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, vinyl, 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 vinyl or aluminum cladding. (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-value 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-factor 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:
- Alpen Windows (Serious Energy)
- Comfort Line
- Great Lakes Windows
- Marvin Windows
- Paramount Windows
- Schuco USA
- Vinyl Kraft
- Weather Shield
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 argon or krypton 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-e glazing. A low-e coating is a thin, nearly invisible metallic coating on glass that lowers the emissivity 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 SHGC.
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 gain, 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 glazing, 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-factor, SHGC, 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-value. 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 Star 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) 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
Shop for your specific climate
Specifying glazing can be daunting. But a few principles will steer you in the right direction.
In all climates, windows with a low U-factor perform better than windows with a high U-factor. The Energy Star 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 SHGC 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:
National Park Service:
Windows must be installed and sealed properly
Windows can make a major contribution to energy conservation and comfort. Today’s windows are available in a wide range of materials. Glazing choices affect how much light they transmit, how much of the sun’s heat they allow in interior spaces, and how well they prevent the flow of heat. Modern doors offer similar choices in construction and performance. But a high-tech window or door won’t give you what you paid for if it isn’t installed properly.
See below for:
A window needs a durable frame and appropriate glazing
Frames and sash. Pultruded fiberglass and aluminum are both rot-resistant materials that stand up to the elements better than wood. But because aluminum conducts heat freely, it is a poor choice for sash and window frames. Like fiberglass and aluminum, vinyl is a rot-resistant material; however, many green builders avoid using it because of the environmental issues raised by the manufacture of PVC. Vinyl is probably less resistant to damage by ultraviolet light than fiberglass. To learn more, see “About Frames and Sash” below.
Glazing. Almost all new windows installed in the U.S. include double glazing, but it varies widely in performance. Some types have a relatively high insulating value, while others leak heat readily. Similarly, some allow a high percentage of the sun’s heat into the house, while others don’t. In colder areas, triple-glazed windows will help keep the indoors comfortable and lower energy bills. To learn more, see “About Window Glazing” below.
It can be a challenge to balance durability, performance and looks
The best energy performers in the spectrum of window- frame materials are foam-filled fiberglass and foam-filled vinyl, followed by wood. However, the slender aesthetic of aluminum appeals to many designers, provoking a conflict between thermal performance and looks. Some designers choose thermally broken aluminum frames, which perform better than aluminum frames without thermal breaks. Fiberglass muntins are very close in width to metal ones.
The vinyl window debate. Designers of affordable housing often face the PVC conundrum: Vinyl windows perform well thermally and are inexpensive, but they are frowned upon by the green building community because of the issues associated with PVC production. Unfortunately, there are no easy (or inexpensive) solutions to this challenge.
The aesthetic of wood windows is perennially popular. The only drawbacks are maintenance and – if maintenance is neglected, as it frequently is – durability. Wood windows, particularly in severe climates or sunny exposures, need to be repainted every few years or they will deteriorate rapidly. Clad windows – that is, wood windows that are wrapped on the exterior with vinyl or metal (usually steel or aluminum, less frequently bronze) are the answer. They are more expensive than non-clad wood windows, but the additional investment will pay off in reduced maintenance costs. A variation on this theme is a vinyl or fiberglass window frame that is clad with wood on the inside.
Casements are better than sliders. Windows are available in a variety of styles, including double-hung, single-hung, horizontal sliders, and casements. Casement windows should be the first choice for an energy-efficient home. Because the cam lock on a casement window pulls the sash tightly against the weatherstripping, a well-built casement will have less air leakage than the best available double-hung window.
Specify different window sizes for different orientations. In an energy-efficient home, it’s common to specify different glazing for different orientations. One drawback to orientation-specific glazing is that it’s possible for window installers to accidentally put a south window on the north side of the house. To prevent this error, a designer can specify differently sized windows depending on the orientation. Builders can’t install the wrong window if it doesn’t fit in the rough opening.
Durable Window Installation Means Letting the Water Drain out
Rule #1 is to keep as much water out as possible. Rule #2 is to let the leaks drain out.
Well-flashed window heads are the first line of defense.
Sloped sills, backdams and pan flashing direct water out.
There are many choices in sill pan flashing; in this video, Mike Guertin shows how to install flexible peel-and-stick pan flashing.
Read the code, then the instructions
IRC provisions related to flashing and installation of windows and exterior glass doors can be found in Section 613. All windows and doors must be installed according to manufacturers’ instructions and be tested, labeled and installed to meet the design wind loads specified in Table 301.2(4).
Bedroom egress windows must be within 44 inches of the floor and have an operable portion at least 20 inches wide and 24 inches high (310.1.2, 310.1.3) with a net opening of at least 5.7 square feet (310.1.1). Egress windows with grade-level access can have a net opening of 5.0 square feet (310.1.1X).
Operating windows that are more than 6 feet above grade must be installed a minimum of 24 inches above the finished floor or be protected by window guards meeting ASTM F2006 or F2090 (613.2). Window guards must open or be removable from inside the room without keys, tools, or special knowledge (310.4).
Window performance is covered in section 1101.5. Windows and doors lacking a U-factor or solar heat gain coefficient (SHGC) rating from an accredited independent laboratory should be assigned the default values in tables 1101.5(1) and 1105(3), respectively. A climate zone map used for fenestration requirements can be found in Table and Figure 1101.2.
Window replacement isn’t the best place to start in energy upgrades. When planning energy improvements to an existing house, replacing windows should show up toward the bottom of the list. It almost always makes sense to improve an existing home’s air tightness and add insulation to the attic and basement. Replacing an old furnace or refrigerator can also be cost-effective. But if the windows work well, it’s usually best to put replacing windows lower on the list. In a cold climate, the best way to improve single-glazed windows is to install exterior storm windows with low-e glass.
PAN FLASHING OPTIONS
How you install windows is even more important than which ones you buy. Allowing water that leaks in to drain out is central to durable window installations.
For more information on this topic, see Installing Windows In a Foam-Sheathed Wall.
GBA has published an 8-part video series on methods for flashing windows. Here is a link to the first episode in the series: Window Sills That Won’t Rot.
The best-looking windows don’t always perform the best
Because the edge spacers separating the panes of double-pane windows are usually made of aluminum, they don’t perform nearly as well as the gas fill between the panes of glass. Windows with true divided lights look the most like old-fashioned double-hung windows, but they leak more heat than windows with undivided panes.
Windows with warm-edge spacers perform better than windows with aluminum spacers. The best windows for performance are casement-style windows with warm-edge spacers and no divided lights.
The best-performing windows don’t always look the best
Not all low-e coatings are created equal. They each distort the proportions of different visible wavelengths making it through the glass resulting in some windows looking “blue” or “red.” This can make wall colors appear slightly different than intended on the interior or give the windows a hue or tinted reflection on the exterior. People really vary in their sensitivity to this effect so the best thing to do is to have your customers tune into the issue BEFORE they select and you order the window package. For more information on this topic, see FURTHER RESOURCES.
Most new windows perform well, and warranty claims are rare. But because warranty coverage varies among manufacturers, it makes sense to read the fine print.
Many window makers have experienced premature failures in the past few decades, however rare. Even major manufacturers have had problems with rot, distorted casement sashes, and condensation between the panes of insulated glazing units.
When selecting windows, it’s always wise to compare warranties. The bigger the manufacturer and the better the warranty, the greater the chance that a manufacturer will stand behind its products and resolve disputes. Most big window manufacturers warrant window glass for 10 years and the frames and hardware for 20.
HISTORIC WINDOW REPLACEMENT ISN’T AN OXYMORON
Many window manufacturers offer historic retrofits (as illustrated in this display by Marvin Windows) that will be more likely to pass the historic commission’s review panel.
Andersen’s Woodwright series of replacement windows features wood jamb liners and traditional sash details that blend gracefully into a historic house.
THE RIGHT GLAZING CAN BE HARD TO FIND
In the U.S., it is much easier to buy low-solar-gain glazing than high-solar-gain glazing. Although high-solar-gain, low-e glazing is available from all major manufacturers of insulated glazing, U.S. window makers rarely offer it. The reason is simple: It’s easier for window manufacturers to stock and sell windows with one kind of glazing from Canada to the Mexican border than to offer glazing choices. Most low-e windows sold in the U.S. have a low SHGC, in the range of 0.27 to 0.35.
Designers of passive solar homes should allow adequate lead times for special-order glazing for south windows.
Good choices for south-facing glazing include the following types of low-e double glazing:
Ask your window rep which types of low-e glazing are offered.
HIGH-PERFORMANCE COLD-CLIMATE WINDOWS
The best performing cold-climate windows are manufactured in Europe, although some energy experts maintain that Canadian fiberglass-frame windows with triple-glazing can match the performance of the best European windows.
High-performance windows are particularly important for Passivhaus builders. In the U.S., some Passivhaus builders insist on using German windows. At least four brands of German windows — Internorm, Optiwin, Pazen, and Unilux — are now being distributed in the U.S. and Canada.
More information on German windows and Passivhaus window standards can be found in a GBA article, “Passivhaus Windows.”
GOOD COLD-CLIMATE WINDOWS PERFORM BETTER THAN WALLS
Although many designers still consider windows to be “energy holes,” it’s actually possible for triple-glazed cold-climate windows to gain more energy than they lose.
To learn more, see “Windows That Perform Better Than Walls.”
WHAT ABOUT SKYLIGHTS?
The GBA Encyclopedia addresses skylights and roof windows in a separate article, “Skylights.”
LEED for Homes Window choice influences thermal performance, potentially affecting 3 to 5 points in EA1 (Energy & Atmosphere), EA2, and/or EA3.
NGBS Under Chapter 7, “Energy Efficiency”: up to 12 points (prescriptive path) based on thermal properties and climate (703.3.1).