Low-Spall Bullet-Resistant Laminates

Low-spall bullet-resistant laminates are security glazing constructions engineered to stop a specified ballistic threat while allowing some protected-side glass fragmentation during impact.

That protected-side fragmentation is known as spall.

The key distinction is important:

Low spall does not mean partial ballistic protection.

A low-spall laminate can successfully stop the projectile and prevent complete penetration while still releasing some glass fragments from the protected-side surface.

Low-spall constructions are often selected for architectural applications where a durable glass interior surface, optical clarity, maintenance, and long-term surface quality are important.

They may be constructed entirely from glass, or they may incorporate polycarbonate within the laminate while still maintaining a glass surface on the protected side.

Engineering Principle

Low spall describes protected-side behavior during ballistic impact. It does not define the laminate materials, thickness, or ballistic threat level by itself.

What Is a Low-Spall Bullet-Resistant Laminate?

A low-spall laminate is a bullet-resistant construction that maintains a glass surface on the protected side of the assembly.

During ballistic impact, the laminate is designed to absorb and dissipate sufficient energy to prevent projectile penetration.

However, because the final protected-side surface is glass, some rear-face glass fragmentation may occur.

This behavior distinguishes low-spall constructions from no-spall systems, which are specifically engineered to prevent hazardous protected-side glass release.

A simplified low-spall construction might include:

Glass / Interlayer / Glass / Interlayer / Glass

or, depending on the design:

Glass / Interlayer / Glass / Interlayer / Polycarbonate / Interlayer / Glass

The exact construction depends on the ballistic threat, weight target, optical requirements, interlayer system, size, and other engineering constraints.

What “Low Spall” Actually Means

Spall refers to fragments of material released from the protected side of a barrier during impact.

In low-spall bullet-resistant glazing, the projectile is stopped, but some glass may separate from the protected-side surface.

This can result from:

• Stress waves traveling through the laminate
• Flexure of the protected-side glass
• Tensile failure of the final glass ply
• Local fracture around the impact area
• Energy transferred through the remaining laminate

The amount, size, and travel of the released glass can vary depending on the construction and test conditions.

The important point is that spall behavior and projectile penetration are separate performance characteristics.

Low Spall Does Not Mean Reduced Ballistic Resistance

The term “low spall” can sometimes sound as though the product provides a lower level of ballistic protection.

That is not what the term means.

A low-spall construction may resist the same defined projectile threat as a no-spall construction.

The difference is what happens on the protected side after the projectile is stopped.

A low-spall product may allow some protected-side glass release.

A no-spall product is engineered to prevent that protected-side fragmentation from entering the occupied space.

Engineering Insight

Ballistic threat resistance tells you whether the projectile is stopped. Spall performance tells you what happens on the protected side while that occurs.

How Low-Spall Laminates Manage Ballistic Energy

Low-spall laminates manage impact energy through the combined behavior of multiple materials and layers.

Depending on the construction, those layers may include:

• Glass
• Transparent interlayers
• Polycarbonate
• Specialty glass or surface technologies

During impact, the threat-side portion of the laminate begins to fracture and absorb energy.

Successive layers continue slowing, deforming, and disrupting the projectile while distributing the remaining energy through the laminate.

Interlayers help hold fractured plies together and allow the construction to continue functioning after individual glass layers have broken.

If polycarbonate is incorporated within the laminate, it can contribute additional toughness and energy absorption while still allowing the protected-side surface to remain glass.

The final glass ply provides a hard architectural service surface but may fracture as residual energy reaches the protected side.

That combination is what creates the characteristic low-spall behavior.

Why Protected-Side Glass Can Produce Spall

Glass performs very well in compression but is much more vulnerable to tensile stress.

During ballistic impact, complex stress waves move through the laminate.

Even though the projectile may stop before reaching the protected surface, the final glass ply can still experience enough tensile stress and flexure to fracture.

When fragments separate from that protected surface, spall occurs.

This is why a laminate does not need to be penetrated for protected-side fragmentation to happen.

Engineering Principle

The projectile can be fully stopped while the protected-side glass still fractures.

Low-Spall Constructions Are Not Necessarily All-Glass

This distinction is important for understanding the product family.

Some low-spall constructions are all-glass laminates.

Others may combine:

• Glass
• Polycarbonate
• Transparent interlayers
• A final glass protected-side surface

In other words:

All-glass describes one possible laminate architecture.

Low spall describes protected-side impact behavior.

The two terms often overlap, but they are not interchangeable.

Why Choose Low Spall Instead of No Spall?

Low-spall glazing can offer several practical advantages.

One of the most important is the protected-side glass surface.

Compared with exposed polycarbonate, glass generally provides:

• Greater scratch resistance
• Better abrasion resistance
• Familiar cleaning procedures
• Less sensitivity to incompatible cleaners
• Long-term surface durability
• Strong architectural appearance

These characteristics can be especially valuable in occupied commercial and institutional buildings where the interior surface is regularly cleaned or touched.

Low-spall construction can therefore provide a useful balance between ballistic performance and long-term architectural serviceability.

The Maintenance Advantage of a Glass Protected Surface

Exposed polycarbonate can provide excellent no-spall performance, but it requires more careful maintenance.

Some cleaners, solvents, or incompatible chemicals can damage polycarbonate surfaces or contribute to stress cracking.

A glass protected-side surface is generally more forgiving.

For building owners and maintenance teams, that means the interior surface behaves more like conventional architectural glass.

This can reduce maintenance complexity over the service life of the glazing.

Typical environments where this can matter include:

• Government buildings
• Corporate offices
• Financial institutions
• Embassies and diplomatic facilities
• Retail environments
• Public facilities
• Other high-traffic architectural spaces

The appropriate cleaning practices should still be followed for any specialty coatings or surface treatments.

Optical and Architectural Considerations

Low-spall constructions can also be attractive where visual quality is important.

A protected-side glass surface provides the optical character and feel of traditional architectural glazing.

Depending on the design, low-spall systems can incorporate:

• Clear glass
• Low-iron glass
• Tinted glass
• Heat-treated glass
• Low-E configurations
• Insulating glass assemblies
• Other architectural glass options

This gives designers flexibility when ballistic protection must be integrated into an architectural facade or interior environment without compromising appearance.

When Protected-Side Spall May Be Acceptable

Whether low-spall performance is appropriate depends on the project requirements.

Relevant considerations can include:

• Occupant proximity to the glazing
• Whether personnel routinely work directly behind the protected surface
• Project specifications
• Security consultant requirements
• Government or agency requirements
• Maintenance priorities
• Architectural goals
• Threat assessment
• Overall security strategy

In some applications, occupants are normally located well away from the protected side of the glazing.

In those cases, a low-spall construction may be appropriate if the project requirements permit it.

However, proximity is not the only factor.

There are also applications where no-spall performance is required regardless of occupant distance because the specification, security doctrine, agency requirement, or operational objective requires protected-side fragment control.

The decision should therefore be based on the complete security requirement rather than on proximity alone.

When No-Spall Performance May Be Preferred or Required

No-spall construction may be preferred when:

• Personnel work directly adjacent to the protected surface
• Protected-side fragment release is specifically prohibited
• UL Listing is required
• Government or agency specifications require no spall
• The glazing protects high-value occupants or critical work areas
• Security doctrine requires maximum fragment control
• The project explicitly specifies no-spall performance

Traditional no-spall glass-clad polycarbonate systems commonly use mar-resistant polycarbonate on the protected surface.

The polycarbonate absorbs residual energy and prevents hazardous glass fragments from entering the occupied area.

That approach can provide excellent fragment control but introduces a softer polymeric service surface that requires different maintenance considerations.

Adding Spall Shield to a Low-Spall Construction

Another approach is to add a transparent spall-control film to the protected-side glass surface.

This can allow the underlying laminate to retain many of the advantages of a glass protected-side construction while adding fragment-retention capability.

A spall shield can be applied to the final glass layer to help retain glass fragments that would otherwise be released during ballistic impact.

This creates an important additional design option.

Instead of changing the entire laminate to a conventional glass-clad polycarbonate no-spall construction, an engineered low-spall laminate may be supplemented with a spall-control layer when appropriate.

Spall-control films can also help maintain a glass-like service surface and avoid some of the maintenance concerns associated with exposed polycarbonate.

The exact configuration should still be validated for the required ballistic and protected-side performance.

Low Spall vs. No Spall

The distinction can be summarized simply.

Low-Spall Glazing

Generally:

• Stops the specified projectile threat
• May release protected-side glass fragments
• Often maintains glass on the protected surface
• Provides a hard, durable service surface
• Can offer strong optical and maintenance characteristics
• Cannot be represented as UL Listed when protected-side spalling occurs

No-Spall Glazing

Generally:

• Stops the specified projectile threat
• Prevents hazardous protected-side glass release
• Commonly uses mar-resistant polycarbonate or another fragment-control strategy on the protected side
• Provides greater protected-side fragment control
• May require more careful cleaning and maintenance when polycarbonate is exposed

Neither is universally superior.

They solve different combinations of engineering requirements.

Low Spall and UL 752

The distinction between low-spall and no-spall performance is especially important when discussing UL 752.

For a transparent glazing product to be represented as UL Listed, protected-side spalling is not permitted.

A low-spall construction that releases protected-side glass fragments therefore cannot be UL Listed as a transparent glazing product.

That does not mean the construction failed to resist the ballistic threat.

Low-spall products can be independently tested by qualified third-party ballistic laboratories using the applicable ballistic threat, projectile, velocity, shot pattern, and general test methodology associated with UL 752.

Under that independent evaluation, protected-side spalling may be permitted while projectile penetration is not.

The correct terminology is therefore important.

UL Listed and independently tested to UL 752 ballistic requirements should not be treated as interchangeable claims.

This distinction is discussed in more detail in PA-TS-001 — Understanding UL 752.

Low Spall Can Be Used in Insulating Glass Units

Low-spall bullet-resistant laminates can also be incorporated into insulating glass units.

In these applications, the ballistic laminate is typically positioned toward the interior side of the IGU, with an exterior glass lite separated by an insulating airspace and spacer system.

This allows the assembly to combine:

• Ballistic resistance
• Thermal performance
• Low-E glass options
• Insulating airspace
• Architectural appearance
• Glass protected-side surface

If no-spall protected-side behavior is required, a spall-control film or another approved construction approach may be incorporated depending on the design.

The ballistic laminate and the insulating glass assembly perform different functions and should be evaluated accordingly.

Typical Applications

Low-spall bullet-resistant glazing is commonly considered for architectural environments where ballistic protection must be balanced with optical quality, maintenance, and surface durability.

Applications may include:

• Government facilities
• U.S. embassies and diplomatic facilities
• Corporate offices
• Financial institutions
• Public buildings
• Law-enforcement facilities
• Retail and luxury stores
• Guard booths
• Security checkpoints
• Other protected architectural openings

The appropriate construction depends on the threat, occupant arrangement, specification requirements, panel size, framing, environmental exposure, and desired maintenance characteristics.

Testing and Performance Documentation

Low-spall glazing should be selected based on documented performance.

Useful test documentation can identify:

• Ballistic standard or test methodology
• Threat designation
• Ammunition
• Projectile velocity
• Shot pattern
• Specimen construction
• Specimen dimensions
• Test laboratory
• Projectile penetration result
• Protected-side spall behavior
• Whether the product is independently tested, certified, or listed

Particular care should be taken with the terminology used in specifications and product literature.

A low-spall construction that is independently tested against a UL 752 ballistic threat should not be described as UL Listed if protected-side spalling occurs.

Accurate terminology helps project teams compare products correctly.

Selecting the Appropriate Construction

A low-spall laminate may be appropriate when a project prioritizes:

• Ballistic resistance
• Glass protected-side surface
• Optical appearance
• Scratch resistance
• Routine cleaning
• Long-term surface durability
• Architectural integration

A no-spall construction may be preferable when:

• Protected-side fragment control is mandatory
• Personnel are positioned close to the glazing
• UL Listing is required
• The project specification explicitly prohibits spall
• Security requirements prioritize maximum fragment containment

A spall-shield approach may offer another option where a glass protected-side surface is desirable but fragment retention is also required.

The correct choice should begin with the actual threat and project requirements.

Engineering Principle

Low-spall and no-spall glazing are not competing grades of the same product. They are different engineering strategies for managing protected-side behavior.

Key Takeaways

• Low spall describes protected-side fragmentation during ballistic impact.
• Low spall does not mean reduced ballistic resistance or partial projectile penetration.
• A low-spall construction can successfully stop the specified projectile while allowing some protected-side glass release.
• Low-spall constructions typically maintain a glass surface on the protected side.
• Low-spall systems are not necessarily all-glass; some may incorporate polycarbonate within the laminate.
• Glass protected-side surfaces offer strong scratch resistance, familiar maintenance, and architectural durability.
• Occupant proximity is one consideration when selecting low-spall performance, but some projects require no-spall performance regardless of proximity.
• No-spall construction provides greater protected-side fragment control.
• Spall-control films can provide another design option by retaining fragments at the protected-side glass surface.
• A low-spall product cannot be represented as UL Listed when protected-side spalling occurs.
• Low-spall products may be independently tested using the applicable UL 752 ballistic threat and methodology without being UL Listed.
• Low-spall laminates can be incorporated into insulating glass units.
• Product selection should consider ballistic threat, spall requirements, occupants, maintenance, optics, panel size, framing, environment, and specification requirements.

Continue Learning

PA-EF-004 — Understanding Spall

Learn the fundamentals of protected-side fragmentation and why spall occurs.

PA-PT-001 — All-Glass Bullet-Resistant Laminates

Explore how multiple glass plies and interlayers are engineered to resist ballistic threats.

PA-PT-003 — No-Spall Glass-Clad Polycarbonate

Learn how glass and polycarbonate are combined to stop ballistic threats while preventing protected-side glass release.

PA-TS-001 — Understanding UL 752

Understand the distinction between UL Listed transparent glazing and independently tested low-spall constructions.

PA-PT-007 — Bullet-Resistant Insulating Glass Units

Explore how ballistic laminates are incorporated into insulating glass assemblies for architectural applications.