Fire-rated glass: Balancing curtain wall performance and design

July 13, 2022

Photos courtesy SAFTI FIRST

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.

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).<span style

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.

After the fire endurance test, the glazing specimen is subjected to a high-pressure water stream from 6 m (20 ft) away at a level of 206 kPa (30 psi). <span style

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.

This private office features a two-hour curtain wall with dynamic testing.

Fire-protective glass

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.

Fire-resistive glass

During the fi re endurance test, thermocouples are placed on the non-fi re side of the glass to measure the radiant heat transmitted through it. The average temperature of the radiant heat calculated cannot exceed 139 C (282 F) above the initial starting temperature.<span style

Fire-resistive glass is tested to CAN/ULC S101, Standard Methods of Fire Endurance Tests of Building Construction and Materials, and is designed to compartmentalize smoke and flames and limit radiant heat transmission. Unlike fire-protective glass, there are no size or application restrictions for fire-resistive glass. When paired with an equally rated fire-resistive framing system, it can be used in wall-to-wall and floor-to-ceiling applications because it is considered a “transparent wall,” allowing for maximum clear views.

Fire-resistive glass is marked with a W for “walls,” and the fire endurance rating is also on the label. It may look like a window or an opening, but the code recognizes this type of material as a wall. It can be used in place of gypsum or masonry where a one- or two-hour fire resistance rating is required. But, unlike gypsum or masonry walls, fire-resistive glazing is transparent—giving additional benefits such as vision, daylight, and direct lines of sight.

Examples of fire-resistive glass include fire-resistive tempered glass and multilaminate glass. During a fire, the intumescent interlayers expand and react to the heat to form a solid wall, effectively containing smoke and flames and significantly limiting the transmission of radiant heat. This gives building occupants a safe path of egress or a haven where they can await rescue.

The one-hour curtain wall on this high-rise includes glazing which is high performance and energy efficient.

National Building Code requirements on exterior fire-rated walls

The NBC provides guidelines on the application of fire-rated glazing for exterior applications. Here are some useful sections:

Section, Determination of Ratings

1) Except as permitted by Sentence (2) and articles and, the rating of a material, assembly of materials, or structural member that is required to have a fire resistance rating, shall be determined on the basis of the results of tests conducted in conformance with CAN/ULC S101.

 Section, Exception for Exterior Walls

1) The limit in the rise of temperature on the unexposed surface of an assembly as required by the tests referred to in Sentence (1) shall not apply to an exterior wall that has a limiting distance of 1.2 m or more, provided correction is made for radiation from the unexposed surface in accordance with Section (9).[7]

When it comes to exterior applications, spatial separation requirements exist to limit the spread of fire from one building to the next. The City of Saskatoon issued a useful bulletin on the determination of spatial separation requirements:

“Requirements are determined through the application of Subsections 9.10.14 and 9.10.15 for buildings falling within the scope of Part 9 of the NBC, and Subsection 3.2.3 for buildings within the scope of Part 3. Buildings are permitted to have a proportion of openings in an exterior [wall] based on limiting distance and size of the exposing building face. The closer an exposing building face is to a property line or the face of another building, the higher the fire-resistance rating required for that building face and the more stringent the construction requirements for that exterior wall.”

In the same bulletin, limiting distance is defined as “the distance from an exposing building face to a property line, the centre line of a street, lane or public thoroughfare, or to an imaginary line between two buildings or fire compartments in the same property, measured as right angles to the exposing building face[8].” Limiting distance covers all materials used in the exterior—including fire-resistive glass.

Since non-rated glazing does not protect against the spread of fire from one building to the next, the NBC imposes limitations on its use. Section, Limiting Distance and Area of Unprotected Openings, outlines the allowable number of doors, windows, and other openings forming the part of the exposing building face which have a fire resistance rating less than what is required. Section has four tables designers can refer to in determining the area of unprotected openings on the exposing building face based on limiting distance: Table and for Unprotected Opening Limits for a Building or Fire Compartment that is Not Sprinklered Throughout; and Table and for Unprotected Opening Limits for a Building or Fire Compartment that is Sprinklered Throughout.

In addition, Table for Maximum Concentrated Area of Unprotected Openings provides some guidance with regards to the size of individual panels:




Maximum Area of Individual Unprotected Openings (m2)
1.2 0.35
1.5 0.78
2.0 1.88


For all these tables, no unprotected openings are allowed at all if the limiting distance is less than 1.2 m (3.9 ft). Further, as the limiting distance increases, so does the allowable area of unprotected openings.

How can these sections of the NBC apply to fire-rated glass? Going back to the exception for exterior walls in Section, building materials or assemblies tested to CAN/ULC S101 are not required for exterior walls if the limiting distance is 1.2 m or more. It then follows that if the limiting distance is less than 1.2 m, architects would have to use building materials or assemblies tested to CAN/ULC S101. Therefore, designers can use fire-resistive glazing tested to CAN/ULC S101 if the limiting distance is less than 1.2 m. Designers can also use fire-resistive glazing to increase both the area of the glazing and the size of the individual glazing panels because it is not subjected to the same area and panel size limitations as unprotected openings.

It is important to note, when it comes to building materials of assemblies tested to CAN/ULC S101, the building code does not distinguish between opaque and transparent building materials. For example, gypsum board, masonry, and fire-resistive glazing all meet CAN/ULC S101 and can be used when the limiting distance is less than 1.2 m. However, only fire-resistive glazing provides vision, transparency, and daylighting—making it the ideal building product for curtain walls in close proximity to property lines.

Beyond fire performance

When fire-rated glass is used as part of a building’s exterior curtain wall, it is expected to perform all the same functions of the adjacent non-rated system. Luckily, today’s high-performance glazing has the ability to do so.

Dynamic curtain wall testing

Static curtain wall testing performed in a chamber is one way to determine a product’s ability to prevent air and water from entering the building. However, the information from this test is limited. This why dynamic curtain wall testing designed to duplicate real-world conditions is preferred by owners, architects, and building envelope consultants.

Today’s advanced fire-resistive curtain wall systems are tested to American Architectural Manufacturers Association (AAMA) 501.1, Water Penetration of Windows, Curtain Walls, and Doors Using Dynamic Pressure, and American Society for Testing and Materials (ASTM) E284, Standard Terminology of Appearance, and E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. In addition to air and water testing, these systems are also tested to AAMA 501.4, Recommended Static Test Method for Evaluating Window Wall, Curtain Wall, and Storefront Systems Subjected to Seismic and Wind-Induced Inter-Story Drift; ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights, and Curtain Walls by Uniform Static Air Pressure Difference; inter-storey vertical displacement; thermal cycling; and condensation evaluation.

Thermal performance

Occupant comfort is a key factor when selecting glazing products for the building envelope. While glass gives the benefit of natural daylight, controlling heat and glare are important considerations as well. Today’s advanced fire-rated glass products can easily incorporate low-emissivity (low-e) or any energy performance glazing. Designers can also find fire-rated glass and framing components in the National Fenestration Rating Council’s (NFRC’s) Component Modeling Approach Software Tool (CMAST) database, enabling them to simulate the U-factor, solar heat gain coefficient (SHGC), and visual light transmission (VLT) of exterior fire-rated glazing assemblies.

Multitasking protection against other threats

Advanced fire-rated glazing systems can multitask to provide additional protection against various environmental and manmade threats. Fire-rated glazing manufacturers have tested and listed systems which also meet:

However, it should be noted the U.S. standards listed are not included in Canadian codes and standards.


With the architectural community pushing the limits of what fire-rated glass can do, manufacturers will continue to respond with innovative products to fulfil their design and performance needs.


[9]Diana San Diego has more than 15 years of experience in the architectural glazing industry and over 17 years of experience in public relations and marketing. As the vice-president of marketing at SAFTI FIRST—a manufacturer of fire-rated glass and framing systems—she oversees the advertising, content management, media relations, promotional activities, and communication initiatives for the company. San Diego is also involved in creating and promoting SAFTI FIRST’s various educational programs, including “Code Considerations in Fire Rated Glass,” “Designing with Fire Rated Glass,” and “Innovative Design Applications with Advanced Fire Rated Glazing Technology,” which are registered with the American Institute of Architects (AIA), the International Code Council (ICC), and the Architectural Institute of British Columbia (AIBC).

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