Energy Efficiency Terminology
The National Fenestration Rating Council (NFRC) is the organization that tests and rates all windows in the US. The NFRC energy performance label can help you determine how well a product will perform the functions of helping to cool your home in the summer, warm your building in the winter, keep out wind, and resist condensation. By using the information contained on the label, consumers can reliably compare one product with another, and make informed decisions about the windows they buy.
NFRC logoNFRC performance labels list the manufacturer, describe the product, provide a source for additional information, and include ratings for one or more energy performance characteristics.
U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value.
Solar Heat Gain Coefficient
Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. 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 in the house.
Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
Air Leakage (AL) is 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 by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.
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. While this rating cannot predict condensation, 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.
* This rating is optional and manufacturers can choose not to include it.
R-Factor is a measure of the resistnace of the insulated glass unit to heat flow. A high R-factor window has a greater resistance to heat flow and thus, a higher insulating value than a window with a low R-Factor.
Ultra Violet Light
The invisible rays of the light spectrum at its short-wavelength violet end. U.V. Ras are found in everyday sunlight and can cause fading and damage to paint, carpet, furniture and fabrics.
This is the fraction of the visible specturm of light weighted by the sinsitivity of your eyes that is transmitted through the window. Simply put, it represents the amount of daylight that the window lets in.
Low-E Stacked Glass keeps you warmer in the winter and cooler in the summer. The overall U-Factor (insulating value) and R-Factor (resistance to heat and cold transfer) of windows with Low-E stacked glass is considerably better than stand clear insulating glass and also better than standard Low-E products.
Argon Gas Filled
Argon is a colorless oderless gas that is 6 times heavier than air. Argon insulates better than just air and also reduces the amount of convection that goes on between the panes of glass.