The R-value is a measure of how much heat can get through the window in an hour.
Windows with a high R-value (greater than five) allow less heat to get in on hot days—and less to leak out when you’re running your furnace on chilly evenings. The greater the R-value, the more efficient the window is.
When buying new windows, it is important to know how efficient they are, thus the R-value. Also, consider buying windows that bear the Energy Star label. Energy Star windows meet strict energy-efficiency guidelines set by the U.S. Environmental Protection Agency and the U.S. Department of Energy.
If you are not replacing windows, but want to increase the R-value, add a low-emissivity (low-E) coating. This is a thin, see-through layer of silver or tin oxide on the glass that prevents heat from penetrating the glass, allowing the window to be more energy efficient.