What Do Window Ratings Mean?

By fiderioandsons Oct 16, 2013

There are many factors to consider when choosing replacement windows; homeowners will need to decide on the appearance, type, and brand that fits their home best. Of course, one of the deciding factors will also be the cost. Consumers often make the mistake of only going by the sticker price at the time of purchase. Arguably, the future performance of the window is of higher importance than the original price itself. Energy efficient replacement windows are a better investment in that they will save you more money long term on energy bills. Not only that, but they will also allow your family to live more comfortably.

Before buying replacement windows for your home, you should be knowledgeable of replacement window ratings and labels that will help you save money. The following post will help you to decode window ratings and labels so you can make an informed decision.

What labels should I look for?

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There are many brands out there that are quick to stick a variety of labels on their products to help increase their selling points. Don’t be fooled. The most important labels to look for are the blue ENERGY STAR label and the black and white NFRC label. ENERGY STAR’ is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy helping homeowners to save money and protect the environment through energy efficient products and practices. Windows that display the ENERGY STAR logo mean they offer excellent energy efficiency and year-round comfort with reduced heating and cooling costs. According to their website, in order to apply the ENERGY STAR label to products, manufacturers must first test their products according to NFRC procedures, which include independent testing at NFRC approved laboratories.

The NFRC testing protocols involve testing of the full window — including glass, frame, spacers, and any other component that is a permanent part of the complete product. This strategy provides you with a more accurate reflection of how the product will perform in the home than testing of just glass, as the framing and other components influence ratings such as U-factor, Solar Heat Gain, and Visible Transmittance.

NFRC Window Rating Definitions 

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measures how well a product prevents heat from escaping a home or building. U-factor ratings generally fall between 0.15 and 1.20. The lower the U-factor, the better a product is at keeping heat inside the building. U-factor is particularly important during the winter heating season in colder climates. This label displays U-factor in U.S. units. Labels on products sold in markets outside the U.S. may display U-factor in metric units.

What’s the Difference between U-factor and R-value?

The biggest difference between U-factor and R-value is that U-factor measures the rate of heat transfer (or loss) while R-value measures the resistance to heat loss. R-value is a measure of conductance and resistance. A product with high conductance will conduct heat quickly, like a hot pan on the stove or a single pane of glass on a cold day. U-factor, on the other hand, takes into account more than conductance. It also is affected by the airflow (convection) around the window and the emissivity (radiated or reflected heat) of the glass. Click here for an article on “Why NFRC Uses U-factors for Windows.”

Solar Heat Gain Coefficient (SHGC)

measures how much heat from the sun is blocked. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the more a product is blocking solar heat gain. Blocking solar heat gain is particularly important during the summer cooling season in hot Southern climates. By contrast, people in Northern climates may want solar heat gain during the cold winter months to lessen the cost of heating the home.
Visible Transmittance (VT) measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the higher the potential for daylighting.

Air Leakage

measures how much outside air comes into a home or building through a product. Air leakage rates typically fall in a range between 0.1 and 0.3. The lower the air leakage, the better a product is at keeping air out. Air leakage is an optional rating, and manufacturers can choose not to include it on their labels. This label displays air leakage in U.S. units; labels on products sold in markets outside the United States may display air leakage in metric units.

Condensation Resistance

measures how well a product resists the formation of condensation. Condensation resistance is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation. Condensation resistance is an optional rating, and manufacturers can choose not to include it on their NFRC labels.

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