Conventional window glass was not designed to resist windblown debris, earthquakes, explosions or terrorist attacks.
Subjected to such stresses, existing glass can break into shards that can endanger building occupants and passers-by. Broken glass can also cause property damage that would not have occurred had the glass remained in its frame.
Security window film can improve the ability of existing glass to mitigate the impact of explosive force and windblown debris. The primary function of security film is to hold glass intact in the event of it being broken, preventing glass from becoming flying projectiles. In some cases, the glass may shatter but remain intact.
Hurricanes and tornadoes produce intense winds, which create damage and injury from flying glass. Earthquakes twist or flex the glass. The intensity of the earthquake will determine whether the glass breaks.
Most injuries from glass are caused by walking into panes of glass. A hurricane can project an object through a window causing shards to strike occupants. The explosion of a bomb creates a shock wave that causes glass to break into lethal projectiles.
How to make window glass safer Windows transmit light and enable building occupants to see outside. In storefronts, windows facilitate objects being seen from the outside. Windows can also generate passive solar energy.
Typical window performance problems include unacceptable air infiltration, poor insulating capability, inability to block solar heat, the transmission of ultraviolet radiation and noise and vulnerability to electronic eavesdropping. Security enhancements to glass become more economically feasible if they do not impede, but actually improve energy and other window performance capabilities.
Existing glass can be replaced with laminated glass, two or more pieces of glass bonded by an interlayer. Compared to conventional glass, laminated glass can provide increased resistance to windblown debris, and seismic and explosive forces.
Security window film Security window film is the alternative to replacing existing glass with laminated glass. Security window film can be optically clear, tinted, or reflective layers of polyester film applied to the interior surface of existing glass. Typical film installations cover the visible portion of the surface of the glass all the way to the edge of the frame, but do not extend to the glass edge within the frame.
Film can be applied to both single-pane and many types of insulating glass. Proper application of appropriate film to insulating glass does not impact the integrity of an insulating glass sealant or generate thermal stress to glass from uneven heat absorption. Applied security window film is available with and without solar control capabilities.
Because security window film has the ability to stretch without tearing, it can absorb a significant degree of the shock wave of an explosion. As this explosive force moves toward the glass and pushes it inwards, the glass eventually cracks and breaks. However, the security film applied to the glass can absorb the shock wave, stretching until it can no longer bear the pressure, at which time it bursts.
The shock wave, when great enough to break the glass, may not be enough to shear the film. This results in the glass being broken, but held intact by the film. In these cases, not only are there no injuries, but there is no damage in the building. In other cases, the shock wave breaks the glass and shears the film. The glass collapses, attached to the security film, causing minimal damage and injuries. In multistory buildings, security film may also prevent glass from falling to the street below.
Security window film vs. laminated glass Both laminated glass and security window film may mitigate the impact of explosions, windblown debris and earthquakes. The performance of both depends on the relationship of each to the existing window frames.
In the case of laminated glass, the window frame must support the weight and thickness of the glass for the total glass and window system to resist stress. Installing laminated glass in existing window frames that are not designed to support the weight of laminated glass may not prevent the glass separating from the frames when the glass is stressed.
Similarly, the ability of security window film to resist force may increase if the film is not only applied to the glass but attached to the frame. Many window film manufacturers market film-attachment mechanisms to secure film to the window frame.
Also, laminated glass is not as energy-efficient as other glass options, resulting in a trade-off between energy and safety/security performance. Its composition and resistance to force impede the ability to break laminated glass for emergency entrance or egress.
Cost of laminated glass vs. security window film The optimum security window film not only provides increased protection from stress; it may reduce a building's energy consumption by blocking solar heat. The cost of disruptions to building occupants in removing and replacing existing glass compared to applying security window film to existing glass also needs to be taken into account when comparing laminated glass and security film.
What to look for when evaluating a security film
Price – The true value of the film is determined by its independently verified performance in terms of mitigating explosive and other forces, life expectancy and energy efficiency. More expensive films should perform better than less expensive films.
Energy benefits – The film's ability to block solar heat and its ability to transmit desirable daylight should be reviewed. Most security films that block heat also block significant amounts of light.
Aesthetic considerations – The ideal security film provides optimum security and energy performance without changing the appearance of the building. Clear, colorless security film is applicable on all or selective windows of a building.
Marty Watts is with V-Kool, Inc.
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The Society of Fire Protection Engineers (SFPE) was established in 1950 and incorporated as an independent organization in 1971. It is the professional society representing those practicing the field of fire protection engineering. The Society has over 4,600 members and 100 chapters, including 21 student chapters worldwide.