If you’ve ever looked at an early 1900s window, you’d notice ripples in the single pane of glass set into a wooden frame - a big difference from the most energy-efficient, double-paned, vinyl framed windows of today.
Although window glass has steadily improved through the decades, insulated windows - two or more pieces of glass with dead air between them - made their first appearance in the 1970s. The windows restricted the change in temperature and cut down on condensation by providing dead air space.
Low-e glass was introduced ten years later. Preventing heat and ultra-violet (UV) rays from coming through , low-e coatings are extremely thin layers of metallic oxide that are bonded to the surface of the glass in the window. This results in windows that do an even better job of keeping heat in during winter and out during summer.
While invisible to the human eye, infrared or UV rays makeup most of the sun’s energy. UV rays can also fade furnishings and finishes inside the home aside from causing those nasty sunburns. Whether on the asphalt outside, or from the furniture and flooring indoors, infrared rays produce heat. This heat is retained and released as the air around them cools, meaning infrared heat can still affect your home after the sun sets, if you let it in.
However, low-e coated glass reflects certain portions of the light spectrum back out through the windows, so the inside of the home stays cooler, not only during the sunny part of the day, but later as well.
The low-e glass comes in hard coat and soft coat. Tin is applied directly to the molten glass to make the hard coat. A thin layer of silver is applied to the glass in a vacuum in the case of the soft coat. Always sandwiched with another piece of glass, soft coat low-e glass is easily damaged. To prevent oxidation and to act as an additional insulator, Argon gas is often used between panes.
The heat mirror is the third type of low-e glass, which includes a thin polyester sheet suspended between the two panes of a dual pane window. The coating reflects radiant heat and the sheet decreases heat loss, making this the best option for extreme climates. The film increases insulation by as much as 100 percent while at the same time blocking as much as 99.5 percent of UV light.
Before buying windows, make sure the ones you think you want are appropriate for the climate you live in. For that, you will need to look for the Energy Star or National Fenestration Rating Council (NFRC) label. The U-value and the solar heat gain coefficient (SHGC) for the window should be included in the NFRC label. To comply with Energy Star standards, in the hottest areas, windows should have an SHGC rating of .4 or lower and a U-factor of .75 or lower; in the colder areas the U-factor should be .35 or less, but there is no SHGC requirement because solar heat gain is not an issue.
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