Bullet-Resistant Insulating Glass Units

Bullet-resistant insulating glass units, or bullet-resistant IGUs, combine an engineered ballistic laminate with insulating glass technology so that a single architectural glazing assembly can address both security and building-envelope performance.

A common configuration consists of:

Exterior Glass Lite / Insulating Airspace / Interior Bullet-Resistant Laminate

A perimeter spacer maintains the insulating airspace between the exterior lite and the interior ballistic laminate. Depending on the project, the assembly may also incorporate Low-E glass, argon gas, solar-control options, specialty glass, and different ballistic laminate technologies.

The result is fundamentally different from simply installing a thick piece of bullet-resistant glass in an exterior opening.

A bullet-resistant IGU must function simultaneously as:

• A ballistic security system
• An insulating glazing system
• An architectural glazing component
• A long-term exterior building-envelope product

Engineering Principle

A bullet-resistant IGU combines two engineered systems in one assembly: a security laminate and an insulating glass system. Each performs a different job.

What Is a Bullet-Resistant Insulating Glass Unit?

A conventional insulating glass unit typically separates two glass lites with a sealed airspace.

A bullet-resistant IGU applies the same general building-envelope concept while replacing one of those conventional lites with an engineered bullet-resistant laminate.

In the common architecture discussed here:

Threat Side / Exterior

Exterior Glass Lite

Insulating Airspace

Perimeter Spacer and Edge-Seal System

Interior Bullet-Resistant Laminate

Protected Side / Interior

Other configurations are possible.

Depending on the project, bullet-resistant IGUs can use:

• Different exterior glass types
• Different ballistic laminate technologies
• Low-spall or no-spall interior laminates
• Low-E glass
• Argon-filled airspaces
• Different spacer technologies
• Specialty architectural glass
• Additional lites in more complex IGU configurations

The final construction should follow the required ballistic, thermal, optical, structural, and architectural performance.

Why Combine Ballistic Protection with an IGU?

Exterior windows must often do much more than provide security.

They may also be expected to control:

• Heat transfer
• Solar heat gain
• Interior comfort
• Condensation
• Exterior noise
• Appearance
• Weather exposure
• Long-term moisture intrusion
• Building energy performance

A stand-alone ballistic laminate can provide security, but it does not automatically provide the thermal characteristics expected from a modern exterior insulating glass assembly.

The IGU architecture allows these two requirements to be addressed together.

The ballistic laminate provides the security function.

The insulating glass system provides the building-envelope function.

Engineering Insight

Ballistic protection does not eliminate the need for building performance, and building performance does not establish ballistic protection. Both must be engineered into the assembly.

The Basic IGU Architecture

The easiest way to understand a bullet-resistant IGU is to divide it into functional zones.

Exterior Lite

The exterior lite forms the outer surface of the building envelope.

Insulating Airspace

The sealed space between the exterior lite and the interior laminate reduces heat transfer and can contribute to acoustic and condensation performance.

Spacer and Edge-Seal System

The perimeter system maintains the separation between the exterior lite and the ballistic laminate while helping preserve the sealed airspace.

Interior Bullet-Resistant Laminate

The interior laminate provides the primary ballistic resistance in this common configuration.

This separation of functions is important.

The exterior lite is not simply another layer in the ballistic laminate.

The airspace is not an interlayer.

The spacer is not part of the laminate stack-up.

The interior ballistic laminate is not simply a conventional interior glass lite.

Each component belongs to the complete IGU, but each performs a different engineering function.

The Exterior Lite

The exterior lite is the first glazing surface exposed to the outdoor environment.

Depending on project requirements, it may use:

• Annealed glass
• Heat-strengthened glass
• Tempered glass where appropriate
• Clear glass
• Low-iron glass
• Tinted glass
• Coated glass
• Low-E glass
• Other architectural glass options

Its selection may be influenced by:

• Wind loads
• Thermal stress
• Solar performance
• Appearance
• Safety-glazing requirements
• Building-code requirements
• Exterior environmental exposure

In the common configuration, the exterior lite is a single glass lite on the threat side, separated from the ballistic laminate by the insulating airspace.

The exterior lite should not automatically be assumed to contribute to the documented ballistic rating unless it was specifically included in the tested ballistic construction.

Engineering Principle

A component being physically in front of the ballistic laminate does not automatically make it part of the validated ballistic construction.

The Insulating Airspace

The airspace is one of the defining characteristics of an IGU.

It separates the exterior lite from the interior ballistic laminate and reduces direct heat transfer across the glazing assembly.

The airspace can contribute to:

• Thermal insulation
• Building energy efficiency
• Condensation resistance
• Acoustic performance
• Interior comfort

The insulating airspace contributes to building performance. It is not part of the solid ballistic laminate itself.

Engineering Insight

The insulating airspace contributes to building performance. It should not be confused with a transparent interlayer inside the ballistic laminate.

The Spacer System and Edge Seals

The spacer system runs around the perimeter of the insulating airspace.

Its basic function is to maintain the required separation between the exterior lite and interior laminate.

Depending on the spacer design, it may also incorporate desiccant to help manage moisture within the sealed cavity.

The perimeter assembly works together with primary and secondary edge seals to help:

• Maintain the insulating airspace
• Limit moisture-vapor transmission
• Retain insulating gas where applicable
• Control moisture inside the cavity
• Support long-term IGU durability

These components are mostly hidden after installation, but they are critical to the long-term performance of the unit.

Engineering Principle

The spacer and edge seals may be visually minor components, but failure at the edge of an IGU can compromise the building-performance function of the entire assembly.

There Is Only One True IGU Spacer System

This point is worth making because a complex bullet-resistant laminate contains many visible layers at its edge.

The actual IGU spacer belongs only in the insulating cavity between the exterior lite and the interior ballistic laminate.

The multiple glass, polycarbonate, and interlayer components visible along the edge of the interior laminate are part of the ballistic laminate itself.

They are not additional insulating cavities and they do not require a second IGU spacer.

Engineering Insight

A layered laminate edge and an IGU spacer are two completely different things.

The Interior Bullet-Resistant Laminate

The interior laminate is the security component of the common bullet-resistant IGU architecture.

It can use many of the ballistic technologies discussed elsewhere in the Engineering Library, including:

• All-glass bullet-resistant laminates
• Low-spall glass/polycarbonate constructions
• No-spall glass-clad polycarbonate
• Other appropriately engineered and tested ballistic constructions

The exact laminate architecture depends on:

• Ballistic threat
• Required spall performance
• Weight
• Overall IGU thickness
• Panel dimensions
• Optical requirements
• Maintenance requirements
• Framing constraints
• Building-envelope requirements

This is why there is no single universal bullet-resistant IGU stack-up.

The insulating glass configuration can remain conceptually similar while the interior ballistic laminate changes to meet different security objectives.

Low-Spall Interior Laminate Options

A low-spall interior laminate typically maintains a glass surface on the protected side.

During a ballistic event, the projectile may be successfully stopped while some protected-side glass fragmentation occurs.

Using low-spall construction in an IGU can provide several architectural advantages:

• Glass interior service surface
• Strong scratch resistance
• Familiar cleaning characteristics
• Long-term surface durability
• Excellent architectural appearance

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

Considerations may include:

• Occupant location
• Security requirements
• Project specification
• Ballistic threat
• Maintenance priorities
• Agency requirements
• Protected-side fragment requirements

No-Spall Interior Laminate Options

A no-spall IGU can use a ballistic laminate designed to prevent hazardous protected-side glass release.

Traditional no-spall glass-clad polycarbonate construction may use mar-resistant polycarbonate as the exposed protected-side surface.

This provides:

• Ballistic resistance
• Protected-side fragment control
• No-spall performance
• A comparatively lightweight energy-absorbing component

The tradeoff is that exposed polycarbonate requires more careful cleaning and chemical compatibility than exposed glass.

As with every ballistic product family:

No spall does not automatically mean UL Listed.

The actual tested and certified construction determines the applicable claim.

Optional Spall Shield

Another possible strategy is to retain a protected-side glass surface while adding a transparent spall-control layer to that surface.

A spall shield can help retain glass fragments that might otherwise be released from the protected-side glass during ballistic impact.

This provides another design option where a project values:

• A glass protected-side surface
• Familiar maintenance
• Surface durability
• Enhanced fragment retention

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

Engineering Principle

Spall control is a design decision within the ballistic laminate. It is separate from the insulating function of the IGU.

Low-E Glass and Solar-Control Options

Bullet-resistant IGUs can incorporate architectural glass technologies used to manage heat transfer and solar performance.

One important example is Low-E glazing.

Low-E coatings can help reduce heat transfer and, depending on the coating selected, influence solar heat gain and overall energy performance.

The specific coating and its position must be compatible with the complete IGU design.

For that reason, Low-E should be treated as a project-specific option rather than assuming that one coating position applies universally.

Available options can depend on:

• Climate
• Required U-factor
• Solar heat gain requirements
• Appearance
• Glass tint
• Coating durability
• Laminate compatibility
• IGU construction
• Project specifications

Engineering Insight

A coating can improve building performance only when it is positioned correctly and remains compatible with the complete glazing system.

Argon and Thermal Performance

The insulating cavity can also be filled with argon rather than ordinary air.

Argon can reduce heat transfer through the cavity and improve the thermal performance of the IGU.

It does not provide ballistic protection.

Its function is building-envelope performance.

This distinction is a good example of why every component in the assembly should be understood by function.

Engineering Principle

Argon helps manage heat. The ballistic laminate helps manage projectile energy. One cannot substitute for the other.

Condensation Resistance

Interior surface temperature is an important consideration in exterior glazing.

An insulating glass design can help keep interior glazing surfaces warmer during cold exterior conditions, reducing the likelihood of condensation compared with less thermally efficient configurations.

Factors that can influence condensation performance include:

• Overall thermal performance
• Spacer technology
• Gas fill
• Glass coatings
• Interior temperature
• Exterior temperature
• Interior relative humidity
• Edge-of-glass conditions

Condensation should be understood as a system-level building-science issue rather than a property of the ballistic laminate alone.

Acoustic Performance

The insulating airspace and increased mass of a security glazing assembly can also contribute to acoustic performance.

Sound transmission depends on the complete construction, including:

• Glass thickness
• Laminate thickness
• Airspace dimension
• Asymmetric glass construction
• Interlayers
• Panel size
• Framing
• Installation conditions

A bullet-resistant IGU may therefore provide acoustic benefits compared with simpler glazing systems.

However, ballistic performance should not be used to infer a specific acoustic rating.

Where acoustic performance is a project requirement, it should be evaluated and specified separately.

Ballistic Performance and Thermal Performance Are Different

One of the most important lessons of this article is that a bullet-resistant IGU contains different performance systems within the same finished product.

The ballistic laminate may be responsible for:

• Projectile resistance
• Energy management
• Fragment control
• Spall performance

The insulating glass system may be responsible for:

• Thermal insulation
• Solar performance
• Condensation resistance
• Acoustic contribution
• Environmental separation
• Building-envelope durability

These systems are integrated physically, but their performance claims should remain technically distinct.

Engineering Principle

Integrating two technologies does not merge their test results. Ballistic performance and building-envelope performance must each be understood on their own terms.

Adding an Exterior Lite Does Not Automatically Increase the Ballistic Rating

It can be tempting to assume that placing another glass lite in front of a bullet-resistant laminate must increase ballistic protection.

That should not be assumed.

Ballistic resistance belongs to the tested construction.

If a ballistic laminate has been tested independently, placing it within an IGU does not automatically create a new or higher ballistic rating.

Likewise, an exterior lite should not be counted as a ballistic layer unless the complete assembly has been evaluated in that configuration and the claim is supported by the applicable test documentation.

Specification Insight

More glass does not automatically mean a higher ballistic rating. Testing determines demonstrated performance.

Edge-Seal Durability and Material Compatibility

The edge of an IGU is exposed to demanding long-term conditions.

It must maintain a sealed insulating cavity while the unit experiences:

• Temperature changes
• Solar heating
• Thermal expansion and contraction
• Moisture exposure
• Building movement
• Wind loading
• Handling and installation
• Long-term service

Material compatibility is therefore critical.

Sealants, setting blocks, gaskets, edge seals, coatings, interlayers, and exposed polymer surfaces should be evaluated for compatibility with one another.

Engineering Insight

A bullet-resistant IGU must survive two very different tests of time: the security threat it may face once, and the environmental exposure it faces every day.

Weight and Overall Thickness

Bullet-resistant IGUs can become substantially heavier and thicker than conventional insulating glass.

Overall construction depends on:

• Ballistic threat
• Ballistic laminate type
• Exterior lite thickness
• Airspace dimension
• Spall requirement
• Low-E or specialty glass options
• Number of lites
• Panel dimensions

Weight and thickness affect:

• Frame depth
• Structural support
• Glazing pocket dimensions
• Setting blocks
• Handling equipment
• Installation procedures
• Shipping
• Maximum practical panel dimensions
• Hardware where movable assemblies are involved

Engineering Principle

The IGU must fit the building physically as well as perform within it technically.

Framing and System Integration

The finished IGU must be retained within a framing or glazing system capable of supporting its:

• Weight
• Thickness
• Edge engagement
• Structural loads
• Security requirements
• Environmental loads

Patriot Armor manufactures the glazing component; the complete framing and installation system may be designed or provided by framing manufacturers, fabricators, contractors, or other system integrators.

This distinction matters when evaluating performance claims.

A ballistic rating for the glass should not automatically be interpreted as a rating for the complete window system unless the complete system has been tested accordingly.

The same principle applies to thermal, structural, forced-entry, and other system-level requirements.

Different IGU Configurations Are Possible

The architecture shown throughout this article is a common configuration, not a universal proprietary stack-up.

Depending on project requirements, other arrangements may be possible, including configurations with:

• Different ballistic laminate families
• All-glass laminates
• Low-spall laminates
• No-spall laminates
• Spall-control films
• Different exterior glass options
• Low-E coatings
• Different airspace dimensions
• Different spacer systems
• Additional glass lites
• More complex insulating assemblies

The goal is not to force every project into one construction.

It is to combine the appropriate technologies for the application.

Typical Applications

Bullet-resistant IGUs are especially useful for exterior architectural openings where both security and building-envelope performance matter.

Applications may include:

• Schools and universities
• Government buildings
• U.S. embassies and diplomatic facilities
• Corporate headquarters
• Healthcare facilities
• Financial institutions
• Law-enforcement facilities
• Critical infrastructure
• Data centers
• Secure commercial buildings
• Public buildings
• Guard facilities
• Other exterior high-security glazing applications

The appropriate configuration depends on the threat, climate, opening size, framing, energy requirements, occupant needs, and architectural goals.

Testing and Performance Documentation

A bullet-resistant IGU can involve multiple independent performance requirements.

Documentation may therefore address different aspects of the assembly.

Ballistic Documentation

May identify:

• Ballistic test standard
• Threat designation
• Ammunition
• Projectile velocity
• Shot pattern
• Tested laminate construction
• Specimen dimensions
• Protected-side behavior
• Test laboratory
• Certification or listing status

Building-Performance Documentation

Depending on the project, may address:

• Thermal performance
• Low-E configuration
• Gas fill
• Spacer system
• Glass type
• Safety-glazing requirements
• Structural requirements
• Environmental requirements
• Other project-specific glazing criteria

The most important point is to understand what was actually tested.

A ballistic test on the laminate does not necessarily constitute a ballistic test of every possible IGU configuration.

Similarly, an IGU thermal-performance value does not establish ballistic resistance.

Specification Insight

The complete product can have many performance characteristics, but each claim needs its own technical basis.

Selecting the Appropriate Bullet-Resistant IGU

The selection process should begin with both the security requirements and the building requirements.

Questions can include:

• What ballistic threat must be addressed?
• Is UL Listing required?
• Is low-spall or no-spall performance required?
• Would a spall-control film be appropriate?
• Will occupants be located close to the protected surface?
• What thermal performance is required?
• Is Low-E glass needed?
• Is solar control important?
• Is argon fill appropriate?
• What condensation conditions are expected?
• Is acoustic performance important?
• What are the maximum panel dimensions?
• What overall thickness can the frame accommodate?
• What is the finished unit weight?
• What framing system will retain the IGU?
• Which setting blocks, gaskets, and sealants are compatible?
• What cleaning and maintenance environment is expected?
• What service-life requirements apply?

The correct IGU is the one that appropriately balances these requirements.

The Wrong Question

What is the best bullet-resistant insulated glass?

A Better Question

What combination of ballistic laminate, exterior glass, insulating airspace, coatings, spacer system, and protected-side performance best satisfies the complete security and building-envelope requirements of the project?

Key Takeaways

• Bullet-resistant IGUs combine an engineered ballistic laminate with insulating glass technology.
• A common configuration places a single exterior glass lite on the threat side, an insulating airspace in the middle, and the ballistic laminate toward the protected interior side.
• The exterior lite, airspace, spacer, and ballistic laminate perform different functions.
• The interior ballistic laminate provides the primary security function in the common configuration discussed here.
• The insulating airspace contributes thermal, acoustic, condensation, and energy performance; it is not part of the solid ballistic laminate.
• The perimeter spacer maintains the airspace, while edge seals and desiccant help preserve long-term IGU integrity.
• There is only one true IGU spacer system in the common configuration; the visible layers within the ballistic laminate are not additional spacer systems.
• Low-spall and no-spall ballistic laminates can both be incorporated depending on the protected-side requirements.
• A spall-control layer can provide another fragment-management option with a protected-side glass surface.
• Low-E glass and argon can improve building-envelope performance but do not establish ballistic performance.
• Adding another exterior glass lite does not automatically increase the ballistic rating.
• Ballistic performance belongs to the tested construction.
• Edge-seal durability and material compatibility are important to long-term IGU performance.
• Bullet-resistant IGUs can become significantly heavier and thicker than conventional insulating glass, affecting framing, handling, and installation.
• The complete glazing and framing system should be considered together.
• There is no single universal bullet-resistant IGU stack-up.
• The appropriate design balances security, spall behavior, thermal performance, optics, weight, architecture, durability, and life-cycle requirements.

Continue Learning

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

Explore how all-glass ballistic laminates can serve as the security component within architectural glazing systems.

PA-PT-002 — Low-Spall Bullet-Resistant Laminates

Understand the advantages and tradeoffs of maintaining a protected-side glass surface.

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

Learn how protected-side polycarbonate can provide ballistic protection with enhanced fragment control.

PA-EF-004 — Understanding Spall

Learn why protected-side fragmentation occurs and how low-spall, no-spall, and spall-control strategies differ.

PA-MC-002 — Understanding Glass in Security Glazing

Explore the architectural glass options that can be incorporated into security and insulating-glass constructions.

PA-ED-002 — Every Layer Has a Job

Understand how ballistic, thermal, optical, environmental, and service-life functions are assigned to different layers within an engineered glazing system.