The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity (k) measured in Btu/hr-ft-oF. It is a measure of a substances ability to transfer heat through a solid by conduction. The thermal conductivity of most liquids and solids varies with temperature. For vapors, it depends upon pressure.
Heat transfer across materials of high thermal conductivity occurs at a faster rate than across materials of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation
Thermal conductivity of materials is temperature dependent. In general, materials become more conductive to heat as the average temperature increases.
The reciprocal of thermal conductivity is thermal resistivity.
In the International System of Units (SI), thermal conductivity is measured in watts per meter kelvin (W/(m·K)).
In the imperial system of measurement thermal conductivity is measured in Btu/(hr-ft⋅-F) where 1 Btu/(hr-ft-⋅F) = 1.730735 W/(m·K). [Perry's Chemical Engineers' Handbook, 7th Edition, Table 1-4]
Other units which are closely related to the thermal conductivity are in common use in the construction and textile industries. The construction industry makes use of units such as the R-Value (resistance value) and the U-Value (transmittance value). Although related to the thermal conductivity of a product R and U-values are dependent on the thickness of a product.
Likewise the textile industry has several units including the Tog and the Clo which express thermal resistance of a material in a way analogous to the R-values used in the construction industry.
Note: R-Values and U-Values quoted in the US (based on the imperial units of measurement) do not correspond with and are not compatible with those used in Europe (based on the SI units of measurement).