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Selecting Windows For Energy Efficiency

New window technologies have increased energy benefits and comfort, and provided more practical options for consumers. This selection guide will help homeowners, architects, and builders take advantage of the expanding window market. The guide contains three sections: an explanation of energy-related window characteristics, a discussion of window energy performance ratings, and a convenient checklist for window selection.

Selecting the right window for a specific home invariably requires tradeoffs between different energy performance features, and with other non-energy issues. An understanding of some basic energy concepts is therefore essential to choosing appropriate windows and skylights.

Three major types of energy flow occur through windows:

(1) non-solar heat losses and gains in the form of conduction, convection, and radiation;
(2) solar heat gains in the form of radiation; and
(3) airflow, both intentional (ventilation) and unintentional (infiltration).

Insulating Value

The non-solar heat flow through a window is a result of the temperature difference between the indoors and outdoors. Windows lose heat to the outside during the heating season and gain heat from the outside during the cooling season, adding to the energy needs in a home. The effects of non-solar heat flow are generally greater on heating needs than on cooling needs because indoor-outdoor temperature differences are greater during the heating season than during the cooling season in most regions of the United States.

For any window product, the greater the temperature difference from inside to out, the greater the rate of heat flow. A U-factor is a measure of the rate of non-solar heat flow through a window or skylight. (An R-value is a measure of the resistance of a window or skylight to heat flow and is the reciprocal of a U-factor.) Lower U-factors (or higher R values), thus indicate reduced heat flow. U-factors allow consumers to compare the insulating properties of different windows and skylights.

The insulating value of a single-pane window is due mainly to the thin films of still air on the interior and moving air on the exterior glazing surfaces. The glazing itself doesn't offer much resistance to heat flow. Additional panes markedly reduce the U-factor by creating still air spaces, which increase insulating value. In addition to conventional double-pane windows, many manufacturers offer windows that incorporate relatively new technologies aimed at decreasing U-factors. These technologies include low-emittance (low-e) coatings and gas fills . A low-e coating is a microscopically thin, virtually invisible, metal or metallic oxide coating deposited on a glazing surface. The coating may be applied to one or more of the glazing surfaces facing an air space in a multiple-pane window, or to a thin plastic film inserted between panes. The coating limits radiative heat flow between panes by reflecting heat back into the home during cold weather and back to the outdoors during warm weather. This effect increases the insulating value of the window. Most window manufacturers now offer windows and skylights with low-e coatings.

The spaces between window panes can be filled with gases that insulate better than air. Argon, krypton, sulfur hexafluoride, and carbon dioxide are among the gases used for this purpose. Gas fills add only a few dollars to the prices of most windows and skylights. They are most effective when used in conjunction with low-e coatings. For these reasons, some manufacturers have made gas fills standard in their low-e windows and skylights.

The insulating value of an entire window can be very different from that of the glazing alone. The whole-window U-factor includes the effects of the glazing, the frame, and, if present, the insulating glass spacer. (The spacer is the component in a window that separates glazing panes. It often reduces the insulating value at the glazing edges.)

Since a single-pane window with a metal frame has about the same overall U-factor as a single glass pane alone, frame and glazing edge effects were not of great concern before multiple-pane, low-e, and gas-filled windows and skylights were widely used. With the recent expansion of thermally improved glazing options offered by manufacturers, frame and spacer properties now can have a more pronounced influence on the U-factors of windows and skylights. As a result, frame and spacer options have also multiplied as manufacturers offer improved designs.

Window frames can be made of aluminum, steel, wood, vinyl, fiberglass, or composites of these materials. Wood, fiberglass, and vinyl frames are better insulators than metal. Some aluminum frames are designed with internal thermal breaks, non-metal components that reduce heat flow through the frame. These thermally broken aluminum frames can resist heat flow considerably better than aluminum frames without thermal breaks. Composite frames may use two or more materials (e.g. aluminum-clad wood, vinyl-clad wood) to optimize their design and performance, and typically have insulating values intermediate between those of the materials comprising them.

Frame geometry, as well as material type, also strongly influences thermal performance properties. Spacers can be made of aluminum, steel, fiberglass, foam, or combinations of these materials. Spacer thermal performance is as much a function of geometry as of composition. For example, some well-designed metal spacers insulate almost as well as foam. Due to their orientation and their greater projected surface areas, domed and other shaped tilted and horizontal skylights have significantly higher U-factors than do vertical windows of similar materials and opening sizes.

Preventing Condensation Air can hold varying amounts of water vapor or moisture. The warmer the air is, the more moisture it can hold. The amount of moisture in the air, expressed as a percentage of the maximum amount the air could hold at a given temperature, is called its relative humidity. For health and comfort, indoor air should contain some moisture. The relative humidity should generally be between 30% and 40% at normal room temperature. The relative humidity of air can be increased by adding more moisture or by reducing the temperature. When the relative humidity reaches 100%, the air can hold no more moisture, and water begins to condense from it. The temperature at which this condensation occurs is called the dew point temperature of the air. When moist air comes in contact with a cold surface in a home, it may be cooled to its dew point temperature, resulting in condensation on the surface.

Windows don't cause condensation; historically they have been the first and most obvious place it occurs. This is because windows generally have lower thermal resistances than insulated walls, ceilings, and floors. As a result, their inside temperatures are usually lower than those of other surfaces in a home during cold weather. If the air in a home is humid enough, water will condense from it when it is cooled at a window surface.

Condensation is most often thought of as a cold climate winter problem. However, in hot humid weather, moisture can condense on the outside surface of a poorly insulatedwindow in an air conditioned building. Left unchecked, condensation can damage window frames, sills, and interior shades. Water can deteriorate the surrounding paint, wallpaper, plasterboard, and furnishings. In severe cases, it can seep into adjoining walls, causing damage to the insulation and framing. The indoor air coming in contact with energy-efficient windows is less likely to be cooled to its dew point temperature because the inside surface temperatures remain higher during cold weather than do those of windows with single glazing, traditional metal spacers, and metal frames. three glazing types with widely varied U-factors. The graph shows clearly that the risk of condensation at the center of the glass is reduced as the insulating value of the glass increases. Even at an outdoor air temperature of -30°F, the indoor air relative humidity must be nearly 50% before condensation will form on the triple glazing with two low-e coatings. On the other hand, at an outdoor temperature of 10#176;F, condensation will form on the single glazing at an indoor relative humidity of only 18%.

Condensation is even more likely to occur at window spacers and frames, which are usually less insulating than the corresponding glazings. With so many insulating glazing types available, efforts to prevent condensation have shifted toward the development of better insulating spacers and frames. Recommendations for Selecting Window U-Factors When shopping for windows and skylights, pay close attention to whether the U-factor listed by the manufacturer applies to the glazing only or to the entire unit. If it is for the glazing only, the overall U-factor may be considerably higher because of the frame and spacer effects. These effects increase with decreasing total window area. Compare different window types or makes by their total U-factors, which are best obtained from NFRC labels. New window energy ratings and the RESFEN computer program (see Window Labeling) can be used to estimate the relative energy usage associated with a particular window type and U-factor. Avoid aluminum-frame windows without thermal breaks if possible. Even in milder climates, these windows tend to have low inside surface temperatures during the heating season, giving rise to condensation problems.

Aluminum-frame windows with properly designed thermal breaks can be used in moderate climates. Wood, vinyl, and fiberglass are the best frame materials for maximum insulating value. Single-pane windows are impractical in heating-dominated climates. In these regions, multiple-pane, low-e, and gas-filled window configurations are advisable. In most climates, glazings with low-e coatings and gas fills will be a choice that provides significant energy savings in a cost-effective product. Low-e and gas fills have now become a common option for many manufacturers, which reduces their added cost. The resultant total window U-factor should be .5 or lower and preferably below .4 for maximum energy savings.

Consumers should select windows with long warranty periods, which indicate sound window design and construction, and a reduced probability of insulating glass seal failure or gas leakage, which would reduce performance. Remember that lower window and skylight U-factors mean less energy consumption, lower utility bills, and greater comfort in the living space. Solar Control Solar transmission through windows and skylights can provide free heating during the heating season, but it can cause a home to overheat during the cooling season. Depending upon orientation, shading and climate, solar-induced cooling costs can be greater than heating benefits in many regions of the United States. In fact, solar transmission through windows and skylights may account for 30% or more of the cooling requirements in a residence in some climates. Because the sun's position in the sky changes throughout the day and from one season to another, window orientation has a strong bearing on solar heat gain. Figure 3 shows the solar heat gain through 1/8-inch clear single glass for various window orientations on very clear days in the heating and cooling seasons at 40°N latitude.

South-facing windows allow the greatest and potentially most beneficial solar heat gain during the heating season, while admitting relatively little of the solar heat that contributes to cooling requirements during the cooling season. The reverse is true for skylights and east- and west-facing windows. North exposures transmit only minimal solar heat at any time. The ultimate importance of these climatic and orientation effects will depend on the type of glazing under consideration. Each glazing layer in a window reduces the amount of solar heat gain to the interior space by absorbing or reflecting energy. The Solar Heat Gain Coefficient (SHGC) is a measure of the rate of solar heat flowing through a window or skylight. (A Shading Coefficient (SC)is the previous standard indicator of a window's shading ability and for simple glazings is approximately equal to the solar heat gain coefficient multiplied by 1.15.) Solar heat gain coefficients allow consumers to compare the solar heat gain properties of different windows and skylights. The solar heat gain coefficient accounts for both the transmissive glazing element, as well as the opaque frame and sash. Additional glazing layers provide more barriers to solar radiation, thus reducing the solar heat gain coefficient of a window. Tinted glazings, such as bronze and green, provide lower solar heat gain coefficients than does clear glass. Low-e coatings can be engineered to reduce window solar heat gain coefficients by rejecting more of the incident solar radiation.

Spectrally selective glazings, including some low-e coated glazings with low solar heat gain coefficients and new light blue and light blue-green tinted glazings, block out much of the sun's heat while maintaining higher visible transmittances and more neutral colors than more heavily tinted bronze and grey glazings. High transmittance low-e coatings, used in conjunction with a tinted outer glass layer, also reduce solar heat gain by preventing the absorbed heat from reaching the interior space. Mirror-like reflective glazings are commonly used in office buildings, but only occassionally chosen for residences. While they may have very low solar heat gain coefficients, they block so much of the light and view that they are not normally desireable in homes. Ultraviolet Protection Ultraviolet radiation is the main component of sunlight that can fade and damage drapes, carpets, furniture, and paintings when transmitted through windows and skylights. Efforts to produce window glazings that transmit less ultraviolet energy have met with some success. In general, windows and skylights with plastic glazing layers or low-e coatings reduce ultraviolet transmission.

Even without any ultraviolet radiation, sunlight can still cause fading of fabrics and other furnishings. Recommendations for Solar Control It may be useful to consider two aspects of window selection to control solar gain - the selection of the window itself, and the choice of interior or exterior shading devices. Traditional windows with clear glass required the use of shading devices to obtain adequate performance, especially when the orientation admitted substantial sunlight in summer However, modern high performance windows can do such a good job of controlling sunlight that the importance of these shading systems is reduced. Window solar heat gain coefficients should ideally be selected according to orientation, but it may not always be practical to do so. If south exposures are to admit beneficial solar heat during the heating season, their solar heat gain coefficients should be high. These high solar heat gain coefficients will not usually result in overheating problems during the cooling season because of the lower solar radiation levels at that time on south-facing windows, especially those with adequate roof overhangs. Skylights and east- and west-oriented windows may warrant lower solar heat gain coefficients since they transmit the most solar heat during cooling periods. In most climates, there is not much point in spending more money to obtain lower solar heat gain coefficients for north-facing windows. In hot, sunny climates, in order to provide low solar heat gain coefficients without loss of light, select windows with spectrally selective glass or spectrally selective low-e coated glazings. Darker tinted glazings also provide lower solar heat gain coefficients, but they will yield somewhat decreased outdoor visibility, particularly at night. Where glare is a concern, this effect may be desired, but under other conditions it may not. In climates where cooling loads are large, look for windows with SHGC of .4 or less.

To maintain good light transmittance and visibility, select windows whose glazings have visible transmittance of .6 or higher. In some hot climates, where winters are mild, it might seem reasonable to select a single glazed window with a low solar heat gain coefficent, rather than a more typical double glazing. However, single glazings have a more limited range of solar control (even if laminated glass and glue-on plastic films are considered), so a double glazed window with appropriate glazing choice as noted above, may be the best overall solution, even in hot climates. Exterior or interior shading devices, such as awnings, louvered screens, sunscreens, venetian blinds, roller shades, and drapes are essential when clear glass is used, and can complement and enhance the performance of windows whose glazings provide low solar heat gain coefficients. One advantage of many shading devices is that they can be adjusted to admit more or less solar heat according to the time of day and the season, if the occupants are conscientious. But windows with "built-in" lower solar heat gain coefficients provide better visibility and require less management and maintenance in todays busy households. Exterior shading devices are more effective than interior devices in reducing solar heat gain because they block radiation before it passes through a window. Light-colored shades are preferable to dark ones because they reflect more, and absorb less, radiation. Horizontally oriented adjustable shading devices are appropriate for south-facing windows, while vertically oriented adjustable devices are more effective for shading windows on east and west orientations. Ventilation and Airtightness Airflow through and around windows occurs by design as ventilation and inadvertently as infiltration.

The use of windows for natural ventilation is as old as architecture itself. Opening windows, particularly on opposite sides of a living space, can cool a home for free. The sash type of a window influences the ventilation airflow rate through the window relative to its size. Some common sash types and their effective open areas for ventilation purposes are shown in Table 3. Casement windows are especially effective for ventilation because they tend to direct the greatest airflow into the living space when fully open. Infiltration is the uncontrolled leakage of air into a building from the exterior through joints and cracks around window and skylight frames, sash, and glazings. This leakage can account for up to 10% of the energy usage in a home. The airtightness o f a window depends on both the properties of the window, (i.e.sash type and the overall quality of the window construction) and on the quality of the installation. Operable windows with compressing seals are generally more airtight than purely sliding seals, because of the way the sash element seals against the framing. An air leakage rating is a standardized measure of the rate of infiltration through a window or skylight under specific environmental conditions. Air leakage ratings allow consumers to compare the airtightness of different windows and skylights as a manufactured product - they do not account for any leakage between the installed product and the wall.

Airflow Recommendations In milder climates, or in spring and fall in more severe climates, operable windows can provide welcome ventilation and improved comfort, with reduced need for air conditioning. Operable windows are often specified to meet building code requirements for emergency egress. Although, operable windows are sometimes useful in household areas with high moisture production, such as bathrooms, kitchens, and laundry rooms, exhaust fans provide more reliable control throughout the year. Select windows with air leakage ratings that meet or exceed standard industry requirements of 0.37 cfm/ft2 to minimize discomfort from uncontrolled infiltration. Even lower values should be selected for particular windy sites or harsh climates. Check the seals between window components for airtightness. To minimize infiltration around installed windows, follow manufacturers installation procedures carefully and seal and caulk joints and cracks. Jeffrey L. Warner, a researcher at Lawrence Berkeley National Laboratory, developed and wrote this document. Michael Wilde coordinated the editorial development and industry review.