R-value (insulation) From Wikipedia, the free encyclopedia
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 m²·K/W
(or equivalently to m²·°C/W). In the United States customary units,
R-values are given in units of ft²·°F·h/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].