While the U.S. Department of Defense has prohibited the use of blast film on windows in most military buildings and facilities, the government has different security standards for federal buildings. The Public Buildings Service Leasing Desk Guide, which details the policies and procedures of federal buildings, states that all exterior windows must feature “shatter resistant material” that meets specific standards:
WINDOWS
SHATTER-RESISTANT WINDOW PROTECTION
The Lessor shall provide and install shatter-resistant material not less than 0.18 millimeters thick on all exterior windows in Government-occupied space meeting the following properties – Film composite strength and elongation rate measured at a strain rate not exceeding 50% per minute shall not be less than the following:
– Yield Strength: 12,000 psi
– Elongation at yield: 3%
– Longitudinal Tensile strength: 22,000 psi
– Traverse Tensile strength: 25,000 psi
– Longitudinal Elongation at break: 90%
– Traverse Elongation at break: 75%
THE ALTERNATE METHOD is for the Lesser to provide a window system that conforms to a minimum glazing performance condition of “3b” for a high protection level and a low hazard level. Window systems shall be certified as prescribed by WINGARD PE 4.3 or later to GSA performance condition 3b (in accordance with the GSA Standard Test Method for Glazing Window Systems Subject to Dynamic Loadings or Very Low Hazard (in accordance with ASTM F 1642, Standard Test Method for Glazing or Glazing Systems Subject to Air Blast Loading) in response to air blast load of 4 psi/28 psi-msec.
GSA Leasing Desk Guide
Chapter 19: Security
Blast Film: Daylight vs. Anchored Installation
Regarding the federal guidelines on blast film, it’s important to understand the differences between the two common types of blast film installation: “daylight” and anchoring. Daylight installation is the simple process of applying blast film only to the window glass. It’s designed to hold the glass together and prevent it from shattering. The problem with daylight installation is that it doesn’t hold the window pane to the frame or wall. In the event of an explosion, the window pane is still likely to fly off the wall and become a high-energy projectile capable of causing severe harm or death.
Blast film is only effective if it’s anchored to the frame or wall. In most anchored installations, the blast film extends beyond the edge of the glass and overlaps with the frame. This not only helps keep the glass from shattering during a blast, but also helps prevent the entire window from detaching.
If you’re going to use blast window film to provide a level of protection against an explosion, anchoring it is the most effective installation method. But daylight installation is easier and less expensive, so it’s more common. And building managers who aren’t aware of the differences between the two may be more likely to choose daylight installation simply because it’s cheaper. This is why it’s important to emphasize the correct installation method whenever discussing blast film.
The Short Lifespan of Blast Film
The federal guidelines also fail to address the short lifespan of blast film. Regardless of whether it was installed correctly or not, blast film deteriorates over time due to exposure to UV light. Typically, blast film must be reapplied every 15 years, depending on how quickly it has deteriorated. If it’s not, the film becomes useless and leaves building occupants as vulnerable to a blast attack as they would be had the windows never been treated in the first place.
This is precisely what’s happened to the hundreds of federal buildings that were treated with blast window film shortly after 9/11. At the time, blast film seemed like a quick and affordable way to increase a building’s physical security. As it turns out, maintaining blast film over the course of a building’s lifetime is quite expensive. To complicate matters, blast film isn’t time-stamped during the installation process, so keeping track of when it needs to be reapplied is difficult and rarely even done. This is one of the main reasons why the Department of Defense finally prohibited the use of blast film altogether in military buildings.
Blast Film Provides No Energy Savings
Another shortcoming of blast film is that it provides no energy benefits. Since windows play an important role in a building’s energy use, any kind of window upgrade – including security –should also make the building more energy efficient. Unfortunately, blast film doesn’t do anything to address solar heat gain, air infiltration, condensation and other factors that affect a building’s energy consumption.
Thermolite’s interior blast window retrofit, on the other hand, provides both bomb blast protection and significant energy savings. Building owners typically see a 20% utility reduction and a payback period that would work with most ESCO projects. Read more about the benefits of incorporating energy efficient bomb blast windows into ESCO projects.
