Anatomy of an energy-efficient window: Quality Frame Materials: A variety of durable, low-maintenance framing materials reduce heat transfer and help insulate better. Multiple Panes: Two panes of glass, with an air or gas-filled space in the middle, insulate much better than a single pane of glass. Some ENERGY STAR-qualified windows include three or more panes for even greater energy-efficiency, increased impact resistance, and sound insulation. Low-E Glass: Special Coatings reflect infrared light, keeping heat inside in winter and outside in summer. They also reflect damaging ultraviolet light, which helps protect interior furnishings from fading. Gas Fills: Some energy-efficient windows have argon, krypton, or other gases between the panes. These odorless, colorless, non-toxic gases insulate better than regular air. Warm Edge Spacers: A spacer keeps a window's glass panes the correct distance apart. Non-metallic and metal/non-metal hybrid spacers also insulate pane edges, reducing heat tansfer through the window.

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