The Thermal Conductivity of Thermal Insulators

This issue, we present an overview of a number of materials that are often used as thermal insulators. The world would have been much easier for thermal engineers if only the creator had provided us with a choice of materials showing the same range in thermal conductivity values as exists for electrical conductivity. Alas, this isn’t the case. Thus, not a single one of the materials with which we must live can really be qualified with the term ‘insulator’. The most important reason why experimental validation of CFD (Computational Fluid Dynamics) codes is a disaster (except for high velocities) is because conjugate heat transfer via the support always plays a role. Consider, for example, a component on a substrate in a natural convection environment. Even the best insulator cannot prevent losses of 10% and more. Hence, adiabatic surfaces, well loved among numerical analysts, cannot be realized in practice.

The accompanying table also shows the density because many insulators consist partly of air. Hence, the density is strongly correlated with the thermal conductivity. Again, all values in the table are defined at room temperature. Due to the presence of air, the temperature dependence is more complex than for plastics and rubbers for which the thermal conductivity increases by a few percent in the range from 0-100oC. The conductivity of air increases by about 30% in this range. However, the major heat path is usually still paved with plastic; hence, the temperature dependence of plastic dominates. Observe the fact that some materials show a value lower than that of air. This can only be realized if the size of the pores filled with air is smaller than the mean free path of the air molecules.

 
 
Thermal Conductivity of Thermal Insulators @ 25oC
Material Density
(kg/m3)
Thermal
conductivity
(W/mK)
Air 1.3 0.025
Al2O3 foam 500 0.042
Balsawood 130 0.05
Calcium Silicate 240 0.051
Ceramic fiber blanket 128 0.032
Cork 150 0.043
Fiberfax paper 324 0.03
Foamed glass 144 0.035
Microtherm 240 0.021
Polystyrene (air) 46 0.026
Polystyrene (vacuum) 46 0.0081
Polystyrene (Styrofoam) 30 0.027
Polyurethane (rigid) 40 0.032
Polyurethane (flexible) 60 0.042
Polyurethane (PIR) 160 0.05
Rubber foam 100 0.042
SiO2 foam 160 0.055
Vermiculite (expanded) 300 0.069

A useful tool to obtain information of this type can be found on the Web at www.tak2000.com/data2.htm#thermo

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