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Energy Efficiency of Windows

Low-E Glass

energy-efficient-windows-diagramLow-emissive glass, otherwise known as Low-E, is window glass that has been treated with an invisible metallic oxide coating. This creates a surface that reflects heat, while still allowing natural light to filter in. The use of Low-E glass results in a reduction of energy consumption and an overall increase in the comfort of your home all year round.

Argon Gas

Argon gas is a clear, odorless, non-toxic gas that is filled in between the window panes to provide added energy efficiency. Used to minimize heat exchange through your windows, argon gas prevents unwelcomed heat from entering your home while the warmth you want stays put. Therefore, regardless of the time of year, the comfort of your home is uncompromised.

NFRC Rating

The National Fenestration Rating Council (NFRC) administers an independent, uniform rating and labeling system for the energy performance of fenestration products, including windows.  Think of this rating system as an energy efficiency report card for your windows.


The rate of heat loss is indicated with the U-Factor. You want your U-Factor to be low because the lower, the greater a window’s resistance to heat flow and the better its insulation. The nationally recognized rating method by the NFRC is for the whole window, including glazing, frame and spacers. Center-of-glass U-factor is sometimes referenced by the NFRC, which describes the performance of the glazing without the effects of the frame. Most energy efficient windows have the whole window U-factor higher than the center-of-glass U-factor.

Solar Heat Gain Coefficient

The Solar Heat Gain Coefficient is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. The nationally recognized rating system by the NFRC is for the whole window, including the effects of the frame. The center-of-glass solar heat gain coefficient is sometimes referenced, which is the effect of glazing alone. Whole window SHGC is lower than glass-only SHGC, and is typically below 0.8.

Visible Transmittance

The visible transmittance is an optical property that indicates the fraction of visible light transmitted through a window. This is separate from the solar heat gain coefficient since many modern windows include spectrally selective coatings that can allow different amounts of visible, infrared and ultraviolet light through. The visible transmittance is a whole window rating and includes the impact of the frame. Since the frame area doesn’t transmit any light, the visible transmittance might be lower than expected, which is done to be consistent with the ratings of U-factor and solar heat gain coefficient. While the VT varies between 0 and 1, most values among double and triple pane windows are between 0.30 and 0.70. The higher the VT, the more light comes through. A high VT on a window is desired to maximize the daylight.

Air Leakage

Heat loss and heat gain can occur by infiltration through cracks in the window. This is indicated by an air leakage rating shown as the equivalent cubic feet of air passing through a square foot of window area. The lower the air leakage, the less air will pass through the cracks in the window. An air leakage rating is required for Energy Star certification. The rating must be 0.30 or less.

Feldco Has Energy Efficient Windows

Feldco has you covered when it comes to energy efficient vinyl windows. With many window style options available, you’ll find just what you’re looking for at Feldco. Our windows are double or triple pane, have argon gas-filled glass and their foam insulated frames are a perfect way to trap heated or cooled air in your home. You’ll feel more comfortable no matter the temperature outside. Not only will you be comfortable but you will save money on your energy bills as well. Make your home more energy efficient, get your window project started today and get a free quote now!

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