R-value (insulation)
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Aerogel is an extremely efficient man-made insulator and has a very high R-value.
The R-value is a measure of thermal resistance [1] used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat transfer per unit area, ) through it or .The R-value being discussed is the unit thermal resistance. This is used for a unit value of any particular material. It is expressed as the thickness of the material divided by the thermal conductivity. For the thermal resistance of an entire section of material, instead of the unit resistance, divide the unit thermal resistance by the area of the material. For example, if you have the unit thermal resistance of a wall, divide by the cross-sectional area of the depth of the wall to compute the thermal resistance. The unit thermal conductance of a material is denoted as C and is the reciprocal of the unit thermal resistance. This can also be called the unit surface conductance and denoted by h.[2] The bigger the number, the better the building insulation's effectiveness.[3] R-value is the reciprocal of U-value.

Around most of the world, R-values are given in SI units, typically square-metre kelvins per watt or mK/W (or equivalently to mC/W). In the United States customary units, R-values are given in units of ftFh/Btu. It is particularly easy to confuse SI and US R-values, because R-values both in the US and elsewhere are often cited without their units, e.g. R-3.5. Usually, however, the correct units can be inferred from the context and from the magnitudes of the values. United States R-values are approximately six times SI R-values [2].