ST&D: Security, Technology, and Design Reprint

Window Film:

The Most Cost-Effective Means to Protect Existing Windows

By Darrell L. Smith

The return on investment for security and facility managers can be high with window film

Existing glass in buildings is generally not designed to resist the stress caused by wind blown debris, earthquakes, gunfire, explosions and terrorist attacks. Subject to such forces, existing glass often breaks into lethal, daggerlike glass shards that fly into a building and strike occupants and damage property or is hurled outward from the window frame, endangering passers-by. Broken glass also causes substantial property damage. The explosion of a bomb, plastic explosive, hand grenade or natural gas creates a shock wave. If the explosion is sufficient, glass may become atomized. As the shock wave causes victims to gasp for breath, they breath in atomized glass particles, often resulting in death.

Security window film can improve the ability of existing glass to mitigate the impact of explosive force, windblown debris and seismic stress. The primary function of security film is to hold the broken glass shards intact. If glass with security window film breaks, the film holds the glass shards together, preventing them from becoming lethal projectiles. The glass will shatter but may remain unbroken in its original frame.

The August 1998 bombings at the American embassies in Nairobi, Kenya, and Dar es Salaam, Tanzania, killed 280 and injured more than 5,000 people. The bombs that destroyed these structures, like the one that brought down the Alfred P. Murrah building in the Oklahoma City bombing of 1995, were of such strength that no window system would have been able to survive. However, the broken glass in adjacent buildings that injured thousands of people may not have done so had the windows in those buildings been protected by security window film.

According to a January 1996 article in Public Works, the bombing of the Murrah Federal Building “underscores… the most significant threat to people and property in bombings arises from the failures of conventional glass.”

Broken glass in the June 25, 1996 terrorist bombing of Khobar Towers, a housing facility at the U.S. Air Force base in Dhahran, Saudi Arabia, resulted in more than 330 injuries and 19 fatalities. According to a report by the Secretary of Defense, 80 to 90 percent of those injuries were caused by broken glass.

Government Takes Action

In response to the Oklahoma City bombing, in June 1995 President Clinton issued Presidential Decision Directive 39, the blueprint for U.S. counterterrorism strategy. A variety of presidential directives, interagency agreements and legislation provide the framework to combat terrorism in more than 40 federal agencies.

In July 1996, the President signed an executive order to develop a national strategy to protect the country’s critical infrastructures from a spectrum of threats, including terrorism. National Defense Authorization Act for Fiscal Year 1997 and the Omnibus Consolidated Appropriations Act of that year also funded anti-terrorist activities. Congress and the White House have nearly tripled the FBI’s counterterrorism budget since 1994.

FBI Director Louis Freeh warned of the increasing danger from terrorist attacks in testimony before the Senate Judiciary Committee. “Terrorism, both international and domestic, threatens us like never before,” he contended. According to U.S. News & World Report, December 29, 1997, 900 terrorist incidents in the US were investigated in 1997, compared to 100 in 1995.

The General Services Administration (GSA), the federal agency responsible for managing federally owned buildings, has identified 8,400 buildings as vulnerable to terrorist attacks. Attention is being paid to decreasing the ability of existing glass to injure building occupants in event of an explosion by increasing the use of security film and other security glazing products.

Security Window Film Defined

Security window film is comprised of either optically clear, tinted or reflective layers of polyester film (from four mils to 15 mils thick) that can be adhered to the interior surface of existing glass. Typical film installations cover the visible portion of the interior surface of the glass all the way to the edge of the frame. This is known as a “daylight” application.

When properly installed, security film forms an invisible protective coating over the glass. Because security window film has the ability to stretch before tearing, it can absorb a significant explosive shock. As this 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 and stretches until it reaches the point that it can no longer bear the pressure, at which time it will burst.

While strong enough to break the glass, the shock wave may not be strong enough to shear the security film. This results in glass being broken but being held intact by the film. Not only are there reduced injuries, but there is also little damage to the property inside the building. If the shock wave is sufficient to break the glass and shear the film, often the glass collapses attached to the security film with minimal damage and injuries. In multi-story buildings, security film also may prevent glass from falling out of its frames to the street below, especially if it is anchored to the window frame.

Building Codes and Performance Tests

Almost all building codes do not require glass to be able to resist windblown debris, forced entry, seismic or explosive force. Only since Hurricane Andrew in 1992 have several counties in south Florida and Texas amended their building codes to require windows in new construction to withstand hurricane force winds without permitting flying debris to pierce the glass.

Revising building codes for glass performance depends on testing the ability of glass and film to withstand explosive and other forms of stress. Tests of the ability of glass and window film to withstand windblown debris and seismic stress have been conducted by several government agencies. Since the Oklahoma City bombing, the GSA and the Department of Defense continue to conduct tests of glass and window film to measure such materials’ ability to withstand explosive force.

The Protective Glazing Association, comprising the International Window Film Association, the Glass Association of North America, AIMCAL and individual building product manufacturers, works with the government on formulating appropriate blast-resistant standards for windows. The object of such testing is to develop performance standards for the ability of glass to mitigate the impact of explosions, resist windblown debris and absorb seismic stress.

It is important to not that results of such tests are not necessarily available to the public for several reasons: 1) To maintain security, government agencies may not identify desired and attainable levels of glass resistance to explosive and other forces. 2) Tests conducted by industry associations are considered confidential for fear of revealing proprietary information to the manufacturers of a competing product. 3) Tests conducted by individual manufacturers often are considered confidential for fear of revealing proprietary information to competing manufacturers of similar products or for fear of compromising the security of purchasers of the product.

Evidence in Support of Security Film

There is evidence that the application of security window film mitigates the impact of terrorist explosions. A January 8, 1996 government vulnerability assessment of Khobar Towers stated, “A blast from a car bomb or other device would shatter windows sending shrapnel into quarters and offices throughout the affected buildings.” The assessment specifically called for the installation of security film on all perimeter glass at Khobar. The subsequent June 25, 1996 bombing of Khobar Towers resulted in 19 deaths, 12 of which were due in large part to glass fragmentation according to the assessment.

The Spradling & Biddinger Real Estate office Oklahoma City is six blocks from the site of the Murrah Federal Building. S&P’s office contained windows with and without film. The windows without film were shattered by the April 19, 1995 explosion and were blown into the office. The windows with the window film did not shatter but fell intact outside on a patio causing no injury or property damage. In a 1996 letter to the Glass Research and Testing Institute, Spencer Brown, director, physical security division with the U.S. Bureau of Diplomatic Security, stated, “Foreign governments that have had the most experience with car bombs, Britian and Israel, use window film to enhance the protection of their government buildings.”

Buildings Protected by Security Film

Several significant government buildings benefit from the protection of security window film. More than 3,700 windows at FBI headquarters and 2,700 windows at the Pentagon are protected by security film. Other government facilities in Washington relying on security film include the U.S. Information Agency headquarters, the Federal Emergency Management Agency headquarters and the Drug Enforcement Agency.

Though all can agree government buildings need protection, it is not difficult to imagine significant commercial and retail structures that might be targeted by terrorists. Corporate headquarters buildings benefiting from security window film include IBM, Chase Manhattan and British Petroleum. Buildings near likely high-risk targets will incur glass breakage for many surrounding blocks, e.g., 10 blocks in Nairobi and six blocks in Oklahoma City.

Although many criticize the General Services Administration for not moving more quickly to apply security window film to the thousands of government buildings under its jurisdiction, security managers of commercial and other non-government facilities need not be impeded by political infighting over limited federal budgets. The question for decision makers interested in enhancing window security is how to choose between security window film and laminated glass– two or more pieces of glass bonded by a polyvinyl butyral plastic interlayer. Laminated glass is often the logical option for new construction but is seldom able to be justified as a replacement for existing glass. A cost benefit analysis is helpful to understand why.

The cost of laminated glass installed is approximately $20 a square foot. The cost of security window film applied to the interior surface of existing glass is approximately $3 to $4 a square foot. Installing 20,000 square feet of laminated glass in a multiple-story building would cost $400,000. The amount of film needed to cover 20,000 square feet of existing glass would cost about $60,000 for the complete installation. A significant measure of extra protection can be provided by attaching the film to the window frame. Attachment mechanisms such as securing bars, edge-to-edge dry lamination techniques, and proprietary frames and sealants increase the cost.

Several additional factors must be considered to determine the true cost of laminated glass compared to security window film. The appropriate security window film not only will provide blast protection, but also may reduce a building’s energy consumption. This is especially true if the existing glass is not energy efficient. Laminated glass might provide equivalent blast protection but fail to be as energy efficient, thereby not providing savings on reduced energy usage.

Security window film is quickly and easily installed with minimal disruption to building occupants. The cost of disruptions to building occupants in removing existing glass and possibly original window frames also should be considered. The return on investment of security/solar control window film can be as high as three years or less depending on the type of film installed. For most existing buildings security window film is the option of choice in enhancing a structure’s ability to protect occupants and withstand damage from explosions, windblown debris and seismic stress.

Darrell L. Smith is manager, Window Film Committee, Association of Industrial Metallizers, Coaters and Laminators (AIMCAL), an organization representing major window film manufacturers. Smith is also executive director of the International Window Film Association, which represents window film installers and dealers.