Pane and Gain: A Guide to Window Performance

About 76% of winter sun that reaches a standard double-pane window enters to become heat, according to the US Department of Energy.

As the season turns to winter in the northern hemisphere, it’s time to check every window to make sure it’s tightly shut. In a superbly sustainable home, closed windows are a passive heating system, allowing winter sun to enter and trapping heat inside. This is possible because incoming solar energy arrives mainly in ultraviolet (UV) and visible light wavelengths shorter than 800 nanometers, while outgoing heat energy radiates in longer infrared wavelengths. High-performance window glazing has coatings that allow visible and some UV light to pass, while blocking infrared. Storm windows, either on the outside or inside of your regular windows, are another passive system that slows down energy flows, keeping homes warmer all winter. You can boost your passive systems by using window attachments, opening blinds or drapes when sunlight is shining through, and closing them the rest of the time.

What Matters Most

To know how well you are inviting free solar heat to come in through your windows and stay for the winter, measure this sustainability indicator: