Years ago, weights were used to impact the specimens.)

Glass may also be tested and receive ratings for other attributes, including impact force.

Fire resistance ratings for building materials and systems are referenced primarily to ASTM E 119, Standard Test Methods for Fire Tests of Building Construction & Materials, a version of which was first drafted in 1918 under a different name. The standard is used to test numerous building systems made of any combination of materials-such as steel, concrete, gypsum, wood and glass-for a variety of building assemblies: columns, beams, roof-ceiling systems, bearing walls and non-load-bearing partitions.

The standard measures the fire-resistive properties as expressed by time: "the period of resistance to standard exposure before the first critical point in behavior is observed."

A controlled fire-usually gas burners-is applied to the system or element. The heat produced follows a fixed time-and-temperature curve given in the standard. At first it rises quickly, and then it gradually increases-simulating the intense early stages of a fire. The test is configured for realistic condition, and the size of the specimen must meet a minimum.

The heat transmitted through the sample assembly is a critical measurement. If the average temperature in any area on the unexposed side rises more than 250 degrees F above ambient conditions on the same side, the test ends. After passing the specified ASTM E119 standard which includes the hose stream test, the specimen is given a fire-resistance classification, or rating based on that test. The ratings are expressed in hours, so that architects can compare building materials and systems and understand their applicability to building codes.

The Hose-Stream Test

One of the testing standards applied to the testing of fire-protective glazing and fire-resistive glazing is the "hose stream test," so named because water under pressure from a hose is used to test the building materials in question. For that reason, an understanding of the test and its significance is important.

The test has been a subject of debate almost since its inception. In the 1950s, the requirement for the hose-stream test was eliminated from the fire testing of floor/roof-ceiling systems. At the time, it was determined that "no record of failure in the hose-stream test" had been found due to holes in floors. In Europe and in other countries, the hose-stream test is not used. British officials eliminated the hose-stream test entirely from fire-testing standards in the early 1960s. But in the United States and Canada it is still used for fire-rated glazing.

The hose-stream test was created back in the late 1800s to test the integrity of cast iron and wrought iron structural systems. Because cast iron becomes brittle when heated by fire and fails as a result of thermal shock after being exposed to even small amounts of water, the first ASTM standard for fire performance-ASTM C19-1917T, now known as standard E119-included a hose stream test. However, according to a recent work summary published by ASTM subcommittee E05.11, no documented reasons were ever given for the inclusion of the hose stream test.

The test was NOT designed to assess how building systems would be affected by firefighting methods. (In fact, weights were originally used to impact the assembly, not water.)

Instead, the test offered a way to apply a consistent exposure to effects that cause failure, applied easily and evenly across the assembly. In this way, the test has been known to indicate two key characteristics:

• an assembly's integrity under fire exposure, and
• the integrity and reliability of materials to perform their intended functions.

For glass, the basic standard involves subjecting a sample of fire-rated glass to a test furnace and then an even hose stream across its surface. To pass the test, the glass should remain intact and not exceed the level of allowable through-openings.

Questions about the Hose Stream- for Fire-Protective Glazing Under 1 Hour

There is considerable debate about the test's value and reliability for predicting the safety of fire glass. First of all, it was designed for evaluating iron and other structurally supporting building members, not glazing. Second, European and British building agencies have not used the hose-stream approach for decades. Third, many codes officials, fire-protection engineers, and glass manufacturers have publicly questioned its efficacy in measuring glass fire safety.

Some observers of the hose-stream test point out that the test helps demonstrate the potentially early failure of glass when in areas with fire sprinklers. While this approach to testing for thermal shock resistance sounds reasonable, there is no evidence or reference in fire-testing standards or other building industry documentation. And the test is applied following a long exposure of the material to a very high temperature-45 minutes at approximately 1,600 degrees F. (Actual temperature at 45 minutes is 1638 degrees F. per NFPA test procedure.) In a real-world building, the sprinklers would activate after only a few minutes of fire.

Moreover, the hose-stream test was simply never intended as a way to rate glazing for thermal shock due to heat and fire sprinklers.

Glazing materials that fail the hose-stream test are not likely to fail during a fire when the sprinklers are activated. In fact, glass generally will resist failure as a result of water exposure from sprinklers during the first few minutes of a fire. Case reports and test data support this conclusion.

Code Considerations for Fire-Protective Glazing: "The 60-Minute Window"

Familiarity with the codes is critical, especially when architects are considering 60-minute fire-rated glazing. Many architects have assumed that 60-minute windows are accepted for use to protect openings as provided by codes, as long as they adhere to allowed uses and size and area limits. In fact, the building codes consistently limit windows to applications rated 45-minutes or less. According to the codes, there is no 60-minute window at all.

So the concept of "60-minute windows" and "90-minute windows"-though popularized in casual building design lexicon-does not exist. What does that mean? It means that when architects design and specify 60-minute and 120-minute fire-rated glazed assemblies, these constructions must be fire-resistive. That is, they must limit temperature and the risk of fire spread on the non-fire side of the wall.

For that reason, the design must limit risk by means other than merely the right choice of glazing. Safe separation distances, careful designs of openings and other design tools are what make the best fire-resistive assemblies-and the best compartmentalized, safest buildings.

Glazing Rated Under 60 Minutes: Key Concerns

There are other considerations that are vital for architects to understand before specifying glazing with fire ratings under 60 minutes.

For example, fire-rated glazing is permitted in sidelight and transom frames rated 45-minutes or less. Glazing are only rated for use in 60-minute assemblies if they are tested as per ASTM E119 as a wall assembly and under door-test standards. The wall test criteria in ASTM E119 includes temperature rise limits. This requirement applies to windows, sidelights and transoms.

This runs counter to the seemingly reasonable belief that if a 45-minute fire-protective rating is the code minimum, then a fire protective rating for 60 minutes must be that much better. However, fire-protection experts and engineers caution that this is not the case. The reason is radiant heat. During that extra 15 minutes, the uncontrolled transfer of radiant heat to the non-fire side of the wall can trigger combustion of flammable materials. This effect does not contain fire; it spreads fire.

Another difficult area related to 60-minute glazing is the question of glazed sidelights and transoms. In one-hour-rated doors and one-hour-rated occupancy walls, many architects believe that glazed sidelights are permitted as long as they meet size and area limits. However, they are not allowed in one-hour-door assemblies.

Listings for fire door frames-in UL, for example-refer to NFPA 80, which requires solid side panels and transoms in frames for assemblies with a rating of 45 minutes or more. This precludes glazed sidelights, although up to 100 square inches of glazing may be permitted in the doors for ratings of up to 90 minutes.

The 45-minute limit for the rating of window assemblies is also given in UBC: Standard No. 43-4, Section 43.301, which is referenced in Section 4306 (i). In NFPA 80, size and area limits are specified for lights in a window assembly rated 45 minutes, given in Chapter 13, Fire Windows. As long as size limits are followed, door assemblies rated 45 minutes could even include more than one pane of glass.

The question of the 60-minute window comes down to radiant heat transfer. For a 60-minute rating as a separated fire-resistive assembly, windows must be tested as per UBC as a wall assembly and meet the requirements for heat transmission. Most transparent glazing will not conform to this standard. Fire-resistive glazing does comply, however, and the specifier and designer should ensure that the 60-minute-rated glazing also meets the requirements for heat transmission.

Radiant Heat: A Matter of Life and Death

So far, we have discussed that fire-rated glass can be classified in two categories:

• Fire-Protective. This glazing can contain flames and flammable gases for short periods, but it will not block heat transfer to the other side of the glass. This category includes wire glass and ceramics. There are fire-protective glazing materials, such as tempered products and laminated products, which can reduce the transfer of radiant heat, but do not meet ASTM E119 and therefore still classified as fire protective. This type of glazing is limited to 25 percent of the wall area due to radiant heat concerns and have fire ratings under 60 minutes (except for wire glass and ceramics, which are rated up to 90 minutes and 3 hours respectively, but limited to 100 square inches in size).
• Fire-Resistive. This type of glazing generally contains flames and flammable gas for longer periods of time and also blocks the transmission of heat through the glass. This type of glazing is rated over 60 minutes and is not subject to wall limitations when combined with an equally rated fire resistive framing system.

Fire-resistive products protect against radiant heat, which is a significant effect of fire events. It is also a poorly understood but critical consideration in specifying glazing. The reason is that fire can pass enough heat through glass and other materials to cause combustion on the non-fire side. This effect is called "non-piloted auto iginition"-or, less technically, spontaneous combustion. The mechanism of heat transfer may be called flux, measured in kilowatts at a standard distance from the non-fire side of the wall or separation.

The transmission of radiant heat occurs through infrared radiation. These electromagnetic waves only carry energy-not temperature-and move in a straight line. When radiant energy is absorbed by matter and converted into heat, the result is fire.

Case histories have shown the benefits of using glass that limits radiant heat transfer to contain fires within interior spaces as well as fires in adjacent structures, brush and trees.

To illustrate this point, independent radiant heat flux testing was conducted on three 45-60 minute fire-protective glazing materials-wire glass, ceramics, and specialty tempered. Using ASTM E119 test standards, mannequins simulating people passing through an exit corridor were placed 3 feet from the glass samples. To get a feel for radiant heat measurements, lower levels of about 5 kilowatts per square meter, are enough to cause serious injury to humans, and spontaneous combustion of wood caused by radiated heat happens at 12 kilowatts per square meter. At 5 minutes, wire glass was at 6.59 kW/square meter, ceramics was at 5.37 kW/square meter and the specialty tempered was at 3.91 kW/square meter and did not reach 5 kW/square meter until approximately 9 minutes.

What the test showed was in a situation where building occupants would have to pass through the non-fire side of an exit corridor glazed with fire protective glass, specialty tempered products can provide up to 5 minutes of additional egress time before it reaches levels of unbearable human pain, making the corridor impassable.

The same ASTM E119 test was conducted on fire resistive glazing materials rated 60 minutes and over. The multilayer laminates and fire retardant-filled intumescents effectively limited the temperature rise to 250 degrees F (as required by ASTM E119) and radiant heat flux to 0 kW/square meter, making these materials safe for use in wall applications.

Where the aim is to protect lives and property, or where combustible materials or contents are located near walls with glazed areas, the architect must be mindful of the risks of radiant heat. When left uncontrolled due to wall design, it can leave egress paths impassable during fire events. For those reasons, architects must carefully consider such interior layout parameters as: corridor width; how quickly occupants will pass by glazed areas; and the amount and type of flammable materials located outside of the spaces with glazing.

In these critical life-safety locations, specifying the safe use of fire-rated glazing requires an understanding of how much radiated energy passes through specific glass products.

To block radiant heat, glass generally must be thick, insulated, or comprise multiple layers -or have a combination of these characteristics. Performance in terms of radiant heat will vary depending on the product design and materials. Generally:

• Some laminated and specialty tempered products can limit radiant heat to some degree, but not enough to pass ASTM E119.
• Multiple-layer laminates and fire retardant filled units are effective in combating radiant heat transfer and pass ASTM E119.

For example, insulated products rated at 60 minutes or more according to the standard ASTM E119 will limit temperature rise to 250 degrees F and reduce radiant heat flux to 0 kilowatts per square meter. Other products are available for use in windows and doors that control radiated flux to one kilowatt per square meter at 60 minutes.

Many glazing products associated with safety applications may not limit radiant heat flux at all. For example, another independent test showed that wired glass allows a radiant heat flux of up to 35 kilowatts per square meter at 37 minutes. At 60 minutes, ceramic glass allows a radiant heat flux of 75 kilowatts per square meter.

How can architects make the best choice? Unfortunately, listings do not always provide information on radiant heat. Codes offer some guidance. For example, because of the dangers of radiant heat, U.S. building codes prohibit the use of "non-temperature rise" window applications rated more than 45 minutes. But other codes and standards are less clear. The limits on glazing size and area in NFPA 80, Standard for Fire Doors and Fire Windows, do not take into consideration radiant-heat hazards. In its Appendix J, however, it does warn that radiant heat should be a factor when selecting large-area glazing materials.

Labeled ratings may provide information on radiant-heat performance, but caution is important for the architect. Labeling for products that do not protect against radiant heat is often indistinguishable from those that do protect against radiant heat. For example, glazing products rated at 60 minutes and above may not necessarily be designed to protect against radiant heat.

For more descriptive information, savvy architects consult technical data sheets and other product literature, which often specifies product performance in terms of radiant heat transfer. Manufacturers will plainly affirm the limitation in the case that their 60-minute glass products do not control radiant heat.