Tempered glass (also called toughened glass) is produced by heating standard float glass to approximately 1200°F, then rapidly cooling it with jets of air. This thermal treatment puts the surface layers of the glass into compression and the interior in tension, dramatically increasing its resistance to mechanical stress and thermal shock.
Where Tempered Glass is Required
The International Residential Code (IRC) mandates tempered or safety glass in specific window locations where breakage risk is elevated:
- Windows within 18 inches of the floor
- Windows within 24 inches of a door
- Shower and bath enclosures
- Skylights and overhead glazing
- Stair landings (within 36 inches of walking surface)
- Windows adjacent to hot tubs or swimming pools
How Tempered Glass Breaks
When tempered glass breaks, the stored internal tension causes it to fracture into thousands of small, roughly cubical pieces with blunt edges—dramatically reducing the risk of laceration compared to standard glass, which breaks into large sharp shards.
Tempered Glass Identification
Tempered glass has a small etched stamp in one corner indicating the manufacturer, thickness, and safety rating (ANSI Z97.1 or CPSC 16 CFR 1201). It cannot be cut or drilled after tempering—if dimensions need to change, a new piece must be cut and then re-tempered.
Cost
Tempered glass costs approximately 20–40% more than standard float glass of the same dimensions and thickness. For a standard 24×36-inch window, the difference is typically $30–$70.
