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These variables include, building material of the envelope, thicknesses of the building materials, day of the year, time of day, orientation of the surface (e.g. wall or roof, 90 degrees or 180), and wall face orientation (cardinal directions, i.e. N, NW, S, SE, etc.), to name a few.
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)
R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions. The measure is therefore equally relevant for lowering energy bills for heating in the winter, for cooling in the summer, and for general comfort.
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In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density [1], heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time. Its SI units are watts per square metre (W/m 2). It has both a direction and a magnitude, and so it is a vector quantity.
Metabolic rate is the rate of transformation of chemical energy into heat and mechanical work by metabolic activities of an individual. It is defined as per unit of skin surface area which equals to 58.2 W/m 2 (18.4 Btu/h·ft 2). This is the energy produced from a unit skin surface area of an average person seated at rest. [1]