Terms such as "low-E glass" and "solar gain" are related to the energy efficiency of windows. These terms -- and similar words -- describe how heat is slowed as it tries to pass from a hot spot to a cooler one. The better a window can stop heat movement, the better its efficiency, which means you will need less energy to heat or cool your house. Here are a few terms you will hear while window shopping:
Insulated glass: An insulated glass window has two or three panes of glass sandwiched together, trapping air and heat between the panes.
Low-E glass: The E stands for emissivity, which describes the ability of glass to allow heat to pass through it. So low-emissivity means that only limited heat passes through the glass. Heat is reflected away by a transparent metallic coating sprayed on one side of the glass. Low-E glass also blocks ultraviolet light, preventing fading of interior fabrics in your home.
U-factor: A U-factor quantifies the amount of heat that flows through a window. The lower the number, the more energy-efficient it will be. A good double-pane window has a U-factor of .5 or lower.
Solar gain: When sunshine hits a window, some heat is transferred inside the house. This heat transfer is called solar gain. A single-pane window will have high solar gain, whereas a low-E window with insulated glass will have much lower gain.
Warm-edge spacer: One avenue for heat to escape is around the edges of the glass. Warm-edge spacers, developed from composite materials, stop the process. Warm-edge spacers improve the thermal performance of windows and reduce condensation.
Energy Star: Keep an eye out for Energy Star windows. Energy Star windows are certified by the National Fenestration Rating Council and meet strict energy efficiency guidelines set by the U.S. Department of Energy. Each window earning this certification will save $125-$450 a year on energy costs for a typical home.