Modifying the glazing in a fire-rated door assembly without verifying the listing can void the door’s fire rating entirely. When glazing exceeds the maximum size permitted for a given fire rating, uses a non-listed glass type, or is installed in an unapproved lite kit frame, the assembly no longer complies with its UL or WHI listing. This means the door fails inspection, the building owner loses code compliance, and the assembly must be replaced or recertified. This guide explains how glazing choices affect fire-rated door assemblies across all five rating levels (20-minute, 45-minute, 60-minute, 90-minute, and 3-hour) and how to specify glazing correctly from the start.
CDF Distributors’ ProBuilder tool at cdfdistributors.com allows you to configure fire-rated door assemblies with listed glazing options built into the selection process. ProBuilder only displays glazing sizes and types that are compatible with your selected fire rating. For assistance specifying glazing for a fire-rated assembly, call (855) 769-9895 or email sales@cdfdoors.com.
About This Guide
This configuration guide is published by CDF Distributors to help architects, contractors, code officials, and facility managers understand how glazing modifications affect fire-rated door assemblies. The information reflects current NFPA 80 (Standard for Fire Doors and Other Opening Protectives), NFPA 252 (Standard Methods of Fire Tests of Door Assemblies), UL 10C (Positive Pressure Fire Tests of Door Assemblies), and UL 9 (Fire Tests of Window Assemblies) requirements as applied in commercial construction. CDF Distributors fabricates and ships complete fire-rated door assemblies from its Nashville, Tennessee headquarters, with all components verified against active UL and WHI listings before shipment.
Maximum Glass Sizes by Fire Rating Level
Fire-rated door assemblies allow vision lites (glass panels), but the maximum permitted glass area decreases as the fire rating increases. This is because higher-rated assemblies must withstand longer fire exposure, and glass is inherently weaker than steel or wood in fire conditions. The following table summarizes the general maximum glazing areas recognized by NFPA 80 and enforced through UL/WHI listings. Actual maximums depend on the specific listing for the door, frame, glass, and lite kit combination.
Fire Rating | Max Glass Area (Fire-Protective) | Max Glass Area (Fire-Resistive) | Common Applications |
20-minute | Up to 2,592 sq in (full lite possible) | Not typically required | Corridor doors, smoke partitions |
45-minute | Up to 1,296 sq in per lite | Up to 1,296 sq in with listed assembly | Corridor walls, room separations |
60-minute | Up to 100 sq in per lite (standard); larger with special listings | Larger areas with fire-resistive glass | Exit enclosures, 1-hour corridors |
90-minute | Up to 100 sq in per lite (standard) | Larger areas with fire-resistive glass | Stairwell doors, 1.5-hour walls |
3-hour (180-minute) | No vision lite permitted (standard) | Limited sizes with fire-resistive glass only | High-hazard separations, vault doors |
The 100 square inch limitation is the default maximum for fire-protective glazing in 60-minute and 90-minute rated doors. This roughly translates to a 10” x 10” vision lite. Doors rated at 20 and 45 minutes allow substantially larger glass areas because the fire exposure duration is shorter. At the 3-hour level, standard fire-protective glazing is generally not permitted at all; only fire-resistive glazing with specific listings may be used.
Fire-Rated Glass Types
Not all fire-rated glass is the same. The glass type determines the maximum size, the temperature performance, and the listing requirements. The four primary categories of fire-rated glass used in commercial door assemblies are described below.
Wired Glass
Wired glass is the oldest form of fire-rated glazing. It contains a wire mesh embedded in the glass that holds the pane together when it cracks during a fire. Wired glass has been the default glazing in fire doors for decades, but it is being phased out of many applications. Wired glass does not meet impact safety standards (CPSC 16 CFR 1201 or ANSI Z97.1) on its own, which means it cannot be used in locations where safety glazing is required, such as doors in educational occupancies, unless it is laminated or filmed to pass impact testing. Many jurisdictions and code editions now restrict or prohibit wired glass in new installations where human impact is possible.
Ceramic Glass
Ceramic fire-rated glass (such as glass-ceramics) withstands fire exposure without the wire mesh. It passes impact safety tests, making it suitable for all door locations including schools and hospitals. Ceramic glass is classified as fire-protective, meaning it blocks flames and smoke but allows radiant heat transfer through the glass. This limits its use to the same size restrictions as wired glass (100 square inches in 60- and 90-minute doors) unless paired with a fire-resistive assembly.
Filmed and Laminated Glass
Some fire-rated glazing products use an intumescent film or laminate layer between glass panes. The film swells when exposed to heat, turning opaque and insulating the unexposed side from radiant heat. Filmed glass can qualify as fire-resistive glazing depending on the product and listing, allowing larger vision lites in higher-rated doors. These products must be tested and listed as a complete assembly with a specific lite kit and door.
Insulated Fire-Rated Glass (Fire-Resistive)
Insulated fire-rated glass consists of multiple layers of glass with intumescent interlayers that block both flames and radiant heat. This glass type meets the hose stream test and the temperature rise requirements of UL 10C. Because it limits heat transmission through the glass, insulated fire-rated glass can be used in larger sizes at higher fire ratings. It is the only glazing option that allows vision lites in 3-hour rated door assemblies (where permitted by the specific listing).
The following table compares the four fire-rated glass types across key performance and compliance characteristics.
Glass Type | Fire Classification | Impact Safety | Status |
Wired glass | Fire-protective | Does not meet CPSC 16 CFR 1201 alone | Being phased out in many jurisdictions |
Ceramic glass | Fire-protective | Meets impact safety standards | Widely used replacement for wired glass |
Filmed/laminated glass | Fire-protective or fire-resistive (per listing) | Meets impact safety standards | Growing adoption for larger lites |
Insulated fire-rated glass | Fire-resistive | Meets impact safety standards | Required for large lites in high ratings |
Fire-Protective vs. Fire-Resistive Glazing
The distinction between fire-protective and fire-resistive glazing is critical for specifying vision lites in fire-rated doors. These two classifications determine maximum glass size, temperature rise compliance, and where the glazing can be installed. The following table summarizes the differences.
Characteristic | Fire-Protective | Fire-Resistive |
Blocks flames and smoke | Yes | Yes |
Blocks radiant heat | No | Yes |
Meets hose stream test | Yes | Yes |
Meets temperature rise limits | No | Yes |
Max area in 60/90-min doors | 100 sq in (standard) | Larger areas per listing |
Usable in 3-hour doors | No | Yes, per specific listing |
Common glass types | Wired glass, ceramics | Insulated fire-rated glass, filmed glass |
Typical cost | Lower | Significantly higher |
Fire-protective glazing (wired glass, standard ceramics) is tested to NFPA 252 or UL 10C without a temperature rise requirement on the glass itself. It prevents flame passage but allows radiant heat to transfer through. Fire-resistive glazing (insulated, filmed) meets the full wall test criteria under ASTM E119 or UL 263, blocking both flames and radiant heat. This makes fire-resistive glazing functionally equivalent to the surrounding wall, which is why it is permitted in larger sizes.
UL and WHI Listing Requirements for Vision Lites
Every component of a fire-rated door assembly must be covered by a listing from a recognized testing laboratory, typically UL (Underwriters Laboratories) or WHI (Warnock Hersey/Intertek). A listing is not a general approval for a product. It is a specific tested combination of door, frame, glass, lite kit, and hardware. The following requirements apply to glazing in fire-rated door assemblies.
- The glass product must appear in the door manufacturer’s listing as an approved glazing option.
- The lite kit (the metal frame that holds the glass in the door) must be listed for use with that specific glass type and fire rating.
- The glass size must not exceed the maximum area specified in the listing.
- The glass thickness must match the listing exactly.
- The installation method (how the glass is set in the lite kit, including glazing beads and setting blocks) must follow the listing.
Substituting any component (different glass type, different lite kit, different thickness) that is not covered by the listing voids the fire rating of the entire assembly. There is no field equivalency for fire-rated glazing. The Authority Having Jurisdiction (AHJ) cannot approve an unlisted substitution.
Exploded view showing: fire-rated door leaf, lite kit frame (steel), intumescent gasket, fire-rated glass panel, glazing bead, and setting blocks in their correct positions. This diagram requires CDF approval before inclusion in the production file.
Temperature Rise Requirements and How Glazing Affects Them
Temperature rise doors are required in specific locations by the International Building Code (IBC) and NFPA 101 (Life Safety Code). A temperature rise rating limits the amount of heat that transfers through the door to the unexposed side during a fire. The most common temperature rise requirement is 450 degrees Fahrenheit maximum rise above ambient at 30 minutes. The following table shows where temperature rise doors are typically required and how glazing interacts with this requirement.
Location | Typical Fire Rating | Temperature Rise Requirement | Glazing Impact |
Exit enclosure doors | 60-minute or 90-minute | 450°F max rise at 30 min | Fire-protective glass exceeding 100 sq in can fail temperature rise test |
Exit passageway doors | 60-minute or 90-minute | 450°F max rise at 30 min | Same limitation as exit enclosure doors |
High-rise stairwell doors | 60-minute or 90-minute | 450°F max rise at 30 min | Fire-resistive glazing required for lites larger than 100 sq in |
Horizontal exit doors | 90-minute | 450°F max rise at 30 min | Limited glazing; fire-resistive required for any significant glass area |
Corridor doors (standard) | 20-minute or 45-minute | Generally none | Larger glass areas permitted with fire-protective glazing |
Fire-protective glazing (wired glass, standard ceramics) transmits radiant heat freely. In a temperature rise door with a 100-square-inch fire-protective lite, the heat radiating through the glass is small enough that the overall door assembly can still meet the 450-degree limit. If the glass area increases beyond 100 square inches using fire-protective glazing, the radiant heat transfer through the larger glass area can cause the assembly to fail the temperature rise test. Fire-resistive glazing blocks radiant heat, which is why it can be used in larger sizes in temperature rise doors without failing the test.
Lite Kit Frame Requirements for Fire-Rated Doors
A lite kit is the metal frame assembly that holds a vision lite in a fire-rated door. The lite kit is not a generic trim piece. It is a fire-tested component that must be listed for use with the specific door, glass type, glass size, and fire rating. The following requirements apply to lite kits in fire-rated assemblies.
Material | Steel is standard for fire-rated lite kits. Aluminum lite kits are not permitted in fire-rated assemblies. |
Listing match | The lite kit must appear in the same listing as the door, frame, and glass. Mixing lite kit brands or models outside the listing voids the rating. |
Glass retention | Lite kits use glazing beads, clips, or snap-in channels to hold the glass. The retention method must match the listing. |
Intumescent material | Most fire-rated lite kits include intumescent gaskets or seals that expand during fire to hold the glass in place and seal gaps. |
Maximum cutout size | The door listing specifies the maximum cutout size for the lite kit. The cutout cannot exceed this dimension regardless of the glass size. |
Minimum stile width | The remaining wood or steel stile width after the lite cutout must meet the listing minimum, typically 6 inches on each side. |
Cross-section diagram showing: fire-rated door panel, lite kit frame (steel), intumescent gasket, fire-rated glass, glazing bead, and setting blocks. This diagram requires CDF approval before inclusion in the production file.
What Happens When Oversized Glazing Voids the Door’s Fire Listing
When a vision lite exceeds the maximum glass area permitted by the door assembly’s UL or WHI listing, the fire rating of the entire assembly is voided. This is not a partial reduction in rating. The assembly becomes unlisted and noncompliant. The following consequences apply when oversized glazing is installed in a fire-rated door.
- The door assembly fails fire inspection. The AHJ will reject the installation during certificate of occupancy inspections or during routine fire safety inspections.
- The building owner loses code compliance. An unlisted fire door assembly is a code violation under the IBC and NFPA 80.
- Insurance implications. Fire-rated assemblies are a condition of many commercial insurance policies. A voided listing may affect coverage.
- Remediation is expensive. The door must either be replaced entirely or the glazing must be removed and replaced with a listed combination. In most cases, the door itself must be replaced because the oversized cutout cannot be repaired to meet the listing.
- Liability exposure. In the event of a fire, a voided fire-rated assembly creates significant legal liability for the building owner, contractor, and specifier.
There is no field correction for an oversized lite cutout in a fire-rated door. Once the door has been cut beyond the listed maximum, it cannot be re-rated. The door must be replaced.
Field-Cutting Fire Doors for Vision Lites
Cutting a fire-rated door in the field to add a vision lite is prohibited on most fire ratings and is severely restricted on all others. The following rules apply based on industry standards and listing requirements.
Fire Rating | Field-Cutting Permitted? | Conditions |
20-minute | Yes, with restrictions | Must use listed lite kit; cutout size per listing; door label must not be removed or damaged |
45-minute | Limited | Some listings permit field preparation; most require factory-prepared doors |
60-minute | Generally prohibited | Most UL/WHI listings require factory preparation of lite cutouts |
90-minute | Prohibited | Lite cutouts must be factory-prepared per the listing |
3-hour (180-minute) | Prohibited | No field modifications permitted; all glazing must be factory-installed |
Even where 20-minute doors permit field cutting, the work must be performed by a qualified installer following the exact lite kit manufacturer’s instructions, using listed components, and without exceeding the listed maximum cutout size. The fire label on the door must remain intact after the modification. If the label is damaged or removed during the cutting process, the door is no longer identifiable as fire-rated and must be replaced.
CDF Distributors factory-prepares all lite cutouts in fire-rated doors before shipment, ensuring the cutout size, position, and lite kit installation comply with the applicable UL or WHI listing. Factory preparation eliminates the risk of field-cutting errors that void the assembly’s fire rating.
How to Properly Specify Glazing for Fire-Rated Assemblies
Specifying glazing for a fire-rated door assembly requires matching every component to a single listing. The following steps ensure the glazing selection maintains the assembly’s fire rating.
- Start with the fire rating. Determine the required fire rating for the opening based on the building code, wall rating, and occupancy type. The fire rating dictates which glass types and sizes are available.
- Determine if temperature rise is required. Check whether the door location requires a temperature rise rating. If so, fire-resistive glazing is needed for glass areas exceeding 100 square inches.
- Select the glass type. Choose between fire-protective (wired, ceramic) and fire-resistive (insulated, filmed) based on the rating, size requirement, and temperature rise needs.
- Verify the listing. Confirm that the specific combination of door, frame, lite kit, glass type, and glass size is covered by a single UL or WHI listing. Do not mix components from different listings.
- Specify the lite kit. Identify the listed lite kit for the selected glass and door. The lite kit must be listed with that specific glass product and fire rating.
- Confirm safety glazing requirements. Verify whether the door location requires impact-rated safety glazing (CPSC 16 CFR 1201 or ANSI Z97.1). If so, wired glass alone does not qualify.
- Document the listing reference. Include the UL or WHI design number on the door schedule and in the project specifications. This allows the AHJ to verify the assembly during inspection.
CDF’s ProBuilder tool automates this process by only presenting glazing options that are compatible with the selected fire rating and door configuration. When you select a fire rating in ProBuilder, the available glass types, sizes, and lite kits automatically filter to show only listed combinations.
Industry Standards Referenced
- NFPA 80: Standard for Fire Doors and Other Opening Protectives
- NFPA 252: Standard Methods of Fire Tests of Door Assemblies
- NFPA 101: Life Safety Code
- UL 10C: Positive Pressure Fire Tests of Door Assemblies
- UL 9: Fire Tests of Window Assemblies
- ASTM E119: Standard Test Methods for Fire Tests of Building Construction and Materials
- UL 263: Fire Tests of Building Construction and Materials
- IBC (International Building Code): Chapter 7, Fire and Smoke Protection Features
- CPSC 16 CFR 1201: Safety Standard for Architectural Glazing Materials
- ANSI Z97.1: Safety Glazing Materials Used in Buildings
Configure Your Fire-Rated Glazed Assembly
CDF’s ProBuilder tool allows architects, contractors, and facility managers to configure fire-rated door assemblies with vision lites online. During configuration, ProBuilder validates glazing selections against the applicable UL and WHI listings, preventing oversized or incompatible glass from being specified. ProBuilder displays the maximum permitted glass area for each fire rating and filters available lite kits to show only listed options.
For assistance specifying glazing for a fire-rated door assembly, call (855) 769-9895 or email sales@cdfdoors.com. CDF’s sales team is available Monday through Friday, 7:30 AM to 4:30 PM CT.
Frequently Asked Questions
What is the maximum glass size allowed in a 60-minute fire-rated door?
The standard maximum is 100 square inches of fire-protective glazing (approximately 10” x 10”). Larger vision lites are possible if fire-resistive glazing is used and the complete assembly (door, frame, glass, and lite kit) is covered by a specific UL or WHI listing that permits the larger size. The listing, not the glass type alone, determines the maximum area.
Can I replace wired glass with ceramic glass in an existing fire-rated door?
Only if the replacement glass, lite kit, and installation method are covered by the same listing as the existing door assembly. You cannot substitute one glass type for another based on general fire ratings. The specific listing must include the ceramic glass product as an approved option for that door and lite kit combination.
What is the difference between fire-protective and fire-resistive glazing?
Fire-protective glazing blocks flames and smoke but allows radiant heat to pass through. Fire-resistive glazing blocks flames, smoke, and radiant heat. Fire-resistive glazing meets wall assembly test criteria (ASTM E119) and can be used in larger sizes at higher fire ratings. Fire-protective glazing is limited to 100 square inches in 60-minute and 90-minute doors.
Is wired glass still allowed in fire-rated doors?
Wired glass is still listed for fire-rated applications, but its use is restricted in locations where safety glazing is required. It does not meet impact safety standards (CPSC 16 CFR 1201 or ANSI Z97.1) on its own. Many code editions and jurisdictions prohibit wired glass in schools, healthcare facilities, and other locations where human impact is likely. Check with the AHJ and current code edition for your project.
Can I field-cut a 90-minute fire-rated door to add a vision lite?
No. UL and WHI listings for 90-minute fire-rated doors require that lite cutouts be factory-prepared. Field-cutting a 90-minute door voids the fire rating. The same prohibition applies to 3-hour rated doors. CDF factory-prepares all lite cutouts in 90-minute and 3-hour rated doors before shipment.
What happens if my fire-rated door has oversized glazing?
The fire rating of the entire assembly is voided. The door will fail inspection, the building loses code compliance, and the assembly must be replaced. An oversized lite cutout cannot be repaired in the field. The door must be replaced with a correctly listed assembly.
Do temperature rise doors require special glazing?
Temperature rise doors can use standard fire-protective glazing at 100 square inches or less. For larger glass areas, fire-resistive glazing is required because it blocks radiant heat and allows the assembly to meet the 450-degree temperature rise limit at 30 minutes. The specific listing must cover the glass type and size for temperature rise compliance.
Why does the fire rating determine the maximum glass size?
Higher fire ratings require the assembly to withstand longer fire exposure. Glass, even fire-rated glass, is a weaker point in the assembly compared to steel or solid wood. The longer the required fire endurance, the less glass area the assembly can tolerate while still passing the fire test. This is why 20-minute doors allow full lites while 90-minute doors are generally limited to 100 square inches with fire-protective glazing.
