Understanding Window Ratings
The National Finestration Rating Council (NFRC) label can be found on all ENERGY STAR certified windows, doors, and skylights, and provides performance ratings in five categories:
U-Factor measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 and are measured in Btu/h-ft²°F. The lower the U-factor, the better the window insulates. (EPA requires that the U-factor for ENERGY STAR labeled products is certified and listed on the product label.)
Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits. (EPA requires that the SHGC for ENERGY STAR labeled products is certified and listed on the product label.)
Air Leakage (AL) measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cf-m/ft². (EPA requires that the AL for ENERGY STAR labeled products is certified.)
Visible Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80. The higher the VT, the more light you see.
Condensation Resistance measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the condensation resistance factor, the less build-up the window allows.