Indeed, aluminum cladding has been embraced by builders and consumers for its reliable performance and low-maintenance characteristics. “It's crisp, stable, and it doesn't expand and contract like other materials,” says Engerman. Unfortunately, it is also a good conductor of heat and cold.

To compensate, manufacturers are engineering their newest models with the highest-performing glazings, such as argon-filled low-E2, and other insulating materials. Consequently, products such as Caradco's Tradition Plus and Pella's ProLine double-hung aluminum-clad wood windows meet or exceed the DOE's tough Energy Star requirements.

“Windows are responsible for quite a bit of energy consumption in a home,” says Joe Wiehagen, the NAHB Research Center's energy-efficiency senior research engineer. “So if you improve the window technology, the benefit can be huge—not just in energy savings. It can also reduce the size of your heating and cooling system, which is an upfront cost you can take advantage of.”

As defined by the DOE, low-E (low-emittance) coatings are highly reflective, transparent coatings applied to the window glazing. Because they are designed to reflect long-wavelength infrared radiation, less heat is transferred through the window either from the home or the outside.

By contrast, the newer low-E2, a spectrally selective or low-solar-gain low-E coating (known under a variety of brand names) reflects the heating energy from the sun (or solar heat gain) to keep homes cooler in hot, sunny climates, particularly in the South and Southwest. Filled with gas—argon or krypton—both become even more thermally efficient, which is de rigueur for colder northern regions.

“Manufacturers must have products that are going to perform with more and more energy codes being put in place by each state,” explains Jeff Kliber, brand manager for Vetter and sister company Peachtree. “We try to put products into place that are going to perform in all four NFRC [National Fenestration Rating Council] zones with our highest glazing option possible.”

At Peachtree, both the aluminum-clad wood 500 series and more custom aluminum-clad or primed-wood 700 series come standard with the same EasyCare low-E2 glazing, which is an option on the more entry-level vinyl-clad wood 300 series. All, however, are outfitted with a flexible warm-edge spacer system for improved insulation and condensation resistance.

ELEMENTAL ADJUSTMENTS According to researcher Christian Kohler of the Lawrence Berkeley National Laboratory Window and Daylighting Group in California, “Every insulated glazing [IG] has a spacer on the edge, which acts as a kind of thermal short circuit. Aluminum is common, but manufacturers are experimenting with different designs and materials to improve performance.”

Weather Shield's flexible Warm Edge I spacer, for one, is made with a non-conductive material that the manufacturer claims reduces conductivity by more than 50 percent over other warm-edge spacers.

“The technology and design used in Warm Edge I increases the glass temperature, making windows warmer. This insulates the frame and edge of the glass better than metal spacers and aluminum spacer bars and significantly minimizes condensation,” explains Peter Lenar, Weather Shield brand manager.

Hurd developed the Energy Plus window with a new warm-edge spacer system in an effort to provide Energy Star-rated extruded aluminum products that don't require a gas-fill. This was done specifically for high-altitude regions where it is necessary to insert a “breather tube” into the glass to prevent damage due to pressure shifts. Although these tubes are crimped and sealed at the site, stopping the release of gas, there's no way to control how much is lost due to varying conditions on jobsites.

“There are two key factors for our Energy Plus unit,” says Hurd design manager Paul Surek. “One is the high-thermal composite core placed within the extruded aluminum frame to reduce the amount of cold and heat conduction. The other is our stainless steel Intercept spacer.” This channel- or C-shaped spacer adds to the unit's efficiency that, again, reduces metal conduction to make it more thermally efficient. “In addition,” Surek contends, “the stainless steel is a better insulating material than aluminum or even other types of steel.”

For states along the Atlantic Ocean and Gulf Coast, hurricane protection in tandem with energy efficiency is fast becoming a code requirement. “South Carolina has a code and we have to prove that our windows meet that rating,” says Daniel Island, S.C.-based custom builder Kevin Kalman of Kalman Construction. “So we continue to use hurricane-rated products with impact-resistant glass.” But, he also notes that his customers are interested in the most energy-efficient windows available.

One product line that fills these two requirements is Andersen's windows and patio doors with the company's Stormwatch protection. These vinyl-clad products with high-performance insulated glass are available in a variety of styles and sizes including double-hung, casement, awning, specialty shapes, patio doors, transoms, and skylights.

Meanwhile, Kolbe & Kolbe says it meets the most stringent criteria in all-wood or extruded aluminum-clad wood with its Sterling double-hung wood windows. Weather Shield LifeGuard—with low-E2 glazing and either an argon- or krypton-filled airspace—may be ideal for colder climates from Maine to North and South Carolina. And Hurd's FeelSafe storm-resistant windows and patio doors bring an Energy Star-rating to the mix.

TOWARD ZERO ENERGY Ultimately, it is the reduction of wasteful energy use and utility bills that drives state-of-the-art window technology. Tucson, Ariz.-based builder John Wesley Clark used Milgard vinyl windows with argon-filled low-E2 glazing for a zero-energy home he built in collaboration with the NAHB Research Center.

“Compared to more expensive wood windows, these perform quite well,” says Clark. “They're easy to install. They require no maintenance, and the quality of vinyl holds up in the harsh desert climate. We've had no problems with sun damage on west-facing windows,” he attests, adding that his carpenters trim them out in the same way they would a wood window on the interior for a more distinctive look.

Palo Alto, Calif.-based Clarum Homes also installed Milgard vinyl windows in its Watsonville Vista Montana zero-energy Enviro-Home community. “We used vinyl windows with SunCoat low-E glazing for its thermal properties, durability, and price,” says senior purchasing agent Linda Schieffelin. “For a production builder they have all the pluses.”

According to Patrick Hooper, Jeld-Wen's marketing manager for vinyl and aluminum windows, “From a thermal or energy-efficiency standpoint vinyl outperforms wood.” The only downside, he admits, has been the overall appearance of vinyl.

Hooper claims Jeld-Wen's Energy Star-rated Premium vinyl line tackles this issue with high-quality extruded vinyl units complete with low-E2 glazing available with simulated divided lights similar to those used in pricier aluminum-clad and wood windows.

For those who require wood interiors, Crestline's CrestWood vinyl-clad wood windows provide the benefits of both materials with specially fade-, dent-, and scratch-resistant PVC exteriors, clear pine interiors, a warm-edge spacer system, and high-performance glazing options. Meanwhile, Marvin's Integrity brand pairs its unique Ultrex pultruded fiberglass with solid pine in its new Energy Star-rated in-swing French door.

FAST FORWARD What's on the horizon? “Over the past 20 years the big development has been in the coatings used on glass to help the performance. That's the state of the art right now,” says Wiehagan of the NAHB Research Center. “But there is some pretty forward-looking research continuing around windows.”

One such technology, electronically controllable electrochromic glazings by Sage Electrochromics, has been applied to a selection of new venting skylights from Velux. These VSE SageGlass skylights are produced by coating the glass at the factory with multiple layers of ceramic thin films that are in total less than 1/50 the thickness of a human hair. When a low DC voltage is applied the active electrochromic layers darken. Reversing the voltage polarity causes the layers to lighten.

“Electrochromic glazings are controllable,” says Wiehagen. “So you could switch the glass from clear to dark electrically by using a remote.” This ability to manipulate accommodates the unpredictable needs of the large mid-heating/mid-cooling climate zones, such as the Middle Atlantic states. These so-called “dynamic” glazings would allow homeowners to control the amount of light and heat that enters a room. The currently available “static” low-E glazings, on the other hand, while effective, were generally designed for either hot or cold climates.

At this point, this sort of cutting-edge innovation doesn't come cheap. The Velux electric venting skylight in size 104 with SageGlass costs $1,800, while the regular model in that size is $650. Still, the company calculates that to operate a whole house full would take less energy than a single 40-watt incandescent bulb.

Other “dynamic” windows on the boards feature integral automated shading systems and light-redirecting devices, the latter of which would allow solar gain in the winter and reflect the sun's heat in the summer. Another technology under consideration is aerogel, an in-pane insulating material that lets light past through the glass.

“These future technology advances [are] the key to zero-energy construction,” writes Lawrence Berkeley's Dariush Arasted in the report “Future Advanced Windows for Zero-Energy Homes.” “They are expected to lay the groundwork for the next generation of window products.”

Energy-Saving Window Guides In addition to individual manufacturer's Web sites, check the following sites for more information about the latest window technologies, government regulations, products, and calculation tools:—The American Architectural Manufacturers Association is a source for performance standards, product certification, and educational programs for the fenestration industry.—DOE's Energy Efficiency and Renewable Energy site has a special section for building technologies.—A helpful resource from the Alliance to Save Energy and the Lawrence Berkeley National Laboratory that provides specifics as to what types of windows are most effective for individual circumstances and locales.—The official site of the Energy Star program.—The official site of the National Fenestration Rating Council.—The official site of the Sustainable Buildings Industry Council.—The official site of NAHB Research Center and one of the best places to learn about zero-energy homes and energy-saving building products.

Integrity. Fabricated with an Energy Star-rated extruded fiberglass-based Ultrex exterior and a solid pine interior, the new jamb-hinged in-swing French patio door boasts tempered, argon-filled low-E2 glazing—said by manufacturer Marvin to provide superior comfort and performance that reduces heating and cooling costs. More aesthetic options include transoms, grilles, and both top-hung and swinging screens. 888-537-8263.

Great Lakes Window. NFRC-certified UniFrame windows exceed the most stringent Northern Zone Energy Star requirements. UniFrame double-hung windows feature internal fiberglass-reinforced meeting rails for strength and durability, according to the maker. Energy performance is further enhanced by an insulated glass system that combines two panes of vacuum-deposition low-E glass with an interior glass substrate and two insulation chambers of krypton gas. 800-666-0000.

SPD Technologies. Infinitint electrically dim-mable windows, skylights, and doors utilize a tinted film that controls the amount of light, glare, and heat transmitted through the glazing. The glazing blocks more than 90 percent of UV radiation and meets all U.S. electrical certifications. 866-773-7627.

Caradco. Energy-efficient and water-resistant Tradition Plus aluminum-clad wood windows are available in a wide variety of sizes and configurations, in both casement and double-hung versions. Standard features include paint- or stain-ready pine interiors, water-resistant exterior cladding—in Hartford green, desert sand, brilliant white, chestnut bronze, French vanilla, and mesa red—and argon-filled low-E2 glazing. 800-877-9482.

Simonton. Designed for new construction, ProFinish Master series windows are made of rigid extruded vinyl with a minimum R-50 performance rating, the firm says. Available with an efficient ¾-inch insulating glass unit and Intercept spacer system, the windows can be ordered to meet Energy Star guidelines for all climate zones. Colors include white, tan, and driftwood. Optional pre-assembled, ready-to-install wood bucks and brick moulds provide a finished look. 800-746-6686.

Capital. The 9555 vinyl double-hung unit from this MI Windows and Doors company comes with the Intercept insulating glass spacer system for enhanced energy efficiency. Low-E glass allows the 9555 to meet Energy Star requirements in most markets; the window also is available with a combination of low-E glass and argon gas filling. In addition, it features removable sashes and easy glass replacement for drywall pass-through. 800-949-3818.

Pella. Notable for its Energy Star rating and affordability, the company's latest ProLine double-hung aluminum-clad wood window features authentic-looking simulated divided light (SDL) grilles permanently applied to the outside and inside of the windows. Available in a selection of white, tan, or brown exterior cladding, the ProLine SDL sports a tilt-to-clean sash for easy maintenance. 888-847-3552.

Republic. The Energy Star-qualified Ramguard vinyl window features durometer glazing that makes replacing glass in the field a breeze, says the firm. The window also has a sloped sill, interlocking meeting rails, spiral balances, and adjustable anti-lift blocks. It comes in numerous styles in white, beige, and cocoa. 800-248-1775.

Superseal. The series 1850B vinyl Brickmould new-construction double-hung window offers a crisp, classic look, and an impressive DP-50 rating, the firm says. It features ¾-inch warm-edge insulated glass, a sloped sill, and a weather-stripped interlock for an airtight fit. Low-E glass and argon gas filling are options. An integral J-channel and nailing fin ensure ease of installation. 800-521-6704.

ViWinTech. Designed for new construction applications, Energy Star-approved 6100 series double-hung vinyl windows feature fully welded frame and sash corners, heavy-duty weather stripping, and integral pre-punched nailing fins for easy and accurate installation, says the maker. The windows come in white or tan and are available with standard- and custom-size grids, glazing options, and mullions. 800-788-1050.

Marvin. The wood Ultimate Insert double-hung window not only is a historically authentic replacement window, but it is energy efficient too, featuring low-E2 glazing, argon-gas filling, and a DP-30 rating. The product was designed to fit an existing wood frame without disrupting the interior millwork or exterior trim. It features an open cam lock, a tilting lever for easy cleaning, and a factory-primed exterior. 888-537-8266.

Weather Shield Said to significantly reduce heat and cold conductivity as well as condensation, the Warm Edge 1 flexible spacer can be used in windows of any shape and size due to its uniquely flexible composite laminate material. This eliminates customer concerns about having dissimilar spacers in special windows. In addition, its high-performance bond line adhesive is said to provide exceptional resistance to argon gas transmission. 800-477-6808.

Hurd. Backed by an Energy Star rating, this aluminum-clad window is said to maintain optimum efficiency at high altitudes, with or without argon gas, due to a thermal composite material at the cladding's core and C-shaped stainless steel warm-edge spacers—better insulators than aluminum. While a breather tube protects the glass, this window, which comes standard with argon-filled low-E glazing, was engineered to override any potential loss of the gas. 800-223-4873.

CertainTeed. Thermally efficient for both hot and cold weather, Bryn Mawr II vinyl replacement windows feature the company's Thermaflect low-E glazing with a low-conductivity spacer system and argon gas. Available in a variety of styles including double-hung, sliders, bays, bows, and geometric shapes, this classic line also offers a cam-type lock and keeper for a weather-tight seal and meets Energy Star guidelines for all climates. 800-233-8990.

Crestline. Easy-to-maintain and energy-efficient, CrestWood vinyl-clad wood windows offer the warmth and charm of wood on the interior. Available in a extensive range of sizes, specialty shapes, and bow and bay configurations, CrestWood windows feature a warm-edge spacer system, multiple weather strips, and high-performance low-E2 glazing options. 800-552-4111.

Kolbe & Kolbe. The new Sterling aluminum-clad double-hung window is designed to provide the traditional aesthetics of fine wood furnishings, in addition to meeting state-of-the-art energy standards and tough coastal building codes. Plus, it has a cleverly concealed jamb liner and block-and-tackle balances. Available in 22 standard exterior colors, as well as a variety of optional interior wood species, the Sterling double-hung unit comes with 7/8-inch argon-filled low-E2 insulating glass. 800-955-8177.

Velux. The VSE SageGlass electric venting skylight features electrochromic glass, which enables the homeowner to control solar heat gain to suit variable weather conditions and preferences. Ideal for regions where the sun patterns and outside temperatures are unpredictable, this cutting-edge product operates on a low DC voltage, and a house full of these units is said to use less energy than a 40-watt incandescent light bulb. 800-283-2831.

Andersen. Advanced glazing, sash, and jamb technologies ensure that windows and patio doors with new Stormwatch protection meet or exceed the code requirements of such coastal states as New York, Florida, and Texas. Available on a wide variety of the company's products, including the 400 series Frenchwood out-swing patio doors (shown), Stormwatch features the company's Perma-Shield vinyl cladding and High-Performance energy-efficient, impact-resistant glass. 888-888-7020.

Vetter. In order to maintain the architectural integrity of this classic Shingle-style four-story Lake Geneva, Wis., residence, while at the same time providing sufficient energy efficiency and comfort, the builder installed this company's aluminum-clad wood double-hung and casement windows, as well as patio doors and transoms, all with its Ashford collection of simulated divided lights and argon-filled low-E2 glazing. 800-838-8372.

Milgard. Sold with standard SunCoat low-E glazing and the company's unique locking system for added security, the vinyl Style Line series—with its slim frame profile—is a contemporary alternative for those who are attracted to the energy- and cost-efficiency of this versatile material. The windows come backed by a lifetime guarantee. 800-645-4273.

Bilco. The series 650 Hopper vinyl window features Intercept Warm-Edge spacers between the two glass panes to create a ¾-inch-thick glass panel. Intercept spacers reduce condensation around the window perimeter. Options include low-E glass and argon-gas filling. 203-934-6363.

Lincoln. This maker of aluminum-clad wood windows and patio doors is an Energy Star partner. Windows offered include double-hung, casement, awning, gliders, and specialty. Energy-efficient low-E2 glazings that retain a high visible light transmission of 72 percent are optional. Also available are simulated divided lights and numerous cladding colors. 715-536-2461.

Peachtree. Featuring energy-efficient low-E2 glazing with ¾-inch warm-edge spacers and a unique EasyCare finish that resists dirt and water, 500 series aluminum-clad windows and patio doors are available in a wide variety of styles and configurations. Natural pine interiors and a choice of five exterior colors, including classic white, ivory, pebble tan, forest green, and Sierra brown, add to the visual appeal. 888-888-3814.

Gorell. The 5200 series premium vinyl windows are available with double- or triple-pane glass, low-E coatings, and argon or krypton gas between the glass panes. Windows with one of these high-performance glass systems qualify for the Energy Star label. The 5200 series features aluminum-reinforced interlocking meeting rails, Insulfil foam-filled master frame extrusions, welded sashes and frames, and PPG's Intercept Warm-Edge spacer system. 724-465-1800.