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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)
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 ...
Values in parentheses are extrapolated, interpolated, or estimated. *It happens that the online record has the thermal conductivity at 30 Kelvins and ∥ {\displaystyle \parallel } to the c axis posted at 1.36 W⋅cm −1 K −1 and 78.0 Btu hr −1 ft −1 F −1 which is incorrect.
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)
Typical R-values are given for various materials and structures as approximations based on the average of available figures and are sorted by lowest value. R-value at 1 m gives R-values normalised to a 1 metre (3 ft 3 in) thickness and sorts by median value of the range.
Typical values range from 0.032 to 0.038 W/(m⋅K) depending on the density of the EPS board. The value of 0.038 W/(m⋅K) was obtained at 15 kg/m 3 while the value of 0.032 W/(m⋅K) was obtained at 40 kg/m 3 according to the datasheet of K-710 from StyroChem Finland. Adding fillers (graphites, aluminum, or carbons) has recently allowed the ...
Greater space increases the insulation value up to a point, but eventually with a large enough gap, convection currents begin to flow carrying heat between the panes within the unit. Typically, most sealed units achieve maximum insulating values using a space of 16–19 mm (0.63–0.75 in) when measured at the centre of the IGU. [12]
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).