UPDATED on July 10, 2015
Good Windows Are Essential to Energy Conservation and Comfort
Choose frames and glazing for energy performance
Windows are among the most complex building components in a house, and at several hundred dollars or more apiece, also among the most expensive. In addition to the important architectural contribution they make, windows have far-reaching energy consequences. Their number, total area, and orientation to the sun can make or break the energy efficiency of a high-performance home.
Window frames do more than hold the glass in place and allow the window to open and close. They are an important part of a window’s overall thermal performance, and the type of frame helps dictate how much maintenance the window will need over its lifetime. Frame materials include wood, fiberglass, vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate)., aluminum, and both vinyl- and aluminum-clad substrates.
Glazing and frame performance are important, but so is the spacer
As the thermal performance of the frame and glazing improves, the performance of the spacer (with thermal effects that can extend out up to more than 2 inches into the glazing) becomes more important. And spacer performance is important in controlling condensation as well. Look to all three elements of a window for high performance.
ABOUT WINDOW MATERIALS
Until World War II, almost all residential windows were made from wood. Older wood windows were usually made from rot-resistant wood — often heartwood from slow-growing trees. Wood windows can last for decades, especially if they are protected from the weather and regularly painted. Some newer wood windows, however, are made from materials that rot faster, such as finger-jointed pine.
If your heart is set on wood windows, it's probably best to choose those that have an vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). or aluminum claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. . (Once a window has been protected with aluminum cladding, it's sometimes hard to tell what the sash or frame is made of. While most aluminum-clad windows are actually made of wood, new composite materials sometimes hide behind aluminum or vinyl cladding. When in doubt, ask the manufacturer.)
If you don't like the look of exterior cladding, design a home with generous roof overhangs and be prepared for a regular maintenance schedule that includes scraping and painting.
Rot-resistant alternatives to wood
In recent decades, window manufacturers have begun using more rot-resistant materials, including aluminum, vinyl, pultruded fiberglass, or some combination of these materials.
Fiberglass and aluminum are likely to be the most durable choices. From an energy perspective, fiberglass is far preferable to aluminum.
Aluminum windows are highly conductive; since they don't insulate as well as vinyl, wood, or fiberglass frames, they are rarely appropriate for an energy-efficient house. (Remember, just because a window has aluminum cladding doesn't mean that the window has aluminum frames. In most cases, aluminum-clad windows are made of wood.)
Foam-filled fiberglass frames perform better than other materials. Foam-filled vinyl frames are a close second, followed by wood frames. Some manufacturers offer composite frames made from a variety of materials; if these include a thermal break, they can perform well.
ABOUT WINDOW GLASS
Single glazing is a very poor insulator, with an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of about 1 (equivalent to U-1). Increasing the number of panes in a window improves the insulating value of the window, so clear double glazing has an R-value of about 2 (equivalent to U-0.5), and clear triple glazing has an R-value of about 3 (equivalent to U-0.33). The values for double or triple glazing can be further improved by including one or two low-e coatings and an inert gas fill between the panes. The best double-glazed windows have a whole-window U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of about 0.27, while the best triple-glazed windows have a whole-window U-factor of about 0.17. Triple glazing has been the standard for residential building in Sweden for many years and recently became mandatory in Germany. (For more information on glazing, see "All About Glazing Options." )
With the possible exception of Hawaii, windows installed in any U.S. state should always have at least double glazing. Triple glazing costs significantly more and only makes sense for colder climates unless a house is facing a very noisy location and needs acoustic isolation.
In addition to saving energy and reducing noise transmission, triple-glazed windows increase comfort by raising the temperature of a room's coldest surfaces in winter. When windows are warmer, the body radiates less heat toward them and feels more comfortable.
Canadian manufacturers are more likely to offer triple glazing than their American counterparts, but more and more U.S. window manufacturers are joining in:
For more information, see "Choosing Triple-Glazed Windows." 
Inert gas fills. In the 1960s and 1970s, most double-glazed, sealed and insulated glazing units had air between the panes. Such units are now called "clear double glazing." Substituting a less conductive, more viscous gas like argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. or kryptonA colorless, odorless inert gas, often used with argon in fluorescent lighting and sometimes used as gas fill in high-performance glazing. for the air between the panes results in better thermal performance (a lower U-factor), and argon- or krypton-filled glazing units are now standard in colder areas of the U.S.
The optimal space between the panes of argon-filled glazing units is 1/2 inch. Increasing or decreasing the thickness of this space degrades performance. For krypton, the optimal space is thinner — only 3/8 inch — so krypton, the more expensive gas, is usually reserved for applications where total glazing unit thickness must be minimized.
Low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. glazing. A low-e coating is a thin, nearly invisible metallic coating on glass that lowers the emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. of the glass. The effect of the coating is to lower a window's U-factor, improving its performance as a thermal insulator. Low-e windows make sense in every U.S. climate, and the cost of upgrading a window to low-e glazing is a cost-effective, energy-saving investment from Florida to California to Alaska to Maine.
There are at least two major categories of low-e coatings: soft-coat low-e (also known as vacuum-deposition or sputtered low-e) and hard-coat low-e (also known as pyrolytic low-e). Within each category, different formulations are possible. Spectrally selective low-e coatings are formulated to achieve a low SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1..
Which type of low-e coating has been applied by the glazing manufacturer is not important as long as the window's NFRC label verifies that the window's U-factor and SHGC are appropriate for the window's purpose.
A low-e window designed for the south wall of a passive solar house should have a low U-factor coupled with a high SHGC. As long as you shop "by the numbers," you'll get the window you need.
Window films. Several manufacturers sell window films that can be applied to the inside of an existing window pane. Because their main purpose is to reduce solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss., they are used mostly in warmer areas where air conditioning is a major expense.
Window films are unlikely to endure for the life of the window. Typical warranties last for five years.
Although window films can be a useful strategy to address a problem in an existing house, they are unnecessary in new construction. New windows can be ordered with low-solar-gain glazing, negating the need for a retrofit film.
ABOUT WINDOW RATINGS
There is no one-size-fits-all standard for choosing the glass, or glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., in windows. The most appropriate glass for a house in the Southwest won’t be the best choice for a house in Maine. Glass on a home's north side should have different characteristics than south-facing glass. Tuning glass for specific applications is an important part of passive solar design.
The National Fenestration Rating Council (NFRC) rates windows on three criteria: U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. , SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1., and visual transmittance (VT). Look for the NFRC label on rated windows (www.nfrc.org ).
U-factor measures how much heat is transmitted through the glass. The U-factor is the inverse of R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. . The lower the U-factor, the more efficiently the glass blocks the passage of heat. In all climates, windows with a low U-factor perform better than windows with a high U-factor. The EPA's Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. guidelines vary by region. In northern climates, an Energy Star–rated window must have a maximum U-factor of 0.35, the equivalent of an R-2.8 insulated wall.
Solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (SHGC) is the fraction of solar radiation admitted through a window, expressed as a number between 0 and 1. Lower numbers mean less of the sun's heat is transmitted through the glass. The lower the SHGC, the greater the shading ability of the glazing.
Visual transmittance (VT) is the fraction of visible light energy that makes it through the window glass. The higher the fraction, the more visible light will reach into the room. Maximizing VT while getting the right combination of U-factor and SHGC, particularly with low-e coatings, can be challenging. All three properties must be considered and balanced to evaluate window performance.
NFRC ratings for U-factor and SHGC are whole-window ratings, not glass-only ratings.
ABOUT CHOOSING WINDOWS
Specifying glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. can be daunting. But a few principles will steer you in the right direction.
In all climates, windows with a low U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. perform better than windows with a high U-factor. The Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. window program has set a low bar for cold-climate windows. To obtain an Energy Star label, these windows must have a maximum U-factor of 0.35. But windows with dramatically better performance are commercially available. Thermotech Windows (www.thermotechwindows.com ) sells triple-glazed casement windows with a U-factor of 0.17.
High-performance windows exceed Energy Star
Builders of energy-efficient homes should look for lower U-factors than Energy Star maximum values, ideally in the teens or twenties. Increasingly, designers of cold-climate homes are improving window U-factors by switching from double glazing to triple glazing.
Different windows for different walls
Designers of passive solar homes need to specify orientation-specific glazing. In a colder or less mild climate, south-facing windows need high-solar-gain glazing, while west-facing windows need low-solar-gain glazing. The SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of north windows doesn't matter much. When it comes to east windows, climate determines which type of glazing makes sense. In regions of North America where air conditioning is rarely used, high-solar-gain glazing is probably a good choice for east windows, since solar heat is welcome on cool mornings. In warmer regions, east windows, which like west windows are hard to shade, should probably have low-solar-gain glazing.
Protect south-facing windows with a roof overhang designed to shade them in summer while allowing the winter sun to enter the house.
In a warmer areas, choosing glazing with an extremely low SHGC — especially for east and west windows — will significantly lower air-conditioning loads. Look for windows with SHGCs that are significantly lower than the Energy Star standard of 0.40.
Cold-climate builders should specify insulated glazing with warm-edge spacers
Glazing spacers are visible at the perimeter of double-glazing units; they maintain the necessary distance between the panes and provide the edge seal. Traditional aluminum edge spacers are the weak thermal link in most double-glazing units. Glazing spacers with a thermal break are called warm-edge spacers, but these cost a little more than basic aluminum spacers and so aren't used by many window manufacturers.
Manufacturers of warm-edge spacers include BayForm, which makes the Thermal Edge spacer; Cardinal, which makes the XL Edge spacer; Edgetech, which makes the Super Spacer; Inex Spacer Industries; PPG, which makes the Intercept spacer; and Truseal Technologies, which makes the Swiggle Seal spacer.
Anyone who is ordering windows should be able to verify the type of glazing spacer used by consulting a representative from the window manufacturer or glazing supplier.
Efficient Window Collaborative:
Window Selection Tool  that compares the potential energy savings by performance characteristic in different parts of the country. Also, extensive background information on window design.
National Renewable Energy Laboratory:
"Passive Solar Design for the Home" 
Singing the "blues" in the key of "low-e" 
National Park Service:
Preservation guidelines 
Green Building Advisor:
All About Glazing Options 
Passivhaus Windows