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

Improving building safety through the use of fire-rated glazing
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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.

When Safety Glass Can Be Unsafe

Ceramic Glass
Due to its material composition and heat treatment during fabrication, the thermal expansion of ceramic glass is reduced, limiting breakage and producing a much higher by softening point than for typical annealed glass. On the other hand, ceramics are more prone to mechanical stresses which can occur during the movement of building assemblies during fires.

Glass ceramics can't be thermally toughened, either. So although ceramic glass may not fail due to fast changes in temperature, movement that occurs in early stages of fires may cause purely mechanical failure.

Cost is another factor with ceramic glass. A 45-minute, 5/16-inch ceramic may cost as much as four times as much as a 1/4-inch alternative non-ceramic.

Some ceramic glass products have listings of 60 minutes and 90 minutes based on testing for impact and fire resistance for window openings. Yet model building codes do not allow this application, and the ceramics were not listed so that they would be considered for fire-rated walls.

Their performance in terms of radiant heat limits their use in fire-resistive construction. For example, when tested at 60 minutes, ceramics allow a radiant heat flux of about 75 kilowatts per square meter.

The 60-minute and 90-minute listings have caused some confusion about the use and application of ceramics in fire-rated constructions.

Traditional Wired Glass (Non-safety)
Unlike heat-shielding glass products, wired glass reacts to sudden changes in temperature during fires by cracking and breaking. The wire inside the glass, however, is meant to hold the broken pieces in place to protect against injuries.

Wire glass does not protect against radiant heat, however. At 37 minutes, wired glass allows radiated flux on the unexposed side of more than 35 kilowatts per square meter. Its heat transmittance is so high that certain codes do not allow its use near emergency exits.

Although wired glass passes the test standard for fire-rated vision panels, the Consumer Product Safety Commission (CPSC) does not classify the material as a safety glazing. But foreign wired-glass products may be sold in the United States under a quarter-century-old exemption to the CPSC rules. The exempted wired glazing is rated according to a less rigorous ANSI standard, ANSI Z97.1, which the CPSC has ruled will only protect building occupants under five years of age.

Due to this and other safety concerns, the 2003 IBC and NFPA 5000 codes eliminated the use of wired glass in the construction of all educational and athletic facilities.

Despite these developments, few architects and building code officials are aware of the CPSC ruling or the exemption. Wired glass is the only glazing materials with such an exemption from federal consumer safety regulations.

SAFTI FIRST has been serving the architectural and building communities as a premier source for fully code-approved fire rated glazing and framing for over 25 years. Having introduced the first transparent fire rated wall system to the USA, SAFTI FIRST continues to develop products that meet the technological and code demands of the construction industry.

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Originally published in Architectural Record.
Originally published in December 2006

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