Prescriptions for Efficient Windows


Inside this Article

NFRC window labels
NFRC window labels include U-factor, solar gain, transmittance, and optionally, air leakage and condensation resistance.

If you’re shopping for windows, look to the National Fenestration Rating Council’s (NFRC) label, which appears on all Energy Star window products. Much like the miles-per-gallon sticker on a new car, the label gives information on how well a window will perform.

The NFRC tests the full window—glass, frame, spacers, and any other component that is a permanent part of the product—providing an accurate reflection of how the product will perform. But the NFRC label can help only if you understand the ratings.

U-factor denotes the rate of heat transfer through the window unit. U-factor values generally range from 0.25 to 1.25, and the lower the number, the better the window resists heat transfer. For northern climates (see map and table), Energy Star recommends a U-factor of 0.30 or below for most window orientations and a maximum of 0.60 for windows in southern climates. Passive House Institute US (PHIUS) has more stringent recommendations to help reach overall energy savings of 60% to 70% and space-heating savings of 90%, compared to a conventionally energy-code-compliant, 2-by-4 stud-framed home. For example, PHIUS recommendations for northern climates (ASHRAE zones 4 through 8) range from a U-factor of 0.11 or less (zone 8) to 0.15 or less (zone 4).

Solar heat gain coefficient (SHGC)—A window’s SHGC signifies how much heat from the sun is blocked. SHGC values typically range from 0.25 to 0.80; the lower the SHGC, the more the product blocks solar heat gain. South-facing windows in passive solar homes should have the highest SHGC possible, while still having an acceptable U-factor (see “Energy Star Qualification Criteria for Residential Windows” table”). In most climates, west-facing windows need to have the lowest SHGCs to prevent heat from the summer sun from entering the home. PHIUS calls for SHGCs of 0.50 or greater for south-facing glass in climate zones 3 through 8.

Visible transmittance (VT) is the measure of how much light comes through a door or window. A number between 0 and 1, the higher the VT, the more light is transmitted. Multiple coatings on glass and suspended films between panes will enhance the unit’s efficiency, but can affect a window’s VT. Windows with a high VT appear relatively clear and provide sufficient daylight and unaltered views; however, they can create glare. For daylighting, choose a VT between 0.5 and 0.7 for good glare control, especially with larger windows.

Air leakage (AL) indicates the rate at which air passes through joints in the window unit. AL rates typically fall between 0.1 and 0.3 cubic feet of air per 1 square foot of window per minute. The lower the AL value, the more airtight a product is. Most industry standards and building codes specify an AL of 0.3 cfm per square foot or less. Providing the AL rating is optional for manufacturers.

Condensation resistance (CR) is how well the window resists water buildup. CR is scored on a scale from 0 to 100. The higher the CR factor, the better a product is able to resist condensation. CR is another NFRC-optional rating.

—Compiled by Kelly Davidson

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Comments (2)

Fred Golden's picture

Care must be used in making a decision for south facing windows. In warm areas, with no need for passive solar heating, then low SHGC is a given. However up north, or high in the mountains, where prevailing winds will cool your home, consider high heat gain windows facing south, protected by 3' deep overhangs to shade those windows in the summer time.

I was reading Passive Solar House: The Complete Guide to Heating and Cooling Your Home by James Kachadorian. He offers great suggestions for storing heat within the cement floor, with active air flow through that floor. Coupled with a two stage heat pump, and you can heat and cool a home very efficiently!

Ben Root's picture

I'm reading Kachadorian's book now. And while his overall attitude is great, and I recognize his approach as ground-breaking at the time, I also recognize that the book is 15 years old and is based on a house design that he developed in the 1970s. Several of his design "features" are no longer considered appropriate. (Kachadorian doesn't even use the south overhangs that you, and the rest of the industry, recommend). The same is true of Edward Mazria's bible "The Passive Solar Energy Book"...great ground-breaking concepts at the time, but many unforeseen kinks have been worked out, and formulas refined, since then. I for-sure plan to read some more contemporary passive design books before building my house.

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