Tom Shafer

U-value demystified

May 31, 2011

I have found that many homeowners purchase windows based on a low U-value rating. Unfortunately, many homeowners have no idea what U-value means.

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U-value may not be my value

Simply put, U-value is a measurement of the rate of heat transfer through a material. A lower U-value means a slower rate of heat transfer from the inside of the window to the outside, meaning less heat loss to the outside.

The introduction of the Energy Star program for windows has heightened U-value awareness. The Energy Star program is a combined effort of the Environmental Protection Agency and the Department of Energy to reduce energy costs for consumers and protect the environment from the effects of unnecessary energy consumption. This map from energystar.gov designates four climate zones across the country:

US climate zones

In the northern areas heating is necessary many days of the year. Therefore, a lower U-value is needed to keep the warmer air inside the house. In more southern areas where losing heat from the house most of the year is not the issue, a higher U-value is fine. In other words a lower U-value is not always desirable; it depends on your climate zone.

Where to find U-value on the National Fenestration Rating Council (NFRC) label

The NFRC provides an energy rating program to help consumers make energy-conscious comparisons of window products. Each new window sports an NFRC label like this one featured on the NFRC website, nfrc.org.

NFRC rating label from nfrc.org website

There are five energy performance ratings on the label: the first is the U-value based on the results of testing thermal transfer across the entire window. The U-value on the sample label is .35.

Advancements in window glass have lowered U-values

Technological advances in glass over the past 50 years have reduced U-values. The once-standard, single lite of glass is now a double-lite, sealed unit. The gas fill that is sealed between the layers of glass has gone from freon to carbon dioxide to argon to now, krypton, which has the best thermal performance. Reflective low-emissivity (low-e) coating has also been added and improved with low-e coatings on layers two and three.

There is no doubt the improvements made to windows can save energy. If you understand U-value you can cash in on the savings by getting the proper Energy Star windows for your geographical area.


About the Author

Tom Shafer has decades of experience in window sales, marketing and product development. He's worked closely with window design engineers in testing, design, and building code interpretation. Past employers include United Windows and Doors and Norandex, MI Windows. He currently works at a home improvement retailer.

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