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Your Replacement Window Performance Rating Explained

Performance Ratings

When you’re buying a big-budget item, it’s easy to get lost in a sea of information. You want to make sure that you’re getting the value for your money, and that your product is going to last you a long time. When buying replacement windows, the best way to do this is to check for a performance rating or Energy Star certification. Replacing old windows with Energy Star-certified windows lowers household energy bills by an average of 12% nationwide. Energy Star ensures quality by holding window manufacturers to strict performance standards on 5 different categories set by the National Fenestration Rating Council. Standards vary on your climate zone. Here in Illinois we are in the Northern Climate Zone, so those standards have an emphasis on durability and insulation. Luckily, most window manufacturers don’t try to hide their performance ratings and windows come with a label. They look like this:

Replacement Window Label

The label looks confusing, but I’ll explain it to you step-by-step.

Performance Rating: U-Factor

The U-Factor measures your the window’s rate of heat loss. Specifically, it measures the rate of heat flow through your glazing system. U-Factors generally range from 0.25 to 1.25 and are measured in Btu/h·ft²·°. Don’t worry, the units don’t matter much to anyone but the manufacturers. All you need to know is that low U-Factor is preferable because it means less heat loss, which is a good thing! A low U-Factor means better insulation and energy efficiency. Illinois winters can be brutally cold and even a small amount of heat loss can make your home feel drafty. To be eligible for the Residential Energy Tax Credit (worth 10% of the product cost up to $200), your window must have a U-Factor equal to or lower than 0.30 for those living in Illinois.

Take Preservation Window’s EnergyMaxx 10 Elite glass package as an example. Most standard replacement window units  have a U-Factor of 0.48. Our EnergyMaxx 10 Elite glass package has a U-Factor value of 0.17, meaning it performs 65% better than its competitors.

Performance Rating: Solar Heat Gain Coefficient (SHGC)

The Solar Heat Gain Coefficient (SHGC) measures the fraction of solar radiation that is transferred through your windows. While the U-Factor measures heat leaving your home, the SHGC measures the amount of heat entering your home. This measurement is especially pertinent in cool seasons. SHGC is measured on a scale from 0 to 1 but most windows have a SHGC values ranging from 0.25 to 0.80. A lower SHGC is preferable because it means less heat is entering your home. During a balmy Illinois summer, this could mean using less AC to cool your home.

We’ll use our EnergyMaxx 10 Elite glass package as an example again. Most traditional clear, insulated glass units have an SHGC of 0.50. Our EnergyMaxx 10 Elite glass has a SHGC of 0.17, meaning it outperforms the other glass package by 66%. Less heat enters a home through our windows with a low SHGC, and our replacement window customers will spend less money cooling their home.

Performance Rating 3: Visible Transmittance (VT)

Replacement Window Light

This rating seems straightforward, but it’s not. VT measures the amount of UV energy that your window allows through. VT can also measure how much visible light passes through your windows. This is where it gets tricky. You WANT an abundant amount of light to pass through your windows to give your home that natural, open character. However, UV energy is responsible for the fading of carpets, hardwood flooring, fabrics and furniture. In addition, to much light can cause glare. VT is measured on a scale of 0 to 1, but values generally range from 0.20 to 0.80.

Our EnergyMaxx 10 Elite glass package has a VT of 0.36 compared to a traditional clear glass package VT of 0.55. This means that the house with traditional windows will be brighter, and will also allow more UV radiation to enter the home. The house with the EnergyMaxx glass package will still have an abundance of visible light inside, but will not suffer the same fading damage as the other home.


Peoria Siding & Window has a number of different glass packages to choose from, all with different performance ratings. Make sure that when you’re choosing a replacement window, you prioritize factors like energy loss, heat gain and visible light. If you’re still confused about window performance ratings, don’t worry. One of our salespeople will be happy to talk you through any questions or concerns. Just call (309) 717-0105 to speak with a representative. If you’re looking to purchase new replacement windows, you can claim up to 30% off your home improvement project by filling out this form. We sell windows with an unbeatable performance at a value you can trust.