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ENERGY EFFICIENT WINDOWS

Windows are the most prominent architectural aspect of residential buildings. They serve a variety of purposes:

  • Building aesthetics
  • View of surroundings
  • Letting in daylight
  • Natural ventilation
  • Filtering of outside noise
  • Emergency exits.

    It is often forgotten that windows have one of the greatest energy impacts of residential building elements.

    HEAT LOSS AND GAIN

    The heat losses and gains of a building occur by the conduction of heat through the solid elements of the building envelope (the walls, ceiling, floor, windows, and doors). This heat is then convected and radiated to the outside or inside depending on the direction of heat flow. Air infiltration (leakage) and ventilation are also important contributors to a building's heat loss or gain. Solar heat gain can offset winter heat losses but contributes to summer cooling loads.

    SOLAR GAIN

    Solar radiation is the most efficient source of heat that can be used to warm a house. However, solar gains are also the source of unwanted heat, especially in the summer.

    Because windows transmit much more solar radiation than the opaque elements of a building, they are the primary focus for controlling the timing and amount of solar gain.

    ORIENTATION

    The orientation of a window greatly affects the times and levels of solar heat gains through the window. South-facing* windows see the sun throughout much of the day.

    East- and west-facing windows face the sun primarily in the early morning and late afternoon hours, respectively.

    North-facing windows view the sun only obliquely during the cooling season and not at all during the heating season.

    * References to north and south in this section are correct for the northern hemisphere; switch them for buildings in the southern hemisphere.

    AIRTIGHTNESS

    The airtightness of a window depends on the sash/frame construction and fit, and installation in the wall. Infiltration, or air leakage between the indoors and outdoors through cracks and joints around window frames, sash, and glazing panes, makes additional heating and cooling necessary.

    CONDENSATION

    Excessive humidity is often a problem in residential buildings because of the many sources of moisture. This is especially the case during the heating season when windows and doors are typically closed to conserve heat. Under such conditions, moisture condensation often occurs on window surfaces which are below the dew point temperature of the surrounding humid air. In addition to poor visibility through windows, severe and repeated condensation may also result in damage to window frames and sills, paint and wallpaper, carpeting, plasterboard, and structural framing in walls.

    Systems and Components

    Glazing type, window frame and sash, weatherstripping, and exterior and interior shading devices all play key roles in defining the overall energy and comfort performance of window systems.

    It is important to weigh individual component performance against total integrated performance to ensure the best overall design.

    GLAZING

    Windows are commercially available with glazing panes of glass, fiberglass, acrylic plastic, or polycarbonate plastic. Fiberglass and plastic glazings are lighter but less scratch-resistant than glass. Various manufacturers offer windows with single, double, triple, or quadruple glazing. Additional glazing panes are one approach to reducing heat flow through windows; however, use of triple- and quadruple-glazed windows can be awkward and more expensive, and is economically justified only in the most severe heating climates.

    Some window manufacturers use advanced glazing technologies to provide maximum window thermal and optical performance. Currently available high performance windows include low-emittance (low-E) coatings and gas fills. Low-E coatings and gas-filled double-glazed windows can achieve many of the benefits of standard triple and quadruple glazings at lower cost.

    Low-E coatings reduce heat flow by making the window less transparent to parts of the solar spectrum. A low-E coating can reject unwanted infrared heat from the sun in the summer and keep infrared heat from radiating out of a home in the winter. Infrared radiation represents just under half of the sun's energy that reaches the earth's surface.

    The interpane spaces of multiple-glazed windows can be filled with gases of greater insulating value than air, such as argon and krypton. Gas fills have proven most effective when used in conjunction with low-E coatings. They also improve the sound deadening properties of windows.

    WINDOW FRAMES

    Window frames can be made of wood, aluminum, steel, vinyl, or some combination of these materials. Conductances of frame materials vary, and they strongly affect heat flow through windows. Some windows are made with thermal breaks between frame segments to reduce heat losses or gains.

    SASH TYPES

    The sash type of a window determines the ventilation air flow rate through the window relative to its size. Shown here are some common sash types and their effective areas for ventilation.

    NATURAL SHADING

    Shading systems of various types can be used to reduce undesirable solar heat gains through windows. The simplest shading systems are properly located trees, shrubs, and vine trellises. Natural vegetation can be very effective in absorbing and/or reflecting solar radiation before it reaches windows.

    EXTERIOR SHADING

    Exterior shading devices for windows may be fixed or movable. Many types can be employed, including overhangs, sidefins, awnings, shutters, roll blinds, and sun screens. Fixed overhangs and side fins are structural elements of buildings which are not easily damaged. Less rugged movable shading devices are more easily adapted to specific situations.

    INTERIOR SHADING

    Interior shading devices for windows are easily operated by the occupants of homes. However, their effective shading coefficients are generally not as low as those of exterior devices. Available types include draperies, venetian blinds, and roll blinds.

    Operation

    HEATING SEASON

    During the heating season, windows should generally be closed to retain heat in the living space. But timely opening of windows, especially those in bathrooms, kitchens, and recreation rooms, can help to alleviate condensation.

    Movable shading devices should be opened or removed from windows to take advantage of beneficial solar heat gains during daylight hours. Closing interior shading devices at night will decrease night time heat losses through windows.

    COOLING SEASON

    The natural ventilation provided by opening windows when outdoor temperature and humidity are within the comfort range is especially important during the cooling season. Shading devices should be employed to reduce undesirable solar heat gains, especially on east, west, and horizontal exposures, which receive the most intense solar radiation.

    Performance

    U-VALUE

    A U-value is a measure of the rate of heat flow through a material. An R-value is a measure of a material's resistance to heat flow and is the reciprocal, or inverse, of a U-value (for example, a window with a U-value of 0.5 has an R-value of 2.0).

    U-values and R-values can be used to describe the insulating abilities of windows. A lower window U-value, or higher R-value, indicates greater resistance to heat flow through the window. If the U-value of a window is halved (or the R-value doubled), the heat loss rate is halved.

    COMPARING U-VALUES

    Note that when comparing different windows, some may be rated in terms of the U-value of the center of the glazing, while others state the U-value for the window as a whole. The whole window U-value is different than the center of glass measurement of the same window since it takes into account losses associated with the window frame and the spacers between the panes. The whole window U-value is therefore more accurate than the center of glazing value, and should be used whenever possible. The National Fenestration Rating Council (NFRC) administers a voluntary window rating program to produce window U-value ratings and labels that can be directly compared because they are subject to identical evaluation procedures.

    SHADING COEFFICIENT OR SOLAR HEAT GAIN COEFFICIENT

    A shading coefficient (SC) is a relative measure of the ability of a window to transmit solar heat compared to that of a clear, double-strength, single glazing.

    Shading coefficients are expressed as numbers between 0 and 1. A lower shading coefficient indicates less solar heat transmission through the window.

    Solar heat gain coefficient (SHGC) is replacing SC in National Fenestration Rating Council ratings as a measure of solar input. It is an absolute measure of the total transmitted solar energy, compared to that incident on the exterior of the window.

    SOLAR TRANSMITTANCE

    Glazing responds to solar radiation in three ways. Some of the rays that strike it are reflected back. Some of the rays are transmitted through to the space. And some are absorbed by the glazing, which will warm up and radiate this heat both into the space and back outside.

    AIRTIGHTNESS

    The airtightness of a window depends on the sash type as well as on the fit of the window frame, sash, and glazing panes. Most swinging sash types close more tightly than do most sliding sash types. The construction, installation, and weatherproofing should be of high quality to control infiltration around a window.

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    Written by: Solstice Sustainable Energy


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