Tempered Glass in VELUX Roof Windows

Tempered glass—heat-treated glass for enhanced strength and safety—is fundamental to both the performance and design innovation of VELUX roof windows. Used on the exterior pane of the insulating glass unit (IGU), it resists hail, debris, and supports rooftop safety. If broken, it shatters into small, blunt fragments that reduce injury risk. Tempered glass also enables advanced design features like curved top units, enamel-printed surfaces, and the step-unit structure in models such as the Heritage Conservation Roof Window, ensuring durability and aesthetic flexibility.

Optical anisotropy - a natural trade-off of tempered glass

Tempered glass may exhibit a visual phenomenon called optical anisotropy — faint non-uniform patterns or slightly darker areas caused by the tempering process. These are typically visible under specific lighting conditions, such as early morning or late afternoon, and may become more pronounced due to reflections from nearby water surfaces—like lakes or the sea—or when two tempered panes are used together. While manufacturers work continuously to reduce its visibility, it cannot be eliminated. However, this effect is not a defect but a natural, documented feature of heat-treated glass, recognized in European standards EN 1863-1 and EN 12150-1.