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Insulation & Windows
Reflecting Heat Away
 Until recently, clear glass was the primary glazing material used in windows. Although glass is durable and allows a high percentage of sunlight to enter buildings, it has very little resistance to heat flow. During the past two decades, though, glazing technology has changed greatly. Research and development into types of glazing have created a new generation of materials that offer improved window efficiency and performance for consumers.
While this new generation of glazing materials quickly gains acceptance in the marketplace, the research and development of even more efficient technologies continues.
Current Options that Increase a Window's Energy Efficiency
Manufacturers usually represent the energy efficiency of windows in terms of their U-values (conductance of heat) or their R-values (resistance to heat flow). If a window's R-value is high, it will lose less heat than one with a lower R-value. Conversely, if a window's U-value is low, it will lose less heat than one with a higher U-value. In other words, U-values are the reciprocals of R-values (U-value = 1/R-value).
Usually, window R-values range from 0.9 to 3.0 (and U-values range from 1.1 to 0.3), but some highly energy-efficient exceptions also exist. When comparing different windows, you should ensure that all U- or R-values listed by manufacturers: (1) are based on current standards set by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), (2) are calculated for the entire window, including the frame, and not just for the center of the glass, and (3) represent the same size and style of window.
Today, several types of advanced glazing systems are available to help control heat loss or gain. The advanced glazings include double- and triple-pane windows with such coatings as low-emissivity (low-e), spectrally selective, heat-absorbing (tinted), or reflective; gas-filled windows; and windows incorporating combinations of these options.
|  Low-e Glazings
Low-e glazings have special coatings that reduce heat transfer through windows. The coatings are thin, almost invisible metal oxide or semiconductor films that are placed directly on one or more surfaces of glass or on plastic films between two or more panes. The coatings typically face air spaces within windows and reduce heat flow between the panes of glass.
When applied inside a double-pane window, the low-e coating is placed on the outer surface of the inner pane of glass to reflect heat back into the living space during the heating season. This same coating will slightly reduce heat gain during the cooling season. Low-e films are applied in either soft or hard coats. Soft-coat low-e films degrade when exposed to air and moisture, are easily damaged, and have a limited shelf life, so they are carefully applied by manufacturers in insulated multiple-pane windows. Hard low-e coatings, on the other hand, are more durable and can be used in add-on (retrofit) applications. But the energy performance of hard-coat low-e films is slightly poorer than that of soft-coat films. Windows manufactured with low-e films typically cost about 10% to 15% more than regular windows, but they reduce energy loss by as much as 30% to 50%.
Although low-e films are usually applied during manufacturing, retrofit low-e window films are also widely available for do-it-yourselfers. These films are inexpensive compared to total window replacements, last 10 to 15 years without peeling, save energy, reduce fabric fading, and increase comfort.
Spectrally Selective Coatings
Spectrally selective (optical) coatings are considered to be the next generation of low-e technologies. These coatings filter out from 40% to 70% of the heat normally transmitted through clear glass, while allowing the full amount of light to be transmitted. Spectrally selective coatings can be applied on various types of tinted glass to produce "customized" glazing systems capable of either increasing or decreasing solar gains according to the aesthetic and climatic effects desired.
Computer simulations have shown that advanced glazings with spectrally selective coatings can reduce the electric space cooling requirements of new homes in hot climates by more than 40%. Because of the energy-saving potential of spectrally selective glass, some utilities now offer rebates to encourage its use.
Heat-Absorbing Glazings
Another technology uses heat-absorbing glazings with tinted coatings to absorb solar heat gain. Some heat, however, continues to pass through tinted windows by conduction and reradiation. But inner layers of clear glass or spectrally selective coatings can be applied with tinted glass to further reduce this heat transfer. Heat-absorbing glass reflects only a small percentage of light and therefore does not have the mirror-like appearance of reflective glass.
Gray- and bronze-tinted windows reduce the penetration of both light and heat into buildings in equal amounts (i.e., not spectrally selective) and are the most common tint colors used. On the other hand, blue- and green-tinted windows offer greater penetration of visible light and slightly reduced heat transfer compared with other colors of tinted glass. When windows transmit less than 70% of visible light, plants inside could die or grow more slowly. In hot climates black-tinted glass should be avoided because it absorbs more light than heat.
Like black-tinted coatings, reflective coatings greatly reduce the transmission of daylight through clear glass. Although they typically block more light than heat, reflective coatings, when applied to tinted or clear glass, can also slow the transmission of heat. Reflective glazings are commonly applied in hot climates in which solar control is critical; however, the reduced cooling energy demands they achieve can be offset by the resulting need for additional electrical lighting.
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Tomorrow's Options for More Efficient Windows
"Superwindows" now coming on the market can attain high thermal resistance by combining multiple low-e coatings; low-conductance gas fills; barriers between panes, which reduce convective circulation of the gas fill; and insulating frames and edge spacers.
Also, optical properties such as solar transmittance can be customized for specific climate zones. The heat from even a small amount of diffuse winter sunlight will convert these super-windows into net suppliers of energy. This first generation of superwindows now available have a center-of-glass R-value of 8 or 9, but have an overall window R-value of only about 4 or 5 because of edge and frame losses.
Also under development are chromogenic (optical switching) glazings that will adapt to the frequent changes in the lighting and heating or cooling requirements of buildings. These "smart windows" will be separated into either passive or active glazing categories.
Passive glazings will be capable of varying their light transmission characteristics according to changes in sunlight (photochromic) and their heat transmittance characteristics according to ambient temperature swings (thermochromic). Active (electrochromic) windows will use a small electric current to alter their transmission properties. Both types should be on the market within 2 to 5 years.
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Heating & Cooling
 Dull, dark-colored home exteriors absorb 70% to 90% of the radiant energy from the sun that strikes the home's surfaces. Some of this absorbed energy is then transferred into your home by way of conduction, resulting in heat gain. In contrast, light-colored surfaces effectively reflect most of the heat away from your home. |
- Roofs
About a third of the unwanted heat that builds up in your home comes in through the roof. This is hard to control with traditional roofing materials. For example, unlike most light-colored surfaces, even white asphalt and fiberglass shingles absorb 70% of the solar radiation.
One good solution is to apply a reflective coating to your existing roof. Two standard roofing coatings are available at your local hardware store or lumber yard. They have both waterproof and reflective properties and are marketed primarily for mobile homes and recreational vehicles.
One coating is white latex that you can apply over many common roofing materials, such as asphalt and fiberglass shingles, tar paper, and metal. Most manufacturers offer a 5-year warranty.
A second coating is asphalt based and contains glass fibers and aluminum particles. You can apply it to most metal and asphalt roofs. Because it has a tacky surface, it attracts dust, which reduces its reflectivity somewhat.
Another way to reflect heat is to install a radiant barrier on the underside of your roof. A radiant barrier is simply a sheet of aluminum foil with a paper backing. When installed correctly, a radiant barrier can reduce heat gains through your ceiling by about 25%.
Radiant-barrier materials cost between $0.13 per square foot ($1.44 per square meter) for a single-layer product with a kraft-paper backing and $0.30 per square foot ($3.33 per square meter) for a vented multi-layer product with a fiber-reinforced backing. The latter product doubles as insulation.
- Walls
Wall color is not as important as roof color, but it does affect heat gain somewhat. White exterior walls absorb less heat than dark walls. And light, bright walls increase the longevity of siding, particularly on the east, west, and south sides of the house.
- Windows
Roughly 40% of the unwanted heat that builds up in your home comes in through windows. Reflective window coatings are one way to reflect heat away from your home. These coatings are plastic sheets treated with dyes or thin layers of metal. Besides keeping your house cooler, these reflective coatings cut glare and reduce fading of furniture, draperies, and carpeting.
Two main types of coatings include sun-control films and combination films. Sun-control films are best for warmer climates because they can reflect as much as 80% of the incoming sunlight. Many of these films are tinted, however, and tend to reduce light transmission as much as they reduce heat, thereby darkening the room.
Combination films allow some light into a room but they also let some heat in and prevent interior heat from escaping. These films are best for climates that have both hot and cold seasons. Investigate the different film options carefully to select the film that best meets your needs.
Note: Do not place reflective coatings on south-facing windows if you want to take advantage of heat gain during the winter.
The coatings are applied to the interior surface of the window. Although you can apply the films yourself, it is a good idea to have a professional install the coatings, particularly if you have several large windows. This will ensure a more durable installation and a more aesthetically pleasing look.
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Blocking the Heat
Two excellent methods to block heat are insulation and shading. Insulation helps keep your home comfortable and saves money on mechanical cooling systems such as air conditioners and electric fans. Shading devices block the suns rays and absorb or reflect the solar heat.
- Insulation
Weatherization measures—such as insulating, weatherstripping, and caulking—help seal and protect your house against the summer heat in addition to keeping out the winter cold.
The attic is a good place to start insulating because it is a major source of heat gain. Adequately insulating the attic protects the upper floors of a house. Recommended attic insulation levels depend on where you live and the type of heating system you use. For most climates, you want a minimum of R-30. In climates with extremely cold winters, you may want as much as R-49. Again, check the above publications for more information.
Wall insulation is not as important for cooling as attic insulation because outdoor temperatures are not as hot as attic temperatures. Also, floor insulation has little or no effect on cooling.
Although unintentional infiltration of outside air is not a major contributor to inside temperature, it is still a good idea to keep it out. Outside air can infiltrate your home around poorly sealed doors, windows, electrical outlets, and through openings in foundations and exterior walls. Thorough caulking and weatherstripping will control most of these air leaks.
- Shading
Shading your home can reduce indoor temperatures by as much as 20° F (11° C). Effective shading can be provided by trees and other vegetation and exterior or interior shades.
- Landscaping
Landscaping is a natural and beautiful way to shade your home and block the sun. A well-placed tree, bush, or vine can deliver effective shade and add to the aesthetic value of your property. When designing your landscaping, use plants native to your area that survive with minimal care. Deciduous trees that lose their leaves in the fall help cut cooling energy costs the most. When selectively placed around a house, they provide excellent protection from the summer sun and permit winter sunlight to reach and warm your house. The height, growth rate, branch spread, and shape are all factors to consider in choosing a tree. Vines are a quick way to provide shading and cooling. Grown on trellises, vines can shade windows or the whole side of a house. Ask your local nursery which vine is best suited to your climate and needs.
Besides providing shade, trees and vines create a cool microclimate that dramatically reduces the temperature (by as much as 9° F [5° C]) in the surrounding area. During photosynthesis, large amounts of water vapor escape through the leaves, cooling the passing air. And the generally dark and coarse leaves absorb solar radiation.
You might also consider low ground cover such as grass, small plants, and bushes. A grass-covered lawn is usually 10° F (6° C) cooler than bare ground in the summer. If you are in an arid or semiarid climate, consider native ground covers that require little water.
- Shading Devices
Both exterior and interior shades control heat gain. Exterior shades are generally more effective than interior shades because they block sunlight before it enters windows. When deciding which devices to use and where to use them, consider whether you are willing to open and close them daily or just put them up for the hottest season. You also want to know how they will affect ventilation.
Exterior shading devices include awnings, louvers, shutters, rolling shutters and shades, and solar screens. Awnings are very effective because they block direct sunlight. They are usually made of fabric or metal and are attached above the window and extend down and out. A properly installed awning can reduce heat gain up to 65% on southern windows and 77% on eastern windows. A light-colored awning does double duty by also reflecting sunlight.
Maintaining a gap between the top of the awning and the side of your house helps vent accumulated heat from under a solid-surface awning. If you live in a climate with cold winters, you will want to remove awnings for winter storage, or buy retractable ones, to take advantage of winter heat gain.
The amount of drop (how far down the awning comes) depends on which side of your house the window is on. An east or west window needs a drop of 65% to 75% of the window height. A south-facing window only needs a drop of 45% to 60% for the same amount of shade. A pleasing angle to the eye for mounting an awning is 45 degrees. Make sure the awning does not project into the path of foot traffic unless it is at least 6 feet 8 inches (2 meters) from the ground.
One disadvantage of awnings is that they can block views, particularly on the east and west sides. However, slatted awnings do allow limited viewing through the top parts of windows.
- Louvers are attractive because their adjustable slats control the level of sunlight entering your home and, depending on the design, can be adjusted from inside or outside your house. The slats can be vertical or horizontal. Louvers remain fixed and are attached to the exteriors of window frames.
- Shutters are movable wooden or metal coverings that, when closed, keep sunlight out. Shutters are either solid or slatted with fixed or adjustable slats. Besides reducing heat gain, they can provide privacy and security. Some shutters help insulate windows when it is cold outside.
- Rolling shutters have a series of horizontal slats that run down along a track. Rolling shades use a fabric. These are the most expensive shading options, but they work well and can provide security. Many exterior rolling shutters or shades can be conveniently controlled from the inside. One disadvantage is that when fully extended, they block all light.
Solar screens resemble standard window screens except they keep direct sunlight from entering the window, cut glare, and block light without blocking the view or eliminating air flow. They also provide privacy by restricting the view of the interior from outside your house. Solar screens come in a variety of colors and screening materials to compliment any home. Although do-it-yourself kits are available, these screens will not last as long as professionally built screens.
Although interior shading is not as effective as exterior shading, it is worthwhile if none of the previously mentioned techniques are possible. There are several ways to block the sun's heat from inside your house.
- Draperies and curtains made of tightly woven, light-colored, opaque fabrics reflect more of the sun's rays than they let through. The tighter the curtain is against the wall around the window, the better it will prevent heat gain. Two layers of draperies improve the effectiveness of the draperies' insulation when it is either hot or cold outside.
- Venetian blinds, although not as effective as draperies, can be adjusted to let in some light and air while reflecting the sun's heat. Some newer blinds are coated with reflective finishes. To be effective, the reflective surfaces must face the outdoors.
- Some interior cellular (honeycombed) shades also come with reflective mylar coatings. But they block natural light and restrict air flow.
Opaque roller shades are effective when fully drawn but also block light and restrict air flow. |
Conclusion
No one type of glazing is suitable for every application. Many materials are available that serve different purposes. Moreover, consumers may discover that they need two types of glazing for a home because of the directions that the windows face and the local climate. To make wise purchases, consumers should first examine their heating and cooling needs and prioritize desired features such as daylighting, solar heating, shading, ventilation, and aesthetic value.
Source List
The following organizations and publications provide more information on advances in glazing technology.
Ask an Energy Expert
The Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) DOE-EREC (363-3732)
Fax: (703) 893-0400
Email:doe.erec@nciinc.com
Consumer Energy Information Web site
EREC provides free general and technical information to the public on the many topics and technologies pertaining to energy efficiency and renewable energy.
Efficient Windows Collaborative
Alliance to Save Energy
1200 18th Street N.W., Suite 900
Washington, D.C. 20036
(202) 530-2245
Fax: 202-331-9588
Email:ewc@ase.org
Provides unbiased information on the benefits of energy-efficient windows, descriptions of how they work, and recommendations for their selection and use.
National Fenestration Rating Council (NFRC)
1300 Spring Street, Suite 500
Silver Spring, MD 20910
(301) 589-6372
Email:info@nfrc.org
Developed the Procedure for Determining Fenestration Product Thermal Properties (NFRC 100-91). These procedures are now being used in NFRC's window certification and efficiency labeling programs, which have already been adopted by three states.
Windows & Daylighting Group
Lawrence Berkeley National Laboratory
1 Cyclotron Road, MS 90-3111
Berkeley, California 94720
Provides technical support to government and industry efforts to help architects, engineers, and other commercial building specifiers choose energy-efficient and cost-effective residential windows.
Window & Door Manufacturers Association
1400 East Touhy Avenue, Suite 470
Des Plaines, IL 60018
(800) 223-2301
Fax: (847) 299-1286
Email:admin@wdma.com
A trade association representing U.S. and Canadian manufacturers and suppliers of windows and doors for the domestic and export markets.
Reading List
- "Low-E Glass—Why the Coating Is Where It Is," Energy Design Update, pp. 5-7, March 1990.
- "No Pane, No Gain (Window Technology: Part One)," Popular Science, pp. 92-98, June 1993.
- "The Elusive Benefits of Low-E and Gas-Filled Windows," Energy Design Update, pp. 7-9, June 1990.
- "Through the Glass Darkly," Popular Science, pp. 80-87, July 1993.
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