The Weakest Link in the Chain: Glass Vulnerabilities and The Active Shooter

As a consultant focused on active shooter risk, there are a number of critical problems that appear as recurring themes during facility assessments—improper use of mag locks, egress obstructions, deficient communication systems, weak emergency response plans, and so on. But when asked to rank issues for purposes of prioritization and cost-benefit analysis, my number one concern is almost always the presence of tempered glass glazing in doors, windows, and indoor partition walls.

Tempered glass is typically employed as safety glass in architectural design. It is made by heating regular glass to a very high temperature and then cooling it rapidly. This process, known as tempering, results in a glass that is stronger and more durable than regular glass and when shattered, breaks into small, rounded pieces rather than sharp, jagged shards.

Although this tempering process produces a safer and more impact resistant glass, tempered glass provides only 4-5 times the impact resistance of annealed glass and offers minimal protection against forced intrusion. Tests documented by Sandia National Laboratories show that 0.25 inch tempered glass delays intruders using a fire axe for only 3-9 seconds, and the mean delay time for penetrating 1/8 inch tempered glass with a hammer is 0.5 minutes.[1]

However, this data is of little value when assessing vulnerability against forced entry by a gunman. The tests documented by Sandia only focused on penetration using hand tools and did not account for the fragility of tempered glass after initial penetration by a firearm projectile. In our penetration tests of 1/4-inch tempered glass windows using a 9mm handgun to penetrate glazing prior to impact by hand, the delay time was only 10 seconds.[2] What happens in this situation is once the glass is penetrated by a bullet, it critically fractures into a spiderweb of cracks and disintegrates.

This critical vulnerability was witnessed in both the 2012 Sandy Hook Elementary shooting and recent attack at The Covenant School in Nashville. As captured in CCTV footage, the tempered glass entrance doors at The Covenant School were completely shattered after impact by several rounds of gunfire resulting in 20 seconds of total penetration time for both the exterior doors and inner vestibule.

Security Glazing and Reinforcement Options

In situations where tempered glass presents a vulnerability concern, we recommend replacement or upgrade with an intrusion-resistant alternative such as laminated glass, polycarbonate, or reinforcing the existing windows with properly-anchored security window (anti-shatter) film.

Laminated Glass

In new construction, laminated glass is the preferred material when budget permits. Laminated glass is a composite material made by sandwiching a layer of polyvinyl butyral (PVB) or other interlayer material between two or more layers of glass. The layers of glass are bonded together under heat and pressure to create a single, laminated pane.

Sandia’s test data shows that 1/4-inch laminated glass delays forced entry by a fire axe for 18-54 seconds, and the mean delay time for penetrating 9/16-inch laminated security glass with hand tools is approximately 1.5 minutes.[1] [2] Although I do not have test data regarding the penetration of laminated glass by a gunman, the construction properties of laminated glass suggest the delay times are considerably higher as the framing would likely fail before the glass-composite material breaks into an aperture.

Most glazing products rated under forced entry standards, such as UL 972 and EN 356, are made of laminated glass.


Polycarbonate is another option. At thinner dimensions, polycarbonate provides decent impact resistance but comparable performance to tempered glass against fire axe attacks. Polycarbonate truly distinguishes its benefit at thicknesses of 1/2-inch or greater. According to tests documented by the Nuclear Security Systems Directorate, 1/2-inch polycarbonate can delay hand tool penetration for up to two minutes.[3] Sandia also cites 2-6 minutes of delay for penetration of polycarbonate by a fire axe and sledgehammer.[4]

Polycarbonate is relatively inexpensive and can be purchased as sheets, cut to size, and edged with sandpaper for retrofitting door vision panels and similar applications. UV-coated polycarbonate can also be useful for constructing intrusion-resistant panels for protecting the inside of windows that cannot be replaced or upgraded with the use of other materials, such stained glass windows in churches or original wood-framed windows in historical buildings.

The main drawbacks of polycarbonate are it’s susceptibility to scratch damage and degradation from UV exposure.

Security Window Film

If budget does not allow for replacing existing tempered glass glazing, security window film (“anti-shatter film”) properly attached and anchored to tempered or annealed glass may be a cost-effective alternative. In 2015, our firm conducted penetration tests of 1/4-inch tempered glass windows with mechanically-attached 14 mil window film using a firearm followed by impact with kicking and rifle buttstock. The resulting delay times ranged from 62 to 94 seconds.

In the following video, Larry (one of my penetration testers) demonstrates the difference in penetration times between unprotected tempered glass and a film-reinforced window. I personally witnessed and timed these tests.

Play Video

If security window film is chosen as an upgrade for existing tempered glass windows, I strongly encourage using a multi-ply film no less than 14 mils in thickness and from a reputable manufacturer. When asked for a brand recommendation, I prefer Solar Gard due to my familiarity with their quality controls and experience testing their products as an independent third-party.

As an important point, specifications should require mechanical or wet glaze attachment to ensure that the film is securely anchored at the frame. The following images illustrate what is meant by mechanical and wet glaze frame attachment.

If security window film is not securely attached to the window frame using a film anchoring device (second mechanical frame) or wet glaze sealant, the film membrane will adhere to the shattered glass as one unit while the window breaks at the edges of the frame. This type of failure compromises any delay benefit from window film. The results of this type of improper installation can be witnessed by viewing videos of the Capitol Hill riot where film-protected windows provided less than 10 seconds of delay after tearing at the edge when kicked by rioters.

I also recommend careful due diligence when selecting a vendor for installation of security window film. I have seen numerous examples over the past fifteen years where organizations spent major sums amounts of money on film upgrades with limited benefit and/or service life due to improper and shoddy installation.

Following are some useful tips for questioning vendors and spotting poor quality installation from one of the top experts on the subject, Scott McCutcheon of Emerald Coast Glass Protection.

    • Bubbles and Wrinkles – One of the most obvious signs of a bad security window film installation is the presence of bubbles and wrinkles in the film. This can be a sign of poor adhesion or improper installation, which can compromise the effectiveness of the film. If you notice bubbles or wrinkles in your security window film, it’s important to address them as soon as possible to prevent further issues.
    •  Gaps and Overlapping Edges – Another sign of poor security film installation is the presence of gaps or overlapping edges in the film. This can indicate that the film was not properly cut or sized for the window, which can compromise the effectiveness of the film’s security features. It’s important to ensure that the film is properly sized and cut before installation to prevent these issues.
    • Inadequate Adhesion – A security window film that is not properly adhered to the window can also be a sign of a bad installation. Inadequate adhesion can cause the film to peel or lift, which can compromise its effectiveness and potentially create a safety hazard. It’s important to ensure that the film is properly adhered to the window to prevent these issues.
    • Poor Quality Film – Not all security window films are created equal, and a poor quality film can also be a sign of a bad installation. Low-quality films may not offer the same level of security as higher quality films, and they may be more prone to bubbling, peeling, and other issues. It’s important to choose a high-quality film from a reputable manufacturer to ensure that you are getting the best possible protection for your property.
    • Lack of Professional Installation – A bad security window film installation may simply be the result of an inexperienced or unqualified installer. It’s important to choose a professional installer who has experience with security window film installations to ensure that the job is done right. A professional installer will have the tools and knowledge needed to properly size, cut, and install the film, which can ensure that it is effective and long-lasting.
    • Exterior Installation – Beware of any vendor proposing the idea of installing film on the exterior of the window! Environmental exposure will quickly degrade the film and reduce its service life. Many inexperienced installers also propose double-sided installation of thinner films due to the difficulty of working with 14 and 16 mil films.

Bullet-Resistant Glass

Bear in mind, none of the aforementioned glazing materials are bullet-resistant with the exception of specially-engineered laminated glass or polycarbonate products that have been tested and certified under appropriate standards. I emphasize this point due to confusion I’ve witnessed in working with clients who have been deceived by misleading marketing efforts or misunderstandings resulting from inaccurate language often used by the public (“ballistic window film,” etc.).

Nevertheless, protection against forced entry by a gunman is usually our top priority when specifying barriers in security designs. Considering a variety of cost-benefit factors, we usually reserve specification of bullet-resistant products for situations where people are positioned in highly vulnerable locations or protecting critical response assets, such as security control rooms and armed officers posted in lobbies. In these types of situations, the vulnerability and importance of the assets often justifies the expense and installation challenges (e.g., weight load, framing requirements, etc.) of installing bullet-resistant products.

If a decision is made to employ bullet resistant glazing and the threat is defined as a gunman with a 5.56mm rifle (AR-15), we recommend only using glazing products that have been properly tested and certified under American standards UL 752 (Level 7) and ASTM F1233 (R1), or European standards EN 1063 (BR5) and EN 1522 (FB5).


[1] Barrier Technology Handbook, SAND77-0777. Sandia Laboratories, 1978.

[2] Critical Intervention Services assisted window film manufacturer Solar Gard in 2015 in conducting a series of timed penetration tests of unprotected tempered glass windows and glazing reinforced with anti-shatter film.

[3] Barrier Technology Handbook, SAND77-0777. Sandia Laboratories, 1978.

[4]  Garcia, Mary Lynn. Design and Evaluation of Physical Protection Systems. Burlington, MA: Elsevier Butterworth-Heinemann, 2007.

[5] Ibid.

[6] Barrier Technology Handbook, SAND77-0777. Sandia Laboratories, 1978.

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