Heat transfer and fire-rated glazing
Confusion reigns when it comes to fire-rated glass, its technical properties and performance. Though I’ve been the North American marketing and advertising manager for Vetrotech Saint-Gobain Corp. of Auburn, Wash., for more than a year now, I still get “glazed” trying to figure them out. The criteria run on endlessly: model codes, test standards, wired glass, intumescents, ceramics, tempered, laminated, fire-resistive, fire-protective. What is a wall? What is a window? Navigating to product selection can be exhausting.
Keep in mind we’re talking life safety here. Manufacturers and distributors of fire-rated glass products must bring clarity to marketing materials to reduce the potential for error.
Heat transfer emerges at the top of the list of topics that require detailed public discussion. Responsible manufacturers and distributors must hold the position that products with no heat-blocking abilities need to be limited in size and placement.
Hows and whys of heat transfer
Consider the three types of heat transfer. First, in “conducted heat transfer,” direct heat flows through matter. An example of conducted heat includes the hot surface of a stove that transfers heat to a cooking pot. Fire-resistive walls and doors are designed to protect against this risk.
Next, in “convected heat transfer,” the transport of heat within a gas or liquid might be caused by the actual flow of the material itself, such as heat traveling upward with the natural upward movement of air.
Lastly, in “radiant heat transfer,” the transmission of electromagnetic radiation occurs in the infrared wavelength of the spectrum through space. These waves are invisible, have no temperature, only energy, and travel in a straight line. Fire emits intensity levels of radiant energy upon striking an object; radiant energy is absorbed and converted into heat.
I recently witnessed the damages that heat transfer through glazing can cause.
During a company retreat in Whistler, British Columbia, last summer, my family and I left our hotel room for roughly 90 minutes to have dinner. Upon our return, we were greeted by the smell of smoke. Thankfully, I found the source almost immediately. The glass-face fireplace—most likely ceramic glass—had been turned on by housekeepers when they turned down our bedding, and a cushion that had been leaning against the glass was smoldering. That’s when I realized the power of heat and its ability to start fires. Had we not come back when we did, the building might have been engulfed in flames. While no lives were lost, our clothes and personal items were saturated with smoke and the room was no longer habitable. Add the cost of our upgrade to a suite and the cost to remove the smoke smell from the room, and a simple accident turned relatively costly.
Blessing and curse of ceramic glass
When it comes to fire-rated glass, ceramics now dominate the market. The physical properties of ceramics aren’t affected by heat. Regular soda-lime-silica glass expands, while glass-ceramics have virtually a zero coefficient of expansion and an extremely high softening point. This means that they resist thermal shock from rapid heating and maintain their integrity for hours. There are three ceramic fire-rated glass product lines in the U.S. market: Vetro-tech Saint-Gobain’s SGG Keralite FR, FireLite by Technical Glass Products Inc. of Kirkland, Wash., and Pyran Star-Crystal by Germany’s Schott AG.
Combining the core properties of ceramic and its ability to allow heat to freely pass permit the development of many great products. These include fireplace inserts and everyday personal-care and kitchen items as well as the cook-top surface– all sharing the beneficial ability to transfer heat. To understand the power of glass ceramics better, refer to the following dictionary definition of ceramic in relation to cook tops: “Glass ceramic is mechanically a very strong material and can sustain repeated and quick temperature changes up to 800-to-1,000 degrees Centigrade. At the same time, it has a very low heat-conduction coefficient and can be made nearly transparent for radiation in the infrared wavelengths.”
If used correctly, fire-rated glass-ceramic is a wonderful product. One example of correct use would be as a replacement for polished wired glass in 45-minute rated fire-window applications. The FireLite line by TGP, Pyran Star-Crystal by Schott and Vetrotech Saint-Gobain’s SGG Keralite line are such products. However, they do not block radiant heat, eexcept for SGG Keralite Ultra. Any of these products can be found with labeling up to 90 minutes.
Common sense needs to be applied when considering ceramics for large openings in areas of human contact and everyday combustibles. Ceramics meet the technical aspects of the standard test method—longer minute ratings, listings with Intertek/ETL Semko of Madison, Wis., or Underwriters Laboratories of Northbrook, Ill.—but even great products have their shortcomings.
A radiant downside of ceramic glass
Radiant heat is the shortcoming of ceramic glass. Developments in recent years with ceramic glass offered us the ability to reshape the importance of fire-rated glass with increased visual aesthetics. But there has been a trade-out in the process: the larger the piece of ceramic glass and the longer the time rating, the more potential a real fire can inflict damage. All the parties involved in selling, specifying, installing and approving the larger ceramic lites in areas of human egress need be concerned and aware of the escalating hidden danger of radiant heat. Minimalists will argue that radiant heat is not required for consideration by the current code, but they ignore common sense. Such applications do not bode well for safety. Furthermore, following a worse-case scenario, their logic would only strengthen a courtroom argument that the product used would contribute to the eventual spread of a fire.
My second experience with heat transfer was well-planned, as Vetrotech Saint-Gobain researchers set out to prove the true power of radiant heat through glazing. In September 2005, I witnessed a fire test at the International Fire Testing Services laboratory in Switzerland.
Fire tests are not inexpensive. We staged 60-minute tests to show the effects of radiant heat because we wanted to know just how much the “more is less” theory could affect the ability of ceramic glass to block radiant heat. A school-oriented setting was created on each side of a wall, as shown in the photo at left. We placed the same items—coats, books, desk, chairs, wastepaper baskets—on each side. For the test, we compared two of our own glass products to see how they would perform. The first was SGG Contraflam-N2 60, a 1-inch intumescent laminated product that blocks radiant heat, and the commonly used 3⁄16-inch ceramic product SGG Keralite FR-R, rated up to 90 minutes.
This test was far from a boring lab exercise. We went in knowing the products would withstand test-furnace temperatures to 1,700 F and remain intact for their specific rating period, and indeed, during the entire 60 minutes, the intumescent SGG Contraflam remained virtually unchanged. Note that the opaque visual of the intumescent product results from its normal reaction: At around the sixth minute, the first intumescent layer reaches the boiling point of water, 212 F, causing bubbles to form as it works to absorb and block the heat.
On the glass-ceramic SGG Keralite side, the story was different. At 17 minutes into the test, a radio in the room had deformed considerably. By 27 minutes, the room’s nylon curtain completely melted, the carpet caught fire, the radio was even more deformed and the coat began melting. Smoke filled the area. At approximately 45 minutes, all contents were damaged and the bookshelf completely engulfed. An effect known as “non-piloted auto ignition,” or spontaneous combustion, occurred with several items and materials. Simply put, the power of the radiant energy, without a pilot flame to ignite it, caused fire to spread from one compartment to another. At approximately 60 minutes, the test was stopped. There was virtually nothing left on the glass-ceramic side, and it was impossible to approach within several yards in front of the test assembly. Amazingly, on the other side of the wall protected by the intumescent product, a person was still able to sit down near the wall and not be harmed.
The test was performed at International Fire Testing & Services Ltd., in Bern, Switzerland, on equipment calibrated to ISO 17025 standards and to testing methods accepted by Underwriters Laboratory and Intertek. Among the witnesses was Hans van de Weijgert of International Fire Consultants Ltd., widely considered to be the leading worldwide expert on radiant heat transmission through glazing.
Hans’ test report concludes: “The test has demonstrated that the radiant heat emitted from a window frame with dimensions 1415 by 3950 millimeters, glazed with Contraflam 60-N2 glass pane, is sufficiently low not to cause non-piloted (auto) ignition of combustibles on the unexposed side of the glazing, when exposed to NFPA 257 fire conditions. The radiant heat emitted by a window frame with dimensions 1415 by 3950 millimeters, glazed with Keralite ceramic glass panels, is sufficiently high to cause non-piloted (auto) ignition of combustibles on the unexposed side after a heating time of 27 minutes when exposed to National Fire Rated Protection Association fire conditions.”
Ceramic fire-rated glass allows most of the energy of fire to pass through almost unrestricted, exactly like the popular cook-top surface, and the fireplace glass in my hotel room. Human beings exiting a burning building could either face extreme heat or burning debris in a structure “protected” by ceramic fire-rated glass.
Comparative tests also were performed on polished wired glass against Vetrotech’s SGG Keralite Ultra to measure radiant-heat properties. The ceramic allowed roughly 10 percent more radiant heat flux than wired glass. Perhaps the most telling story about radiant heat can be found in the chart at right.
The data represent radiant heat flux in kilowatts per square meter measured at 1-meter from each of the glass products tested and time of exposure to fire measured in minutes.
To put the chart in perspective, consider:
0.7 kilowatts—the maximum amount of solar radiation in Miami Beach on a summer afternoon
15 kilowatts—results in second-degree burns after five seconds of exposure, and can cause hardwood to ignite after prolonged exposure
30 kilowatts—results in unbearable pain after two seconds.
The critical intensity level of 15 kilowatts per square meter is attained at about 25 minutes for both wired glass and glass ceramic, and at 45-minutes more than 20 kW/m2 is recorded. When pushed to 60 and 90 minutes, respectively, the glass-ceramic records 26 kW/m2 and 32 kW/m2.
Approved ratings, the key
Architects use similar products to the ones we tested in 45-minute fire-protection windows typically installed within 60-minute fire-resistive walls. These approved glass ratings lay at the heart of the radiant-heat matter. While architects and specifiers often have a wide-held belief that 60- and 90-minute windows exist, there is no such thing. Simply put, 60- and 120-minute fire-rated glazed assemblies require “fire-resistive” performance that limits temperature on the non-fire side, protects occupants and limits the risk of fire spreading through glazed openings. The principles of limiting risk through openings and the creation of “safe separation distances” remain fundamental to fire-resistive building-design practice, and the basis for modern model building codes. However, architects, specifiers, fabricators, glazing contractors and glass-shop owners alike often either do not understand or ignore these principles.
On the one hand, in 45-minute fire-protective openings, glass ceramics can provide an effective alternative to polished wired glass where, due to an area limitation of 25 percent of a wall area, the risk of fire spread and danger of escaping occupants through heat radiation is limited. Based on the results of the fire test, glass-ceramic products such as SGG Keralite FR, FireLite, and Pyran fall into this category. In addition, intumescent products such as Pyrobel by InterEdge Technologies LLC of Sausalito, Calif., Superlite II-XL by Safti First of San Francisco, Pyrostop by Pilkington in Germany, and SGG Swissflam by Vetrotech are excellent choices to reduce radiant heat in 45-minute openings near human contact or reduce chance of auto-ignition combustibles.
On the other hand, openings in walls calling for fire-resistive assemblies rated at 60 minutes and greater require intumescent-type products that can block conducted, convected and radiant heat to satisfy temperature rise limitation requirements. These products also allow the designer to glaze more than 25 percent of a wall area. Products that meet 60- and 120-minute requirements include Vetrotech’s SGG Contraflam, SGG Swissflam, Pyrobel, Pyrostop and Superlite II-XL.
Beware of the technicalities
You might be wondering why ceramic products are listed as 60 and 90 minutes. Simply put, these are technicalities. The ratings were listed based on impact and fire resistance for window openings, despite the fact that no model building codes allow them. From a sales and marketing standpoint, the concept that a product with a higher time rating must be a superior product is logical. In reality, the same lites of ceramic glass that meet the 45-minute rating also meet the 90-minute rating. It’s conceivable to have a 180- or 240-minute rating with the same piece of glass. The ceramic products were not listed with the intent to move them into fire-rated walls, but the opportunity for applying the same principles for windows to walls was too hard to resist. Unfortunately, the danger of radiant heat was overlooked.
As manufacturers, installers, specifiers, building officials and purchasers of fire-rated glass, we have a responsibility to consider the dangers of heat transmission as it affects fire-resistive building construction and the safe egress of human occupants. Replacing one problem with another provides no progress toward creating safe environments in public and private buildings alike. To protect human life, both impact-resistance and heat transmission need to be fully embraced by glaziers and architects.
In short, when used in areas of direct human contact, ceramic fire-rated glazing products replace the potential that wired glass might have to cut with the potential to cook, and introduces another danger into the mix.