Protecting Your School Exterior with Security Window Film
By: Marty Watts
Conventional window glass was not designed to resist windblown debris, earthquakes, explosions, and terrorist attacks. Subject to such stresses, existing glass often breaks into lethal shards to be hurled from the window frame, endangering school building occupants and passersby. Broken glass causes 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 lethal flying projectiles. In some cases, the glass may shatter but remain intact.
The 1998 bombings at the embassies in Kenya and Tanzania injured more than 5,000 people, many due to broken glass. Broken glass in the 1996 terrorist bombing of Khobar Towers at the U.S. Air Force base in Saudi Arabia resulted in more than 330 injuries, 80 to 90% of which were caused by broken glass.
Obviously, the destruction of the World Trade Center was of such magnitude that no window system would have been able to survive. However, the broken glass in adjacent buildings may not have occurred had those windows been equipped with security window film.
Glass Damage from Natural Disasters
Broken glass also results from natural disasters such as the Northridge and Kobe, Japan earthquakes and Hurricane Katrina.
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. In earthquakes of significant magnitude, thousands of panes of glass can be broken.
The insurance industry has adopted a grim new phrase — mega catastrophe —one in which insured losses exceed $1 billion. Before 1990, there were no mega catastrophes. Since then, there have been seven. Of the 25 largest insured catastrophes in the U.S., 21 have occurred in the last decade. Global warming indicates volatile weather may cause more large-scale property damage and glass-related fatalities and injuries.
How Broken Glass Causes Injury and Death
Most injury from glass is caused by walking into a pane of glass. When broken, glass falls, causing injuries to the knee and upper leg. The next most likely area of injury is to the head, neck, and shoulders. If not fatal, these injuries can lead to severe damage to tendons and loss of limbs or limb function.
A hurricane can project an object through a window, causing dagger-like glass shards to strike occupants. An explosion of a bomb creates a shock wave that causes glass to break into lethal projectiles. If the explosion is sufficient, glass may become atomized. As the explosive shock wave causes victims to gasp, they breathe in atomized glass particles, often resulting in death.
Glass penetrating the body assumes the color of the bodily organ. Because glass particles cannot be detected by X-rays, emergency room physicians have difficulty finding glass inside the body cavity.
How to Make Window Glass Safer
Windows transmit light and enable building occupants to see outside. On the first floor of school buildings, windows facilitate being seen from the outside. On a building’s south exposure, windows generate passive solar energy.
Typical window performance problems include unacceptable air infiltration, poor insulating capability, the 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 a polyvinyl butyral plastic interlayer. Compared to conventional glass, laminated glass can provide increased resistance to windblown debris, seismic and explosive force.
Security Window Film
Security window film is the alternative to replacing existing glass with laminated glass. Security window film is either 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 crack and breaks. However, the security film applied to the rear of the glass continues to 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, is not 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 with minimal damage and injuries. In multi-story 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.
Independent tests verify that many security window films provide equivalent (or in some cases, superior) ability to withstand stress compared to laminated glass.
Also, laminated glass is not as energy-efficient as other glass options, resulting in a tradeoff between energy and safety/security performance. Its composition and resistance to force impedes the ability to break laminated glass for emergency entrance or egress.
Cost of Laminated Glass vs. Security Window Film
The cost of laminated glass installed is approximately $20 a square foot. The cost of conventional security window film applied to the interior surface of existing glass is approximately $5 to $6 a square foot, though the most energy-efficient security films cost from $10 to $13 a square foot installed. A system to physically attach the film to the window frames would cost approximately $6 to $16 a square foot.
The optimum security window film not only provides increased protection from stress, but it also may reduce a building’s energy consumption by blocking solar heat. The cost of disruptions to school 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.
Marty Watts is president and chief executive officer of V-Kool, Inc., a sales and marketing distribution company of security and spectrally selective applied films for architectural, automotive, and specialized vehicular applications, www.v-kool-usa.com.
