How to Make Sense of a Window Label

CR decodes the NFRC label that you'll see on most replacement windows

two people installing replacement window Photo: Monty Rakusen/Getty Images

Replacement windows have energy-efficiency labels, just as you’ll find on home appliances like refrigerators and dryers. But these labels, created by the National Fenestration Rating Council (NFRC), an industry group, don’t estimate how much money you’ll spend annually on energy when your windows are in place, as the bright yellow EnergyGuide labels on appliances do. Nor do they indicate that a window meets EPA standards for efficiency (for that, look for the Energy Star logo alongside). The NFRC label can give you an idea of how energy-efficient a window is relative to other windows on the market.

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To do this, the NFRC window label gets into the nitty-gritty of how a window performs, including how well it insulates, how much of the sun’s heat and light it allows in, and how well it limits air leakage when closed, information that can be useful when comparison shopping.

NFRC ratings are based on data from third-party labs, and past tests by CR found them to be accurate and reliable. As a result, we devised our replacement window test procedure to be complementary.

Here, we break down the meaning of each figure on the NFRC sticker. Hover over each rating for a description, or see below.

Glass Type and Product Information

You’ll find a frame type, along with any coatings on the glass (this window has a low-E, or low-emissivity, coating) and the gas that fills the cavity between glass panes (in this case, argon), contributing to the product’s overall energy efficiency.

U-Factor

This number represents the heat transfer coefficient, or how much heat the window’s coatings help keep inside the home. The U-factor ranges from 0.2 to 1.2; the lower the number, the less heat escapes.

Solar Heat Gain Coefficient

This number measures how well a product can keep solar heat from penetrating the window. The range is 0 to 1. If you live in a hot, sunny climate, such as Arizona’s, you will want a low number to block out heat; if you live in a colder climate, you’ll want a high number. If you live in a balanced climate, where winters are cold and summers are hot, you’ll want a solar heat gain coefficient rating of around 0.3 (the same goes for the U-factor). And you may want windows with different ratings for different parts of your house, depending on whether particular windows get a lot of sun or shade.

Air Leakage

This is how much outside air will come through, according to the NFRC’s testing. The lower the number, the better. (The range here is less than or equal to 0.1 to 0.3.) “In our tests, we take this a step further by testing at very low temperatures with high wind conditions to see if there are any changes in performance due to materials expanding and contracting,” says Enrique de Paz, the engineer who leads replacement window tests for Consumer Reports.

Condensation Resistance

Not every label will carry this optional rating. But if you live in a humid climate and/or are concerned with mold growth, you may want to look at the condensation resistance rating, which is on a scale from 1 to 100. The higher the number, the better the window is at resisting condensation.

Visible Transmittance

This is how much visible light will come in through the window during the daytime. The range is 0 to 1, and the higher the number, the more visible light passes through. Clear glass without any coatings has the highest VT rating. But coatings that you may add to improve energy efficiency can sacrifice some visible light.

Editor’s Note: This article also appeared in the May 2019 issue of Consumer Reports magazine.