Rating Window Performance: What It All Means

Last Updated on February 24, 2022 by Kravelv

Given the role windows play to improve your home’s form and function, it is necessary to ensure that you pick out the right ones. While your personal preferences will weigh a lot in determining which window will be right for you, a window’s performance rating must also be taken into consideration. They are meant to be functional, after all, and performance ratings give you a good measure of what a certain window is capable of. But what do these performance ratings gauge in the first place? That’s what I’m here to talk about with you.

Window Ratings: What They Are

Window ratings are found on labels from ENERGY STAR and the National Fenestration Council. They are formulated with input from agencies like the Environmental Protection Agency and the U.S. Department of Energy, meant to assess how a window performs given certain criteria. In the case of ENERGY STAR, its label determines how energy efficient a window is. A number is assigned to a window and the lower this number is, the more cost-efficient it is to use that window. The NFRC label offers a more comprehensive assessment, taking into consideration several factors that can affect a window’s overall performance.

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Factors to Consider

A typical window has many parts, but because its glass and frame make up the majority of the unit, this mostly affects a window’s overall rating. The ENERGY STAR rating assesses a window overall, factoring what all the parts have to offer. NFRC ratings, on the other hand, use considerations mostly dictated by window glass and frames.

Natural light, for instance, lets in more than just the visible light that you see, also streaming in infrared light that produces heat and ultraviolet light that can facilitate fading in home interiors. These affect the following NFRC ratings:

  • Visible Transmittance (VT). This refers to the level of light that a window can let in. The higher the rating then, the higher the level of light let in. A high VT rating is great for homes looking to take advantage of daylighting.

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  • Solar Heat Gain Coefficient (SHGC). This refers to the level of heat from the sun that a window can block. As a window blocks more heat, less heat will find its way inside your home, warranting a low SHGC rating. The lower the rating, the higher the level of heat you can expect a window to block.

How well a window insulates, on the other hand, is affected by the kind of frame it has. Window glass also has a hand in determining insulative properties but frames are mostly responsible in terms of addressing air leakage, which greatly affects insulation. Where air leaks are concerned, the following NFRC ratings are affected:

  • U-Factor. This refers to the level of indoor heat loss that a window can prevent. Air leaks can lead to continuous heat loss and that’s not a good thing as it strains heating systems to keep up with demands to maintain your comfort. As U-factor is proportional to the level of heat lost, the lower the rating, the better a window is at locking in heat inside the home.

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  • Air Leakage. This refers to the level of outdoor air that a window can let in. If the U-factor is all about keeping indoor heat in, the Air Leakage rating is concerned with keeping warm and cool outdoor air out of the home, which also contributes to stabilizing indoor environments.

Combined with ENERGY STAR’s rating, these NFRC scores paint a picture of how capable a window is. Aside from choosing windows specifically suited for your local climate, make sure you have them installed by a reputable contractor to make the most out of your investment.


Author Bio:

Dave Steffes is the local owner of Renewal by Andersen of Des Moines. A trusted name in the local home improvement industry, Dave never tires of sharing his knowledge with his clients, family, and friends, writing about them through the company blog.


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