Types of Window Glass
If you’re planning a major window installation or replacement job, you have a number of options when it comes to the type of glass to use. There are many ways to treat glass that will affect its strength, energy efficiency, and appearance:
Float Glass – float glass is created by passing molten glass onto a pool of molten tin, creating large, thin, flat panels. The resulting glass panels will be very smooth with a highly consistent thickness. Almost all window glass starts out as float glass before being treated further.
Annealed Glass – annealed glass is float glass that has been cooled slowly, in a controlled manner. The process of slowly cooling the glass reduces internal stressed within the sheet, making it stronger. Almost all float glass is annealed; this annealed glass is the starting point for further treatment. If it is not treated, annealed glass will break into large, sharp shards. For this reason, it is rarely used in buildings.
Heat Strengthened Glass – heat strengthened glass starts out as a piece of annealed glass that is then reheated beyond the annealing point (1200 F) and cooled fairly rapidly. This cooling is not as rapid as it would be with tempered glass, meaning the compressive and tensile stresses won’t be quite equal across a section of glass. This results in glass that’s about twice as strong as annealed glass.
Heat strengthened glass will break into pieces smaller than annealed glass shards, but still sharp enough to cause injury. As a result, it’s rarely used in buildings unless laminated.
Fully Tempered Glass – fully tempered glass is formed through a process that makes it four times stronger than annealed glass. First, the annealed glass has to be cut and finished to size (tempered glass cannot be cut). Next, it is heated past the annealing point (1200 F) and cooled rapidly enough that the internal portion of the glass remains fluid longer, forming equal tensile and compressive stresses across the glass. This makes it extremely strong, and the resulting glass pane, if broken, will shatter into small granular pieces, vastly reducing the risk of injury.
Tempered glass is commonly used as a safety glass where the glazing may need to be broken out of the frame during an emergency, for instance in car windows that may need to be broken in the event of an accident.
Heat Soaked Tempered Glass – heat soaking is a way to test tempered glass for unstable nickel sulfide inclusions, which can cause spontaneous breakage of the pane. Panes of glass are put into an oven and heated to a temperature around 550 F for a few hours. This causes any nickel sulfide inclusions to expand faster than the glass, causing it to break.
The idea behind heat soaked glass is to force unstable glass panes to fail in the plant before they fail in the field. This is hugely important for things like glass railings and other uses where the glass pane is critical for safety.
Laminated Glass – laminated glass, another safety glass, is made by fusing two or more layers of glass with inter-layers of a material called polyvinyl butyral (PVB) using heat and pressure.
If the laminated glass is made of heat strengthened glass, the PVB layers will hold the glass sheets together in the frame, so if they happen to shatter, shards of glass will stay in place and won’t fly everywhere. If it is made using tempered glass, the sheet may fall out of the frame but will still stay together.
Laminated glass is best used as a safety glass where the glazing must remain intact if the glass is broken, for instance in a car windshield – in the event that an object hits it, you don’t want the object passing through or the glass to shatter in the faces of the passengers. Laminated glass for home windows is primarily used in impact / hurricane resistant windows
Low-Emissivity Glass – low-emissivity (Low-E) glass has a special coating that reflects infrared radiation (heat) while still allowing light from the visible spectrum to pass through. The benefits are two-fold: heat from the outside is kept out, reducing cooling load during the summer, and heat from the inside is kept in, preventing heat loss during the winter. Low-E glass is created by coating the glass in either tin oxide or silver.
Low-E glass can make any window significantly more energy efficient. Whether you plan to purchase wood windows, vinyl windows, fiberglass windows, aluminum windows, or composite windows, Low-E glass can be added to improve the energy efficiency or your selected unit.
At Quality Window & Door, our window installers can walk you through the pros and cons of each type of glass so you can choose the best replacement windows for your job. Call us today!
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