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Understanding Window Energy Efficiency Standards

Courtesy Champion Windows, Sunrooms, Roofs and Home Exteriors

Why you should be concerned about window efficiency...

Millions of houses built before energy-efficient windows entered the market have high utility bills. Heat gain from the sun's UV rays, leaking air around the frames and condensation are also problematic. Houses with bow windows, casement windows or bay windows from should consider replacements with high energy efficient ratings. Over time, the energy savings can pay for the windows and the installation. In some areas, choosing windows with the ENERGY STAR® certification can qualify for grants, tax rebates or incentives.

Champion energy efficient windows today have ratings awarded by the NFRC, the National Fenestration Rating Council. Each window is rated in four different areas; it can be confusing to recognize a highly rated window from a poorly rated one. Here is an explanation of the ratings to help consumers choose the right replacement for their home and budget.  (See sample label below.)

Condensation resistance factor (CR) 

The condensation resistance factor, or CR, measures the window's ability to prevent condensation from forming and dripping down the glass panes. Condensation forms because of the temperature difference between the warm air inside and cold air outside coming in contact through the glass pane. The moisture in the inside air condenses on the glass and accumulates to drip down the pane and cause problems. The best rating is 100.  CR is an optional rating and manufacturers may choose to not display it on their label.


Sample NFRC window labelThe U-factor...

The U-factor is the same as the R-value of insulation. The U-factor rating explains how well the window and its components will insulate against heat transfer. Unlike searching for a high R-value in insulation, you should seek the lowest U-factor you can find because the rating is given according to how much heat transfer takes place.  Typical range for the U-factor is from 0.15 and 1.20. 

Solar Heat Gain Coefficient (SHGC)

The SHGC, or solar heat gain coefficient will tell you how much of the sun's UV rays are blocked by the window. As with the U factor, the rating is given according to how much gets through. The ratings are given on a scale of 0 to 1. Look for the lowest number you can find.

Visible Transmittance (VT)

To make the most use of daylight, windows have been tested and given a rating called the VT, or visible transmittance. It is rated on a scale of 0 to 1; higher numbers mean the window allows more daylight to pass through.

Air Leakage (AL)

Air Leakage (AL) refers to how well the window, including the insulating components such as the frame and weatherstripping, keeps out air.  The rating range is from 0.1 to 0.3.  The lower the AL, the better! AL is an optional rating and all manufacturers do not have this on their window's label

In conclusion...

Most manufacturers offer free in-home evaluations and estimates for your window replacement projects. Many factories are open seven days a week; the staff is happy to help you find the most energy efficient replacements for any brand of window you have.

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