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For one square foot of the insulation, one Btu per hour of heat flows across the insulator for each degree of temperature difference across it. 1 therm is defined in the United States as 100,000 Btu using the Btu 59 °F definition.
Measured in square feet, the reference standard for EDR is the mattress radiator [1] invented by Stephen J. Gold in the mid 19th century. One square foot of EDR is able to liberate 240 BTU per hour when surrounded by 70 °F (21 °C) air and filled with steam of approximately 215 °F (102 °C) temperature and 1 psi of pressure.
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)
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kelvin square-metre per watt (K⋅m 2 /W or, equally, °C⋅m 2 /W), whereas the I-P (inch-pound) unit is degree Fahrenheit square-foot hour per British thermal unit (°F⋅ft 2 ⋅h/BTU). [13] For R-values there is no difference between U.S. and Imperial units, so the same I-P unit is used in both.
The British thermal unit (Btu) is defined as the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. [47] It was in use before 1859 as a unit of heat based on imperial units rather than the metric units used by the French [ 48 ] — Clément-Desormes having defined the calorie in terms of the kilogram and ...
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