Energy Performance Ratings

Energy Performance Ratings for windows, doors, and skylights indicate the potential for gaining or losing heat as well as transmitting sunlight into the home. Heat loss or gain can occur in a number of ways:

• Conduction,
• Radiation, and / or
• Air leakage.

These properties are typically measured and rated according to a number of performance characteristics.

Performance Characteristics

The National Fenestration Rating Council (NFRC) tests, certifies and labels windows, doors and skylights, strictly on a voluntary basis, providing a means to determine the energy efficiency of specific products, and compare them with alternatives. These performance ratings are based on the following characteristics:

• U-Factor measures the rate at which the product conducts non-solar heat flow. Though possible to refer to the glass or glazing alone when looking at windows, skylights and glass doors, NFRC ratings also include the frame and spacer material. A lower U-Factor is indicative of better energy efficiency.
• Solar Heat Gain Coefficient (SHGC) reports the amount of solar radiation admitted through a window, door or skylight. A lower SHGC reflects the transmittal of less solar heat and better shading ability (i.e. better able to reduce cooling loads in the summer); where as a higher SHGC rating reflects a product that is more effective at collecting solar heat during the winter. Therefore, in using the SHGC rating as a criteria for selecting one product over another, one should consider climate and orientation to the sun.
• Air Leakage, described in units of cubic feet per minute per square foot of frame area, measures and reports on the rate of air infiltration around a window, door or skylight.
• Sunlight Transmittance measures the product’s ability to transmit sunlight into a home, based on visible transmittance (VT where a higher VT connotes the transmittal of more visible light), and Light to Solar Gain (LSG measuring relative efficiency of transmitting sunlight without excessive amounts of heat). The primary factor in selecting one product over another applying these measures are day lighting requirements, amount of desired glare, and the amount of solar generated heat one desires.