Understanding Energy-Efficient Window Ratings

In a world intent on becoming more energy-efficient, windows are often the subject of confusion.


Windows are one of the most effective ways to improve your home’s insulation. Choosing energy-efficient windows will help reduce your energy use, both heating and cooling. And, as a result, they can help reduce your electric bill significantly.

Understanding the insulating properties of different types of glass and frame materials will help you select the best energy-efficient windows for your home.

Energy-Efficient Windows and the U-Factor

The u-factor rating addresses thermal conductivity, which is the window’s ability to retain heat during the colder months of the year. This rating ranges from 0.20 to 1.20. The lower the number, the better the product is at keeping heat inside your home.

Products with the best (lowest) u-factor include wood and vinyl frames with multiple panes of Low-E coated reflective glass.

Solar Heat Gain Coefficient Ratings

The solar heat gain coefficient (SHGC) rating refers to how much heat an energy-efficient window allows in. This rating applies to the summer months and how much excessive heat is allowed to penetrate the interior of your home.

The SHGC rating ranges from 0.0 to 1.0. The lower the rating, the better the product is at keeping heat out of your home.

Logic tells us that you want to minimize the heat coming into your home during the summer, so that your cooling bills can be reduced. Keep in mind, however, that this will prevent the sun from warming your home during the winter months, resulting in higher heating costs.

Visible Transmittance and Air Leakage Ratings

Visible transmittance (VT) ratings tell you how much light can permeate the glass to light your home, a process known as daylighting. Visible sunlight is rated on a scale of 0.0 to 1.0.

Air leakage (AL) refers to how much air can move around (not through) the window under a specific level of pressure. The most energy-conscious windows allow the least amount of AL. This rating spans from 0.1 to 0.3. The lower the rating, the better the window is at reducing air leakage. Not all manufacturers list this rating on their products, however.

Look for the Energy-Efficiency Label

The National Fenestration Rating Council (NFRC), a non-profit agency that provides performance labeling for new windows, doors and skylights, lists the u-factor, SHGC and air-leakage ratings on their labels. The NFRC also provides fact sheets to manufacturers and industry professionals to help them educate their customers.

Keep in mind the best energy efficiency values for a Utah home are different than those in other geographic locations. The best way to determine which factors are best for you is to talk with a local professional window contractor.

In Utah, the experienced team at Rocky Mountain Windows & Doors understands all the important details. We can assist you in selecting the most energy-efficient windows for your home and budget.