The value of energy-efficient windows is clear

A window should do more than provide light and air flow to your home. It should also keep heat in and cold out in the winter, and help keep heat out in the summer. That's a lot to ask of the humble window.
Every window leaks heat - it can't be as thermally efficient as an insulated wood or concrete wall. No matter how good a window is, its R-value won't be as high as those barriers. Warm air leaks out in cold weather and heat seeps in in the hot months. And, year-round, UV light passes through the glass.
Old windows were made with a single pane of glass, set in a wooden frame and secured with putty. Things improved when storm windows were placed over these, which created an air layer between the two windows and helped with insulation.
Later windows had double-glazing - two sheets of glass together in the frame. Many older homes still have single-glazed or old-style double-glazed windows, and they lose a lot of heat. It's like having a hole in your wall.
Today, better-quality windows have an inert gas injected between the layers of glass. This helps provide insulation and almost doubles the R-value of a window.
Another feature to look out for is low-E glass - low-emissivity glass. This has been treated with a microscopic metallic oxide spray that reduces the amount of UV light passing through the glass. It lets in light but reflects heat in summer and helps retain it in winter.

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