The U-Factor measures the rate of heat transfer, or heat loss, of the window. This value reflects the conductivity if the window, as well as the windows ability to absorb certain types of energy. The U-Factor is usually a number between 0.2 and 1.20. The lower the number, the less heat is lost through the window and the better its insulation properties. (from National Fenestration Rating Council).
R-Value measures the windows resistance to heat loss, or its conductivity. While the R-Value and the U-Factor are similar, the R-Value does not take the window's ability to absorb energy into account. This makes it a less accurate measure of heat transfer. Highly conductive products allow heat within a building to escape to the outdoors, resulting in higher energy costs. A high R-value indicates that the product is more efficient (from NFRC).
SHGC, or the solar heat gain coefficient, measures how well the window blocks heat from the sun. The SHGC, a value between 0 and 1, can be thought of as a ratio. For example, s SHGC of .4 would indicate that the window allows 40% of solar heat to pass through it. A low SHGC indicates that the window blocks more solar heat. Spectrally selective glass has become increasingly popular as a means of decreasing a windows solar heat gain (from NFRC).
Emissivity coatings (Low-e), or spectrally selective coatings, are usually used with dual paned windows or in insulated glass units (from NFRC). Low-e coatings can be applied in several ways. One option is to directly apply the coating to the windowpane. The coating can also be applied to a thin plastic sheet, which is suspended in the air cavity between the interior and exterior panes. Use of Low-e coatings can reduce the amount of heat transferred through the pane by a factor of 5 to 10. In heating based climates, Low-e coatings can be used to let solar heat enter the building and trap it inside. In this way, the house can be passively solar heated. In cooling climates, the Low-e coating keeps solar heat from entering the building (source: City of Austin Sustainable Building Sourcebook).
Visible Transmittance (VT) refers to the amount of possible light that a window conducts. Represented as a number between 0 and 1, a higher VT signifies that the window has a higher potential for day lighting. When used in conjunction with lighting controls, day lighting can save between 30% and 60% in building energy. Day lighting is directly related to the number of windows in a building and whether the glass is tinted or reflective. While tinted and reflective glass decreases the amount of solar gain, they also limit the amount of daylight entering the building (from NFRC).
Air Leakage (AL), represented as a number between 0.1 and 0.3, measures how much outside air comes through the windows. The lower the AL, the more effective the window is at keeping out air (from NFRC).
The number of glass panes used also greatly affects the window unit's energy efficiency. A single pane window, characterized by a single sheet of glass or plastic, provides little to no protection from heat transfer. In contrast, double and triple pane windows have two or three sheets of glass that are separated by a layer of air or inert gas, typically argon or krypton. The layer of gas acts as insulation, dramatically decreasing the amount of heat transferred through the window. Low-emittance (Low-e) coatings for windows have decreased the need for triple pane windows (source: City of Austin Sustainable Building Sourcebook).
The material used for the frame and sashing also affects a window unit's efficiency. Wood, aluminum, vinyl, and a combination of wood and vinyl or aluminum can be used. While wood minimizes heat transfer, it is susceptible to damage from moisture and insects. In contrast, aluminum is an inexpensive, long-lasting material, but it transmits the most heat of all the possible options. The use of thermal breaks can be effective in lowering the rate of aluminum's heat transfer. Vinyl frames also minimize the heat transfer and have the benefit of being insect and rot resistance. Vinyl can be used as an exterior casing for wood frames and wood can be used to clad aluminum. Both of these options are durable and provide protection from heat transfer (source: City of Austin Sustainable Building Sourcebook).
Because of the sun's east-west path, the sides of a building are exposed to different amounts of solar heat. Exposure to east-facing windows should be minimized, while exposure to west-facing windows should be eliminated. This is due to the fact that the east and west sides of structures receive the majority of solar heat (source: City of Austin Sustainable Building Sourcebook).. While solar exposure can be reduced with the use of awnings, using reflective film or glazing can also be effective. On west facing windows, a low SHGC, or .40 or less, is most desirable (source: Portland's Green Building Resource).
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