A Guide to Understanding Energy Efficiency Ratings for Windows


Energy efficient windows are practically the standard these days, with businesses and homeowners eager to invest in money-saving, eco-friendly items. Informed consumers and contractors alike often wonder how to compare different manufacturers’ products. Here’s a quick reference for the most common window ratings.

NFRC Certification

Look in the upper lefthand corner of the glass. The National Fenestration Rating Council (NFRC) provides ratings for window manufacturers. Look for the NFRC logo; that label will have the certified numbers listed below.


The U-factor is a three-digit number, generally between 0.20 and 1.20. The U-Factor is an indication of how quickly heat escapes through the window. The lower the U-Value, the better the window is at insulating.

Solar Heat Gain Coefficient

The SHGC measures a window’s ability to block heat from the sun. It is displayed as a number between zero and one, the closer to zero the better.

Visible Transmittance

VT is similar to SHGC, but it measures the amount of visible light from all sources. It is similarly expressed between zero and one with a lower number being better.

Air Leakage

The air leakage rating measures how much air will enter a room through a window. Look for a low number in the 0.1–0.3 range.

If you’re still curious about energy efficient windows, go with a reputable and knowledgeable window installation contractor who can answer all your questions.