Thermal Conductivity Thermal conductivity is a material property. It indicates the heat flow (W or J/s) occurring in 1m of material length at a temperature differential of 1 Kelvin on both ends of this length.
The Greek symbol commonly used for thermal conductivity is lower case lambda λ. Sometimes, lower case k is used instead. The unit for thermal conductivity is W/(mK). The parentheses are not optional, as this example might illustrate: A material might conduct 30 Watt over the length of 1 meter at a temperature differential of 30 Kelvin. Without parentheses the calculation goes: 30/1*30=900; with parentheses it's 30/(1*30)=1.
Usually, a material would have a measured thermal conductivity (from a sample) and a design thermal conductivity fit for calculations, containing a safety margin, thus acknowledging inconsistencies in the production process.
The smaller the thermal conductivity, the lesser the amount of heat that is conducted, or in other words: the better the insulation properties of the material.
Metals usually have a high thermal conductivity (aluminium for example of at about 220 W/(mK)).
Wood usually has a low thermal conductivity, but can be anisotropic: having different thermal conductivities in line with the grain and perpendicular to the grain (lower). By default the lower value perpendicular to the grain is given, thus if the piece of wood is used with the grain in line with the heat flow (e.g. end grain boards), adjustments have to be made.
As a rule of thumb, thermal conductivity will be relative to the gross density of a material: high gross density = high thermal conductivity. Water content and temperature of the material are furthermore relevant for its thermal conductivity.
ISO 10456:2007 and NZS 4214:2006 give tabulated values for the thermal conductivity of generic materials. Where ever possible, more accurate, specific values should however be used for calculations. | 