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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)
NFRC defines a standard size for each type of window and door. This is the size that is modeled and tested. [4] NFRC maintains a list of standard materials whose u-factor is used in modeling. [5] Window and door manufacturers, and their suppliers, may submit proprietary materials to be added to the list as well.
U = Overall heat transfer coefficient in Btu/(ft2-hr-deg F) A = Area in square feet T1 = outdoor temperature in degrees F T2 = indoor temperature in degrees F For heat gains due to people, equipment (hooded and unhooded), and lighting
The U-value is used to refer to the amount of heat that can pass through a window, called thermal transmittance, with a lower score being better. [1] 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.
A standard IGU consisting of clear uncoated panes of glass (or lights) with air in the cavity between the lights typically has an RSI-value of 0.35 K·m 2 /W. Using US customary units, a rule of thumb in standard IGU construction is that each change in the component of the IGU results in an increase of 1 R-value to the efficiency of the unit ...
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Architects and engineers call the resulting values either the U-Value or the R-Value of a construction assembly like a wall. Each type of value (R or U) are related as the inverse of each other such that R-Value = 1/U-Value and both are more fully understood through the concept of an overall heat transfer coefficient described in lower section ...
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