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A Quick Guide to Energy Performance Ratings for Replacement Windows in Des Moines

Everyone wants energy efficient windows for their Des Moines homes, but how can you really gauge the energy efficiency of replacement windows? You can and should ask the sellers, but they may be biased as they want to sell the windows. And unless you’re a window expert yourself, energy efficiency might not be immediately evident. Fortunately, there’s already an impartial rating system in place, created by the National Fenestration Rating Council.

The NFRC is a non-profit organization created to rate and certify energy efficient windows based on a number of comprehensive factors. You may have already noticed their energy performance label if you’ve gone shopping for energy efficient replacement windows in Des Moines in the past 5 years. This label includes four different categories: U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage. The NFRC also has a condensation rating that is optional for manufacturers to include, so you may or may not see it on the label.


U-Factor measures how well the window holds heat inside the home without allowing it to escape. This is especially important in winter, when homeowners will turn up the heat to escape the cold outside. If that warm air inside leaks through the window, the room can feel drafty, and you’ll have to turn the heat up to feel comfortable, thus raising energy bills.

U-Factor is rated on a range of 0.20 to 1.20. The lower the rating, the less heat escapes. One way for replacement windows in Des Moines to keep their U-Factor low is through the use of double- or triple-pane windows with argon or krypton gas fillings. These gases are non-toxic and help slow the flow of air through the window, keeping hot air in.

Solar Heat Gain Coefficient (SHGC)

Meanwhile, the movement of heat through window glass can go both ways. Not only can heat escape from inside, but it can leak in from outside. SHGC judges how well the window resists heat from outside, and is most relevant in summer. This has a range of 0-1, and like U-Factor, the lower the numbers, the more the window resists heat. Generally, 0.4 or lower SHGC is ideal for your replacement windows in Des Moines. External factors can often impact SHGC, such as the position of the window and potential shading nearby. Homeowners can also add a shading mesh over the window to reduce SHGC, though this will darken the view.

Visible Transmittance (VT)

While the other two categories address heat, VT addresses light. This scale measures how much light your windows let into your home. This is an important factor for energy efficiency, because the more light your windows let in, the less you have to spend on artificial lighting. The range here is also 0-1, but in this case, the higher the number, the more efficient. Some homeowners will raise their VT by daylighting their home — placing clerestory windows high on the wall so that light pours down into the room.

Air Leakage (AL)

Finally, AL determines how much air leaks through a window. This is most important during the colder months, but it’s good to keep in mind throughout the year. If windows have a high rate of AL, your indoor living spaces can feel drafty and uncomfortable. The less air a window leaks through, the less homeowners will have to rely on their HVAC system. AL is rated on a scale of 0.1 – 0.3. Here again, gas filled windows can help to reduce AL.

Condensation Resistance (CR)

This is more important in colder climates where conditions are more likely to present the possibility of condensation (extreme cold outside, warmer and more humid inside). This has a range of 1 – 100. A higher the rating means that the window has a better chance of resisting condensation from forming on the inside of the glass.

Before you decide on an energy efficient replacement window in Des Moines, check the NFRC label so you can feel confident in the whole spectrum of the window’s energy performance. And if you have any questions about what replacement windows to best trust in Des Moines, contact us at Zen Windows.