March 9, 2026
- Standards & Certifications
No Spall vs. Low Spall: Understanding Ballistic Glazing Performance
However, these terms do not describe the same performance condition. Understanding the distinction requires a clear explanation of how ballistic standards define spall and how different glazing configurations respond to ballistic impact.
As a manufacturer of UL 752-tested and UL-listed ballistic glazing systems, Insulgard regularly collaborates with architects, security consultants, and facility owners to clarify these distinctions. In life-safety applications, accurate terminology and a clear understanding of testing standards are essential.
Why Spall Matters in Ballistic Protection
When evaluating ballistic glazing, attention is typically focused on whether a bullet penetrates the system. While penetration resistance is critical, ballistic performance standards also address what happens on the protected side of the glazing.
Spall refers to fragments of glazing material that may break free from the interior side after a ballistic impact. Even if the projectile itself does not pass through the glazing, interior glass fragments can pose a risk to occupants positioned near the window or door.
Effective ballistic protection therefore considers both penetration resistance and fragment containment.
What UL 752 Requires
UL 752 is one of the most widely recognized standards for evaluating bullet-resistant glazing. It defines eight protection levels based on specific firearms and ammunition types, ranging from handguns to high-powered rifles.
To meet UL 752 requirements, a glazing product must satisfy two primary criteria:
- The projectile must not penetrate the glazing.
- No spall may contact the designated witness panel (defined below) placed behind the test specimen.
During testing, a thin sheet of cardboard known as a witness panel is positioned at a specified distance behind the glazing. The witness panel simulates the presence of a person on the safe side of the glazing. If interior fragments strike or penetrate that panel during ballistic impact, the product does not meet the UL 752 criteria for that level.
It is important to clarify that UL 752 does not include a “low spall” rating category. Under the standard, products either meet the no-spall requirement or they do not. Understanding this distinction helps ensure that specification language aligns with documented test outcomes.
How No-Spall Glazing Is Achieved
Many glazing systems achieve no-spall performance by incorporating a layer of polycarbonate on the interior side. Polycarbonate is a high-impact-resistant material capable of absorbing energy and containing fragments created during ballistic impact.
In glass-clad polycarbonate assemblies, exterior glass layers provide rigidity and hardness, while the interior polycarbonate layer serves as a containment shield. When properly engineered and tested, this configuration can prevent both penetration and interior spall, allowing the system to comply with UL 752 requirements.
Products tested and listed under UL 752 must demonstrate that no interior fragments contact the witness panel during testing.
Practical Considerations of Exposed Polycarbonate
While exposed interior polycarbonate plays a critical role in achieving no-spall performance, it also introduces practical considerations in real-world environments.
Compared to glass, polycarbonate is more susceptible to surface scratching. It may also be sensitive to certain cleaning agents, sealants, solvents, or construction chemicals. During installation, glazing systems are often installed early in the construction process to secure and weather-in a building. Subsequent exposure to incompatible materials used by other trades can result in surface crazing or cracking before occupancy.
Routine maintenance can also affect long-term appearance if cleaning personnel are unaware of product-specific care requirements. While surface damage does not automatically affect ballistic rating, improper maintenance can influence long-term durability and appearance.
What “Low Spall” Means in Practice
The term “low spall” is commonly used to describe glazing configurations that aim to improve maintenance durability while still providing ballistic resistance.
A low-spall configuration is created by adding a thin layer of glass to the interior side of a glass-clad polycarbonate assembly. This additional layer of interior glass protects the polycarbonate from direct exposure, reducing the likelihood of scratching or chemical damage and simplifying cleaning procedures.
However, by adding interior glass, the system may permit some fragment release on the protected side during ballistic impact. Because UL 752 requires that no spall contact the witness panel, a modified configuration that allows fragment release would not qualify as UL-listed under that specific standard, even if the base assembly had previously met the criteria.
It’s important to note that while a low-spall configuration fails to meet the UL 752 no-spall requirement, it passes the UL 752 ballistic requirements by preventing a bullet from penetrating the glazing. A low-spall assembly maintains the same level of ballistic resistance as its unmodified no-spall counterpart. Meeting one requirement but not the other explains why modified low-spall configurations are ineligible to carry the UL 752 label.
This distinction is not a judgment of one approach over another. Rather, it reflects how specific configurations perform under defined testing conditions. Accurate product descriptions should align with the test method used.
Selecting the Appropriate Configuration
Choosing between no-spall and low-spall configurations involves evaluating the specific conditions of a project.
Considerations may include:
- Occupant proximity to glazing.
- Level and type of ballistic threat.
- Compliance requirements tied to UL listing.
- Long-term maintenance expectations.
- Construction sequencing and exposure risks.
For glazing located at eye level in frequently occupied areas, strict no-spall compliance may be preferred. In other settings, such as elevated glazing, secured vestibules, or locations where occupants are not positioned directly adjacent to the glazing, maintenance durability may influence the selected configuration.
Each project presents a unique combination of risk profile, performance requirements, and operational realities.
Insulgard works closely with design teams and facility owners to evaluate these factors and determine which ballistic glazing configuration best meets the unique needs of each application.
By aligning testing standards, environmental conditions, and long-term use considerations, projects can achieve both documented protective performance and practical durability.
Understanding how glazing systems are constructed, and how they are tested, ensures that terminology, expectations, and performance remain aligned in life-safety specifications.