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Security Film Catches Broken Glass

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Security window film protects building occupants from flying glass generated by explosions, storms, and accidental breakage.

 

By Marty Watts, V-Kool Inc.

 

Conventional window glass was not designed to resist wind-blown debris, earthquakes, or explosions. When subjected to such stresses, glass often breaks into lethal shards that shoot from the window frame endangering building occupants and passers by. Broken glass causes property damage that would not have occurred had the glass remained in its frame.

When security window film is applied to glass it prevents glass shards from becoming potentially dangerous flying objects in the event of violent glass breakage. Properly designed film will transmit sufficient light and block heat.

Security window film can improve the ability of existing glass to mitigate the impact of explosive force and wind-blown 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.

How to make window glass safer

Windows transmit light and enable building occupants to see outside. In storefronts, windows facilitate being seen from the outside. On a buildings south exposure windows generate passive solar energy.

Typical window performance problems include unacceptable air infiltration, poor insulating capability, inability to block solar heat, 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 properties.

One way to improve window safety and security is to replace existing glass with laminated glasstwo or more pieces of glass bonded by an interlayer. Compared with conventional glass, laminated glass can provide increased resistance to wind-blown debris, seismic stresses, and explosive forces.

Another approach is to use security window film. The film can be 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 glass surface all the way to the edge of the frame but do not extend to the glass edge within the frame.

The film can be applied to both single-pane and many types of insulating glass. Proper application of the appropriate film to insulating glass does not compromise 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.

What makes good security film?

Consider the following when choosing 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.

Review not only the films ability to block solar heat but also its ability to transmit desirable daylight. Most security films that block heat also block significant amounts of light. Look for security film that provides optimum security and optimum energy performance.

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.

Because security window film has the ability to stretch without tearing, it can absorb a significant amount of the shock wave of an explosion. As the explosive force moves toward the glass and pushes it inward, the glass eventually cracks and breaks. However, the security film applied to the rear of the glass continues to absorb the shock wave. The shock wave, while 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, wind-blown 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 from separating from the frames when it 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 with laminated glass.

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 impedes the ability to break laminated glass for emergency entrance or egress.

The cost of laminated glass installed is approximately $20/sq. ft. The cost of conventional security window film, applied to the interior surface of existing glass, is $5 to $6/sq. ft. The most energy efficient security films cost from $10 to $13/sq. ft., installed. A system to physically attach the film to the window frames would cost approximately $6 to $16/sq. ft.

The optimum security window film provides increased protection from stress and may reduce a buildings energy consumption by blocking solar heat. The cost of disruptions to building occupants in removing and replacing existing glass compared with 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 CEO of V-Kool Inc., Houston, a sales and marketing distributor of spectrally selective applied films.