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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)
The radiant exitance (previously called radiant emittance), , has dimensions of energy flux (energy per unit time per unit area), and the SI units of measure are joules per second per square metre (J⋅s −1 ⋅m −2), or equivalently, watts per square metre (W⋅m −2). [2] The SI unit for absolute temperature, T, is the kelvin (K).
The SI unit of power for heating and cooling systems is the watt. Btu per hour (Btu/h) is sometimes used in North America and the United Kingdom - the latter for air conditioning mainly, though "Btu/h" is sometimes abbreviated to just "Btu". [18] MBH—thousands of Btu per hour—is also common. [19] 1 W is approximately 3.412142 Btu/h [20]
It quantifies how effectively a material can resist the transfer of heat through conduction, convection, and radiation. It has the units square metre kelvins per watt (m 2 ⋅K/W) in SI units or square foot degree Fahrenheit–hours per British thermal unit (ft 2 ⋅°F⋅h/Btu) in imperial units. The higher the thermal insulance, the better a ...
Thermal conductivity and resistivity. The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by , , or and is measured in W·m −1 ·K −1. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity.
The R-value (in K ⋅ m 2/ W) is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive [2] flow of heat, in the context of construction. [3]. R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the ...
Also the copy is blurred up enough to give you the impression that maybe what it really means is 1.36 W −1 cm −1 K −1 and 78.6 Btu hr −1 ft −1 F −1 and a type-head that got overdue for its cleaning since the secretary had a tall heap of papers on her desk and if that is the case then the multilingual expression is perfectly ...
The general definition of the heat transfer coefficient is: where: : heat flux (W/m²); i.e., thermal power per unit area, : difference in temperature between the solid surface and surrounding fluid area (K) The heat transfer coefficient is the reciprocal of thermal insulance. This is used for building materials (R-value) and for clothing ...