More information

MORE INFORMATION
For a glossary of window terns, visit www.efficientwindows.org/glossary.cfm.
Research the ratings of window styles by 343 manufacturers through the Certified Products Directory Search at http://cpd.nfrc.org/pubsearch/psMain.asp.
WEB SITES
Efficient Windows Collaborative: www.efficientwindows.org
Energy Star Windows Program: www.energystar.gov
National Fenestration Rating Council: www.nfrc.org
U.S. Department of Energy: www.energy.gov
National Renewable Energy Lab: www.nrel.gov
Center for Sustainable Building Research: www.csbr.umn.edu
Source: National Fenestration Rating Council
When you shop for replacement or new-construction windows for your home, look for the label that gives you energy and other performance ratings, including:
U-Factor. Measures how well a product prevents heat from escaping. U-Factor ratings generally fall between 0.20 and 1.20. The insulating value is indicated by the R-value, which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.
Argon is an inert, nontoxic gas used in insulating glass units to reduce heat transfer.
Low-emittance (Low-E) coating. Microscopically thin, virtually invisible, metal or metallic oxide layers are deposited on a window to reduce the U-factor by suppressing heat flow. Low-e windows cost about 10 to 15 percent more than regular windows, but they reduce energy loss by 30 to 50 percent.
Solar Heat Gain Coefficient (SHGC). Measures how well a product blocks heat caused by sunlight. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat-gain coefficient, the less solar heat it transmits, which ultimately affects your cooling bills.
Visible Transmittance (VT). Measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
Air Leakage (AL). Indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly. Optional rating.
Condensation Resistance (CR). Measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. This rating cannot predict condensation, but it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100. Optional rating.