In 2005 the National Fenestration Rating Council (NFRC) developed a label to provide consumers with energy performance data on windows, doors, and skylights. This label helps you compare one product to another, listing the manufacturer, a product description, a source for additional information, and ratings on the following energy performance characteristics (windows and doors are rated as whole systems, glazing and frame).
U-Factor: A window’s U-Factor measures how well a unit prevents heat from escaping (the inverse of its R-value). U-Factor ratings are usually between 0.25 and 1.25. The lower the U-value, the greater its ability to resist heat transfer and better insulate the home.
Solar Heat Gain Coefficient (SHGC): The Solar Heat Gain Coefficient rating indicates how well a window, door, or skylight blocks solar heat by measuring solar radiation released into the home. The lower the coefficient value, the less solar heat is transmitted into the home. Numbers generally range from 0 and 1, typically ranging from 0.25 to 0.8.
Visible Transmittance (VT): Visible Transmittance measures how much visible light is transmitted through the glazing. The higher the number, the more light is transmitted.
Air Leakage (AL): Air leakage ratings test the equivalent cubic feet of air that passes through one square foot of window area. Heat can be lost and gained through seals in the window frame and assembly. The lower the air leakage number, the less air passes through the unit. Most industry standards require an AL value to be 0.3 cfm/ft2.
Condensation Resistance (CR): The condensation resistance rating measures the units ability to resist condensation forming on the inside of the product. Ratings are expressed in numbers from 0 to 100. The higher the rating, the better the product resists condensation.
Consult with your architect on the best windows for your green home project that will meet your energy efficiency needs, site specific requirements, code requirements and budget. Lawrence Berkley National Laboratories offers a free software program, Resfen (PC-compatible only) which lets you calculate how windows with different glazing systems and frame materials affect your energy bills.
