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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 ...
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)
The construction industry makes use of measures such as the R-value (resistance) and the U-value (transmittance or conductance). Although related to the thermal conductivity of a material used in an insulation product or assembly, R- and U-values are measured per unit area, and depend on the specified thickness of the product or assembly. [note 2]
To account for other components in a wall such as framing, first calculate the U-value (= 1/R-value) of each component, then the area-weighted average U-value. An average R-value is 1/(average U-value). For example, if 10% of the area is 4 inches of softwood (R-value 5.6) and 90% is 2 inches of silica aerogel (R-value 20), the area-weighted U ...
Quantity (common name/s) (Common) symbol/s Defining equation SI unit Dimension General heat/thermal capacity C = / J⋅K −1: ML 2 T −2 Θ −1: Heat capacity (isobaric)
Thermal insulance (R-value) is a measure of a material's resistance to the heat current. It quantifies how effectively a material can resist the transfer of heat through conduction, convection, and radiation.
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A r (Ar) is the relative atomic mass of argon and M u = 10 −3 kg⋅mol −1 as defined at the time. However, following the 2019 revision of the SI, R now has an exact value defined in terms of other exactly defined physical constants.