Photo © Robert Brayton
Insulating glass
The non-insulating fire-protective-rated glazing products mentioned above will not reduce heat radiation emitted by a fire or satisfy fire wall insulation criteria for more than a few minutes. If the glass has to comply with the full fire wall criteria of integrity and insulation, as it may if it is installed in a compartment wall, then a clear multi-ply insulating glass—either intumescent laminated or gel glass—is required.
Intumescent laminated glazing consists of layers of float glass interleaved with clear hydrated sodium silicate intumescent material. Increasing the number of intumescent layers within the glass increases the performance to a higher fire rating, resulting in overall glass thicknesses from 19 to 51 mm (¾ to 2 in.) and greater. When heated, the exposed layer of glass cracks, activating the first layer of intumescent. With time, the exposed intumescent is degraded by the hot gases in the fire, and the heat is conducted through the opaque layer, causing the next layer of glass to crack and the second layer of intumescent to be activated.
This procedure continues until there are no more layers of intumescent to activate. One by one, the intumescent interlayers turn opaque and expand to form a fully insulating heat shield, thus reducing the transmission of radiated and conducted heat through the glass for as much as 180 minutes in some cases. Another benefit of the opaque insulating interlayer is it blocks the view of the fire, which minimizes panic as occupants exit the building.
Intumescent gel glazings consist of two or more sheets of toughened safety glass. The cavity between the glass sheets is filled with a transparent gel that crystallizes into an opaque heat-absorbing char when exposed to fire. The protective interlayer is based on nanotechnology; it also offers high light transmission and can meet the demands of ‘impact-safe materials’ required for areas where there are people.
Photo © Brent Drury
Intumescent gaskets
When specifying fire-rated glazing materials, one must consider not only the glass itself, but also the entire glazing system of frame and gaskets as tested by the manufacturer. Gaskets and seals introduced to restrict the passage of noise and odors could generate significant volumes of smoke if incorrectly specified and could also compromise both the noncombustibility and zero surface spread of flame characteristics of the glass. As the fire temperature rises and the glazing system is put under stress, the hot gases will readily exploit a poorly designed glazing detail. An intumescent-based glazing seal can ensure the natural attributes of the glass are maintained by preserving the integrity of the glazing pocket. Intumescent seals can expand to accommodate any deterioration in the glazing pocket and, therefore, play a significant role in achieving fire safety.
The case against wired glass
Wired glass gives the illusion of safety, but in reality, it is weaker than nonwired glass due to the incursions of the wire into the crystalline structure of the glass. Wire mesh glass also can be dangerous—if a person strikes the glass with enough force to puncture through it, the wire amplifies the irregularity of the fractures, making injuries more severe. In fact, wired glass has been banned in many jurisdictions in the United States, and recently lost its safety designation
in Canada.
Wired glass is still well-rated to withstand both heat and hose stream tests. The wire prevents the glass from falling out of the frame even if it cracks under heat stress, impeding flames and smoke from passing through and spreading throughout the building quickly. However, unless modified to achieve a higher impact safety rating required by building code regulations, wired glass is limited in size and to areas and applications away from people in any type of building.
Innovations in glazing technology, such as ceramic and intumescent laminated glazing, have dramatically changed the industry and brought numerous benefits over wired glass, including an expanded field of use with longer fire ratings and larger openings, application-adapted performance, and safer alternatives in the case of accidental human impact.
