# Thermal conductivity coefficient

The thermal conductivity coefficient (λ) expresses the quantity or heat flow that passes through the surface unit of a sample of the material, of infinite extension, parallel plane faces and unit thickness, when a difference of temperatures equal to unity, under stationary conditions.

## Determination of the coefficient of thermal conductivity

An empirical formula found by statistical means is used since deducing it by the laws of Physics presents great difficulties since it is necessary to consider the cell dimensions, the thickness of the layers of water molecules on the internal surface, the cosines in the direction with respect to the fibers and the heat conductivity coefficients of each of the component parts.

• λ = 0.168 ρ0 + 0.022 [Kcal / mh ºC]
• In the International System of Units (SI) it is measured in watt / meter × kelvin (W / (m · K)). The coefficient varies with the material conditions, mainly the humidity it contains and the temperature at which the measurement is made, so the conditions must be created to do it, generally for dry material and 15 ° C and on other occasions, 26 , 84 ° C.

## Thermal conductivity of different materials

 Thermal conductivity values ​​of some materials materials Thermal conductivity (W · m-1 · K-1) Steel 68-98 Water 0.98 Air 0.01 Aluminum 209.3 Asbestos 0.04 Zinc 106-140 Cork 0.04-0.30 Tin 64.0 Fiberglass 0.03-0.07 Common brick 0.80 Firebrick 0.47-1.05 Brass 1.14 Wood 0.13 Lead 35.0 Polyurethane 0.018-0.025 Glass 1.0