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In the case of materials, it is often expressed in terms of R-value per metre. R-values are additive for layers of materials, and the higher the R-value the better the performance. The U-factor or U-value is the overall heat transfer coefficient and can be found by taking the inverse of the R-value. It is a property that describes how well ...
The U-factor of a window can often be found on the rating label of the window. Although the concept of U-value (or U-factor) is universal, U-values can be expressed in different units. In most countries, U-value is expressed in SI units, as watts per square metre-kelvin: W/(m 2 ⋅K) In the United States, U-value is expressed as British thermal ...
Although the concept of U-value (or U-factor) is universal, U-values can be expressed in different units. In most countries, U-value is expressed in SI units, as watts per square metre-kelvin: W/(m 2 ⋅K) In the United States, U-value is expressed as British thermal units (Btu) per hour-square feet-degrees Fahrenheit: Btu/(h⋅ft 2 ⋅°F)
This takes into account a combination of the heat loss through the window (U value, the reciprocal of R-value), the solar gain (g value), and loss through air leakage around the frame (L value). For example, an A Rated window will in a typical year gain as much heat from solar gain as it loses in other ways (however the majority of this gain ...
The organization gained credibility and recognition with the passage of the 1992 National Energy Policy Act and the 1997 launch of the Energy Star program for windows. [2] In 2003, NFRC implemented one-size testing, moving away from the dual u-factor ratings that it had previously used (which were a compromise between founding manufacturers). [2]
With six-pane glazing, a U g value as low as 0.24 W/(m 2 K) [R-value 24] was reported. [1] This brings several advantages, such as: Energy efficient buildings without modulated sun shading The desired overall window thermal transmittance value of lower than about 0.4 W/(m 2 K) is possible
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Using the European standard a window with an ideal gap of 17 mm has a simulated U-Value of about 2.8 W/(m 2 K), a window with a much smaller gap of 6 mm has a U-value of about 3.3 W/(m 2 K), while a single glazed window has a U-value of about 5.5 W/(m 2 K). [2] (see also insulated glazing).