Wednesday Wisdom - Tempered Glass vs annealed glass

There are 2 main types of glass that make up the thermal pane units we replace in existing windows as well as come in newly manufactured replacement windows. People are often unaware of the differences between tempered and annealed glass and their appropriate uses.

For normal installations, glass is annealed. In cases where a window is less than 18 inches from the floor or in a shower, glass must be tempered by code, even if the glass being replaced is not tempered. Patio doors must have tempered glass as well as windows in an entry door or glass in a storm door. Also any window within three feet of an open door must be tempered.

Annealed glass is slow cooled to relieve internal stresses so that it isn't apt to break during extreme temperature changes. When annealed glass breaks, it breaks into large shards which can be very dangerous.


Tempered glass is one of the hardest glass types. It is created by using chemicals and heat to balance the eternal stress rating of the glass. It breaks into small pieces when broken. Because it has to go through more processes in its creation, tempered glass is more expensive than annealed.