When choosing your windows, pay careful attention to the energy rating numbers. They can tell you a lot.
Window manufacturers voluntarily participate in an energy-performance labeling program from the National Fenestration Rating Council (NFRC) to help both homeowners and contractors choose the most energy-efficient window for their needs and a particular climate.
The very first thing you should look for is an energy-star-rated window. Each new window contains a label with numbers and codes. The upper left corner contains the NFRC certification, which means the council has certified that particular window and its manufacturer according to NFRC standards. The upper right side of the label shows you the materials the window is made from, the manufacturer, model, and style.
You’ll notice energy performance ratings on the label. The U-factor ranges from .20 to 1.20 and shows its resistance to heat flow. The lower the number, the better. The window’s R-value measures its insulating value.
Another rating you’ll notice is the Solar Heat Gain Coefficient (SHGC). This number tells you how well the window blocks heat from the sun. Numbers range between 0 and 1, and as with the U-factor, the lower the number the better, meaning it lets in less solar heat.
You’ll also want to look for a window with a higher Visible Transmittance (VT) number. The VT is rated between 0 and 1. The higher the number, the more light is transmitted. The light coming through the panes: good. The heat coming through the panes: bad.
You’ll also see other ratings for Air Leakage (you’ll want a low AL number) and Condensation Resistance. Look for a higher CR rating, which goes from 0 to 100.