Same furnace, different quench. One is safety glazing and one stays in the frame — and the code decides which you need where.
Both are heat-treated in the same furnace; the difference is how fast they are cooled and therefore how much compression is locked into the surface. Tempered is stronger and breaks into cubes, which is what safety glazing means; heat-strengthened is half as strong, breaks into large pieces that stay in the frame, and almost never breaks on its own. Where the code requires safety glazing — a door, a shower, a railing, a sidelite, glass near the floor — it must be tempered or laminated, and heat-strengthened does not qualify. Where nobody can walk into the glass and thermal stress is the risk — a fixed lite in a facade, a spandrel — heat-strengthened is the better and usually cheaper answer. The three-way comparison with laminated glass is here.
| Feature | Heat-strengthened | Tempered (fully tempered) |
|---|---|---|
| Surface compression | 3,500–7,500 psi | ≥10,000 psi |
| Strength vs annealed | ~2× | ~4× |
| Break pattern | Large pieces, stay in the frame | Small blunt cubes, fall out |
| Safety glazing (ANSI Z97.1) | No | Yes |
| Spontaneous (NiS) breakage | Negligible | Possible; heat-soak to screen |
| Optical distortion | Less roller wave | More roller wave and anisotropy |
| Thermal stress resistance | Excellent | Excellent |
| Can be cut after | No | No |
| Typical use | Facade fixed lites, spandrels, laminated overhead | Doors, showers, railings, storefront, anything within reach |
| Cost | Similar; often slightly less | Similar |
Both start as annealed glass, cut to final size, heated to about 620°C. Tempered glass is then quenched fast with air jets, locking in surface compression above 10,000 psi. Heat-strengthened glass is cooled more slowly, to between 3,500 and 7,500 psi. Compression is what gives heat-treated glass its strength, so tempered is stronger; but it is also what makes it shatter completely when the surface is breached, and what makes a nickel-sulphide inclusion able to trigger a spontaneous break years later. Heat-strengthened glass has too little stored energy for either.
A fixed lite forty feet up wants three things: to resist the thermal stress of sun on one half and shade on the other, to resist wind, and to stay in the frame if it ever breaks so it does not fall on the sidewalk. Heat-strengthened glass does all three, with less optical distortion than tempered and no spontaneous-breakage risk. Tempered glass at that height is a liability: a break sends a shower of cubes down and leaves a hole. Where the code also requires safety glazing on a facade lite — near a walkway, in a door — the answer is laminated heat-strengthened. See heat-soaked glass for the screening test when tempered is unavoidable.
Safety glazing is defined by the break: if a person falls into it, it must not produce shards that cut. Tempered glass passes because it breaks into cubes; laminated passes because it stays in one piece. Heat-strengthened produces large sharp pieces and fails the test, so it cannot be used where the code calls for safety glazing however strong it is. Doors, sidelites, showers, railings, storefront at sidewalk level, glass within 18 inches of the floor: tempered or laminated, never heat-strengthened.
No. It breaks into large sharp pieces and does not meet ANSI Z97.1. Tempered and laminated glass do.
Tempered, about four times annealed strength against two for heat-strengthened.
Heat-strengthened for fixed lites nobody can walk into: it stays in the frame when broken, has less distortion and no spontaneous-breakage risk. Laminated heat-strengthened where safety glazing is also required.
Almost never; its stored energy is too low for a nickel-sulphide inclusion to trigger a break. That is one of its advantages over tempered.
About the same; heat-strengthened is often slightly less because the quench is gentler. The application, not the price, decides.
About this page. Written by Alex GC Inc. d/b/a Alex Glass Construction and reviewed before publication by Aleksandr Chernovolenko, Managing Partner, with our design and estimating lead and a field foreman. Our foremen hold the NYC DOB Site Safety Training supervisor card and our crews the 40-hour worker card; we are certified to fabricate, install and service Alutech and Everest aluminum systems. Facts are drawn from our own projects, manufacturer test data and the applicable codes; where we are giving an opinion, we say so. Last reviewed September 2026. Credentials
Tell us where the glass goes. We will specify tempered, heat-strengthened or laminated to the code, and quote.
