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Builder's Guide to Windows, Images 1-6

Builder's Guide to Windows, Images 1-6

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    High-performance windows increase energy efficient and reduce fading damage from ultraviolet radiation.

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    The NFRC label makes it easy to assess a window’s energy performance. Which U-factor and solar heat-gain coefficient (SHGC) are optimal depends on climate. Optional ratings for air leakage (AL) and condensation resistance (CR) are included on this sample rating. AL indicates how many cubic feet of air leak through a square foot of window area, and generally ranges from 0.1 to 0.3. CR, which uses a scale of 1 to 100, indicates how well a window resists forming condensation on the inside of the glass. Higher numbers indicate better resistance.

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    The Energy Star label uses color shading to show the climate zones where a particular window or skylight will perform well.

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    Builders can turn to the Energy Star program for help selecting the best windows for their climate. In northern regions, windows must be good at keeping in wintertime heat; in the south, they should be able keep heat out in the summer. Windows installed in the middle of the country must balance summer and winter performance. Note that all values in the table are based on NFRC whole-window ratings.

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    The inside surface of the low-E windows (left and center panes) are much warmer than that of the clear double-pane (right pane). If this window had three bays of clear double-pane glass, the room’s heating thermostat would have to be 2°F to 3°F warmer to provide the same level of comfort achieved with low-E glass.

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    Spectrally selective low-E coatings do a good job of preventing winter heat loss and reducing summer heat gain while still allowing most of the visible light to enter the space (as indicated by high center-of-glass VT ratings). The chart compares the performance of standard clear glass with that of three types of low-E. Note the performance options available with advanced low-E. Going from high to low solar gain cuts the SHGC by nearly two-thirds with only a modest loss of visible light.

There are more than 100 million homes and approximately 20 billion square feet of clear-glass residential windows in the U.S. Most of those homes are more than 30 years old, and as a result, the market is growing for replacement windows with energy-efficient insulating glass. High-performance options are now readily available in all regions of the country. Windows with U-factors and solar heat-gain coefficients less than 0.3 are now standard. And the cost of upgrading from conventional double-pane windows to double-pane low-E units with argon gas is less than $1 per square foot. Heating and cooling energy savings of 10 percent to 20 percent or more are possible, depending on the climate region and the replacement-window choice. Similar savings can be achieved in new construction by upgrading to high-performance glazing. Compared with clear glass, almost all of these new products reduce ultraviolet (UV) radiation and cut fading damage due to UV by 50 percent or more.

A Profusion of Choices

Since there are hundreds of window manufacturers, each with many different product offerings, it's easy to get confused trying to sort through the trade names and marketing hype. Window options used to be limited to single-pane versus double-pane and wood frame versus aluminum frame. Today, there are three or four basic frame types, double- and triple-pane glass, and warm-edge insulating glass spacers. The low-E coatings used on most windows can be formulated for low, medium, or high solar gain. The terminology alone is enough to make your head spin, and most of this technology is hidden inside the window so it can't readily be seen.

To make matters even more challenging, consumers have grown to expect high performance from their windows even when they don't understand the technologies involved. I once received a call from a homeowner who had bought tinted windows. A few sunny days after the installation, she wanted the windows removed because she didn't feel they were doing anything to reduce heat gain, as the salesperson had so zealously promised. After a few questions, I discovered that she had a 10-foot-wide covered porch wrapping all the way around her house. She was right: Her windows never received direct sunlight, so there was no way a tinted window was going to affect her energy costs. She had been sold the wrong product.

Better Labeling

Thanks to the efforts of the National Fenestration Rating Council, most building codes now require windows to be labeled. (The NFRC is a collaborative effort between manufacturers, the Department of Energy, utility companies, and others.) The NFRC label gives you specific information about the whole window performance, not just the glass or components. For instance, a single pane of clear glass transmits about 90 percent of the visible light striking it, giving it a visible transmittance rating of 0.90. For the whole window, including sash and frame, however, that number drops to about 0.60 or less, depending on the specific window. (See below for a more complete explanation of visible transmittance ratings.)

The NFRC label makes it easy to assess a window's energy performance. Which U-factor and solar heat-gain coefficient (SHGC) are optimal depends on climate. Optional ratings for air leakage (AL) and condensation resistance (CR) are included on this sample rating. AL indicates how many cubic feet of air leak through a square foot of window area, and generally ranges from 0.1 to 0.3. CR, which uses a scale of 1 to 100, indicates how well a window resists forming condensation on the inside of the glass. Higher numbers indicate better resistance.

U-factor measures how well a product prevents heat transfer; the lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value. The U-factor is the inverse of the more common R-value measurement. For example, a window with a U-factor of 0.25 has an R-value of 4 (1/.25 = 4).

Solar heat-gain coefficient (SHGC) measures how well a window blocks heat from incoming sunlight. The number, from 0 to 1, is the fraction of incident solar radiation admitted through a window. The lower a window's SHGC, the less solar heat it transmits into the house.