Q–My husband and I will be building a new home soon. Our windows are available with optional low-E glass. Does it really work? Are there different types? Will it block ultraviolet (UV) light? Do you think it is worth the extra money?
A–Window glass was revolutionized in the 1970s. Insulated glass (two or more pieces of glass with a dead air space in between) made its debut in the early ’70s. Low-E glass (the “E” stands for emissivity) was introduced in 1979. It utilizes an ultra thin metallic coating on or in the glass to reflect heat back toward the source.
Sunlight contains visible light, UV light, and infrared (IR) light. Visible light enables us to see things. Ultraviolet light damages skin, wood and fabrics and causes colors to fade. Infrared light is basically heat. Low-E glass has the ability to allow visible light to pass while blocking certain amounts of UV and IR light.
The infrared light in sunlight heats up the objects it strikes, including your tile floors, furniture, sidewalks and patio furniture. As these objects cool, they emit a low-powered form of IR light. Low-E glass reflects this form of energy. In the summer, this helps to keep your house cooler, as the heat from objects outside is kept outside. In the winter, all objects in your home are heated (by either the sun or your furnace). This heat is also bounced back into your house by the low-E glass.
There are two types of low-E glass: hard coat and soft coat. To make hard coat low-E glass, tin is applied directly to the molten glass. The soft coat process commonly involves the application of a thin layer of silver while the glass is in a vacuum. This coating is delicate and can oxidize if exposed to air, so soft coat low-E glass is always sandwiched with another piece of glass. Argon gas is sometimes used to prevent this oxidation and also acts as an additional insulator.
Low-E coatings help to reduce condensation on glass. The inside surface remains warmer than the outside and the difference can be dramatic. Imagine a cold night with an outdoor temperature of 0 degrees and a 15 m.p.h. wind. The inside temperatures of different types of glass would be: single pane, 26 degrees; regular double-pane, 35 degrees; hard coat, 49 degrees; and soft coat, 62 degrees.
Some glass manufacturers have gone even farther. They have suspended thin, low-E transparent films in between pieces of glass. This system has excellent performance characteristics. Some of these films can block 99.5 percent of UV light. Some boast an insulating value twice that of soft coat low-E glass.
Low-E glass is worth the price, especially because houses tend to lose 25 percent of their heat through windows. Purchase the highest quality low-E glass you can afford.
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Have questions about the remodeling process? Write to Tim Carter, c/o The Chicago Tribune, P.O. Box 36352, Cincinnati, Ohio 45236-0352. Questions will be answered only in the column.
