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Welcome.
In this post, I'll be going over the different types of glass used in glazing most residential and commercial windows and doors.
Before we get started, I should say that this is not a complete guide. Before purchasing, installing or re-glazing any windows, make sure to discuss your needs with a general contractor or architect, or feel free to contact us to learn what window and glass options are available.
Types of Glass
Annealed
In this context, Annealed glass is just regular glass. Annealing is a part of the manufacturing process during which hot glass is cooled slowly to relieve internal stresses after it was formed. This strengthens the glass and prevents shattering from temperature changes and physical shock. Annealed glass can be cut, machined, polished, or otherwise altered, and so is typical in the glass used in windows.
Heat-Strengthened Glass
For increased strength, glass may be subjected to a controlled heating and cooling cycle during the manufacturing process. Once heat strengthened, glass becomes about twice as strong as annealed glass, and has a higher tolerance to thermal stress or wind loads. When broken, the shards are typically larger than those of tempered glass and may stay in the window opening.
It should be noted that heat-strengthened glass is not the same as tempered or "safety glass", and should not be used anywhere that tempered glass is a requirement. Heat-strengthened glass cannot be cut, sandblasted, etched, drilled or otherwise altered without the risk of failure.
Tempered Glass
When broken, tempered glass shatters into small pieces that are less likely to cause serious injury.
Like annealed glass, tempered glass is another standard for residential windows. Glass is tempered by heating it to over 1,000 degrees Fahrenheit then rapidly cooling it to lock the surface in a state of compression, and the core in a state of tension.
Once tempered, the glass becomes four times stronger than annealed glass (twice as strong as heat-strengthened), and when broken will shatter into thousands of tiny pieces that are less likely to cause serious injury. For these reasons it's often referred to as "safety glass" by the various code organizations who set building standards for safety.
Some applications for tempered glass include bath and shower enclosures, bathroom windows, sliding doors, and windows near swinging doors or along staircases (it's also used in the passenger and rear windows of your car).
Laminated Glass
Laminated glass consists of two lites permanently adhered to a plastic interlayer.
When it is necessary to prevent objects (or people) from penetrating glass, laminated glass is used. Laminated glass is two or more panes of glass (also called "lites") permanently bonded together with one or more plastic interlayers in between. When broken, shards and fragments adhere to the plastic layer to keep the laminated sheet in one piece, reducing the risk of injury. Laminated gass is typically used anywhere with risk of impact, whether by a human or object (such as a treebranch) falling through. Car windshields and skylight glazing are typical applications.
Laminated glass is also considered "safety glass", and is often incorporated with tempered glass for increased impact resistence. Some uses include hurricane resistence, bomb blast protection, sound attenuation, and entrance security.
Insulating Glass
In this cross-section of insulated glass, we can see that there's much more to insulating glass than just two glass panes stuck together. The spacer is often made of aluminum with a moisture absorbing dessicant inside. To prevent heat transfer between the glass surface and the aluminum spacer, a "warm edge spacer" may be used to break the thermally conductive metal-to-glass contact point, thus increasing the insulation provided by the glass.
Insulating glass refers to two or more panes sealed around the edges with an open air space in between, forming a single unit. The windows in your home are likely glazed with insulating glass.
Insulating glass is the most effective way to reduce air-to-air heat transfer through glass, and is often used in conjunction with Low-E and/or reflective glass for increased insulation. The most common architectural insulating glass unit configuration is 1/4" glass - 1/2" air space - 1/4" glass.
Tinted and Low-E Coated Glass
To increase energy efficiency and supplement insulating glass, glass is often given a "low-e" or "tinted" coating. Low-E coatings reduce the amount of solar energy passing through the glass (and in turn reduce how hot the carpet gets in your cat's favorite spot in the living room).
Before Low-E technology was invented, tinted or reflective coatings were often used, but were much less effective at heat insulation. Tinted glass radiates the absorbed thermal energy, making the glass and air around it hot (defeats the purpose of insulating at all). Reflective coatings are somewhat effective in reducing the amount of heat transmitted, but limit the amount of visible light to pass through the glass).
Nobody likes a sunburned cat. A low-E coating on your windows reduces the amount of heat transmitted through your windows, saving you money on energy and sun-screen.