Fire Rated Windows Guide – 1 Hour vs 2 Hour Ratings
Everything the specifier needs to know: rating grades from 20 minutes to 2 hours, what "E" vs "EI" means, the three categories of fire-resistant glass, why frames matter, and a step-by-step specification workflow — with code references.
Before we start: This guide covers architectural fire rated windows — glazing assemblies certified by ASTM E119 or EN 1363 for building envelope performance during fire exposure. If you were looking for decorative crushed glass for fire pits and fireplaces, that is our companion Fire Glass Calculator — a completely different product, different purpose, different certification. The two are easy to confuse in search results, so we link them here explicitly.
What Fire Rated Windows Actually Are
A fire rated window is not just fire resistant glass — it is a complete assembly. The glass, the frame, the glazing gaskets, the perimeter seals, even the screws and anchors all contribute to the certification. Change any one of them and the rating no longer applies. This is the single most important point to understand about specifying fire rated glazing: it is an assembly certification, not a material certification.
The test standards are ASTM E119 in North America and EN 1363-1 / EN 13501-2 in Europe. A full-size assembly is placed in a furnace and heated to the ISO 834 standard time-temperature curve, which rises from ambient to about 1000°C over 60 minutes. The assembly is considered to have passed if it maintains integrity (no flaming or significant smoke on the cold side) and, for insulation ratings, keeps cold-side glass temperatures below specified limits. The results are certified by third-party laboratories (UL, Warnock Hersey, SGS) and manufacturers are required to permanently mark certified units with their rating.
Two rating prefixes appear frequently and are often confused:
- E = Integrity only: The assembly must maintain its shape and prevent flame passage for the rated duration. Cold-side temperature limits are not required. Used in low-risk applications.
- EI = Integrity + Insulation: Everything required for "E", plus the cold side must stay below 140°C average and 180°C at any point. The more stringent rating used in escape routes and rated walls.
Fire Rating Grades — 20, 45, 60, 90, 120 Minutes
The number of minutes in a fire rating is not arbitrary. Fire duration codes are tied to building construction type and escape route travel distance. Below is the complete breakdown of what each grade means and where it appears.
| Grade | Min | Requirement | Typical Glass | Typical Use |
|---|---|---|---|---|
| E20 / 20-min | 20 | Integrity only (flame resistance, no structural failure for 20 min) | Wired glass or tempered annealed | Non-rated assemblies, decorative openings, low-risk interiors |
| E45 / 45-min | 45 | Integrity + limited radiation temperature (≤110°C on cold side) | Wired glass or low-expansion ceramic | Apartment corridors, service shafts, secondary escape routes |
| EI60 / 1-hour | 60 | Integrity + insulation (cold side ≤140°C avg, ≤180°C point) | Intumescent gel-filled units or ceramic | Primary escape routes, stairwells, 2-hour corridor walls |
| EI90 / 90-min | 90 | Same as EI60 for 30 min longer — critical assembly duration | Multi-layer intumescent or high-performance ceramic | Long travel distances in high-rise, basement escape routes |
| EI120 / 2-hour | 120 | Integrity + insulation for full 2-hour exposure duration | Multi-pane insulated intumescent with ceramic layers | Fire wall penetrations, high-rise primary escape, tunnel portals |
💡 Test environment note: These are center-of-glass grade descriptions. Whole-assembly performance can vary by ±20% depending on frame, gasket, installation precision and temperature curve variations. Always reference the specific third-party test report for your chosen assembly — the test conditions and results are authoritative, not the grade letters alone.
The Three Categories of Fire Resistant Glass
Not all glass is created equal when it comes to fire performance. Three distinct families meet certification standards, each with different capabilities, manufacturing methods, and ideal applications. Standard float glass, tempered glass and ordinary laminated glass do not qualify as fire rated — they fail integrity within a few minutes of exposure.
1. Wired Glass
Wired glass is the oldest fire-rated glazing. It is annealed float glass with a galvanized steel wire mesh embedded during manufacturing. When exposed to fire, the glass cracks but the wire mesh holds the panes together, preventing flame passage for 20–45 minutes. It does not provide insulation — cold-side temperatures climb throughout the exposure.
Pros: Low cost, proven technology, widely available.
Cons: Only E rating (no insulation), reduced visibility from wire mesh, cannot be used where EI rating is required, wire eventually corrodes in coastal environments.
Where you'll see it: Service shafts, electrical room doors, decorative openings in non-rated partitions.
2. Low-Expansion Ceramic Glass
Ceramics expand and contract much less than glass when heated — the coefficient of thermal expansion (CTE) of fire-rated ceramics is near zero. This means they do not crack or shatter under rapid thermal shock and maintain both integrity and insulation throughout the rated duration. Some ceramics are also tougher and more impact-resistant than annealed glass, useful where both fire and vandalism resistance are needed.
Pros: Achieves full EI ratings, zero thermal expansion, high clarity, impact resistance in some grades.
Cons: Significantly more expensive than wired glass, limited thickness range, minimum and maximum size restrictions per certification.
Where you'll see it: Escape stairwell windows, corridor glazing, high-rise rated partitions, 1-hour and 2-hour assemblies.
3. Intumescent Gel-Filled Units
Intumescent glass is an insulated glazing unit with two regular or tempered glass panes separated by a clear polymer gel. When the outer pane heats up and breaks, the gel absorbs water (from atmospheric moisture) and foams into a thick, opaque, insulating foam that blocks heat and flame passage for up to two hours. The gel layer is the magic — it goes from clear and invisible to a robust foam barrier in minutes.
Pros: Best cold-side insulation of any fire glass, clear when installed (views preserved), achieves full EI120 ratings, can combine with laminated panes for safety.
Cons: Expensive, thicker than other types (requires deeper frame rebates), size restricted per certification, requires moisture to activate (dry buildings may need moisture access provision).
Where you'll see it: High-rise escape route glazing, tunnel portals, two-hour fire wall penetrations, premium mixed-use developments.
Steel vs Aluminum — Why Frame Material Makes or Breaks the Rating
A fire rated window is only as good as its weakest link, and frame material is the most common failure point. Aluminum melts at about 660°C; steel melts at about 1500°C — more than twice as high. In a 60-minute fire exposure (peak furnace temperature around 950°C), an aluminum frame softens, deforms and loses connection integrity. A galvanized steel frame maintains its shape and connection strength throughout.
Aluminum Frames
Acceptable only for E20 and E45 (integrity-only) ratings. Must be reinforced with steel inserts in some codes. Not acceptable for EI60+ insulation ratings. Advantage: lighter, lower cost, available in more finishes.
Galvanized Steel Frames
Required for EI60+ (insulation) ratings. Often powder-coated for corrosion resistance. Heavier and more expensive than aluminum but mandatory for code-compliant escape route assemblies. Steel frames are certified together with the glass as one assembly.
⚠️ Assembly integrity: Never mix a listed fire glass with an unlisted frame. The certification applies to the complete assembly — glass + frame + gasket + installation method. If any component is changed, the certification is void. This is the most common field inspection failure in fire window installations.
Step-by-Step Fire Rated Window Specification
Specifying a compliant fire rated window assembly follows a disciplined workflow. These six steps — from code reference to certification documentation — cover everything you need to avoid field rejections and ensure the rated assembly actually performs as specified.
- 1
Confirm the code requirement first
CRITICAL ~5 minutesBefore selecting any glass, check your local building code and plan approval documentation to find the required rating grade and location. Typical code references: IBC (US), BC (Canada), BS 9990 (UK), EN 1990 (EU). The code specifies the minimum rating and whether it requires insulation (EI) or integrity only (E).
Risk if skipped: Skipping this step leads to field rejections and expensive replacements.
- 2
Determine the correct frame material
CRITICAL ~10 minutesEI60+ ratings almost always require steel frames. E20–E45 may allow reinforced aluminum in some codes. Consult the glazing manufacturer's listing — the frame must be listed together with the glass in order for the assembly to maintain its certification. Mixing listed glass with an unlisted frame invalidates the rating.
Risk if skipped: Using the wrong frame invalidates the test certification.
- 3
Select the glass type matching your rating
~15 minutesMatch rating to glass type: E20/E45 = wired or low-expansion ceramic; EI60 = single intumescent or ceramic; EI120 = multi-layer intumescent or high-performance ceramic. Note that wired glass only achieves E ratings (integrity, no insulation), while ceramic and intumescent achieve both E and EI.
Risk if skipped: Selecting glass below the required rating fails inspection.
- 4
Confirm glazing gasket and bead specifications
CRITICAL ~10 minutesFire rated glass must be glazed with intumescent gaskets that expand when heated to fill gaps and prevent smoke and flame passage. Standard EPDM gaskets are acceptable for low ratings (E20) but NOT for EI60+. Intumescent beads are applied around the glass perimeter between glass and frame. Verify the gasket type is listed with your assembly certification.
Risk if skipped: Incorrect gaskets are a common field inspection failure.
- 5
Verify size limitations and edge treatments
~5 minutesEvery fire glass certification has maximum and minimum size limitations. Some ceramic glass must have polished edges to maintain temperature uniformity during exposure. Intumescent units require specific edge clearances in the glazing rebate. Check the manufacturer's product datasheet before finalizing dimensions.
Risk if skipped: Specifying outside certified dimensions voids the rating.
- 6
Request third-party certification documentation
CRITICAL ~5 minutesAsk your supplier for the current certification test report from UL, Warnock Hersey, SGS or another recognized third party. The report must specifically name the glass, frame, gasket and installation method. Keep this documentation — building inspectors will ask for it during final inspections.
Risk if skipped: Installing uncertified assemblies stops the inspection process.
✅ Verification step: Before signing off, cross-check your specification against the manufacturer's current UL listing or EN certification. Verify the listing specifically names your chosen glass, frame material, glazing gasket, edge clearance and maximum pane size. If any parameter differs, the listing does not apply.
Do You Actually Need Fire Rated Windows in a Residential Home?
This is one of the most common questions about fire rated glazing, and the answer is almost always "no" for single-family detached homes — but there are important exceptions. Fire rated windows are primarily an assembly for multi-family construction (apartments, condominiums, townhouses) and commercial buildings where windows adjacent to escape routes or rated walls create a fire spread risk.
You PROBABLY don't need it if
- ✓ Single-family detached home
- ✓ Windows are not adjacent to another dwelling's escape route
- ✓ Garage-to-house wall has no window opening, or it's in a non-rated position
- ✓ Local code does not have wildfire-specific glazing requirements
You MIGHT need it if
- ✗ Apartment or townhouse adjacent to another dwelling
- ✗ Window opens onto a shared escape stair or corridor
- ✗ Building code requires a rated wall and your window is in it
- ✗ Wildfire-prone areas may require tempered or impact-resistant glazing
Always confirm with your local building code official or a certified fire protection engineer. Requirements vary significantly by jurisdiction and site conditions.
Glass in Fire Rated Doors — The Rules
Fire rated doors with vision panels are among the most common applications for fire rated glass. The glass panel in a fire door must meet or exceed the door's fire rating, and must be properly glazed with intumescent gaskets — standard door glazing clips are not acceptable. Maximum glass panel size is also limited by most codes.
Typical Fire Door Glass Rules (per IBC and EN 1634)
- EI60 door Glass must be EI60 or better. Maximum panel size: typically 1,290 cm² (200 sq in).
- EI120 door Glass must be EI120 or better. Maximum panel size may be smaller per door listing.
- Any fire door Glazing perimeter must have intumescent gaskets (not standard EPDM). Verify with door manufacturer.
- Non-rated door Can use standard wired or tempered glass, but it does not contribute to the door's rating.
As with fire windows, the door-glass-gasket assembly is certified as a unit. Changing the glass or gasket after installation voids the door's listing. For high-security applications, some fire doors also combine fire rated glass with bullet-resistant laminates — check the manufacturer's listing for combined certifications.
Fire Rated Windows FAQ
What is a fire rated window?▼
A fire rated window (also called fire resistant window or protected glazing) is a window assembly — glass, frame and glazing — that has been tested to maintain its integrity and, in higher ratings, its insulating performance for a specified duration when exposed to standard fire conditions. Ratings are expressed in minutes (20, 45, 60, 90, 120) and often include a letter code: E = integrity only, EI = integrity + insulation. A 60-minute EI window maintains its shape and keeps the cold side glass below 140°C for one hour of fire exposure.
What does a 1-hour vs 2-hour fire rating mean?▼
The number of minutes is how long the assembly must withstand standard fire conditions (ASTM E119 in North America, EN 1363-1 in Europe). A 1-hour (60-minute, EI60) window must keep its shape and maintain cold-side glass temperatures below 140°C for the full hour. A 2-hour (120-minute, EI120) window does the same for twice as long. The difference matters when the building code requires a 2-hour rated wall — any openings (windows or doors) in that wall must also be 2-hour rated. In practice, a 2-hour window uses thicker intumescent layers, additional ceramic panes, and a heavier steel frame.
Which glass types count as fire rated?▼
Three categories meet fire rating requirements: 1) Wired glass — the old standard, annealed float with a wire mesh embedded that holds the pane together if it breaks; rated E20–E45 only. 2) Ceramic glass — high-temperature-resistant ceramics that expand minimally when heated; rated EI45–EI120; the current high-performance standard. 3) Intumescent gel-filled units — two panes with a clear polymer gel between them that foams into an insulating foam when heated; rated EI60–EI120; best insulating performance. Note: tempered glass is NOT fire rated — it breaks into small pieces and falls out during exposure, losing integrity immediately. Laminated glass with PVB may provide short-duration integrity but is not certified.
Can fire rated glass go in a regular aluminum frame?▼
It depends on the rating. Low-rated E20/E45 windows (integrity only) may use reinforced aluminum frames in some codes. For EI60+ (insulation) ratings, the frame must be steel — almost always galvanized steel with intumescent gaskets in the glazing rebate. Aluminum melts at 660°C; steel melts at 1500°C. In a 60-minute fire, steel frames maintain their shape and connection integrity. Aluminum frames lose strength rapidly in that time. Most fire window specifications call for steel frames as standard.
Do I need fire rated windows in a residential home?▼
In most jurisdictions, single-family detached homes do not require fire rated windows — even between the garage and the house. However, some areas (particularly those with wildfire risk) require tempered or impact-resistant glass near property lines or high-risk areas. Townhouses and apartments are different: any window adjacent to another dwelling's escape route may require E45 or EI60 rating. Always check your local building code and plan approval requirements.
What glass rating goes in a fire rated door?▼
Fire rated doors with vision panels (glass) require that the glass maintain at least the same rating as the door. A 1-hour door needs EI60 or better glass. A 2-hour door needs EI120. The glass panel must also be properly glazed with intumescent gaskets around the perimeter — standard door glazing clips are NOT acceptable. In some codes, the maximum glass panel size in a fire door is limited to 1,290 cm² (200 sq in).
How is fire rated glass tested and certified?▼
The test standard is ASTM E119 in North America and EN 1363-1 / EN 13501-2 in Europe. A full-size assembly (glass + frame) is placed in a furnace and heated to a time-temperature curve (ISO 834) matching real-fire conditions. The test passes if the assembly maintains integrity (no flaming on cold side) and insulation (cold side glass temperatures within specified limits) for the full rated duration. Certifications are issued by third-party labs (UL, Warnock Hersey, SGS). Manufacturers mark certified units with a permanent label showing the rating.
Is fire rated glass the same as fire pit glass?▼
No — these are completely different products with zero functional overlap. Fire rated glass is a structural or glazing assembly designed and certified to maintain building envelope performance during a fire. Fire pit glass (the subject of our companion calculator) is decorative crushed or tumbled tempered glass placed inside fire pits and fireplaces as a bed for flames. Our site has both: the Fire Glass Calculator covers decorative fire pit glass, and this guide covers architectural fire rated windows. Be careful not to confuse them when specifying.
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