Should You Use Laminated or Tempered Glass in a Curtain Wall?
Both are safety glass. They fail in completely different ways, and that difference is the whole basis for choosing between them.
Tempered glass shatters into small blunt fragments and the entire pane leaves the opening. Laminated glass cracks but the interlayer holds the pieces in place, so the pane stays in the frame.
On a ground-floor shopfront, either outcome is survivable. Twenty floors up, a pane that empties itself out of its frame is a very different event from one that cracks and stays put, and that is why most tall buildings end up specifying laminated glass on at least one surface regardless of what the strength calculation says.
This article covers how each type behaves, where each belongs, and why the combination is so common. PRANCE supplies glass curtain wall systems with laminated, tempered, and heat-strengthened glazing.
Tempered Glass in Curtain Wall Systems
Tempered glass, also called fully toughened, is heated to around 620 degrees Celsius then cooled rapidly. The surfaces contract and set while the core is still hot, which puts the surfaces into compression and the core into tension.
That locked-in stress is what makes it strong. Approximately four to five times the bending strength of annealed glass of the same thickness, which is why it handles wind load and thermal stress that would break ordinary glass.
Governed by ASTM C1048 in North American practice and EN 12150 in Europe.
How It Breaks
When the compressive surface layer is penetrated, the stored energy releases all at once. The pane disintegrates into small roughly cubic fragments, usually described as dicing.
Those fragments are far less dangerous than the long shards annealed glass produces, which is the safety benefit. What they are not is retained. The pane empties, and at height the fragments fall.
Nickel Sulfide Inclusions
The failure mode that makes specifiers cautious about tempered glass in facades.
Nickel sulfide particles occasionally occur in float glass as microscopic inclusions. In annealed glass they are harmless. In tempered glass they sit inside the tension zone, and over time a phase change causes them to expand slightly. If an inclusion sits in the wrong place, that expansion is enough to trigger failure.
The result is spontaneous breakage with no impact, sometimes years after installation.
Heat soak testing reduces the risk substantially. Panes are held at approximately 290 degrees Celsius for several hours, accelerating the phase change so that susceptible panes fail in the factory rather than on the building. Specified to EN 14179 in European practice.
Heat soaking does not eliminate the risk entirely. It reduces it to a level most projects accept, and on any tall building with tempered glass it should be specified rather than assumed.
Heat-Strengthened Glass
A middle option that gets overlooked.
Heat-strengthened glass is cooled more slowly than tempered, producing roughly twice the strength of annealed rather than four times. It breaks into larger fragments that tend to stay in the frame longer, and it is far less susceptible to nickel sulfide failure because the core tension is lower.
It cannot be used alone as safety glazing in most codes. Laminated into a unit, it is extremely common in curtain walls, offering good strength with better post-breakage behavior than tempered.
Laminated Glass in Curtain Wall Systems
Two or more panes bonded with a polymer interlayer under heat and pressure. Governed by ASTM C1172 and EN 14449.
The defining property is post-breakage retention. When the glass cracks, the interlayer holds the fragments, so the pane remains in the opening and continues to provide a weather barrier and a fall barrier until it can be replaced.
Interlayer Types
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PVB, polyvinyl butyral: the standard interlayer, typically 0.76 mm, good retention and widely available
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Acoustic PVB: softer formulation adding 2 to 4 STC points, covered under acoustic interlayers and sound reduction
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SGP, ionoplast: considerably stiffer and stronger, used where structural post-breakage capacity is required
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EVA: used in some applications and for encapsulating elements within the laminate
SGP interlayers matter where the glass has to keep performing structurally after breaking, such as balustrades, canopies, and overhead glazing. They cost more and the stiffness genuinely changes the engineering.
Thickness Notation
Laminated glass is usually described by its components. A 10.8 mm laminate means two 5 mm panes with a 0.76 mm interlayer. An 11.52 mm laminate means two 5 mm panes with a 1.52 mm interlayer, which is two layers of PVB.
Doubling the interlayer improves retention and acoustic performance, and it is a common upgrade on upper floors.
Breakage Behavior Compared
| Property | Tempered | Heat-strengthened laminated | Annealed laminated |
|---|---|---|---|
| Relative strength | 4 to 5 times annealed | Approximately 2 times annealed | Same as annealed |
| Fragment size | Small dice | Larger fragments, retained | Large cracks, retained |
| Stays in opening | No | Yes | Yes |
| Spontaneous breakage risk | Present, reduced by heat soaking | Low | None |
| Thermal stress resistance | High | Moderate to high | Low |
| Typical facade use | Outer pane, lower floors | Outer or inner pane, any height | Inner pane, low rise |
Indicative comparison. Code requirements and structural calculations govern the actual selection.
Where Each Type Belongs？
Most curtain walls use both, in different positions in the same unit.
Outer Pane
Takes the wind load, the thermal stress from solar gain, and any impact from outside. Tempered or heat-strengthened, selected from the structural calculation.
Where the outer pane is tempered and the building is tall, heat soak testing to EN 14179 is worth specifying as standard rather than as an option.
Inner Pane
Faces the occupants. This is where laminated glass earns its place, because a cracked inner pane that stays in the frame does not fall into the room.
Many jurisdictions require laminated inner panes above a certain height, and even where they do not, it is common practice on tall buildings.
Overhead and Sloped Glazing
Laminated is effectively mandatory. Glass above an occupied space that fails must not fall, which means retention is the controlling requirement rather than strength.
Codes typically require laminated glass on the underside of any sloped or overhead assembly, frequently with heat-strengthened rather than tempered plies.
Balustrades and Guarding
Where glass forms a fall barrier, laminated with a stiff interlayer is standard. SGP interlayers are common because the assembly has to retain structural capacity after one ply breaks.
Spandrel Zones
Opaque areas take high thermal stress from solar absorption behind the panel. Heat-strengthened or tempered glass is normal here, and laminated construction is less often required since there is no occupied space directly behind.
Safety Code Requirements
Requirements vary by jurisdiction and should be checked against the governing code rather than assumed.
In North American practice, CPSC 16 CFR 1201 and ANSI Z97.1 define safety glazing requirements, with the IBC setting out hazardous locations where safety glazing is mandatory. European practice uses EN 12600 for impact classification.
Common triggers for mandatory safety glazing include glazing in and adjacent to doors, glazing below a defined sill height, glazing adjacent to walking surfaces, glazing in wet areas, and overhead glazing.
Curtain walls frequently meet several of these at once, which is part of why both glass types appear in a single facade.
Typhoon, Hurricane and Impact Requirements
Jurisdictions with severe wind events add requirements that effectively mandate laminated glass.
Impact-resistant glazing must withstand a missile impact followed by cyclic pressure loading simulating a storm. Tempered glass alone fails this, since the pane leaves the opening after impact and the building envelope is then breached, allowing internal pressurization.
Laminated glass with an appropriate interlayer passes because it retains the opening after impact. Documenting glazing for a typhoon jurisdiction covers the submittal requirements that follow.
Processing and Coordination
Three practical points that cause problems when missed.
Tempered glass cannot be cut or drilled after tempering. All fabrication happens before the heat treatment, so late changes mean new panes and new lead time.
Low-E coatings interact with tempering. Standard soft coats cannot survive the furnace, so either a heat-treatable coating is specified or the glass is coated after tempering. Low-emissivity coatings and their surfaces covers coating selection.
Roller wave distortion. Tempered and heat-strengthened glass passes through a roller furnace, which introduces slight waviness. On a highly reflective facade this shows as visible distortion in reflections, and tighter tolerances can be specified where appearance matters.
Verifying any of this at submittal requires evidence, and how assemblies are proven before approval sets out what testing applies. Published certification documentation is a reasonable place to start when assessing a supplier.
Frequently Asked Questions
1. Which is safer, laminated or tempered glass?
Both are classified as safety glass, but they are safe in different ways. Tempered produces small blunt fragments rather than sharp shards. Laminated keeps the broken pane in the opening. At height, retention usually matters more, which is why laminated inner panes are standard on tall buildings.
2. Why does tempered glass sometimes break with no impact?
Nickel sulfide inclusions. Microscopic particles within the glass expand slightly over time, and in tempered glass the core is in tension, so an inclusion in the wrong location can trigger failure years after installation. Heat soak testing to EN 14179 reduces the risk substantially.
3. Is laminated glass required on high-rise curtain walls?
Frequently, and in many jurisdictions it is mandatory above a defined height. Even where codes permit tempered glass, laminated inner panes are common practice on tall buildings because a cracked pane that stays in the frame avoids falling glass.
4. Can a curtain wall use both laminated and tempered glass?
Yes, and most do. A typical unit uses a tempered or heat-strengthened outer pane selected for wind and thermal load, with a laminated inner pane providing post-breakage retention toward the occupied space.
5. What is heat-strengthened glass and when is it used?
Glass cooled more slowly than tempered, giving roughly twice the strength of annealed rather than four times. It breaks into larger retained fragments and carries much lower nickel sulfide risk, which makes it common as a ply within laminated units in curtain walls.
Choosing Between Them
Take the structural calculation first, since it sets the minimum strength. Then apply the code requirements for the location, which often mandate safety glazing regardless. Then ask what happens when a pane breaks at that position in the building, because retention is the question the calculation does not answer.
On most commercial facades the answer is a heat-strengthened or tempered outer pane with a laminated inner pane, heat soaked where tempered glass sits at height.
PRANCE supplies curtain wall systems with tempered, heat-strengthened, and laminated glazing including acoustic and structural interlayers, with processing and test documentation for the configuration supplied. Send your elevations and wind load data and request a glazing specification.