By Diana San Diego
Curtain walls, particularly exterior glass curtain walls, are part of the building envelope designed to protect the interior from outdoor elements. They are preferred by designers because they are typically lightweight—usually made with aluminum and glass—while keeping the building airy and watertight. Today’s fire-resistive framing is a combination of steel framing and aluminum cover caps, to give it a decorative look. It also has aluminum covers which can be anodized or painted to match non-fire-rated framing.
The emergence of energy-efficient glazing has also made it possible to let natural light in while controlling heat and glare. This is why in the last decade or so, there have been curtain wall systems where glazing is the preferred infill compared to opaque panels. Today’s advanced glazing has really good energy performance and can be incorporated into many different systems. This can include high-performance coating, photovoltaics, vacuum glass, and triple-insulated glass. It is not as opaque as sheet rock or gypsum, but the ability to harness natural daylight while keeping out heat and glare also contributes to overall energy performance.
This trend has carried over to fire-rated glazing as well. Advancements in fire-rated glazing technology have paved the way for exterior fire-rated glazing applications which go beyond typical punched openings and into fully engineered, multifunctional curtain wall systems. These advanced fire-rated glazing systems can perform like the rest of the materials in the building envelope and even match the esthetic of adjacent non-rated glazing systems.
Using fire-rated glass in a building’s exterior has the same rationale as using any fire-resistive building material in the exterior—to prevent the spread of fire from one building to another.
Understanding fire-rated glazing testing requirements
The discussion of fire-rated glazing in curtain walls or other exterior applications starts with an understanding of what fire-rated glazing is. All fire-rated glass products used in the field must be tested, listed, and currently under the follow-up service of a nationally recognized testing laboratory. Manufacturers of fire-rated glass products hire these labs to test their products to certain standards and durations. If the product passes, it is given a listing.
Fire endurance test
The fire endurance test determines the time a glazing product can withstand fire and extreme heat, with temperatures reaching in excess of 1000 C (1832 F). During this test, the prime source of heat—in this case, the test furnace—follows a fixed time and temperature curve designed to stimulate a fire, where the temperature rises quickly, then gradually continues to increase. If the glass remains in the frame for the duration of the test, it is certified with an endurance rating, ranging from 20 minutes to three hours.
Hose stream test
After the fire endurance test, the glazing test specimen is subjected to a high-pressure water stream from 6 m (20 ft) away at 206 kPa (30 psi). All fire-rated glazing applications in Canada require a hose stream test.
Radiant heat test
During the fire endurance test, thermocouples are placed on the surface of the glass on the non-fire side to measure the radiant heat transmitted through the glass. The average temperature of the radiant heat calculated from these readings cannot exceed 139 C (282 F) above the initial starting temperature—even when the temperature on the fire side reaches more than 1000 C at two hours.
Understanding fire-protective versus fire-resistive glazing
Following the discussion of the different ways fire-rated glass is tested, it is easier to understand the two types of fire-rated glass products. Knowing the difference between the two will help project teams choose the correct and code-approved product for their application.
Fire-protective glass is tested to Underwriters Laboratories of Canada (CAN/ULC) S104, Standard Method for Fire Tests of Door Assemblies, and S106, Standard Method for Fire Tests of Window and Glass Block Assemblies, and is designed to compartmentalize smoke and flames—not radiant heat. Therefore, it is subject to application, area, and size limitations under the National Building Code of Canada (NBC). Fire-protective glass is typically used in doors and openings up to 45 minutes and cannot exceed 25 per cent of the total wall area, or as doorlites in 60- and 90-minute doors but limited to 0.0645 m2 (0.694 sf).
Fire-protective glass is etched with either a D for “door” or an O for “openings.” Sometimes an H marking is added to show the fire-protective glass product meets hose stream. The fire endurance rating is also indicated on the label.
Examples of fire-protective glass include fire wire glass, ceramics, and specialty fire-protective glass. Of all these options, ceramics are the most expensive, with laminated ceramics costing as much as $100 per square foot.